Mechanistic approaches for crosstalk between nanomaterials and plants: plant immunomodulation, defense mechanisms, stress resilience, toxicity, and perspectives

Abstract

Plants are challenged with unexpected and diverse environmental stresses in the era of climate changes. Plant development and metabolism are significantly hindered by both abiotic and biotic stresses, which lead to a reduction in the crop yield by 50–88% worldwide. Fortunately, plants have developed diverse defence mechanisms across multiple levels in response to environmental challenges. Plant defence mechanisms range from molecular-level modifications to morphological, physiological, anatomical, and biochemical characteristics. In addition, nanotechnology is a promising area of innovations in the field of plant sciences, and it is generating novel concepts for comprehending the optimal survival mechanism of stressed plants. Nanomaterials are considered regulatory molecules for plants owing to their ability to modulate an extensive array of physiological and biochemical processes, the plant immune system, stress-related gene expression, hormonal regulation, and the activation of anti-oxidative defence systems. However, the intricacies of interactions between nanomaterials and plants in terms of antioxidative and immunomodulatory effects are not yet fully explored. Thus, the present review elucidates the potential antioxidative and immunomodulatory regulation of nanomaterials in plants via an enhanced antioxidative system, reduced oxidative stress levels and reactive oxygen species (ROS) generation, upregulation of defense related gene expression, phytohormone regulation, and miRNA regulation. Further, the toxicity behaviour of nanomaterials in plants and developmental prospects are discussed to provide future directions in the area. Overall, this review provides new insights for the development of nanomaterials with potential immunomodulatory effects in plants for resistance against biotic and abiotic stresses.

---

Ragini Singh

Ragini Singh received her Ph.D. in 2018 from the University of Ahmedabad, India. Currently, she is working as Associate Professor in KL Deemed to be University, Guntur, India. She is a former professor at Liaocheng University, Liaocheng, China. Her current research interest includes the synthesis of nanomaterials and establishing their applications in therapeutic and biosensing purposes. She is a lifetime member of the Materials Research Society of India (MRSI), Indian Society of Nanomedicine, and a senior member of IEEE society. Dr. Singh is the active reviewer of MDPI, 3 Biotech, Biosensors and Bioelectronics, IEEE sensors journal, and Biomedical Optics Express.

Pinky Choudhary

Pinky Choudhary is pursuing her doctoral research at Nanomedicine and Nanotoxicity Research Laboratory, Central University of Rajasthan, India. She obtained her master's degree in biotechnology from Jamia Millia Islamia, New Delhi, India. She has worked as a project associate at Amity University Rajasthan, Jaipur, and focused on developing functional nanomaterials for drug delivery to enhance their therapeutic efficacy. Currently, her research objective revolves around the innovative design and synthesis of metal oxide nanoparticles, tailored for diverse applications in both biomedical and agricultural domains.

Santosh Kumar

Santosh Kumar, currently a professor in the Department of Electronics and Communication Engineering, KL Deemed to be University, Guntur, India, is a former professor at Liaocheng University, Liaocheng, China. He has published more than 380 research articles in national and international SCI journals and conferences. His current research interests include optical fiber sensors, nanophotonics and biophotonics, waveguide, interferometers, and Internet of things. To date, he has reviewed more than 2100 SCI articles. Dr. Kumar is a Fellow of SPIE and a Senior Member of OPTICA and IEEE. He is also Associate Editor of IEEE Sensors Journal and Biomedical Optics Express.

Hemant Kumar Daima

Hemant Kumar Daima is an Indian scientist, academician, and administrator. Daima's group at Central University of Rajasthan is focused on the development of functional nanomaterials for drug/gene delivery, biosensing, management of cancer, control of MDR bacteria, nanozyme activities and medical devices. His research has demonstrated the importance of surface functionalization in regulating nanomaterial exterior corona, which dictates nanomaterials’ interaction at a nano–bio interface. His findings have revealed guiding principles involved in rational nanoparticle design strategies for biomedical and agricultural applications. He is member of numerous scientific/professional bodies and recipient of several international fellowships/awards. He obtained his Ph.D. from RMIT University, Melbourne, Australia.

---

Environmental significance

This study investigates the complex interactions between nanomaterials and plants, casting light on their potential to modulate the plant immune system and regulate antioxidative processes, which is of the utmost environmental significance. Gaining an understanding of these effects facilitates the advancement of sustainable agricultural practices. Through a comprehensive analysis of the pros and cons, including enhanced antioxidative systems and diminished oxidative stress, this study establishes the groundwork for environmentally sustainable implementations of nanomaterials in the agricultural sector. In conclusion, this critical review could potentially pave the way for more environmentally conscious methodologies in the realm of plant-nanomaterial interactions, thereby promoting farm ecosystems that are resilient and sustainable.

1. Introduction

The environmental transformations due to the growing human population and the concurrent climatic changes have adversely impacted nutritive food, biodiversity, and environmental sustainability. Therefore, achieving food security, preserving worldwide biodiversity, and promoting sustainability is necessary, and an urgent attention is required from all the stakeholders. Herein, plant nanoengineering and plant biotechnology are expected to play critical roles as they can provide advanced machinery and technology to enhance crop nutrition, confer resistance against biotic/abiotic stresses, and improve the crop yield. Moreover, recent technological developments can reduce resource consumption and offer real-time sensing of plant stress.^1,2 It is important to state that the adverse effects in the form of pathogen infestation, herbivore predation as well as pest and insect proliferation pose significant obstacles to food preservation and agriculture. Plants, being physiological, genetic, and ecological entities, are capable of enduring the existence of diverse microorganisms, which may be detrimental, neutral, or advantageous.³

As reported by the Food and Agriculture Organization (FAO), 40% of global crops have been damaged by pests, causing high economic losses.⁴ International agencies have declared that almost 50% of the total crop yield in the world has been damaged by biotic stresses. Further, it has been estimated that worldwide biotic and abiotic stresses in plants cause a total loss of billions of dollar every year.⁵ However, the scavenging of toxic oxygen species induced by stress might be associated with a plant's capacity to withstand unfavourable oxidative stress conditions and maintain its productivity. Owing to the multifunctional nature of ROS, cellular regulation of their concentration is critical to prevent oxidative damages and their total elimination. In contrast, plants exhibit a remarkable diversity of defence mechanisms to combat oxidative stress, such as cell organelle-distributed enzymatic and nonenzymatic antioxidant systems. Enzymes like catalase, superoxide dismutase (SOD), and peroxidase are mainly involved in antioxidant defence mechanism. On the other hand, lipid soluble (lycopene, carotenoids, and tocopherol) and water soluble (flavonoids, glutathione, ascorbate, and phenolic compounds) metabolites are nonenzymatic antioxidant.⁶ Interestingly, plants' natural defence response against pathogenesis is contingent on their ability to identify pathogens in an early stage. Indeed, throughout the course of evolution, plants have devised a multitude of defence mechanisms against newly emerging organisms. The initiation of a natural defence response encompasses various mechanisms, such as heightened synthesis of cell walls, accumulation of phenolic compounds, and overexpression of defence-related genes and enzymes.⁷ It has been demonstrated that plants treated with various biotic elicitor molecules can elicit this innate immune response by imitating distinct pathogens.⁸

In recent times, scientific attention has been directed towards investigating alternative crop protection strategies in response to evolving the adaptability of phytopathogens caused by the unregulated use of synthetic chemicals and the growing interest and demand for organic or non-polluted food and crops among health-conscious consumers. In this context, the pursuit of alternative disease management strategies, facilitated by the progress of nanotechnology, has created opportunities for investigating the potential of nanomaterials as a candidate for plant disease control.⁹ In comparison to the classical approach, nanomaterials have been profoundly used in plant pathology due to their long shelf life, tiny size, high efficiency, and easy transportation within the plant cells. Furthermore, the broad applicability of nanomaterials in plant pathology is facilitated by their high efficiency, which also offers environmentally sustainable and efficacious strategies for managing biotic stresses, as shown in Fig. 1. Further, nanotechnology has the potential to mitigate ecological challenges and support agriculture by enabling the sustainable applications of nano-pesticides, nano-fungicides, and nano-bactericides to combat plant diseases.¹⁰ Various nanomaterials like iron oxide (Fe₃O₄), zinc oxide (ZnO), chitosan, copper (Cu), silver (Ag), and silica (SiO₂) nanoparticles are important in managing plant diseases and plant stresses.^11–14 The greatest advantage of nanomaterials is their high activity and larger surface area, which results from their minuscule size. This enhanced efficacy can be applied extensively in the field of plant and human pathology.¹⁵ Additionally, organic solvents in the form of nanomaterials are introduced into fungicides that are less water-soluble. Fungicides that are poorly soluble in water can be transported by nanomaterials, which also increase stability and diminish volatilization.¹⁶ Therefore, continuous research and development in nanotechnology is expected to support and shape the subsequent phase of excellence in the agriculture sector pertaining to genetically modified crops, precision farming techniques, and chemical pesticides.

Fig. 1 Schematic representation of different types of biotic and abiotic stresses on plants, and the potential impact of nanomaterials to counter these stresses together with the possible action mechanisms.

Recently, two manganese-based nanomaterials, i.e., manganese oxide (Mn₃O₄) and manganese dioxide (MnO₂), are compared in order to analyse their effects over radish growth, nutrient availability and antioxidant responses.¹⁷ The nanomaterial's exposure leads to an enhancement in plant length with increased vitamin C and soluble sugar content. It has been shown that MnO₂ nanoparticles at higher concentration cause 58% increase in malondialdehyde (MDA) activity, whereas Mn₃O₄ nanoparticles at the same concentration reduces the MDA activity by 14%. Additionally, MnO₂ and Mn₃O₄ nanoparticles accumulated in plant leaves and roots, respectively, suggesting the importance of nanoparticles' choice depending upon requirements. Next, the application of selenium (Se) nanoparticles enhanced the beneficial microorganisms in the rhizosphere of plants, which further leads to the accumulation of molecular compounds, enhanced photosynthesis (in plant leaves), and carbohydrate content.¹⁸ Se nanoparticles exposure has also shown the enhanced expression of carbohydrate transport-related genes in Brassica chinensis. Recently, the use of urea nanoparticles is demonstrated in field for agricultural application, which minimizes 25% recommended dose in comparison to conventional urea fertilizer.¹⁹

Further, the evaluation of the environmental risks posed by nanomaterials towards resistance and stability must explore the plant-nanomaterial interactions due to the significance of plants in determining ecosystem functions.²⁰ Various reports have demonstrated the effects of nanomaterials over plant's antioxidant enzymatic activity, oxidative stress, photosynthetic processes, and DNA expression. However, variations in plant growth and metabolic functions can be observed in response to nanomaterial exposure.²¹ Contradictory results regarding nanomaterials' effect have also been reported, which majorly depend on plant growth medium, exposure timing, plant species, and experimental parameters, i.e., laboratory or field.²² The positive impact of titanium dioxide (TiO₂) nanoparticles on plants like enhanced antioxidant enzymatic activities, higher photosynthetic rate, increased chlorophyll content, and tolerance of various stress have been demonstrated,^23,24 whereas, other studies show the chlorophyll degradation, generation of ROS and cellular toxicity following TiO₂ nanoparticles exposure.²⁵ The exposure of multi-walled carbon nanotubes (MWCNTs) reduces the growth of tomato and spinach seedling with decreased biomass, though it enhanced the germination and seedling root elongation of wheat.²⁶ When single-walled carbon nanotubes (SWNTs) are surface-functionalized with positively charged polymers, they induce cellular death and become toxic.²⁷ The interplay between nanomaterials and plant cells leads to the modulation of gene expression and related biological pathways, influencing the antioxidant responses, development, and growth of plants.²⁸ As a consequence of the inconsistency between the findings of various studies, it is imperative to meticulously examine the properties of nanomaterials to attain the intended results. Further, the behaviour of same plants towards different varieties of nanomaterials should also be taken into consideration.

In this perspective, field studies and controlled experiments can provide effective insights into the capacity of plants to identify and respond to nanomaterials. Nanomaterials have the potential to enhance plant health and productivity as well as minimise crop loss through efficient and targeted applications such as nano-fertilizers, pesticides, or immune-stimulating agents.²² On the contrary, nanomaterials' physiochemical interaction with different plant varieties may lead to the generation of detrimental molecules, including ROS.²⁹ The ROS can induce oxidative damage³⁰ in plants, followed by lipid peroxidation³¹ and the alternation of ion transport across the membranes.³² Nonetheless, the interactions between nanomaterials and plants are mediated by several factors, including nanomaterial physicochemical characteristics, plant species, and environmental conditions. Furthermore, the entry of nanomaterials into plants is influenced by multiple particle trafficking channels and entry pathways, including atmospheric deposition, direct application, contaminated soil, and accidental release. However, several key regulatory components operate at critical checkpoints to control this entry process. Nanomaterials with suitable pore size can effectively cross the cell wall by acting as a structural sieve and reach the plasma membrane that complements the pore size of the cell wall. Nanomaterials of smaller size enter through the root system, while larger nanomaterials penetrate via openings such as flower stigma, hydathodes, and stomata. Metal-based nanomaterials undergo conversion into reactive metal ions, engage with diverse functional groups within plant cells, and induce changes in their biochemical and metabolic processes. The large surface area of nanomaterials facilitates the adsorption of different molecules and ions from nutrient medium and soil. This interaction between nanomaterials and ion molecules leads to toxicity symptoms, such as chlorosis and wilting, as nanomaterials are taken up and accumulate in the plants.^33,34

The influence of nanomaterial exposure on the plant immune system represents a notable gap in our understanding of their impact. Unlike animals, plants do not possess specialized immune cells; instead, all cells exhibit innate immune reactivity and transmit signals systemically from injured sites.³⁵ Plants have ‘innate immune memory’, which helps in recognizing the re-exposure of foreign particles and activates the immediate protective effect.³⁶ Despite all these, a limited number of articles are available exploring the plant immune and antioxidant responses upon their exposure to nanomaterials. Therefore, in this review, the recent advancements in the immune responses and antioxidant defence systems of plants towards nanomaterials have been discussed along with their potential phytotoxicity effects. Further, an in-depth discussion and outlook have also been deliberated for a clear understanding about this important area and possible future applications.

2. Immunomodulatory effects of nanomaterials, and defense mechanism in plants

Plants lack specific immune cells; however, they can mount a defense against pathogen invasion by activating inducible defense mechanisms that have limited self-reactivity and frequently boost resistance to reinfections.^37,38 The immunity of plants operates independent to specialized tissues or organs; rather, it is a process that can be triggered in any cell. The innate immune response relies on the receptors, which are mostly present inside the plasma membrane. In contrast to specialized phagocytic cells, the particle assimilation can occur via interactions with any cell in plants.³⁹ Endocytosis, channels, and pores are majorly responsible for nanomaterials uptake in plant cells, whereas various physical barriers like plasma membrane, cell wall, and cuticle, inhibit nanomaterial's internalization within the plant cells.⁴⁰ The cell wall serves as the first defense barrier to phytopathogen attack, while the plant defense system can be triggered by pattern-recognition receptors (PRRs), which in turn activates the pattern triggered immunity (PTI).^41,42 Further, PTI has also been employed by plants to fight multicellular pathogens like other parasitic plants, insects and nematodes.⁴³ Here, the resistance proteins serve as the second defense layer, which lead to the activation of effector-triggered immunity (ETI). ETI is characterized by a hypersensitive response capable of safeguarding plants against subsequent infections till 20 days, and this phenomenon is known as ‘systemic acquired resistance (SAR)’.⁴⁴ Plant's natural defense system predominantly relies on the early detection of pathogens. Indeed, throughout the course of evolution, plants have devised a multitude of defense mechanisms against newly emerging organisms. Plants exhibit the induction of natural immune system by increasing the expression of different enzymes and genes involved in plant defense mechanism as well as cell wall synthesis and enhancing the accumulation of phenolic compounds.⁷ The innate immune system of plants can be stimulated by different biotic elicitor molecules, which mimics different pathogen behaviors,⁸ as discussed below.

2.1 Nanomaterials-induced plant immune responses under biotic stresses and their role in plant defense

Plant viruses are considered the most destructive pathogens, and a range of different types of nanomaterials have been employed to counter the plant viruses. Significantly, beyond the direct interaction of nanomaterials with phytopathogens, nanoparticles also trigger plant defense responses in vivo through indirect pathways. The nanoparticles-based plant pathogen defense is facilitated by the activation of diverse signaling pathways and associated mechanisms.⁴⁵

(i) Upon infection with pathogenic microbes, the host's protective mechanism triggers a rapid ROS burst. Owing to the imbalance between ROS and antioxidants, oxidative products are produced (when ROS concentrations surpass a certain threshold). Thus, by inducing oxidative stress, nanomaterials can influence cellular redox homeostasis and contribute to plant defense mechanisms. The application of SiO₂ nanoparticles to cucumber results in the sustained activation of phenylalanine ammonia lyase (PAL) and peroxidases for varying durations, thereby bolstering the plant's resistance to papaya ring spot virus (PRSV) infection.⁴⁶ The application of nickel oxide (NiO) nanoparticles to plants infected with cucumber mosaic virus (CMV) results in an upregulation of peroxidase gene expression in cucumber.⁴⁷ Comparable to healthy controls, Ag nanoparticles may enhance the plant immunity by increasing the antioxidant enzyme activity and decreasing MDA and hydrogen peroxide (H₂O₂) levels. Further, Ag nanoparticles exhibit sustained antibacterial activity against Xanthomonas oryzae.⁴⁸ Antioxidant levels can be increased by CNTs, thereby stimulating plant defense mechanisms against tobacco mosaic virus (TMV) infection. Similarly, the concurrent utilization of Se nanoparticles and Cu nanoparticles diminishes the intensity of Alternaria solani infection through the augmentation of enzymatic and non-enzymatic compounds in leaves. This is accomplished primarily through the stimulation of glutathione peroxidase (GPX), SOD, PAL, and ascorbate peroxidase.⁴⁹

(ii) Plant hormones serve as signaling molecules that regulate resistance responses in plants. Among plant hormones, ethylene, jasmonic acid (JA), and salicylic acid (SA) are primary plant resistance hormones. Typically, each hormone contributes to the plant's immune response against pathogen interference through a complex network of signaling pathways. Specific phytohormone expression depends on specific interactions among plants, pathogens, and nanomaterials.⁵⁰ Treatment of TMV infected tobacco plants with MWCNTs, carbon-60 (C₆₀ or fullerene), TiO₂ or iron(III) oxide (Fe₂O₃) nanoparticles induces a change in the levels of phytohormones; specifically, abscisic acid (ABA) is diminished, while zeatin-riboside (ZR) and brassinosteroids (BR) are increased. Likewise, the treatment of plants with ZnO nanoparticles increases SA and ABA levels by 162% and 517%, respectively, and activates the plant's immune system to prevent viral infection.⁵¹ Similarly, the use of magnesium oxide (MgO) nanoparticles in the roots of tomato plants can stimulate various hormone pathways for the induction of PR1, LoxA, and Osm by SA, JA, and ethylene to promote resistance for Ralstonia solanacearum.⁵²

(iii) Furthermore, disease resistance-associated genes have the potential to serve as molecular diagnostic markers for plant defense signaling pathways. Exposure to various nanoparticles like carbon nanoparticles, SiO₂, and Ag significantly induces the innate immune response in plants and upregulates the defense-related genes like β-1,3-glucanase, TLP, PR1, PR2, and PR5.^53,54 Fe₃O₄ nanoparticles improve plant defense response against TMV by upregulating the PR1 and PR2, SA responsive genes.

(iv) Plants can also be protected from pathogenic microbes by secondary metabolites, i.e., phytoalexins or phytoanticipins. Nanomaterials can sensitize plants to generate specialized metabolites prior to the entry of viral particles as a defense mechanism.^55,56 For example, chitosan (Cs) nanoparticles exposure enhances total phenol content in rice and thus provide resistance against Rhizoctonia solani.⁵⁷ Tomato plant (hypocotyls and roots) can also be protected from R. solanacearum infection upon exposure to MgO nanoparticles, which induces phenoxyl radical production by the deprotonation of the phenolic hydroxyl residue of polyphenols.⁵² PAL is an enzyme that serves as a critical link between the primary metabolism of plants and the phenylpropanoid specialized metabolism. The concentrations and stability of PAL have a direct impact on the levels of specialized metabolites in plants and the degree to which plants can withstand stress. The treatment of PRSV-infected plants with SiO₂ nanoparticles and CMV-infected plants with NiO nanoparticles significantly increase PAL expression.^46,47 The resistance of maize to Fusarium oxysporum and Aspergillus niger can also be enhanced through nano-silica, which is correlated with elevated concentrations of saponins, total phenols, PAL, and polyphenol oxidase.⁵⁸

2.2 Nanomaterials-induced plant innate immune responses and their role in defense

The effect of nanomaterials on the plant immune system has been evaluated using gold (Au) nanoparticles due to their inert and non-toxic nature. Diverse and occasional antagonistic impacts of Au nanoparticles on plants have been documented, wherein the responses of plants to Au nanoparticles are dependent upon particular circumstances and interactions.^59,60 However, the effects of stable and well-characterized Au nanoparticles on the immune system of plant have been the subject of a restricted number of studies, and the majority have concentrated on oxidative burst phenomena. The immunostimulatory effect of Au nanoparticles on Arabidopsis thaliana with growth-promoting effect rather than stress induction has been reported.⁶¹ Utilizing moderate concentrations of Au nanoparticles on plants can enhance the growth characteristics, including longer primary roots, an increasing number of lateral roots, and a larger diameter of rosette. Additionally, the plants exhibit reduced oxidative stress activity in response to the immune-stimulating PAMP flg22 (pathogen-associated molecular pattern). Lipid peroxidation and ROS generation within the apoplast are customary cellular occurrences that are initiated by the plant surveillance system, which employs pattern-triggered immunity to identify persistent molecular patterns (M/PAMPs) associated with microbes or pathogens via cell surface-located pattern-recognition receptors (PRRs).⁶² Specifically, the genes involved in defense response, metal response, disease resistance, and response to oxidative stress exhibit downregulation following short-term exposure with increase in growth response genes. This indicates that exposure to Au nanoparticles results in detrimental effects on the immune and oxidative stress responses. This indicates that the equilibrium between immunity and growth may shift towards growth due to the presence of Au nanoparticles. Likewise, it was found that Au nanoparticles alone cannot induce classical plant defense activity. Additionally, increasing the concentration of Au nanoparticles leads to reduced ROS production, which is yielded in response of bacterial flagellin as PAMP. In agreement to above, the lipid peroxidation result has also been recorded in accordance with the reduction of PAMP-triggered ROS burst.^61,63 Au nanoparticles at a concentration of 80 mg L⁻¹ show enhanced ROS scavenging activity in Arabidopsis seedling by upregulating the defense-related enzymes,⁶⁴ whereas a high concentration of Au nanoparticles (100–400 mg L⁻¹) exposure leads to reduced growth, which might be attributed to high free radical production. The short-term exposure (6 h) of Au nanoparticles at 10 mg L⁻¹ leads to the downregulation of peroxidase at both transcriptome and proteome levels. All these reveal that Au nanoparticles can effectively decrease the stress induced by immune stimulatory peptides. Comparable to the consequences observed in the case of exogenous Au nanoparticles exposure, the intracellular formation of Au nanoparticles and treatment with Au ions result in the downregulation of genes governing oxidative stress and defense responses.⁶⁵ The different interaction of Au nanoparticles with plants has been depicted in Fig. 2.

Fig. 2 Gold nanoparticles (AuNPs) and their possible interactions with plants. Au nanoparticles are capable of penetrating plants via natural leaf apertures or absorption via the roots. (a) The cell wall' pore size, ranging from 3 to 6 nm, acts as a barrier, preventing the passage of pathogens and large engineered nanomaterials to the plasma membrane and uptake apparatus. (b) The Au nanoparticles are enveloped by a bio-corona consisting of molecules adsorbed by the environment. The bio-corona has the potential to modify the cell wall barrier capability and facilitate passage. (c) Au nanoparticles can cross the cell wall via endocytosis, apertures, or channels across the plasma membrane. Despite the absence of uptake, physiological alterations in plants exposed to Au nanoparticles can be identified. (d) Potential causes include cell surface pattern recognition receptors recognizing Au nanoparticles and their effects; (e) blockage of cell wall or membrane pores; or (f) disruption of external environmental factors (pathogen-associated molecular patterns, PAMPs), and (g) stress responses as a whole or more particular immune responses may be directly or indirectly impacted. Adapted with permission from ref. 39.

Chitosan exposure is well known to elicit the plant's systemic resistance; however, its insoluble behavior in aqueous environment limits its application. Thus, chitosan nanoparticles have been used as plant defense elicitor with significantly high immunomodulatory effect at 10 times lower dose in comparison to chitosan.⁹ In comparison to chitosan, the chitosan nanoparticles require less time to provide resistance against pathogens, which might be attributed to their nano size, and facilitates rapid movement through tissues and develop the necessary defence action.⁶⁶ The activity of various defence related enzymes, i.e., catalase, SOD, peroxidase, phenylalanine amino lyase, and polyphenol oxidase, in plants has also been upregulated following exposure to chitosan nanoparticles and reached its maximum level upon exposure to pathogen. One of the studies has demonstrated the induction and augmentation effect of chitosan nanoparticles on the immune system of Camellia sinensis (tea).⁹ Revealing chitosan nanoparticles to leaves induces and upregulates the activity of defence related enzymes, gene expression, various antioxidant enzymes, and level of total phenolic components. Additionally, chitosan nanoparticles exposure elevates the level of nitric oxide, which is an important plant defence signalling molecule, whereas, upon the inhibition of nitric oxide production, the immune stimulatory effect of chitosan nanoparticles also mitigates, thus proving the crucial role of nitric oxide in the stimulation of the immune system. It has been suggested that exposure to chitosan nanoparticles shows a remarkable enhancement in defence response by enhancing the defence enzyme accumulation like β-1,3-glucanase, peroxidase, thaumatin like protein (TLP), PAL, and polyphenol oxidase. Here, β-1,3-glucanase enzyme and TLP are reported to provide protective effect against pathogens and enhance immunity against pathogens including insects.⁶⁷ The results together suggest that in comparison to natural chitosan, chitosan nanoparticles play a more effective role as a disease controlling agent or phytosanitary.

Mohamed et al. have proven the comparative effect of Cu nanoparticles and chitosan nanoparticles to evaluate their potential effect in modulation of date palm (Phoenix dactylifera L.) innate immune system.⁶⁸ It has been observed that exposure of both Cu and chitosan nanoparticles significantly enhances the innate immune system of the plant. The exposure of Cu nanoparticles at a concentration of 1 g L⁻¹ reduces the total phenol level by 28.7% and enhances the level of antioxidant enzymes like catalase and peroxidase by 4.15% and 199%, respectively, whereas the treatment of chitosan nanoparticles at the same concentration and under the same conditions increases the level of total phenols, catalase, and peroxidase enzyme by 7.83%, 12.4%, and 12.5%, respectively. The result confirms the applicability of both Cu and chitosan nanoparticles in enhancing the innate immune response of date palm seedling. In comparison to chitosan nanoparticles, Cu nanoparticles show higher potency in enhancing the peroxidase level, which may be attributed to Cu since copper acts as a cofactor of peroxidase enzyme, whereas compared to Cu nanoparticles, chitosan nanoparticles are majorly involved in enhancing the effect of catalase enzyme, which shows response to nitric oxide production.

Further, the influence of Ag nano-formulations on the enhancement of plant innate immune system, expression of plant defence modulator, and to provide resistance against fungal infection in rice plant has been demonstrated.⁶⁹ Eco-friendly Ag nanoparticles were formulated using turmeric rhizome extract, as shown in Fig. 3A. The exposure of Ag nanoparticles at a concentration of 100 ppm inhibits the growth of Rhizoctonia solani both in vitro and in vivo up to 82% and 77%, respectively. Further, Ag nanoparticles treatment enhances ROS accumulation, and it reduces the mitochondrial membrane potential (MMP) in fungal mycelia by 1.5-fold and 2-fold, correspondingly (Fig. 3B). Cytochrome oxidase 3 and NADH dehydrogenase 5 expression has been upregulated by 2-fold and 5.25-fold. Additionally, Ag nanoparticles exposure also upregulates the activity of PAL, peroxidase and SOD enzyme, as shown in Fig. 3C, as well as the gene expression of PAL and pathogenesis related protein (PR-protein). The root system of nano-primed rice seedlings is robust, with a 2.5-fold increase in callose accumulation. Collectively, the findings indicate that Ag nanoparticles impede Rhizoctonia growth while concurrently enhancing the innate immunity of plants, enabling them to develop resistance against the fungal pathogen.

Fig. 3 Panel-A. Schematic representation of the green synthesis of Ag nanoparticles and their application in resistance against pathogens. Panel-B. Images of reactive oxygen species (ROS) and mitochondrial membrane potential (MMP) estimation of mycelia post-treatment with turmeric rhizome extract, silver ions, and Ag nanoparticles. Panel-C. Phenylalanine ammonia lyase (PAL), peroxidase and superoxide dismutase (SOD) activity assessment in seeds primed with Ag nanoparticles (100 ppm), and 100 ppm turmeric rhizome extract. Reprinted with permission from ref. 69. Panel-D. DAB staining for H₂O₂ detection in DI water and Fe₃O₄ nanoparticles-treated leaves, and Panel-E. Catalase and peroxidase enzyme content in plant treated with DI water and Fe₃O₄ nanoparticles treatment. Reprinted with permission from ref. 70.

Likewise, the absorption and physiological impacts of Fe₃O₄ nanoparticles has been investigated along with the plant's resistance response to TMV subsequent to foliar application.⁷⁰ To determine the potential of Fe₃O₄ nanoparticles as an elicitor for inducing innate immunity in plants, authors have assessed the cellular defence responses of plants treated with Fe₃O₄ nanoparticles. The findings indicated that applying Fe₃O₄ nanoparticles to tobacco leaves through foliar treatment triggers the generation of ROS, as shown in Fig. 3D-i. An abundance of research and data indicate that a significant rise in ROS signifies a critical resistance mechanism in plants against both biotic and abiotic stresses.⁷¹ The activities of the two antioxidant enzymes, i.e., catalase and peroxides has been enhanced by Fe₃O₄ nanoparticles. It has been shown that in comparison to the deionized (DI) water group, the catalase and peroxidase activities significantly increase by 40.18% and 32.37%, respectively, upon exposure to Fe₃O₄ nanoparticles (Fig. 3D-ii). Taking cues from elevated levels of H₂O₂ accumulation and heightened antioxidant enzyme activity, it can be postulated that the activation of the tobacco plant's elongator-regulated immune system is induced by Fe₃O₄ nanoparticles, considering the current results and previous studies.

Moreover, the effectiveness of chitosan nanoparticles against downy mildew disease caused by Sclerospora graminicola in pearl millet is also assessed.⁶⁶ In contrast to the control group, investigations have discovered that seed treatment with chitosan nanoparticles substantially increases the germination percentage and seedling vigour of pearl millet. Compared to the untreated control group, seed treatment with chitosan nanoparticles initiates systemic and enduring resistance, providing substantial protection against downy mildew in greenhouse conditions. Herein, the gene expression profiles were found to be significantly upregulated in response to seed treatment with chitosan nanoparticles, explicitly for catalase, polyphenoloxidase, PAL, peroxidase, and SOD. The expression profiles and activation of pathogenesis-related proteins PR-1, PR-2, and PR-5 have been traditionally recognized as vital markers for systemic acquired resistance governed by the SA pathway. The findings demonstrate that seedlings treated with chitosan nanoparticles exhibit early and substantially improved activation of both PR-1 and PR-5 in comparison to the untreated seedlings. Moreover, the elevation in activity is more noticeable after the introduction of the downy mildew pathogen. Here, the resistance induction by chitosan nanoparticles was mediated by nitric oxide signalling; pearl millet seedlings treated with nano-chitosan produced more nitric oxide than the untreated control; seedlings treated with nano-chitosan in combination with the nitric oxide scavenger cPTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide] produced significantly less nitric oxide. The observed discrepancy in nitric oxide production between seedlings treated with nano-chitosan and chitosan nanoparticles and cPTIO-treated exhibited a positive correlation with the expression levels of PR proteins and diverse defence enzymes, as well as the level of downy mildew resistance attained in field conditions. Additionally, a comparison between chitosan nanoparticles and chitosan demonstrated that even the most minute quantity of chitosan nanoparticles exhibits comparable efficacy to the recommended dose of chitosan in the management of downy mildew. Another study indicates that the introduction of chitosan nanoparticles into leaves may have induced a defence mechanism that inhibits the pathogen's development in leaves.⁷² Tomato early blight and Fusarium wilt have been substantially mitigated by Cu–chitosan nanoparticles.⁷³ Moreover, the seed-causing blast disease of finger millet (Eleusine coracana) and rice have been inhibited by chitosan nanoparticles.⁷²

2.3 Nanomaterials-induced RNA silencing in plants and their role in plant defense

RNA silencing, also referred as RNA interference (RNAi), is an additional antiviral defense mechanism of plants. It is triggered by double-strand RNA (dsRNA), a replication intermediate generated by RNA virus during plant infection. The commencement of RNA-silencing pathways happens as viral dsRNA undergoes cleavage, forming duplexes of small interfering RNA (siRNA) or microRNA (miRNA). The argonaute protein, a component of the RNA-induced silencing complex (RISC), facilitates the unwinding of double-stranded siRNAs, thus producing an antisense strand and a sense strand. An RISC that incorporates the antisense strand identifies a complementary mRNA transcript through base-pairing interactions, leading to mRNA degradation and, consequently, the inhibition of protein biosynthesis,⁷⁴ while the sense strand undergoes degradation. The induction of silencing effects occur via cytoplasmic RNA viruses, and this process can be transferred to endogenous plant genes that possess similar sequences.⁷⁴ In mammals, an analogous antiviral effect against the encephalomyocarditis virus, Nodamura virus, and influenza A virus has been reported in the recent past.^75–77 Mammalian antiviral RNA interference is not restricted to specific cell types or viruses, therefore opening new avenues for the development of RNA interference-based therapies. RNA interference delivery has been shown to be effective in suppressing genes associated with cancer and several viral diseases.⁷⁸ However, the delivery and stability of RNAi molecules have been proven to be formidable barriers to therapeutic efficacy. Thus, RNA modification and nanoparticle formulation have demonstrated their utility in the non-viral transportation of RNAi molecules.⁷⁹

In a few cases, the regulatory function of miRNAs in relation to exposure to nanoparticles has been documented.⁸⁰ A class of small regulatory RNAs such as miRNAs regulates genes post-transcriptionally, and they are crucial in governing plant responses to numerous environmental stresses including exposure to nanomaterials. Interestingly, nanomaterials can modulate the expression levels of plant miRNAs. Plants can be protected by miRNAs against biotic stress, including pathogens that cause powdery mildew. Hydroponic cultivation has been conducted on ‘KWS Olof’ and ‘Marthe’ varieties of barley (Hordeum vulgare L.) treated with Fe₃O₄ and copper oxide (CuO) nanoparticles. An examination on the expression of miR156a, plant morphology, and genotoxicity showed that different concentrations of CuO nanoparticles increased the miR156a expression exclusively in the Marthe variety, thereby enhancing their resistance. In contrast, the treatment of the KWS Olof variety with CuO nanoparticles shows a reduction in plant resistance and an adverse impact on miRNA expression. When two distinct varieties of barley are treated with Fe₃O₄ nanoparticles at different concentrations, the KWS Olof variety exhibited a favorable response, whereby miRNA expression levels are increased at 35 mg L⁻¹ and 70 mg L⁻¹, ultimately leading to enhanced plant resistance. An investigation confirmed the upregulation of WRKY27, a regulator of ABA phytohormone biosynthesis and the signaling pathway, in the leaves of Glycine max L. when subjected to dehydration stress.⁸¹ Under harsh conditions, it has been established that endogenous phytohormone levels can be maintained through the overexpression of transcription factors, miRNAs, and genes involved in signaling and biosynthesis using nanoparticles. However, additional research is required to delve into the intricate mechanisms governing nanoparticles and phytohormones.

2.4 Nanomaterials-induced phytohormones-mediated immune responses and their role in plants defense

The physiological performance of plants is directly correlated with the modulation of their hormone reservoirs, and these signaling molecules are also intimately linked to the regulation of plant growth in response to abiotic and biotic stresses.⁸² Under stressful conditions, plants develop a defense mechanism that increases their tolerance capacity through the activation of diverse plant growth regulators. Plant hormones like, ABA, ethylene, SA, and JA play crucial roles in overseeing plant defense mechanisms against a spectrum of biotic and abiotic stresses.⁸³ The assessment of plant toxicity is frequently reliant on the concentration and performance of hormones.⁸⁴ The complex web of interplay between nanoparticles and phytohormone signaling is just beginning to be comprehended. Exposure to nanomaterials induces synergistic or antagonistic crosstalk between plant hormones and nanomaterials, which is essential for the stress response of plants.^85,86 The mechanism by which nanomaterials regulate the expression of genes associated with phytohormones that are crucial for plant immunity has yet to be investigated in greater detail. Current understanding indicates that ROS production is a crucial process in modulating plant development and defense responses.⁸⁶ Abiotic and biotic stressors disrupt the equilibrium between ROS elimination and production in plants, which is maintained under typical metabolic circumstances. Optimal ROS concentration promotes the biosynthesis of phytohormones such as JA, ethylene, ABS, and SA. However, under unfavorable conditions, plant hormones are identified as catalysts for plant development by promoting the activation of the plant's defense mechanism and inhibiting excess ROS.⁸⁷

SA is produced via the shikimate pathway, and it is an important defense-related plant hormone essential for plant immunity regulation. It is also important for modulating seed germination, fruit yield, DNA damage and repair, and resistance to abiotic stress. SA is a crucial signaling compound that is accountable for the activation of pathogenesis-related genes, and it plays a major role in systemic acquired resistance in plants.⁸⁸ It has been reported that nanomaterials can influence the endogenous concentration of SA in plants. Results have proven an significant increase in SA content in the foliage of A. thaliana upon exposure to ZnO nanoparticles.⁸⁹ Further, in order to ascertain the role of Fe₃O₄ nanoparticles as an elicitor to stimulate innate immunity in plants, Cai et al. have assessed the cellular defense responses of Nicotiana benthamiana treated with Fe₃O₄ nanoparticles, and it was revealed to be phytohormone-mediated defense in response of nanoparticles exposure.⁷⁰ As reported in Fig. 4A-(i), the Fe₃O₄ nanoparticles' exposure to tobacco leaves enhanced the abscisic acid and SA phytohormones level by 223.27% and 292.52%, respectively, whereas the level of JA did not get altered after nanoparticles exposure. Undoubtedly, the synthesis of SA was increased by Fe₃O₄ nanoparticles, demonstrating a persistent tendency towards the SA-mediated defense genes. To analyze the potential mechanism by which Fe₃O₄ nanoparticles regulate plant resistance, authors examined the genes governing resistance at the transcript levels, including salicylic acid-responsive PR genes (PR1 and PR2) that are responsible for inducing the upregulation of signaling molecules in response to a variety of abiotic stresses (PR1 and PR2). Explicitly, the application of Fe₃O₄ nanoparticles resulted in a 7.37% and 32.80% increase in the expression levels of PR1 and PR2, respectively. As stated in Fig. 4A-(ii), the expression levels of these proteins are considerably high in tobacco compared to DI water (0.987 and 1.01, respectively). In contrast, the infection did not exhibit any inhibition after inoculation with the pretreated TMV mixtures. Therefore, Fe₃O₄ nanoparticles deposition resulted in the buildup of endogenous SA, and this accumulation is found to be associated with the plant's resistance to TMV infection. Notably, the expression of these genes would manifest as proteins that operate to further obstruct the plasmodesmata, thereby impeding the ingress and dissemination of phytopathogens within the plant.

Fig. 4 Panel A. Effect of Fe₃O₄ nanoparticle exposure on (i) phytohormones content and (ii) salicylic acid (SA)-regulated PR1 and PR2 genes. Reprinted with permission from ref. 70. Panel B. SiO₂ nanoparticles function by gradually releasing Si(OH)₄ into cells, thereby inducing SA, local defense, and SAR; they obstruct stomata, thereby inducing SA and subsequent defenses. Panel C. The gene expression of the salicylic acid-regulated (i) PR-1 and (ii) PR-5 genes of A. thaliana analyzed using RT-qPCR in wild-type Arabidopsis in response to SiO₂ nanoparticles and Si(OH)₄ exposure for 48 h. Reprinted with permission from ref. 54.

Further, as illustrated in Fig. 4B, salicylic acid-dependent plant immunity has also been activated in response to SiO₂ nanoparticles. This is partially due to the slow release of orthosilicic acid [Si(OH)₄] from nanoparticles entering the plant through the stomata and distributing within the spongy mesophyll and possibly due to salicylic acid-dependent response induced by intact nanoparticles.⁵⁴ SiO₂ exposure induces the salicylic acid-responsive marker genes PR protein (PR1 and PR5). Analogous to the impact of avirulent P. syringae treatment, the introduction of SiO₂ nanoparticles and Si(OH)₄ led to a 6-fold and 30-fold rise in the transcript abundance of AtPR-1 and AtPR-5, respectively, in comparison to control treatments (Fig. 4C, i and ii). Therefore, defense reactions dependent on SA are triggered by both SiO₂ nanoparticles and Si(OH)₄. While the induction effect of SiO₂ nanoparticles was comparatively lower than that of avirulent P. syringae-infiltrated plants and Si(OH)₄-treated plants, it is still adequate to grant SAR. With the application of ZnO nanoparticles, there is an observed elevation in both endogenous SA levels and the expression of the salicylic acid-binding protein 2 (SABP2) gene in tobacco plants. The introduction of SA enhances the germination rate of tobacco seeds, leading to improved chlorophyll content and increased root length in tobacco seedlings. Treatment with ZnO nanoparticles and the overexpression of the LcSABP2 gene in transgenic tobacco result in a significant boost in SA levels.⁹⁰ Compared to wild-type plants, transgenic tobacco plants treated with ZnO nanoparticles exhibit enhanced chlorophyll content, net photosynthetic rate, related gene expression, and increased activities of SOD, peroxidase, and catalase, and decreased H₂O₂, O²⁻, and MDA contents. The outcomes suggest that LcSABP2 could potentially regulate plant resistance to ZnO nanoparticles toxicity by increasing endogenous SA levels, facilitating the operation of the photosynthetic system, and upregulating the antioxidant enzymes activity.

The other phytohormone JA serves as a pivotal signaling molecule, playing a crucial role in initiating the antioxidant defense system (peroxidase, superoxide anion radical, NADPH-oxidase) under stress conditions.⁹¹ On exposure to nanomaterials, it functions as a stress inducer and alters the endogenous concentration of JA within plants. SA and JA production in the shoots has been modulated by copper sulfide (CuS) nanoparticles, thereby enhancing the plant's defense mechanisms against G. fujikuroi infection.⁹² According to a study, grey mold disease was more prevalent in JA-deficient mutant tomatoes compared to the wild-type, indicating that JA plays a crucial role in disease suppression.⁹³ Results indicate that the CuS nanoparticles treated seeds with high JA concentration may enhance the phytohormone's ability to eradicate pathogens, leading to a subsequent decrease in the occurrence of rice's bakanae disease.

Abscisic acid is acknowledged for its capacity to modulate both the SA and JA pathways, which in turn facilitates a suitable immune response against invading micro-organisms.⁹⁴ The elevated concentration of abscisic acid can activate the JA signaling pathway during fungal infections. By modulating phytohormone concentrations, Cu-based nanoparticles appear to function as a positive promoter as an abiotic effector, aiding the plant in its defense against pathogenic infection. By altering the concentration of endogenous hormones, nanomaterials increase the resistance of plants to fungal infections. Collectively, Cu-based nanoparticles facilitate intercellular communication among jasmonic acid, salicylic acid, and abscisic acid, regulate the plant's reaction to G. fujikuroi infection, and balance the immune response while stimulating plant development.⁹⁵

2.5 Nanomaterials-induced rhizospheric microorganisms-mediated plant immune responses and their role in defense

Rhizosphere serves as a place to provide interaction among microorganisms and plants. Rhizosphere soil exhibits the greatest diversity and activity of microorganisms. Plant health and growth are influenced by the composition and activities of the rhizosphere microbial community.⁹⁶ Rhizospheric microorganisms are reported to enhance the plant growth via several mechanisms like nitrogen fixation, phosphate solubilization, phytohormone regulation, and enzyme regulation in favor of nutrition provided by plants.⁹⁷ Biogenic nanoparticles are well known to interact with the various microorganisms present in the rhizosphere and thus provide resistance against plant disease. The concentration-dependent effect of Au nanoparticles over phytohormone indole-3-acetic acid (IAA) production from rhizospheric Pseudomonas monteilii as well as plant probiotic effect have been demonstrated⁹⁸ to show a significant increase in IAA production with increasing concentration of Au nanoparticles with maximum production at 50 μg mL⁻¹. Additionally, an increased growth of the plant has been observed when its seedling was treated with Au nanoparticles. Likewise, it has been shown that the presence of rhizospheric microorganisms can significantly alleviate the stress induced by molybdenum (Mo) nanoparticles in soybean plant,⁹⁹ wherein the exposure of Mo nanoparticles induces significant toxic effect in soybean plant in terms of reduction in plant height and biomass, with complete loss of nitrogen reduction capacity. It has been revealed that the addition of several rhizospheric bacterium can circumvent the Mo nanoparticles toxic effect with improved nitrogen fixation ability, enhanced root/shoot lengths and decreased Mo content in leaves and stem. Sequencing result revealed that Mo nanoparticles exposure can significantly affect the structure of microbial community. Further, it is observed that the presence of abundant Flavisolibacter and Caulobacter plays a crucial role in relieving stress in soybean plant. Iron is another crucial element in the establishment of microbial niches in the rhizosphere.¹⁰⁰ The positive impact of Fe nanoparticles on nodulation and nitrogen fixation within the rhizobium legume symbiosis system has been validated.¹⁰¹ Furthermore, ureides serve as the primary nitrogen carriers from nodules to roots and foliage in legumes and found to be increased in Fe nanoparticles-treated plant. Fe nanoparticles are observed to exhibit greater beneficial effects on alfalfa growth and nitrogen fixation capacity than iron(II) chloride (FeCl₂) at optimal 10 mg L⁻¹ concentration by seed soaking plus leaf sprinkling method. Moreover, co-occurrence networks and the correlation of fungal communities in Fe nanoparticles were more intricate in comparison to FeCl₂, whereas Fe nanoparticles attracted a greater number of beneficial rhizosphere microorganisms.

In addition to other nanomaterials, Se nanoparticles have also attracted considerable attention for enhancing crop health and plant growth. This is primarily attributed to their exceptional bioavailability, bioactivity, and safety, when compared to alternative Se forms, including sodium selenite and selenate. Se nanoparticles could substantially improve the abundance of Pseudomonas and Bacillus in the rhizosphere of Brassica chinensis. By regulating the crosstalk between root exudates and rhizobacteria in the rhizosphere, the soil application of Se nanoparticles has significantly enhanced the growth of rice.¹⁰² Furthermore, the structure of rhizosphere microbial communities is notably altered by the nanomaterials. The greatest increase in the relative abundances of Streptomyces and Sphingomonas is reported to occur during the third week, whereas the relative abundance of Pseudomonas is notably increased by Se nanomaterials during the fourth week, while Bacillus' abundance increased considerably during the fifth week. Additionally, it is shown that Se nanomaterials recruit Bacillus and Pseudomonas and directly increase malic and citric acid secretion from rice roots by upregulating the biosynthesis and transporter genes of organic acids in plants. Furthermore, the expression of chemotaxis and flagellar genes in motile Sphingomonas is increased by Se nanoparticles in order to facilitate greater interaction with rice plants, which stimulates rice growth and secretion of root exudates. The coordinated control of rhizosphere microorganisms and root exudates facilitates increased nutrient mobilization and absorption, thereby fostering rice growth.

Nevertheless, there is significant concern regarding the potential impacts of engineered metal oxide nanoparticles on bacterial nitrogen fixation. The effect and mechanism of increasing concentrations of metal oxide nanoparticles, specifically TiO₂, aluminium oxide (Al₂O₃), and ZnO nanoparticles on nitrogenase activity, has been investigated.¹⁰³Pseudomonas stutzeri was utilized in the associative rhizosphere nitrogen-fixing bacteria experiment. The inhibitory effect of metal oxide nanoparticles on nitrogen fixation capacity increased with the increasing concentrations of TiO₂, Al₂O₃, and ZnO nanoparticles. The introduction of metal oxide nanoparticles substantially inhibits the expression of nitrogenase synthesis-related genes, including nifA and nifH, as determined by real-time qPCR. Herein, ROS not only altered the permeability of the membrane but also inhibits the expression of nifA and biofilm formation on the surface of the roots. Metal oxide nanoparticles could induce an intracellular ROS eruption. The inhibition of nif-specific gene transcriptional activation by the repressed nifA gene and the reduction in root surface biofilm formation caused by ROS had an adverse impact on the plant's resistance to environmental stress. These findings indicated that the utilization of metal oxide nanoparticles in the rice rhizosphere impedes bacterial biofilm formation and nitrogen fixation, which may potentially disrupt the nitrogen cycle within the bacteria-rice system.

3. Nanomaterials as regulators: enhancing plant growth and mitigating plant abiotic stress

In recent years, tremendous efforts have been dedicated to improving stress tolerance in crops, addressing challenges faced by the growing world population without harming the environment and living systems. Various biotic and abiotic factors, including drought, salinity, temperature fluctuations, and exposure to heavy metals, induce stress, resulting in substantial alterations in plant physiology. Plants employ diverse strategies to bolster their stress tolerance, involving the up-regulation of protective elements, both functional and structural. This includes the synthesis of compatible solutes and antioxidants.¹⁰⁴ In agriculture sectors, nanotechnology has emerged as a promising strategy, demonstrating the potential to impart tolerance against stress, enhancing productivity, and boosting plant morphologies when employed as herbicides, nano-pesticides, and nano-fertilizers. Due to their sessile nature, plants confront various environmental stressors such as salinity, drought, temperature extremes, heavy metals, flooding, fluctuating light intensities (high and low), and ultraviolet (UV) radiations. Plants consistently generate ROS within diverse cellular compartments, such as chloroplasts, mitochondria, and peroxisomes, as part of metabolic processes like respiration and photosynthesis. At lower concentrations, ROS functions as signaling molecules, actively engaging in processes such as growth, development, and defense. However, under stress conditions, elevated ROS levels in plants can cause damage to cell membranes, DNA, proteins, and other cellular components.¹⁰⁵ Various reports have shown that the utilization of antioxidant enzyme-mimicking nanomaterials promote plant resistance to abiotic stress and mitigate crop yield losses by augmenting the plant's ROS-scavenging potential. In recent years, several studies have highlighted that under abiotic stress conditions, nanomaterials, including cerium(IV) oxide (CeO₂), C₆₀, and Fe₂O₃ demonstrate ROS scavenging capabilities due to their inherent enzyme-like behavior, and such nanomaterials are identified as ‘nanozymes’. Additionally, nanoparticle-engineered plants exhibit enhanced resistance to stress conditions.¹⁰⁶

Other than this, comprehensive investigations have also been conducted on the role of metal (Au, Ag, Cu, Al, and Fe) and metal oxide-based (TiO₂, CeO₂, Fe₂O₃, Al₂O₃, and ZnO) nanomaterials in crop improvement. Most of these studies have centered on enhancing plants resilience against abiotic and biotic stress conditions.^107–109 To combat these stressful conditions, plants employ adaptive measures at morphological, physiological, biochemical, and molecular levels. Scientific literature suggests that nanoparticles facilitate plants to overcome adverse environmental conditions in a concentration-dependent manner and influence plant growth and development.¹¹⁰Table 1 describes the role of various nanomaterials in treating plant abiotic stress condition.

Table 1 Various categories of nanomaterials and their involvement in mitigating abiotic stresses in plant species, along with the responses elicited by these nanomaterials

Nanomaterials	Plant species	Abiotic stress	Responses	Ref.
TiO2 nanoparticles	Triticum aestivum	Drought	Enhanced growth and yield, higher gluten and starch content of wheat	127
Fe nanoparticles	Carthamus tinctorius		Lower drought effect, and higher yield due to foliar application	172
TiO2 nanoparticles	Linum usitatissimum		Improved carotenoids and chlorophyll, higher growth, and yield, reduced H2O2 and malondialdehyde	173
SiO2 nanoparticles	Crataegus sp.		Higher photosynthetic rate, and plant biomass, good stomatal conductance, minor effect on plant chlorophyll and carotenoid	126
Zerovalent Fe nanoparticles	Arabidopsis thaliana		Stimulation of plasma membrane H^+-ATPase, stomatal opening, improved chlorophyll and biomass, normal drought sensitivity, intensified CO2 assimilation	123
Ag nanoparticles	Lens culinaris		Decreased adverse effects on germination rate and germination percentage, enhanced root length, root fresh and dry weights	174
Fullerenol	Beta vulgaris		Drought effects disappeared after foliar application	121
Chitosan nanoparticles	Hordeum vulgare L.		Enhanced relative water content, higher weight and protein in grains, proline levels, and induced SOD and catalase activities	175
SiO2 nanoparticles	Ocimum basilicum	Salinity	Fresh and dry weight enhancement, higher chlorophyll, and proline	176
CuO nanoparticles	Triticum aestivum		Better root and shoot growth	177
ZnO and Fe3O4 nanoparticles	Moringa peregrina		Na^+ and Cl^− reduction, improved N, P, K^+, Ca^2+, Mg^2+, Fe, Zn, total chlorophyll, carotenoids, carbohydrates, proline, crude protein, and higher enzymatic and non-enzymatic antioxidants	178
ZnO nanoparticles	Helianthus annuus L.		Better growth, higher net CO2 assimilation rate, sub-stomatal CO2 concentration, chlorophyll, Fv/Fm and Zn content, and reduced Na^+ in leaves	179
Fe2O3 nanoparticles	Mentha piperita L.		Declined accumulation of ROS and proline	180
Mn nanoparticles	Capsicum annum L.		Redistributed potassium, sodium, manganese, and calcium in root and shoot	181
Cu nanoparticles	Solanum lycopersicum		Improved lycopene, and carotenoid, higher SOD activity	182
ZnO nanoparticles	Lupinus termis		Modulate growth, antioxidant responses, and photosynthesis	107
ZnO nanoparticles	Lycopersicon esculentum		Enhanced roots and shoots growth, higher biomass, chlorophyll, protein levels, and photosynthetic parameters, and good catalase, SOD, and peroxidase activities	183
ZnO nanoparticles	Trigonella foenum-graecum		Upregulated levels of protein and proline, improved antioxidants activities, decreased H2O2 and malondialdehyde levels	184
CeO nanoparticles	Gossypium hirsutum L.		Ionic equilibrium, greater root growth, decreased ROS generation	144
Fe3O4 nanoparticles	Fragaria x ananassa Duch		Upgraded growth, pigment, relative water content, total soluble sugar; increased membrane stability	185
TiO2 nanoparticles	Cicer arietinum L.	Cold	Improved Rubisco expression and chlorophyll binding protein genes, declined H2O2, enhanced phosphoenolpyruvate carboxylase activity	186
SiO2 nanoparticles	Agropyron elongatum L.		Overcame seed dormancy, greater seed germination and seedling weight	187
Ag nanoparticles	Triticum aestivum	Heat	Protection against heat and improved plant growth	151
Multi-walled carbon nanotubes	Lycopersicon esculentum		Upregulated expression of stress-linked genes including HSP90	188
TiO2 nanoparticles	Glycine max L.	Cadmium (Cd)	Better uptake of Cd, lower Cd-toxicity, better chlorophyll and relative water content, photosynthetic rate, and growth parameters, declined lipid peroxidation and proline	189
Graphene oxide nanoparticles	Microcystis aeruginosa	Cd	Escalated Cd toxicity symptoms	190
Na2SiO3 nanoparticles	Pisum sativum L.	Cr(VI)	Pea seedlings defense against Cr(VI) phytotoxicity, lower Cr(VI) uptake and oxidative stress, up-regulated antioxidant defense system and increased nutrient elements led to improved growth	191
Fe nanoparticles	Oryza sativa	Arsenic (As) stress	Lowered arsenic uptake and oxidative stress	161
ZnO nanoparticles	Oryza sativa	Cu and lead (Pb)	Reduced metal uptake	192
Cu nanoparticles	Triticum aestivum L	Cd	Increased weight and growth, reduced Cd accumulation, raised ion contents and antioxidative properties	193
Cu nanoparticles	Triticum aestivum L	Chromium (Cr)	Greater biomass and growth, decreased Cr uptake, increased uptake of nutrient and higher antioxidative properties	194
ZnO nanoparticles	Triticum aestivum L	Drought and Cd	Increased growth, chlorophyll, SOD, and peroxidase activities	195
Fe nanoparticles	Triticum aestivum L.		Enhanced growth, photosynthetic activities, uptake of Fe; decreased Cd accumulation	196
Fe3O4 nanoparticles	Triticum aestivum	Cd, Pb, Cu and Zn	Restricted the toxicity of heavy metals, enhanced the SOD and peroxidase activities	153

---

(i) Nanozymes

Various inorganic nanomaterials such as Fe₃O₄, fullerene C₆₀, CeO₂, Au, Mn₃O₄ and platinum (Pt) nanoparticles, and molybdenum disulfide (MoS₂) nanosheets have been reported to possess antioxidant enzyme-mimetic activities such as SOD, catalase or peroxidase.^111–117 In the recent past, nanozymes have been extensively utilized for safeguarding plants against diverse abiotic stresses such as salinity, drought, and osmotic stress.

It has been reported that under high saline conditions, polyacrylic acid-coated CeO₂ nanoparticles (35% Ce^3+/Ce⁴⁺, 10 nm, 50 mg L⁻¹), exhibit SOD and catalase activities, and they are able to maintain the photosynthetic ability of Arabidopsis.¹¹⁸ Polyacrylic acid-coated spherical CeO₂ nanozymes improve plant photosynthesis by internalization into chloroplasts and scavenging ROS induced by abiotic stress, as demonstrated in a separate study. Furthermore, under various stresses such as excessive light, heat, dark, and cold, CeO₂ nanozymes-exposed Arabidopsis plant exhibit increased the quantum yield of photosystem-II (PS-II), carbon assimilation rate, and Rubisco carboxylation by 19%, 67%, and 61%, respectively.¹¹⁹ Likewise, in sorghum subjected to drought stress, the application of foliar-sprayed CeO₂ nanozymes (15 ± 5 nm, 10 mg L⁻¹) has been demonstrated to alleviate oxidative damage by reducing superoxide radicals (41%), hydrogen peroxide levels, and lipid peroxidation in the cell membrane (37%). These nanozymes result in a notable improvement in 31% seed yield per plant, 38% leaf carbon assimilation, and 31% pollen germination.¹²⁰ Additionally, these nanozymes enhance plant growth and stress tolerance through mechanisms beyond ROS scavenging. For instance, under drought, fullerol nanoparticles alleviate oxidative stress in sugar beet by providing an additional intercellular water supply.¹²¹ Mn₃O₄ nanoparticles are also considered highly potential nanozymes for agricultural applications, particularly for improving stress tolerance. It has been reported that Mn₃O₄ nanozymes exhibit a higher Mn²⁺ : Mn³⁺ ratio of 1 : 2, resulting in stronger in vivo ROS-scavenging ability compared to CeO₂ nanozymes.¹¹¹ Therefore, Mn micronutrients can be employed as Mn₃O₄ nanozymes to improve plant stress tolerance. Furthermore, nanozymes also enhance stress tolerance by increasing the levels of various antioxidant molecules and inducing stress tolerance-associated metabolic pathways. For instance, Zhang et al. reported that the foliar application of nanoceria induces the upregulation of ascorbate peroxidase (APX) gene expression.¹²² Various nanozymes with their specific effects on plant defense system have been evaluated in the subsequent section.

(ii) Non-nanozymes

In addition to the above, it has also been indicated that various nanoparticles confer abiotic stress tolerance without exhibiting enzyme-mimicking activities. As reported, nano zerovalent iron (nZVI) enhances the functionality of H⁺-ATPase, resulting in a reduction in apoplastic pH, increased leaf surface area, and widened stomatal aperture.¹²³ To date, the mechanism underlying the abiotic stress tolerance induced by Fe-based nanoparticles remains unknown. It has been described that certain nanoparticles trigger the upregulation of antioxidant defense-associated genes, thereby boosting stress resistance in plants without directly scavenging the ROS. TiO₂ nanoparticles-mediated induction facilitates several strategies including (i) enhancing the functionality of antioxidant enzymes, thereby increasing ROS scavenging, and (ii) osmotic balance is regulated by the proline and soluble sugar content in plant cells. Moreover, the application of mesoporous silica nanoparticles in wheat and lupin exhibited a notable enhancement in seed germination rate, chlorophyll content, and protein levels. Intriguingly, no significant alterations were observed in H₂O₂ levels, MDA (malondialdehyde) content, or electrolyte leakage, indicating a selective positive impact on specific physiological aspects.¹²⁴

3.1 Drought

Amid various stress conditions, drought is the most prominent factor that causes insufficiency of water for plant uptake, followed by elevated temperatures. Literature indicates that in early-stage growth of plants, encompassing seed germination to seed setting is substantially affected in dry and semiarid regions.¹²⁵ Numerous studies have validated the stress tolerance potential of several nanomaterials. For instance, under drought conditions, silica nanoparticles promote seedling growth and physiological parameters in hawthorns.¹²⁶ Similar results are observed in the case of Triticum aestivum in which nanoparticles improve the starch and gluten content, thereby enhancing growth and crop yield.¹²⁷ In plants, TiO₂ nanoparticles facilitate seed germination, increase biomass and relative water content (RWC), and enhance antioxidative enzymes under drought conditions.¹²⁸ According to an experiment, the enhanced activity of gibberellins was observed to prompt seed germination percentage and accelerate dry weight through the efficient consumption of seed reserve at a faster rate. Likewise, Fe₂O₃ alters carbohydrate metabolism and stomatal movements to enhance tolerance against drought conditions.¹²⁹ Nevertheless, ZnO nanoparticles mediate the downregulation of photosynthetic pigment degradation, consequently increasing the photosynthesis rate and influencing stomatal movement. Additionally, under drought stress, ZnO nanoparticles enhance the performance by manipulating the key enzymes such as UDP glucose (uridine diphosphate glucose), pyro phosphorylase, phosphoglucoisomerase, and cytoplasmic invertase that regulate the synthesis of starch and sucrose.¹³⁰ These observations make ZnO nanoparticles a promising nano-agent for mitigating the effect of drought conditions in plants. Further, under stress conditions, Cu nanoparticles exert a positive influence on the pigment system and ROS scavenging mechanism in maize.¹³¹ At lower concentrations, these nanoparticles augment the functionality of chlorophyll and photosynthetic enzymes, including RubisCo. As shown in Fig. 5(A–C) under drought condition Cu nanoparticles exposed plant exhibited less wilting and rolling leaves in comparison to control plant, as well as enhanced shoot biomass. It has also been observed that in comparison to Cu nanoparticles treated plant, only water exposed plants showed enhanced accumulation of O₂˙⁻ and H₂O₂ (Fig. 5D). Additionally, under drought condition there is significant increase in SOD and APX enzyme expression in nanoparticle treated plant (Fig. 5E). Thus, all results in combination indicate the adverse drought response in plant can be controlled by exposure of Cu nanoparticles via ROS repression.¹³¹ Additionally, it is evidenced that under drought stress in sorghum, foliar-spray of CeO₂ nanozymes declines superoxide radicals (O₂˙⁻) by 41% and H₂O₂ by 36% compared to a water-sprayed control.¹²⁰ Ferromagnetic iron oxide nanomaterials have been significantly employed in protecting plants against environmental stress conditions by effectively eliminating ROS.¹³² For instance, it has been demonstrated that under drought conditions, exposure to yttrium-doped γ-Fe₂O₃ nanoparticles on Brassica napus leaves resulted in a 45% reduction in H₂O₂ content and membrane lipid peroxidation by 28%, thereby relieving drought stress.¹³³ Furthermore, it was found that foliar-sprayed Fe₃O₄ nanoparticles also amend the H₂O₂ scavenging capacity and reduce lipid peroxidation rates in comparison to the control in calcareous soil-grown maize.¹³⁴ In another experiment, it has been observed that in cotton plants, foliar-spray of TiO₂ nanoparticles (50 mg L⁻¹) or SiO₂ nanoparticles (3200 mg L⁻¹) augments tolerance to drought stress.¹³⁵ Furthermore, under drought stress conditions, the colloid solutions of Cu and Zn nanoparticles not only enhance the antioxidant enzyme activity but also maintain the level of photosynthetic pigment content.¹⁰⁸ This has significant potential for nanoparticles in fortifying stress resilience, thereby promoting a positive impact on plant growth and yield.

Fig. 5 Panel A and panel B. Effect of copper (Cu) nanoparticles on plants with drought stress. Panels C. Dry weight of shoots exposed to Cu nanoparticles and water after 7, 14 and 21 days of drought stress. Panel D. Representative histochemical detection of ROS accumulation of the maize leaf under well-watered and at 7 days of drought stress. Panel E. The SOD and APX enzyme activity of the maize leaf under well-watered and at 7 and 14 days after drought stress. Reprinted with permission from ref. 131.

3.2 Salinity

Among various stress conditions, stress in the presence of elevated salt concentrations emerged as a universal concern, impacting crop growth and productivity. The salinity condition induces cytotoxicity and creates nutritional imbalance through enhanced sodium (Na⁺) and chloride (Cl⁻) concentrations. Further coupling with oxidative stress subsequently leads to ROS production. Under osmoregulation conditions, the accumulation of organic compounds, such as quaternary ammonium compounds, sugars, amino acids, glycine betaine, and polyols, results in a reduction of osmotic potential. Additionally, for combating elevated ROS levels and activating initiating enzymatic machinery, the cell underwent a reduction in sodium (Na⁺) and an increase in potassium (K⁺), thereby maintaining ion homeostasis.^136,137 Numerous nanomaterials have been documented to mitigate stress by eliciting the expression of specific genes, accumulating osmolytes, and facilitating the delivery of nutrients and amino acids. For instance, in Cucurbita pepo subjected to salinity stress, exposure to SiO₂ nanoparticles induces changes in transpiration rate and water use efficiency (WUE), along with an enhancement in carbonic anhydrase enzyme activity.¹³⁸ In an experiment carried out with Abelmoschus esculentus, it is observed that the foliar spray of ZnO reduces the impact of salinity stress by escalating photosynthetic functionality, improving the efficacy of photosystem-II and modulating the enzymatic machinery.¹³⁹ Furthermore, the simultaneous foliar application of zinc oxide (ZnO) and silicon (Si) in mango seedlings enhances carbon assimilation and nutrient uptake, consequently amending the growth conditions.¹⁴⁰ Literature indicates that for mitigating salinity impact, SiO₂ nanoparticles facilitate various strategies, including the enhancement of the epicuticular wax layer, proline accumulation, and the up- or downregulation of salinity-associated genes in various plants such as Solanum lycopersicum, Ocimum basilicum, and strawberry.^141,142 Similarly, in Triticum aestivum, exposure to Ag nanoparticles leads to increased peroxidase activity, proline, and sugar accumulation, consequently resulting in increased germination.¹⁴³ Furthermore, in cotton and Catharanthus roseus, the application of CeO, CNTs, and graphene nanoparticles demonstrates significant tolerance to salinity stress by enhancing photosynthetic carbon assimilation, augmented protein levels, and enhanced amino acids content at the reproductive stage.^144,145 Furthermore, in lupin plants, ZnO enhances salt tolerance by diminishing MDA and Na⁺ concentrations and promoting cumin seed germination. The detrimental impacts of sodium chloride (NaCl) are alleviated by n-ZnO through enhanced photosynthetic efficiency, effective osmoregulation, and reduced malondialdehyde and Na⁺ levels.¹⁰⁷

Nevertheless, CeO₂ nanozymes exhibit significant ˙OH scavenging potential under salinity stress and induce the antioxidant defense in plants.¹¹⁸ Studies have demonstrated that anionic CeO₂ nanozymes effectively neutralize increased ROS levels, particularly hydroxyl radicals, and regulate the functions of diverse channels, including outward-rectifying K⁺ channels and non-selective cation channels. These alternations may lead to reduced K⁺ efflux from mesophyll cells and enhance the salt tolerance potential of plants. Fig. 6(A–C) demonstrate the CeO₂ nanoparticles-mediated salinity stress tolerance in plants via K⁺ retention. Herein, the nanoceria exposed plant leaves are visibly healthier with high chlorophyll content after treatment with 100 mM NaCl for three days (Fig. 6D and E), and 30% enhanced PS-II quantum yield has been observed with 85% increase in carbon assimilation rate and 26% in quantum efficiency of carbon dioxide (CO₂) (Fig. 6F–H). Numerous studies have also highlighted the dual nature of CeO₂ nanozymes as scavengers of free radicals and activators of antioxidant enzymes.¹¹⁸

Fig. 6 Schematic illustration of the effect of poly(acrylic acid)-coated nanoceria [PNC] particles on NaCl salinity stress in Arabidopsis thaliana. Panel A. Depiction of PNC internalization in the leaf mesophyll of A. thaliana through stomatal pores. Panel B. Mechanism of PNC particle-mediated hydroxyl radical scavenging is achieved by K⁺ retention. Panel C. Impact of high salinity stress on plants after treatment with PNC particles and buffer with no nanoparticles (NNP or control). Cerium oxide nanoparticle (nanoceria) treated leaves of A. thaliana (panel-D) have healthy leaves; (panel E) high chlorophyll content; (panel F and G) high quantum yield for photosystem II (PS-II) and (panel-H and I) improved carbon assimilation rate and high CO₂ quantum efficiency (φCO₂) after treatment with PNC, and buffer with NNP, respectively. Reprinted with permission from ref. 118.

In an investigation conducted under saline conditions, a down-regulation of the SOD and GPX genes was identified in tomato plants (Solanum lycopersicum). Notably, the introduction of ZnO nanoparticles at concentrations of 15 and 30 mg L⁻¹ counteracted this effect, manifesting a positive modulation in the plant's metabolic processes.¹⁴⁶ ZnO nanoparticles augment salinity stress tolerance by regulating the resistance-associated proteins and enzymes, whereas in the case of finger millet, foliar spray of ZnO nanoparticles alleviates salinity stress.¹⁴⁷ Similarly, the effect of SiO₂ nanoparticles on Cucurbita pepo under highly saline conditions is investigated to confirm that these nanoparticles induces the up-regulation of antioxidant-associated enzymes (catalase, SOD, peroxidase) as well as increased expression of APX and GR genes. Additionally, it led to improvements in chlorophyll content, enhanced the photosynthetic system, and contributed to increased plant biomass.¹³⁸ Although limited studies have investigated the interaction between Mn₃O₄ nanoparticles and plant systems, the antioxidant enzyme-mimicking potential of Mn₃O₄ nanoparticles has been investigated in detail by Lu et al. (2020).¹⁴⁸ It is reported that under salinity stress in cucumber plants, 1 mg per plant concentration of Mn₃O₄ nanoparticles effectively reduced oxidative stress and maintained biomass, as depicted in Fig. 7. As shown in Fig. 7A and B, no toxic response on cucumber leaves is observed following foliar exposure to Mn₃O₄ nanoparticles. Fig. 7C results show the increased production of carotenoids, chlorophyll a, and chlorophyll b, and by 15.6%, 17.7%, and 20.9%, respectively, in plants treated with low dose of Mn₃O₄ nanoparticles (1 mg per plant). These findings also indicated that the application of low and high concentrations of Mn₃O₄ nanoparticles significantly (p < 0.01) increased the net photosynthetic rate of cucumber leaves by 12% and 8%, correspondingly, in comparison to the control group (Fig. 7D). The potential reason for the enhanced photosynthesis facilitated by Mn₃O₄ nanoparticles could be attributed to their ability to antioxidant enzyme-mimic activities. At an optimal dosage, Mn₃O₄ nanoparticles collectively display a positive impact on photosynthesis and biomass accumulation. The biomass of roots remains unaltered when subjected to Mn₃O₄ nanoparticles (Fig. 7F); however, both doses of Mn₃O₄ nanoparticles substantially increased the length of roots (Fig. 7E and G), suggesting a beneficial influence on the growth of underground tissues. While the exact mechanism underlying the increase in root length induced by Mn₃O₄ nanoparticles remains unknown, this effect may result in improved nutrient assimilation, consequently enhancing plant growth in both stress and normal conditions. However, further investigation regarding the antioxidant properties of manganese oxide nanomaterials in plants needs to be done.

Fig. 7 Panel A. Cucumber plants exposed to Mn₃O₄ nanoparticles at concentrations of 0, 1 and 5 mg per plant representing control, low and high, respectively. Panel B. Fresh biomass; panel C. photosynthetic pigment (chlorophyll A, chlorophyll B and carotenoid) content. Panel D. Photosynthetic rate. Panel E. Cucumber roots exposed to Mn₃O₄ nanoparticles at concentrations of 0, 1, and 5 mg per plant for 7 days. Panel F. Roots biomass. Panel-G. Root length. Reprinted with permission from ref. 148.

3.3 Extreme temperature

Heat stress is a condition in which temperature surpasses the threshold level, while temperature below the minimum threshold is classified as cold stress. These temperature fluctuations create cell homeostasis imbalance, ultimately resulting in plant mortality. Extreme temperatures induce oxidative stress, resulting in the generation of reactive species and perturbing the physiological and biochemical functions of osmolytes and heat shock proteins (HSPs). The role of HSPs is crucial in defending cell structure and enhancing the antioxidant mechanism to restore redox potential, thereby maintaining cellular homeostasis.¹⁴⁹ In an experiment, it is reported that in sorghum, selenium (Se) nanoparticles alleviate heat stress by facilitating the antioxidant machinery to scavenge ROS.¹²⁰ Similarly, in L. esculentum, Se nanoparticles prompt tolerance to both high and low-temperature stress.¹⁵⁰ Additionally, it was reported that in wheat plants, heat significantly impacts the morphological characteristics, including root length and numbers, as well as fresh and dry weight, leaf area, and number.¹⁵¹

3.4 Soil metal toxicity, and phytoremediation by nanomaterials

The proficient interaction of nanomaterials with plant metabolites and metal ions also makes them a viable technique for phytoremediation. Phytoremediation is a sustainable technique in which potent plants are employed to remove hazardous waste from the environment.¹⁵² In recent times, global apprehensions have escalated concerning heavy metal contaminations, leading to diminished crop yield and heightened risks to food safety. Nanoparticles mitigate oxidative stress induced by heavy metals in various plant species exposed to heavy metal toxicity and promote their growth.^153,154 In A. pygmaea, treatment with 100 μM silicon dioxide enhances the tolerance to cadmium (Cd), manganese (Mn), and copper (Cu) stress by increasing biomass accumulation and the activity of various biocatalysts within the plant system.¹⁵⁴ Moreover, silicon dioxide inhibits the translocation of heavy metals to the leaves by enhancing their absorption and accumulation.¹⁵⁴ Nanomaterials exhibit significant metal chelation efficiency, facilitating the removal of chromium (Cr) and Cd ions by immobilizing them and nanofibrous composite membranes composed of polyvinyl alcohol and polyacrylonitrile.¹⁵⁵ This study evidenced that the positive and negative surface charge of nanomaterials play a pivotal role in metal chelation efficacy. Similarly, Fe₃O₄ nanoparticles mitigate oxidative stress in tomatoes and protect them from cadmium toxicity.¹⁵⁶ Experimental evidences have been provided that under oxidative stress, Fe₃O₄ nanoparticles possess superior antioxidant potential than γ-Fe₂O₃ in Citrus maxima seedlings. In plants, numerous studies have confirmed the protective role of Fe₃O₄ nanoparticles and γ-Fe₂O₃ nanoparticles.¹⁵⁷ Another study investigated the impact of Fe nanoparticles on wheat growth and their potential in alleviating oxidative stress induced by Cd. Findings reveal that concentrations of Fe nanoparticles below 20 mg L⁻¹ demonstrated a reduction in electrolyte leakage and an augmentation of antioxidant enzyme efficiency in the leaves of Cd-stressed wheat.¹⁵⁸ Furthermore, in maize leaves, the application of ZnO nanoparticles via foliar spray significantly diminishes the uptake of Cd and alleviates the oxidative stress induced by Cd.¹⁵⁹ Collectively, these observations suggest that nanoparticles exert a substantial influence on stress tolerance mechanisms in agricultural plants.

Furthermore, the literature suggests that nanomaterials prevent the degradation of plant membranes exposed to metal stress by reducing the accumulation of low malondialdehyde accumulation. In Leucaena leucocephala, under Cd and lead (Pb) stress conditions, ZnO nanoparticles induce an increase in the activity of SOD, catalase, and APX, attributed to the reduction of malondialdehyde concentration.¹⁶⁰ Moreover, the application of magnetic nano-Fe₃O₄ into Pb, Zn, Cd, and Cu (10 mM)-contaminated growth media for wheat seedlings increased the activity of SOD and peroxidase as well as reduced the accumulation of malondialdehyde content.¹⁵³ Furthermore, in rice, iron oxide nanoparticles boost the potential tolerance to arsenic by upregulating the catalytic efficacy of glyoxalase and antioxidant enzymes by accumulating glutathione and phytochelatins.¹⁶¹ In finger millet, exposure to nanoparticles regulates the biosynthesis of the photosynthetic pigment and assists mineral acquisition.¹⁶² Konate et al. documented that Fe₃O₄ nanoparticles diminish the uptake and accumulation of heavy metals, such as lead, zinc, cadmium, and copper, while concurrently alleviating the toxic effects by enhancing the activity of antioxidant enzymes in wheat seedlings.¹⁵³ However, when exposed to low concentrations of heavy metals, wheat seedlings manifest a decrease in SOD and peroxidase activity. Several other nanoparticles such as Fe₃O₄ elicit a protective response in plants by initiating mechanisms that reduce oxidative stress. This indicates a correlation between the enhanced enzymatic activity of SOD and peroxidase in wheat seedlings as well as a reduction in lipid peroxidation.

3.5 Others abiotic stress conditions

Several studies unveiled that cobalt oxide (Co₃O₄) nanoparticles possess dual inherent enzyme-mimicking properties (peroxidase and catalase).¹⁶³ The peroxidase-mimicking potential of Co₃O₄ nanoparticles is primarily attributed to electron transfer from substrates and H₂O₂, whereas the catalase-mimicking activity is excavated during the study of peroxidase-mimic activity, and it can be regulated by adjusting the pH from highly acidic to alkaline conditions.¹⁶⁴ It has been reported that lower concentrations (<100 mg L⁻¹) of foliar-sprayed Co₃O₄ nanoparticles induce plant growth without causing oxidative damage. In contrast, higher concentrations (>250 mg L⁻¹) of Co₃O₄ nanoparticles limit plant growth and photosynthesis by inducing oxidative stress.¹⁶⁵ However, the reaction mechanism underlying enzyme-mimicking function and the interaction between Co₃O₄ nanoparticles and plants needs to be further explored under environmental stresses for more understanding.

In recent years, CeO₂ nanoparticles have gathered significant attention as antioxidant enzymes-mimic to improve plant environmental stresses. For example, in the context of abiotic stress in A. thaliana, anionic poly(acrylic acid)-coated CeO₂ nanoparticles, characterized by a low Ce³⁺/Ce⁴⁺ ratio (35%), demonstrated a 52% reduction in ROS.¹¹⁹ According to the literature, it is reported that the plants' innate enzyme system is not capable of scavenging the ˙OH, which is the most disruptive ROS.¹⁶⁶

Various studies have revealed the enzyme-mimicking activities of manganese oxides (MnO, MnO₂, and Mn₃O₄) nanomaterials and their potential to efficiently eliminate excess ROS.^111,167,168 It has been observed that MnO nanoparticles possess higher intrinsic SOD-mimicking activity compared to the native Mn-SOD, whereas MnO₂ nanoparticles exhibit multiple enzyme-mimicking properties encompassing SOD, catalase, peroxidase, and oxidase.^167,169 Yao et al. found that Mn₃O₄ nanoparticles demonstrate substantial ROS scavenging efficacy, particularly for the removal of hydroxyl radicals (˙OH). However, the remarkable intrinsic enzyme-mimic activity of Mn₃O₄ nanoparticles is attributed to the rapid redox transition between Mn²⁺ and Mn³⁺ oxidation states.¹⁷⁰ Moreover, the Mn²⁺/Mn³⁺ transition metal couple displays a higher affinity to H₂O₂ and O₂˙⁻ compared to other transition metal couples. Additionally, Mn₃O₄ nanoparticles demonstrate more prominent ROS-eliminating efficacy than CeO₂ nanoparticles.¹¹¹ Therefore, these manganese oxide-based nanoparticles display significantly considerable therapeutic potential for treating ROS-linked diseases.^111,148,171

4. Nanotoxicity and the plants antioxidant machinery activation to mitigate the effect

Over the last decade, concerns regarding bioavailability, nanomaterials-associated toxicity, and inadequacies in the regulatory framework have faced challenges in the comprehensive adoption of nanotechnology in the agricultural sector.¹⁹⁷ The utilization of nanomaterials, in an inadequate manner, possesses a dual challenge. Nanomaterials application on plants has demonstrated a negative impact on plant growth¹⁹⁸ and inhibits the synthesis of plant protein and pigments.¹⁹⁹ Numerous studies have reported that the overuse of nanomaterials induces oxidative stress through ROS production.²⁰⁰ Recently, nanoparticles such as Ag, Au, CeO, TiO₂, ZnO, and Cu surged in unprecedented popularity to their remarkable ability to induce oxidative stress in plants.²⁵ However, for alleviating the nanoparticle-induced oxidative stress, plants generate various enzymatic and non-enzymatic antioxidants, such as catalase, peroxidase, SOD, thiols, phenolic compounds, and ascorbate.²⁰¹ Moreover, plants biosynthesize various compounds such as metallothioneins and phytochelatins to mitigate the detrimental impacts of oxidative stress induced by nanomaterials. Varied concentrations of nanomaterials elicit differential responses in plants, influencing physiological, genetic, and proteomic aspects.²⁰² Furthermore, in the soil, the bioavailability and toxicity of nanoparticles are governed by their bio/geo transformations.¹⁹⁷ Scientific literature indicates that various factors influence the phytotoxicity of nanomaterials, encompassing the type of nanomaterials, test plant species, nanoparticle concentration, and size distribution, as well as morphology and surface charge. Multiple studies have reported that nanoparticles impact plant growth in a concentration-dependent manner.^203,204 Nanomaterials are distinct and they have unconventional catalytic properties that make them potential toxic materials. The bioactive surface and smaller size of nanoparticles enable them to move across the cell membrane and interact with various cellular structures and biomolecules within plant cells. According to the available literature, several nanomaterials have shown phytotoxicity, including ZnO,¹¹² CeO₂,²⁰⁵ TiO₂,²⁰⁶ NiO,²⁰⁷ CuO,²⁰⁸ Ag, Au,²⁰⁹ SiO₂,²¹⁰ fullerenes,²¹¹ and graphene oxide.²¹²

For assessing the presence or absence of any stress inducer in plants, ROS detection and activation of antioxidant machinery is one of the well-established methods. Additionally, in ecotoxicological analysis, the presence of nanoparticles is evaluated by determining the alternations in plant defense responses as biomarkers.²¹³ In Oryza sativa, exposure to Ag nanoparticles enhanced the ROS production level. The presence of nanoparticles initiates the activation of antioxidant machinery, which leads to an increase in carotenoid levels to alleviate the negative impact of ROS on the plant.^214,215 Numerous studies have reported the elevated levels of ROS, including superoxide radicals and H₂O₂, in Allium cepa exposed to Ag nanomaterials, which results in the oxidative burst.²¹⁶ Under nanoparticle stress, various enzymes including SOD and peroxidase play a crucial role in reducing oxidative stress.²¹⁷ The elevated antioxidant enzyme concentration indirectly suggests an increase in ROS levels within cells. Furthermore, in plants, phenol and phenolic acids protect the cellular components from oxidative stress induced by exposure to nanoparticles.^217–219 The presence of Ag nanomaterials in Triticum aestivum, amplified levels of oxidized glutathione and up-regulation of the metallothionein gene are associated with detoxification.²²⁰ Similarly, the treatment of Brassica juncea with Au nanoparticles possesses a 29% higher concentration of ROS compared to the control plants.²²¹ A considerably significant 61% increase in H₂O₂ production in green leaves is observed when exposed to zinc oxide nanoparticles at a concentration of 500 mg kg⁻¹.²²² An experiment conducted by Wang et al. assessed the impact of Fe₃O₄ nanoparticles on ROS production in Cucurbita mixta and Lolium perenne. The findings indicated that elevated ROS production influenced membrane stability.²²³ Additionally, exposure of Ag nanoparticles and ZnO nanoparticles to Spirodela punctata induced the production of reactive nitrogen species, H₂O₂ and ROS.²²⁴ The cytotoxic impact of aluminium oxide nanoparticles by exposing to increasing doses (0.01, 1, and 100 μg mL⁻¹) of nanoparticles has also confirmed their impact on plant root tips.²²⁵

Additionally, in A. thaliana, exposure to carbon nanotube triggered ROS accumulation in both the root tips of A. thaliana seedlings²²⁶ and Nicotiana tabacum cell culture.²¹¹ Recent studies have shown that various nanoparticles including CeO₂, Fe₃O₄, cobalt ferrite (CoFe₂O₄), and TiO₂ induce ROS production in plants.^227–229 Furthermore, not only ZnO nanoparticles but also the nanoparticle-release zinc ions triggered the ROS production in A. cepa.²³⁰ An investigation on the concentration-dependent ROS generation ability of NiO nanoparticles in tomato roots and isolated protoplast from roots has also been performed.²⁰⁷ Likewise, several investigations have documented the elicitation of oxidative stress attributable to either CuO nanoparticles or the liberation of copper ions.^231,232 Within plant cells, the swift solubilization of copper oxide nanoparticles occurs, leading to the liberation of copper ions and subsequently initiating oxidative damage through the redox reaction involving Cu²⁺ and Cu⁺.²³¹ In red spinach, tomato, and cabbage leaves, the increase in the H₂O₂ level in a concentration-dependent manner has also been reported.²³³

Plants exhibited several defense strategies including enzymatic antioxidative defense mechanisms and non-enzymatic components to counter oxidative damage induced by nanomaterials exposure. These induced defense systems effectively scavenge or detoxify the ROS generated in nanomaterial-exposed plants. Essential enzymes, including glutathione reductase (GR), SOD, dehydro-ascorbate reductase, and catalase, and various peroxidases such as ascorbate peroxidase (APOX) and guaiacol peroxidase (GPOX) play pivotal roles in the enzymatic antioxidant defense mechanism. Additionally, the non-enzymatic defense system includes low molecular weight antioxidants, such as ascorbate and thiols (e.g., glutathione).²³⁴ The phytotoxicity level generated from the interaction between engineered copper-oxide and zinc-oxide nanoparticles with the sand-grown plants has been investigated, wherein a sand-based growth system was employed with 500 mg kg⁻¹ nanoparticles incorporated in autoclaved sand.²³⁵ The assessment of the enzymatic performance of peroxidase and catalase was done using root samples. The results demonstrated a notable rise in catalase and peroxidase activity in plants subjected to Cu nanoparticles, in contrast with control plants. Conversely, plants treated with Zn nanoparticles displayed no significant alterations. In a distinct investigation, the influence of magnetite iron oxide nanoparticles on wheat development was studied to assess the oxidative damage. It evaluated the cellular internalization of citric acid-coated iron oxide nanoparticles (5, 10, 15, and 20 mg L⁻¹) and their physiological effects on a hydroponic-grown wheat plant system.²³⁶ It has been indicated that nanoparticles contribute to increased enzymatic functionality in a concentration-dependent manner. This increment in antioxidant enzymatic activities, including catalase, APOX, DPOX, and SOD, followed in both root and aerial parts of the plant. The impact of copper oxide nanoparticles with varying concentrations (50, 100, 200, 300, 400, and 500 mg L⁻¹) on the change in growth, redox status, and the expression levels of stress-linked genes in Cicer arietinum is assessed, and the biochemical assay demonstrated a substantial increase in lipid peroxidation levels in a concentration-dependent manner of nanoparticles.²³⁷ Moreover, plants treated with copper oxide nanoparticles displayed increased mRNA expression levels of SOD, catalase, and APOX in comparison to their respective control group. In a separate investigation, different concentrations of titanium oxide nanoparticles (0.01, 0.1, 1, and 10 mg L⁻¹) were applied, and the enzymatic activities of catalase and GR were evaluated, revealing a concentration-dependent enhancement in both enzymes. In another study, the role of Ag nanoparticles on the growth enhancement of B. juncea was evaluated. Herein, the seeds were allowed to germinate and grow with exposures of 25, 50, 100, 200, and 400 ppm of Ag nanoparticles, and the findings revealed an increment in the functionality of antioxidant enzymes including GPOX, peroxidase, and catalase in a dose-dependent manner. It was reported that 7 day-old seedlings demonstrate higher enzymatic activity at elevated concentrations of Ag nanoparticles.²³⁸B. juncea treated with varying amounts of copper oxide and titanium oxide nanoparticles show enhancement in APOX activity in both the roots and shoots of the plants.¹⁹⁸ Similarly, catalase displayed elevated enzymatic activity in a dose-dependent manner with copper oxide nanoparticles, whereas no alternation was noted in the treatment with titanium oxide nanoparticles. The oxidative stress induction potency of green-synthesized ZnO nanoparticles and their ability to activate antioxidant machinery in the Lathyrus sativus (grass pea) has been evaluated.²³⁹ The seedling of grass pea was allowed to germinate with varying concentrations (10, 20, 40, 80, and 100 mg L⁻¹) of green-synthesized ZnO nanoparticles through root-immersion in a medium containing nanoparticles. The findings indicated a dose-dependent increment in SOD, GPX and ascorbate peroxidase APOX activities while a reduction in catalase activity.

5. Physicochemical properties of nanomaterials and their effects on plants

Nanoparticles' distinctive physicochemical properties play a crucial role in enhancing plant growth and stress resistance, and the bioavailability and toxicity of nanoparticles are determined by their bio/geo-transformations. The biological functions of nanoparticles can be modulated by their physicochemical properties, application methods (soil, foliar, or hydroponics), and concentration. Nanoparticles absorption by plants is mediated by size, and it enables their entry through cell wall pores or stomata. Furthermore, the size of nanoparticles directs their accumulation, toxicity, and transport kinetic within plant cells.²⁴⁰ Similarly, nanoparticle agglomeration, surface area, and reactivity on the cell surface or within the plant cell are correlated by their shape.²⁴¹ The nanoparticles' attachment to plant cell surface depends on their surface charge, and the negative charge of the cell wall.

Physicochemical factors like size, surface charge, and hydrophobicity significantly impact nanoparticles' uptake and translocation process and complement their absorption, translocation, and aggregation within the plants.^242,243 Therefore, a detailed examination of the nanoparticle's nature is crucial for determining the precise movement and localization within the plant cells. It has been reported that nanoparticles employ an active transport mechanism, involving multiple cellular processes such as signaling, recycling, and regulation of the plasma membrane.²⁴⁴ In recent years, significant progress has been made in understanding the translocation kinetics and the mechanisms governing the uptake of molecule/ions and facilitating the tracking of nanoparticle agglomeration kinetics.²⁴⁵ After exposure to plants, nanoparticles undergo a complex series of events, penetrating the cell wall and entering the plant vascular bundle, with subsequent symplastic movement to the stele, and translocation to the leaves.²⁴⁶ Numerous studies have indicated that nanoparticles within 3 to 5 nm size-range penetrate the plant root system through osmotic pressure, capillary forces, or by passing through the epidermal cell, which restricts the transfer of larger nanoparticles due to their semipermeable nature.^247,248 Further, nanoparticles travel across the leaves through the cuticle, which inhibits the entry of particles <5 nm and that of nanoparticles >10 nm through the stomata. Furthermore, nanoparticles between 10 and 50 nm size-range travel through the symplastic route (adjacent cell cytoplasm), while those sized between 50 and 200 nm translocate via the apoplastic route (between cells).²⁴⁹ It has also been observed that among TiO₂ nanoparticles ranging from 14 to 655 nm, only the smallest ones have the potential to translocate through the entire plant. Moreover, nanoparticles with a size smaller than 140 nm penetrate the root epidermis, while those below 36 nm transfer through the root parenchyma and translocate from root to shoot.²⁵⁰

It is witnessed that nanoparticles with distinct crystalline structures and identical compositions follow variance in uptake and translocation within plants. For instance, in cucumber plants, TiO₂ nanoparticles in anatase form primarily accumulate in roots, while those in rutile crystalline form exhibit differential translocation.²⁵¹ Interestingly, in the case of foliar application, the adsorption of nanoparticles onto plants is contingent on several factors such as application method, size, concentration, and climatic conditions.²⁴¹ In plants, the cell wall serves as a specific barrier, regulating the entry of nanoparticles into the cells.²⁵² The nature of nanoparticles mediates their penetration across the cell wall and stimulates attachment to the radical surface.²⁵³ Positively charged nanoparticles exhibit higher adhesion to the cell wall thorough electrostatic interaction as the cell well possesses negative charge. The morphology (shape) and surface coatings of nanoparticles also play an important role in regulating their action within plants. In plant systems, nanoparticles enter through various paths, especially though roots and leaves, and they alter the physiological processes of plants, consequently influencing growth and development.²⁵⁴

It has been indicated that nanoparticles can impact the rate of plant germination and biomass accumulation at different concentrations, and they may exhibit potential toxic effects.²⁰⁴ For instance, it is observed that iron oxide nanoparticles enhance wheat seedlings' growth at 100 ppm concentration; however, higher concentrations of these nanoparticles are associated with reduced-germination rate, root biomass, and meantime of germination.²⁵⁵ Nanoparticles at certain concentrations show toxic effects in plants, including alterations in morpho-anatomical, physiological, biochemical, and genetic constitutions and reduce crop production.^25,256 The toxicity of nanomaterials stimulates the production of ROS, leading to oxidative stress, lipid peroxidation, and damage to proteins and DNA.^257,258 It has been observed that lower concentrations of SiO₂ nanoparticles enhanced the seed germination in Lycopersicon esculentum. To predict phytotoxicity of nanoparticles, parameters such as retardation in growth potential, biomass accumulation, and leaf surface area are considered integral assessments.²⁵⁹ The accumulation of nanoparticles within the plants starts reducing the crop quality, characterized by reduced seed germination rate, fresh and dry biomass, altered root and shoot lengths, and modulate the transpiration rate. Additionally, the accumulation induces lipid peroxidation and regulates stress-related genes, resulting in apoptosis.²⁶⁰

Nonetheless, as discussed in earlier sections, plants inherently possess several defense mechanisms, encompassing enzymatic and non-enzymatic machineries to overcome the phytotoxic effects associated with nanoparticles.²⁵ The phytotoxicity is highly dependent on the physicochemical attributes of nanoparticles, including size, aggregation, composition, concentration, shape, porosity, surface area, hydrophobicity, and electrical and magnetic properties.^261–263 In conclusion, the interaction between nanoparticles and plants is mediated by multiple factors, encompassing nanomaterial characteristics, and plant-associated variables, and they all must be considered while designing nanomaterials for plants to secure desired outcomes without any potential nanotoxicity.

6. Outlook and concluding remarks

Research to date on the application of nanomaterials to enhance plant defense through the induction of resistance has primarily concentrated on the protective impacts of nanomaterials spraying prior to pathogen challenge. However, concerning the broad and minute effects of nanomaterials on plant defense responses, further investigations are necessary to ascertain the potential therapeutic effects of nanomaterials application after infection. In addition to this, reconnaissance is needed into the fundamental mechanisms that govern the interactions between nanomaterials and pathogens in plants. Moreover, clarification is needed regarding the most effective concentrations and application times for field usages. Specialized research providing insights into these inquiries will facilitate the development of innovative nanomaterials with controlled physicochemical properties to support the agricultural management strategies.²⁶⁴

However, despite the attention given to the prospective benefits of nanomaterials in plant management, there remain numerous obstacles that must be surmounted to fully exploit the capabilities of nanomaterials-enabled technologies for plants. For example, nanomaterials effect on plants majorly depends on exposed concentration and it may cause adverse effects if exceeded beyond a certain limit.²⁶⁵ Thus, it is of major concern to review the concentration of nanomaterials applied in plants. Further, the impact of nanomaterials on the equilibrium between stress resistance and plant growth promotion, as well as the mechanisms underlying their absorption, translocation, and accumulation in various plant species, remain poorly understood. As a novel approach to administration, nanomaterials have yet to undergo extensive research regarding their persistence and specificity in various plants, as well as the variations in efficiency induced by distinct application techniques. Additional research is also required to obtain a comprehensive understanding of biochemical or molecular interactions between nanomaterials in terms of antioxidant enzymes and miRNA expression in plants.

Hitherto, certain laboratory-stage investigations have yielded favorable outcomes; however, the progress of large-scale field applications or commercialization of nanomaterials has faced hindrances due to materials cost and the necessity for specialized equipment tailored for large-scale production. Consequently, there is an urgent need to develop facile and sustainable methodologies for synthesizing nanomaterials applicable in agricultural settings. Irrespective of the specific purpose of nanomaterial application on plants, interactions with the plant and its environment are inevitable. Numerous studies have explored the potential impacts of nanomaterials on both biotic and abiotic stresses. However, there has been relatively less attention towards the risks that nanomaterials pose to the diversity and composition of microorganisms in the phyllo-sphere and soil. It has been shown that several nanomaterials can induce detrimental alterations in the structure of soil communities. Nevertheless, microbial cells have been documented to degrade nanoparticles through processes such as coagulation with extracellular polymeric substances or modification of nanoparticle morphologies. Therefore, greater emphasis should be placed on the interactions that occur in the natural environment between microorganisms and nanoparticles.

In summary, a systematic and thorough analysis of diverse applications of nanotechnologies in the domain of plant production, underscoring the pivotal challenges that necessitate resolution before the commercial deployment of nanomaterials in plant sciences, have been discussed. Further, insightful recommendations are provided for the optimal delivery of nanomaterials to plants, ensuring that they do so in the correct form, dosage, and location, thereby capitalizing on their respective strengths and minimizing their weaknesses. Concurrently, additional investigation into the mechanisms underlying the interactions between nanomaterials and pathogens on plants may advance a more sustainable paradigm of agricultural production amidst an unforeseeable future, thereby safeguarding the food security of future generations.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

HKD acknowledges Central University of Rajasthan, India for a Seed Grant (2023), and Indian Council of Forestry Research and Education (ICFRE), Dehradun, India for a research grant (ICFRE/RFRI/2023-24/SFM-2). HKD also acknowledges Department of Science and Technology, Government of India, New Delhi for providing FIST grant (SR/FST/LS-I/2023/1153) to Department of Biochemistry, School of Life Sciences, Central University of Rajasthan, India.

References

1. L. Zhao, L. Lu, A. Wang, H. Zhang, M. Huang, H. Wu, B. Xing, Z. Wang and R. Ji, J. Agric. Food Chem., 2020, 68, 1935–1947.
2. R. Singh, A. Umapathi, G. Patel, C. Patra, U. Malik, S. K. Bhargava and H. K. Daima, Sci. Total Environ., 2023, 854, 158771.
3. A. Kumari, M. S. Bhinda, B. Sharma and M. Parihar, in Sustainable Agriculture Reviews 53: Nanoparticles: A New Tool to Enhance Stress Tolerance, ed. M. Faizan, S. Hayat and F. Yu, Springer International Publishing, Cham, 2021, pp. 33–60.
4. P. Pandey, V. Irulappan, M. V. Bagavathiannan and M. Senthil-Kumar, Front. Plant Sci., 2017, 8, 537.
5. A. C. Oluwaseun and N. B. Sarin, Pollut. Res., 2017, 24, 13700–13709.
6. S. Sharma, V. K. Singh, A. Kumar and S. Mallubhotla, in Molecular Plant Abiotic Stress, 2019, pp. 315–333.
7. A. Sánchez-Estrada, M. E. Tiznado-Hernández, A. J. Ojeda-Contreras, A. I. Valenzuela-Quintanar and R. Troncoso-Rojas, J. Phytopathol., 2009, 157, 24–32.
8. H. C. McCann, H. Nahal, S. Thakur and D. S. Guttman, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 4215–4220.
9. S. Chandra, N. Chakraborty, A. Dasgupta, J. Sarkar, K. Panda and K. Acharya, Sci. Rep., 2015, 5, 15195.
10. A. Hazarika, M. Yadav, D. K. Yadav and H. S. Yadav, Biocatal. Agric. Biotechnol., 2022, 43, 102399.
11. K. K. Garg, D. Jain, D. Rajpurohit, H. S. Kushwaha, H. K. Daima, B. J. Stephen, A. Singh and S. R. Mohanty, Comments Inorg. Chem., 2022, 42, 209–225.
12. D. Jain, H. S. Kushwaha, K. S. Rathore, B. J. Stephen, H. K. Daima, R. Jain and A. Singh, Part. Sci. Technol., 2022, 40, 97–103.
13. D. Jain, Shivani, A. A. Bhojiya, H. Singh, H. K. Daima, M. Singh, S. R. Mohanty, B. J. Stephen and A. Singh, Front. Chem., 2020, 8, 778.
14. A. Kaphle, P. N. Navya, A. Umapathi and H. K. Daima, Environ. Chem. Lett., 2018, 16, 43–58.
15. A. Marwal, M. Mishra, R. Verma, R. Prajapat and R. K. Gaur, in In Silico Approach for Sustainable Agriculture, ed. D. K. Choudhary, M. Kumar, R. Prasad and V. Kumar, Springer Singapore, Singapore, 2018, pp. 69–90.
16. E. A. Worrall, A. Hamid, K. T. Mody, N. Mitter and H. R. Pappu, Agronomy, 2018, 8, 285.
17. W. Zhao, T. Ma, P. Zhou, Z. Wu, Z. Tan and Y. Rui, Plant Physiol. Biochem., 2024, 207, 108428.
18. C. Wang, L. Yue, B. Cheng, F. Chen, X. Zhao, Z. Wang and B. Xing, Environ. Sci.: Nano, 2022, 9, 302–312.
19. P. P. Singh, A. Priyam, J. Singh and N. Gupta, Sci. Hortic., 2023, 315, 111988.
20. A. Pinsino, N. G. Bastús, M. Busquets-Fité, L. Canesi, P. Cesaroni, D. Drobne, A. Duschl, M.-A. Ewart, I. Gispert, J. Horejs-Hoeck, P. Italiani, B. Kemmerling, P. Kille, P. Procházková, V. F. Puntes, D. J. Spurgeon, C. Svendsen, C. J. Wilde and D. Boraschi, Environ. Sci.: Nano, 2020, 7, 3216–3232.
21. V. Mishra, R. K. Mishra, A. Dikshit and A. C. Pandey, in Emerging Technologies and Management of Crop Stress Tolerance, ed. P. Ahmad and S. Rasool, Academic Press, San Diego, 2014, pp. 159–180.
22. C. O. Dimkpa, Curr. Opin. Environ. Sci. Health., 2018, 6, 1–8.
23. G. Gohari, A. Mohammadi, A. Akbari, S. Panahirad, M. R. Dadpour, V. Fotopoulos and S. Kimura, Sci. Rep., 2020, 10, 912.
24. A. A. H. Abdel Latef, A. K. Srivastava, M. S. A. El-sadek, M. Kordrostami and L. S. P. Tran, Land Degrad. Dev., 2018, 29, 1065–1073.
25. C. M. Rico, J. Peralta-Videa and J. Gardea-Torresdey, Nanotechnology and plant sciences: nanoparticles and their impact on plants, 2015, pp. 1–17.
26. P. Miralles, T. L. Church and A. T. Harris, Environ. Sci. Technol., 2012, 46, 9224–9239.
27. E. González-Grandío, G. S. Demirer, C. T. Jackson, D. Yang, S. Ebert, K. Molawi, H. Keller and M. P. Landry, J. Nanobiotechnol., 2021, 19, 431.
28. Y. Zhang, G. Qi, L. Yao, L. Huang, J. Wang and W. Gao, J. Agric. Food Chem., 2022, 70, 916–933.
29. A. Nel, T. Xia, L. Mädler and N. Li, Science, 2006, 311, 622–627.
30. S. Foley, C. Crowley, M. Smaihi, C. Bonfils, B. F. Erlanger, P. Seta and C. Larroque, Biochem. Biophys. Res. Commun., 2002, 294, 116–119.
31. J. Kamat, T. Devasagayam, K. Priyadarsini and H. Mohan, Toxicology, 2000, 155, 55–61.
32. M. Auffan, W. Achouak, J. Rose, M.-A. Roncato, C. Chaneac, D. T. Waite, A. Masion, J. C. Woicik, M. R. Wiesner and J.-Y. Bottero, Environ. Sci. Technol., 2008, 42, 6730–6735.
33. D. L. Slomberg and M. H. Schoenfisch, Environ. Sci. Technol., 2012, 46, 10247–10254.
34. P. Begum and B. Fugetsu, J. Hazard. Mater., 2012, 243, 212–222.
35. J. D. G. Jones and J. L. Dangl, Nature, 2006, 444, 323–329.
36. H. Yakura, Vaccines, 2020, 8, 541.
37. S. Magadán, I. Mikelez-Alonso, F. Borrego and Á. González-Fernández, Adv. Drug Delivery Rev., 2021, 175, 113821.
38. S. H. Spoel and X. Dong, Nat. Rev. Immunol., 2012, 12, 89–100.
39. D. Boraschi, L. Canesi, D. Drobne, B. Kemmerling, A. Pinsino and P. Prochazkova, Biol. Rev., 2023, 98, 747–774.
40. F. Schwab, G. Zhai, M. Kern, A. Turner, J. L. Schnoor and M. R. Wiesner, Nanotoxicology, 2016, 10, 257–278.
41. S. Rakoff-Nahoum, J. Paglino, F. Eslami-Varzaneh, S. Edberg and R. Medzhitov, Cell, 2004, 118, 229–241.
42. P. C. Ronald and B. Beutler, Science, 2010, 330, 1061–1064.
43. R. N. G. Miller, G. S. Costa Alves and M.-A. Van Sluys, Ann. Bot., 2017, 119, 681–687.
44. A. Kachroo and G. P. Robin, Curr. Opin. Plant Biol., 2013, 16, 527–533.
45. P. Dutta, A. Kumari, M. Mahanta, K. K. Biswas, A. Dudkiewicz, D. Thakuria, A. S. Abdelrhim, S. B. Singh, G. Muthukrishnan, K. G. Sabarinathan, M. K. Mandal and N. Mazumdar, Front. Microbiol., 2022, 13, 935193.
46. M. M. Elsharkawy and K. M. Mousa, Int. J. Pest Manage., 2015, 61, 353–358.
47. A. S. Hamed Derbalah and M. M. Elsharkawy, J. Biotechnol., 2019, 306, 134–141.
48. T. Ahmed, M. Shahid, M. Noman, M. B. K. Niazi, F. Mahmood, I. Manzoor, Y. Zhang, B. Li, Y. Yang, C. Yan and J. Chen, Pathogens, 2020, 9, 160.
49. T. Quiterio-Gutiérrez, H. Ortega-Ortiz, G. Cadenas-Pliego, A. D. Hernández-Fuentes, A. Sandoval-Rangel, A. Benavides-Mendoza, M. Cabrera-de la Fuente and A. Juárez-Maldonado, Int. J. Mol. Sci., 2019, 20, 1950.
50. M. L. Berens, H. M. Berry, A. Mine, C. T. Argueso and K. Tsuda, Annu. Rev. Phytopathol., 2017, 55, 401–425.
51. L. Cai, C. Liu, G. Fan, C. Liu and X. Sun, Environ. Sci.: Nano, 2019, 6, 3653–3669.
52. K. Imada, S. Sakai, H. Kajihara, S. Tanaka and S. Ito, Plant Pathol., 2016, 65, 551–560.
53. M. Adeel, T. Farooq, J. C. White, Y. Hao, Z. He and Y. Rui, J. Hazard. Mater., 2021, 404, 124167.
54. M. El-Shetehy, A. Moradi, M. Maceroni, D. Reinhardt, A. Petri-Fink, B. Rothen-Rutishauser, F. Mauch and F. Schwab, Nat. Nanotechnol., 2021, 16, 344–353.
55. G. Marslin, C. J. Sheeba and G. Franklin, Front. Plant Sci., 2017, 8, 832.
56. S. Anjum, I. Anjum, C. Hano and S. Kousar, RSC Adv., 2019, 9, 40404–40423.
57. K. Divya, M. Thampi, S. Vijayan, S. Varghese and M. S. Jisha, Microb. Pathog., 2020, 149, 104525.
58. R. Suriyaprabha, G. Karunakaran, K. Kavitha, R. Yuvakkumar, V. Rajendran and N. Kannan, IET Nanobiotechnol., 2014, 8, 133–137.
59. K. S. Siddiqi and A. Husen, Nanoscale Res. Lett., 2016, 11, 400.
60. I. Sanzari, A. Leone and A. Ambrosone, Front. Bioeng. Biotechnol., 2019, 7, 120.
61. E. Ferrari, F. Barbero, M. Busquets-Fité, M. Franz-Wachtel, H.-R. Köhler, V. Puntes and B. Kemmerling, Nanomaterials, 2021, 11, 3161.
62. Y. Saijo, E. P.-i. Loo and S. Yasuda, Plant J., 2018, 93, 592–613.
63. H. S. El-Beltagi and H. I. Mohamed, Not. Bot. Horti Agrobot. Cluj-Napoca, 2013, 41, 44–57.
64. V. Kumar, P. Guleria, V. Kumar and S. K. Yadav, Sci. Total Environ., 2013, 461–462, 462–468.
65. M. Tiwari, S. Krishnamurthy, D. Shukla, J. Kiiskila, A. Jain, R. Datta, N. Sharma and S. V. Sahi, Sci. Rep., 2016, 6, 21733.
66. C. N. Siddaiah, K. V. H. Prasanth, N. R. Satyanarayana, V. Mudili, V. K. Gupta, N. K. Kalagatur, T. Satyavati, X.-F. Dai, J.-Y. Chen, A. Mocan, B. P. Singh and R. K. Srivastava, Sci. Rep., 2018, 8, 2485.
67. M. Iriti and E. M. Varoni, Environ. Sci. Pollut. Res., 2015, 22, 2935–2944.
68. E. A. Mohamed, Arch. Phytopathol. Plant Prot., 2019, 52, 1276–1288.
69. S. Chintala, R. Laishram, P. Mondal, K. Pal, P. Kantamraju, S. Ghosh, K. Karmakar, H. Chakdar, R. Mukhopadhyay, R. Sen, A. Choudhury, S. Mandal and N. Sahana, Ind. Crops Prod., 2023, 206, 117616.
70. L. Cai, L. Cai, H. Jia, C. Liu, D. Wang and X. Sun, J. Hazard. Mater., 2020, 393, 122415.
71. T. Yang, R. Lv, J. Li, H. Lin and D. Xi, Plant Cell Physiol., 2018, 59, 2381–2393.
72. A. Manikandan and M. Sathiyabama, Int. J. Biol. Macromol., 2016, 84, 58–61.
73. V. Saharan, G. Sharma, M. Yadav, M. K. Choudhary, S. S. Sharma, A. Pal, R. Raliya and P. Biswas, Int. J. Biol. Macromol., 2015, 75, 346–353.
74. I. P. Calil and E. P. B. Fontes, Ann. Bot., 2016, 119, 711–723.
75. Y. Li, J. Lu, Y. Han, X. Fan and S.-W. Ding, Science, 2013, 342, 231–234.
76. P. V. Maillard, C. Ciaudo, A. Marchais, Y. Li, F. Jay, S. W. Ding and O. Voinnet, Science, 2013, 342, 235–238.
77. Y. Li, M. Basavappa, J. Lu, S. Dong, D. A. Cronkite, J. T. Prior, H.-C. Reinecker, P. Hertzog, Y. Han, W.-X. Li, S. Cheloufi, F. V. Karginov, S.-W. Ding and K. L. Jeffrey, Nat. Microbiol., 2016, 2, 16250.
78. S. Kadelka, H. Dahari and S. M. Ciupe, Sci. Rep., 2021, 11, 200.
79. B. Kim, J.-H. Park and M. J. Sailor, Adv. Mater., 2019, 31, 1903637.
80. T. P. Frazier, C. E. Burklew and B. Zhang, Funct. Integr. Genomics, 2014, 14, 75–83.
81. T. M. Linh, N. C. Mai, P. T. Hoe, L. Q. Lien, N. K. Ban, L. T. T. Hien, N. H. Chau and N. T. Van, J. Nanomater., 2020, 2020, 4056563.
82. Z. Xiao, L. Yue, C. Wang, F. Chen, Y. Ding, Y. Liu, X. Cao, Z. Chen, S. Rasmann and Z. Wang, J. Hazard. Mater., 2021, 411, 124971.
83. N. Khan, A. Bano, S. Ali and M. A. Babar, Plant Growth Regul., 2020, 90, 189–203.
84. J. Yang, W. Cao and Y. Rui, J. Plant Interact., 2017, 12, 158–169.
85. T. Vinković, O. Novák, M. Strnad, W. Goessler, D. D. Jurašin, N. Parađiković and I. V. Vrček, Environ. Res., 2017, 156, 10–18.
86. D. Tripathi, M. Singh and S. Pandey-Rai, Plant Stress, 2022, 6, 100107.
87. X.-J. Xia, Y.-H. Zhou, K. Shi, J. Zhou, C. H. Foyer and J.-Q. Yu, J. Exp. Bot., 2015, 66, 2839–2856.
88. Y. M. Koo, A. Y. Heo and H. W. Choi, Plant Pathol. J., 2020, 36, 1–10.
89. R. Vankova, P. Landa, R. Podlipna, P. I. Dobrev, S. Prerostova, L. Langhansova, A. Gaudinova, K. Motkova, V. Knirsch and T. Vanek, Sci. Total Environ., 2017, 593–594, 535–542.
90. D. Peng, Y. Zhang, Q. Li, Y. Song, J. Ji, G. Wang, C. Guan and X. Li, Plant Soil, 2020, 450, 443–461.
91. M. Ghorbel, F. Brini, A. Sharma and M. Landi, Plant Cell Rep., 2021, 40, 1471–1494.
92. H. Shang, C. Ma, C. Li, J. C. White, T. Polubesova, B. Chefetz and B. Xing, Environ. Sci.: Nano, 2020, 7, 2632–2643.
93. L. Navarro, R. Bari, P. Achard, P. Lisón, A. Nemri, N. P. Harberd and J. D. G. Jones, Curr. Biol., 2008, 18, 650–655.
94. A. M. Shigenaga and C. T. Argueso, Semin. Cell Dev. Biol., 2016, 56, 174–189.
95. Y. Hao, P. Fang, C. Ma, J. C. White, Z. Xiang, H. Wang, Z. Zhang, Y. Rui and B. Xing, Environ. Res., 2019, 170, 1–6.
96. N. Ling, T. Wang and Y. Kuzyakov, Nat. Commun., 2022, 13, 836.
97. T. S. Walker, H. P. Bais, E. Grotewold and J. M. Vivanco, Plant Physiol., 2003, 132, 44–51.
98. J. Panichikkal, R. Thomas, J. C. John and E. K. Radhakrishnan, Curr. Microbiol., 2019, 76, 503–509.
99. Y. Zhou, J. Ma, J. Yang, Z. Lv, Z. Song and H. Han, Chemosphere, 2023, 310, 136784.
100. M. Li, L. Gao, J. C. White, C. L. Haynes, T. L. O'Keefe, Y. Rui, S. Ullah, Z. Guo, I. Lynch and P. Zhang, Nat. Nanotechnol., 2023, 18, 688–691.
101. M.-X. Zhang, L.-Y. Zhao, Y.-Y. He, J.-P. Hu, G.-W. Hu, Y. Zhu, A. Khan, Y.-C. Xiong and J.-L. Zhang, Bioresour. Technol., 2024, 391, 129987.
102. L. Jiao, X. Cao, C. Wang, F. Chen, H. Zou, L. Yue and Z. Wang, Sci. Total Environ., 2023, 878, 163175.
103. C. Lu, R. Hei, X. Song, Z. Fan, D. Guo, J. Luo and Y. Ma, Chemosphere, 2023, 336, 139223.
104. W. Wang, B. Vinocur and A. Altman, Planta, 2003, 218, 1–14.
105. B. C. Tripathy and R. Oelmüller, Plant Signaling Behav., 2012, 7, 1621–1633.
106. M. F. Khalid, R. Iqbal Khan, M. Z. Jawaid, W. Shafqat, S. Hussain, T. Ahmed, M. Rizwan, S. Ercisli, O. L. Pop and R. Alina Marc, Nanomaterials, 2022, 12, 3915.
107. A. A. H. Abdel Latef, M. F. Abu Alhmad and K. E. Abdelfattah, J. Plant Growth Regul., 2017, 36, 60–70.
108. N. Taran, V. Storozhenko, N. Svietlova, L. Batsmanova, V. Shvartau and M. Kovalenko, Nanoscale Res. Lett., 2017, 12, 1–6.
109. N. M. Alabdallah and M. M. Hasan, Saudi J. Biol. Sci., 2021, 28, 5631–5639.
110. C. O. Dimkpa, P. S. Bindraban, J. Fugice, S. Agyin-Birikorang, U. Singh and D. Hellums, Agron. Sustainable Dev., 2017, 37, 1–13.
111. J. Yao, Y. Cheng, M. Zhou, S. Zhao, S. Lin, X. Wang, J. Wu, S. Li and H. Wei, Chem. Sci., 2018, 9, 2927–2933.
112. S. S. Lee, W. Song, M. Cho, H. L. Puppala, P. Nguyen, H. Zhu, L. Segatori and V. L. Colvin, ACS Nano, 2013, 7, 9693–9703.
113. F.-y. Liu, F.-x. Xiong, Y.-k. Fan, J. Li, H.-z. Wang, G.-m. Xing, F.-m. Yan, F.-j. Tai and R. He, J. Nanopart. Res., 2016, 18, 1–13.
114. P. Jawaid, M. U. Rehman, Y. Yoshihisa, P. Li, Q. L. Zhao, M. A. Hassan, Y. Miyamoto, T. Shimizu and T. Kondo, Apoptosis, 2014, 19, 1006–1016.
115. Y. Liu, H. Wu, M. Li, J.-J. Yin and Z. Nie, Nanoscale, 2014, 6, 11904–11910.
116. J. Shah, R. Purohit, R. Singh, A. S. Karakoti and S. Singh, J. Colloid Interface Sci., 2015, 456, 100–107.
117. S. Bhagat, N. S. Vallabani, V. Shutthanandan, M. Bowden, A. S. Karakoti and S. Singh, J. Colloid Interface Sci., 2018, 513, 831–842.
118. H. Wu, L. Shabala, S. Shabala and J. P. Giraldo, Environ. Sci.: Nano, 2018, 5, 1567–1583.
119. H. Wu, N. Tito and J. P. Giraldo, ACS Nano, 2017, 11, 11283–11297.
120. M. Djanaguiraman, R. Nair, J. P. Giraldo and P. V. V. Prasad, ACS Omega, 2018, 3, 14406–14416.
121. M. Borišev, I. Borišev, M. Župunski, D. Arsenov, S. Pajević, Ž. Ćurčić, J. Vasin and A. Djordjevic, PLoS One, 2016, 11, e0166248.
122. H. Zhang, L. Lu, X. Zhao, S. Zhao, X. Gu, W. Du, H. Wei, R. Ji and L. Zhao, Environ. Sci. Technol., 2019, 53, 6007–6017.
123. J.-H. Kim, Y. Oh, H. Yoon, I. Hwang and Y.-S. Chang, Environ. Sci. Technol., 2015, 49, 1113–1119.
124. D. Sun, H. I. Hussain, Z. Yi, J. E. Rookes, L. Kong and D. M. Cahill, Chemosphere, 2016, 152, 81–91.
125. M. F. Seleiman, N. Al-Suhaibani, N. Ali, M. Akmal, M. Alotaibi, Y. Refay, T. Dindaroglu, H. H. Abdul-Wajid and M. L. Battaglia, Plants, 2021, 10, 259.
126. P. Ashkavand, M. Tabari, M. Zarafshar, I. Tomásková and D. Struve, Leś. Pr. Bad., 2015, 76, 350–359.
127. A. Jaberzadeh, P. Moaveni, H. R. T. Moghadam and H. Zahedi, Not. Bot. Horti Agrobot. Cluj-Napoca, 2013, 41, 201–207.
128. J. Faraji and A. Sepehri, J. Soil Sci. Plant Nutr., 2020, 20, 703–714.
129. M. Sedghi, M. Hadi and S. G. Toluie, Ann. West. Univ. Timisoara. Ser. Biol., 2013, 16, 73.
130. L. Sun, F. Song, J. Guo, X. Zhu, S. Liu, F. Liu and X. Li, Int. J. Mol. Sci., 2020, 21, 782.
131. D. Van Nguyen, H. M. Nguyen, N. T. Le, K. H. Nguyen, H. T. Nguyen, H. M. Le, A. T. Nguyen, N. T. T. Dinh, S. A. Hoang and C. Van Ha, J. Plant Growth Regul., 2021, 1–12.
132. Z. Chen, J.-J. Yin, Y.-T. Zhou, Y. Zhang, L. Song, M. Song, S. Hu and N. Gu, ACS Nano, 2012, 6, 4001–4012.
133. N. M. Palmqvist, G. A. Seisenbaeva, P. Svedlindh and V. G. Kessler, Nanoscale Res. Lett., 2017, 12, 1–9.
134. M. Jalali, F. Ghanati, A. Modarres-Sanavi and A. Khoshgoftarmanesh, J. Agron. Crop Sci., 2017, 203, 593–602.
135. M. A. Shallan, H. Hassan, A. A. Namich and A. A. Ibrahim, Res. J. Pharm., Biol. Chem. Sci., 2016, 7, 1540–1551.
136. S. Isayenkov, Cytol. Genet., 2012, 46, 302–318.
137. S. V. Isayenkov and F. J. Maathuis, Front. Plant Sci., 2019, 10, 80.
138. M. H. Siddiqui, M. H. Al-Whaibi, M. Faisal and A. A. Al Sahli, Environ. Toxicol. Chem., 2014, 33, 2429–2437.
139. N. M. Alabdallah and H. S. Alzahrani, Saudi J. Biol. Sci., 2020, 27, 3132–3137.
140. N. I. Elsheery, M. N. Helaly, H. M. El-Hoseiny and S. M. Alam-Eldein, Agronomy, 2020, 10, 558.
141. Z. M. Almutairi, Plant Omics, 2016, 9, 106–114.
142. S. Avestan, M. Ghasemnezhad, M. Esfahani and C. S. Byrt, Agronomy, 2019, 9, 246.
143. A. K. S. Mohamed, M. F. Qayyum, A. M. Abdel-Hadi, R. A. Rehman, S. Ali and M. Rizwan, Arch. Agron. Soil Sci., 2017, 63, 1736–1747.
144. J. An, P. Hu, F. Li, H. Wu, Y. Shen, J. C. White, X. Tian, Z. Li and J. P. Giraldo, Environ. Sci.: Nano, 2020, 7, 2214–2228.
145. D. L. McGehee, M. Alimohammadi and M. V. Khodakovskaya, Nanotechnology, 2019, 30, 275102.
146. H. F. Alharby, E. M. Metwali, M. P. Fuller and A. Y. Aldhebiani, Saudi J. Biol. Sci., 2016, 23, 773–781.
147. P. Haripriya, P. Stella and S. Anusuya, SCIOL Biotechnol., 2018, 1, 20–29.
148. L. Lu, M. Huang, Y. Huang, P. F.-X. Corvini, R. Ji and L. Zhao, Environ. Sci.: Nano, 2020, 7, 1692–1703.
149. M. Hasanuzzaman, K. Nahar, M. M. Alam, R. Roychowdhury and M. Fujita, Int. J. Mol. Sci., 2013, 14, 9643–9684.
150. M. Haghighi, R. Abolghasemi and J. A. T. da Silva, Sci. Hortic., 2014, 178, 231–240.
151. M. Iqbal, N. I. Raja, Z.-U.-R. Mashwani, M. Hussain, M. Ejaz and F. Yasmeen, Iran. J. Sci. Technol., 2019, 43, 387–395.
152. V. Shah and A. Daverey, Environ. Technol. Innovation, 2020, 18, 100774.
153. A. Konate, X. He, Z. Zhang, Y. Ma, P. Zhang, G. M. Alugongo and Y. Rui, Sustainability, 2017, 9, 790.
154. A. Emamverdian, Y. Ding, F. Mokhberdoran, Z. Ahmad and Y. Xie, Glob. Ecol. Conserv., 2020, 24, e01306.
155. X. Liu, B. Jiang, X. Yin, H. Ma and B. S. Hsiao, Sep. Purif. Technol., 2020, 233, 115976.
156. R. Rahmatizadeh, S. M. J. Arvin, R. Jamei, H. Mozaffari and F. Reza Nejhad, J. Plant Interact., 2019, 14, 474–481.
157. J. Li, J. Hu, L. Xiao, Y. Wang and X. Wang, Sci. Total Environ., 2018, 625, 677–685.
158. M. Rizwan, S. Ali, B. Ali, M. Adrees, M. Arshad, A. Hussain, M. Z. ur Rehman and A. A. Waris, Chemosphere, 2019, 214, 269–277.
159. M. Rizwan, S. Ali, M. Z. ur Rehman, M. Adrees, M. Arshad, M. F. Qayyum, L. Ali, A. Hussain, S. A. S. Chatha and M. Imran, Environ. Pollut., 2019, 248, 358–367.
160. P. Venkatachalam, N. Priyanka, K. Manikandan, I. Ganeshbabu, P. Indiraarulselvi, N. Geetha, K. Muralikrishna, R. Bhattacharya, M. Tiwari and N. Sharma, Plant Physiol. Biochem., 2017, 110, 118–127.
161. H. Bidi, H. Fallah, Y. Niknejad and D. B. Tari, Plant Physiol. Biochem., 2021, 163, 348–357.
162. M. Sathiyabama and A. Manikandan, Carbohydr. Polym., 2021, 258, 117691.
163. J. Mu, Y. Wang, M. Zhao and L. Zhang, Chem. Commun., 2012, 48, 2540–2542.
164. J. Mu, L. Zhang, M. Zhao and Y. Wang, J. Mol. Catal. A: Chem., 2013, 378, 30–37.
165. M. Jahani, R. A. Khavari-Nejad, H. Mahmoodzadeh and S. Saadatmand, Acta Biol. Cracov., Ser. Bot., 2019, 61, 29–42.
166. V. Demidchik, Environ. Exp. Bot., 2015, 109, 212–228.
167. W. Li, Z. Liu, C. Liu, Y. Guan, J. Ren and X. Qu, Angew. Chem., Int. Ed., 2017, 56, 13661–13665.
168. R. Ragg, A. Schilmann, K. Korschelt, C. Wieseotte, M. Kluenker, M. Viel, L. Völker, S. Preiß, J. Herzberger and H. Frey, J. Mater. Chem. B, 2016, 4, 7423–7428.
169. X. Liu, Q. Wang, H. Zhao, L. Zhang, Y. Su and Y. Lv, Analyst, 2012, 137, 4552–4558.
170. N. Singh, M. A. Savanur, S. Srivastava, P. D'Silva and G. Mugesh, Angew. Chem., 2017, 129, 14455–14459.
171. S. Kumar, I. M. Adjei, S. B. Brown, O. Liseth and B. Sharma, Biomaterials, 2019, 224, 119467.
172. Z. F. DAVAR, A. Roozbahani and A. Hosnamidi, Int. J. Adv. Biol. Biomed. Res., 2014, 2(4), 1150–1159.
173. M. T. B. Aghdam, H. Mohammadi and M. Ghorbanpour, Braz. J. Bot., 2016, 39, 139–146.
174. S. S. Hojjat and A. Ganjali, Int. J. Farm Allied Sci., 2016, 5, 208–212.
175. F. Behboudi, Z. Tahmasebi Sarvestani, M. Z. Kassaee, S. A. M. Modares Sanavi, A. Sorooshzadeh and S. B. Ahmadi, J. Water Environ. Nanotechnol., 2018, 3, 22–39.
176. M. Kalteh, Z. T. Alipour, S. Ashraf, M. Marashi Aliabadi and A. Falah Nosratabadi, J. Chem. Health Risks, 2018, 4, 49–55.
177. A. Fathi, M. Zahedi, S. Torabian and A. Khoshgoftar, J. Plant Nutr., 2017, 40, 1376–1385.
178. A. S. Soliman, S. A. El-feky and E. Darwish, J. Hortic. For., 2015, 7, 36–47.
179. S. Torabian, M. Zahedi and A. H. Khoshgoftar, J. Plant Nutr., 2016, 39, 172–180.
180. M. Askary, S. M. Talebi, F. Amini and A. D. B. Bangan, Biologija, 2017, 63, 65–75.
181. Y. Ye, K. Cota-Ruiz, J. A. Hernández-Viezcas, C. Valdes, I. A. Medina-Velo, R. S. Turley, J. R. Peralta-Videa and J. L. Gardea-Torresdey, ACS Sustainable Chem. Eng., 2020, 8, 1427–1436.
182. H. Hernández-Hernández, S. González-Morales, A. Benavides-Mendoza, H. Ortega-Ortiz, G. Cadenas-Pliego and A. Juárez-Maldonado, Molecules, 2018, 23, 178.
183. M. Faizan, J. A. Bhat, C. Chen, M. N. Alyemeni, L. Wijaya, P. Ahmad and F. Yu, Plant Physiol. Biochem., 2021, 161, 122–130.
184. Z. Noohpisheh, H. Amiri, A. Mohammadi and S. Farhadi, Plant Biosyst., 2021, 155, 267–280.
185. A. a. Mozafari, F. Havas and N. Ghaderi, Plant Cell, Tissue Organ Cult., 2018, 132, 511–523.
186. H. Hasanpour, R. Maali-Amir and H. Zeinali, Russ. J. Plant Physiol., 2015, 62, 779–787.
187. R. Azimi, M. J. Borzelabad, H. Feizi and A. Azimi, Pol. J. Chem. Technol., 2014, 16, 25–29.
188. M. V. Khodakovskaya, K. de Silva, D. A. Nedosekin, E. Dervishi, A. S. Biris, E. V. Shashkov, E. I. Galanzha and V. P. Zharov, Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 1028–1033.
189. J. Singh and B.-K. Lee, J. Environ. Manage., 2016, 170, 88–96.
190. Y. Tang, J. Tian, S. Li, C. Xue, Z. Xue, D. Yin and S. Yu, Sci. Total Environ., 2015, 532, 154–161.
191. D. K. Tripathi, V. P. Singh, S. M. Prasad, D. K. Chauhan and N. K. Dubey, Plant Physiol. Biochem., 2015, 96, 189–198.
192. N. Akhtar, S. Khan, S. U. Rehman, Z. U. Rehman, A. Khatoon, E. S. Rha and M. Jamil, Toxics, 2021, 9, 113.
193. M. Noman, T. Ahmed, S. Hussain, M. B. K. Niazi, M. Shahid and F. Song, J. Hazard. Mater., 2020, 398, 123175.
194. M. Noman, M. Shahid, T. Ahmed, M. Tahir, T. Naqqash, S. Muhammad, F. Song, H. M. A. Abid and Z. Aslam, Ecotoxicol. Environ. Saf., 2020, 192, 110303.
195. M. Adrees, Z. S. Khan, M. Hafeez, M. Rizwan, K. Hussain, M. Asrar, M. N. Alyemeni, L. Wijaya and S. Ali, Ecotoxicol. Environ. Saf., 2021, 208, 111627.
196. M. Adrees, Z. S. Khan, S. Ali, M. Hafeez, S. Khalid, M. Z. ur Rehman, A. Hussain, K. Hussain, S. A. S. Chatha and M. Rizwan, Chemosphere, 2020, 238, 124681.
197. S. Ali, A. Mehmood and N. Khan, J. Nanomater., 2021, 2021, 1–17.
198. S. Rao and G. S. Shekhawat, 3 Biotech, 2016, 6, 244.
199. A. Singh, N. á. Singh, S. Afzal, T. Singh and I. Hussain, J. Mater. Sci., 2018, 53, 185–201.
200. L. V. Nhan, C. Ma, Y. Rui, S. Liu, X. Li, B. Xing and L. Liu, Sci. Rep., 2015, 5, 11618.
201. P. Sharma, A. B. Jha, R. S. Dubey and M. Pessarakli, J. Bot., 2012, 2012, 217037.
202. A. Dev, A. K. Srivastava and S. Karmakar, Environ. Chem. Lett., 2018, 16, 85–100.
203. A. Rastogi, M. Zivcak, O. Sytar, H. M. Kalaji, X. He, S. Mbarki and M. Brestic, Front. Chem., 2017, 5, 78.
204. B. Van Aken, Curr. Opin. Biotechnol., 2015, 33, 206–219.
205. M. I. Morales, C. M. Rico, J. A. Hernandez-Viezcas, J. E. Nunez, A. C. Barrios, A. Tafoya, J. P. Flores-Marges, J. R. Peralta-Videa and J. L. Gardea-Torresdey, J. Agric. Food Chem., 2013, 61, 6224–6230.
206. J. Gao, G. Xu, H. Qian, P. Liu, P. Zhao and Y. Hu, Environ. Pollut., 2013, 176, 63–70.
207. M. Faisal, Q. Saquib, A. A. Alatar, A. A. Al-Khedhairy, A. K. Hegazy and J. Musarrat, J. Hazard. Mater., 2013, 250, 318–332.
208. S. Lee, S. Kim, S. Kim and I. Lee, Environ. Sci. Pollut. Res., 2013, 20, 848–854.
209. A. Rajeshwari, S. Suresh, N. Chandrasekaran and A. Mukherjee, RSC Adv., 2016, 6, 24000–24009.
210. V. N. Le, Y. Rui, X. Gui, X. Li, S. Liu and Y. Han, J. Nanobiotechnol., 2014, 12, 1–15.
211. Q. Liu, X. Zhang, Y. Zhao, J. Lin, C. Shu, C. Wang and X. Fang, Environ. Sci. Technol., 2013, 47, 7490–7498.
212. N. A. Anjum, N. Singh, M. K. Singh, I. Sayeed, A. C. Duarte, E. Pereira and I. Ahmad, Sci. Total Environ., 2014, 472, 834–841.
213. N. A. Anjum, S. S. Gill, A. C. Duarte, E. Pereira and I. Ahmad, J. Nanopart. Res., 2013, 15, 1–26.
214. B. P. Chew and J. S. Park, J. Nutr., 2004, 134, 257S–261S.
215. F. Mirzajani, H. Askari, S. Hamzelou, M. Farzaneh and A. Ghassempour, Ecotoxicol. Environ. Saf., 2013, 88, 48–54.
216. K. K. Panda, V. M. M. Achary, R. Krishnaveni, B. K. Padhi, S. N. Sarangi, S. N. Sahu and B. B. Panda, Toxicol. In Vitro, 2011, 25, 1097–1105.
217. J. Yasur and P. U. Rani, Environ. Sci. Pollut. Res., 2013, 20, 8636–8648.
218. B. Winkel-Shirley, Curr. Opin. Plant Biol., 2002, 5, 218–223.
219. C. Krishnaraj, E. Jagan, R. Ramachandran, S. Abirami, N. Mohan and P. Kalaichelvan, Process Biochem., 2012, 47, 651–658.
220. C. O. Dimkpa, J. E. McLean, N. Martineau, D. W. Britt, R. Haverkamp and A. J. Anderson, Environ. Sci. Technol., 2013, 47, 1082–1090.
221. S. Arora, P. Sharma, S. Kumar, R. Nayan, P. Khanna and M. Zaidi, Plant Growth Regul., 2012, 66, 303–310.
222. A. Mukherjee, J. R. Peralta-Videa, S. Bandyopadhyay, C. M. Rico, L. Zhao and J. L. Gardea-Torresdey, Metallomics, 2014, 6, 132–138.
223. H. Wang, X. Kou, Z. Pei, J. Q. Xiao, X. Shan and B. Xing, Nanotoxicology, 2011, 5, 30–42.
224. M. Thwala, N. Musee, L. Sikhwivhilu and V. Wepener, Environ. Sci.: Processes Impacts, 2013, 15, 1830–1843.
225. A. Rajeshwari, S. Kavitha, S. A. Alex, D. Kumar, A. Mukherjee, N. Chandrasekaran and A. Mukherjee, Environ. Sci. Pollut. Res., 2015, 22, 11057–11066.
226. Q. Liu, Y. Zhao, Y. Wan, J. Zheng, X. Zhang, C. Wang, X. Fang and J. Lin, ACS Nano, 2010, 4, 5743–5748.
227. S. Mingyu, W. Xiao, L. Chao, Q. Chunxiang, L. Xiaoqing, C. Liang, H. Hao and H. Fashui, Biol. Trace Elem. Res., 2007, 119, 183–192.
228. M. Ursache-Oprisan, E. Focanici, D. Creanga and O. Caltun, Afr. J. Biotechnol., 2011, 10, 7092.
229. C. M. Rico, M. I. Morales, R. McCreary, H. Castillo-Michel, A. C. Barrios, J. Hong, A. Tafoya, W.-Y. Lee, A. Varela-Ramirez and J. R. Peralta-Videa, Environ. Sci. Technol., 2013, 47, 14110–14118.
230. M. Kumari, S. S. Khan, S. Pakrashi, A. Mukherjee and N. Chandrasekaran, J. Hazard. Mater., 2011, 190, 613–621.
231. J. Shi, A. D. Abid, I. M. Kennedy, K. R. Hristova and W. K. Silk, Environ. Pollut., 2011, 159, 1277–1282.
232. S. Lee, H. Chung, S. Kim and I. Lee, Water, Air, Soil Pollut., 2013, 224, 1–11.
233. P. Begum, R. Ikhtiari and B. Fugetsu, Carbon, 2011, 49, 3907–3919.
234. T. Khare, V. Kumar and P. K. Kishor, Protoplasma, 2015, 252, 1149–1165.
235. C. O. Dimkpa, J. E. McLean, D. E. Latta, E. Manangón, D. W. Britt, W. P. Johnson, M. I. Boyanov and A. J. Anderson, J. Nanopart. Res., 2012, 14, 1–15.
236. M. F. Iannone, M. D. Groppa, M. E. de Sousa, M. B. F. van Raap and M. P. Benavides, Environ. Exp. Bot., 2016, 131, 77–88.
237. P. M. G. Nair and I. M. Chung, Environ. Sci. Pollut. Res., 2014, 21, 12709–12722.
238. P. Sharma, D. Bhatt, M. Zaidi, P. P. Saradhi, P. K. Khanna and S. Arora, Appl. Biochem. Biotechnol., 2012, 167, 2225–2233.
239. K. K. Panda, D. Golari, A. Venugopal, V. M. M. Achary, G. Phaomei, N. L. Parinandi, H. K. Sahu and B. B. Panda, Antioxidants, 2017, 6, 35.
240. R. Nair, S. H. Varghese, B. G. Nair, T. Maekawa, Y. Yoshida and D. S. Kumar, Plant Sci., 2010, 179, 154–163.
241. W.-N. Wang, J. C. Tarafdar and P. Biswas, J. Nanopart. Res., 2013, 15, 1–13.
242. A. Kaphle, P. Navya, A. Umapathi and H. K. Daima, Environ. Chem. Lett., 2018, 16, 43–58.
243. R. Raliya, C. Franke, S. Chavalmane, R. Nair, N. Reed and P. Biswas, Front. Plant Sci., 2016, 7, 198920.
244. E. Etxeberria, P. Gonzalez and J. Pozueta, Plant Sci., 2009, 177, 341–348.
245. P. Zhang, Y. Ma and Z. Zhang, Nanotechnology and Plant Sciences: Nanoparticles and Their Impact on Plants, 2015, pp. 77–99.
246. X. Ma, J. Geiser-Lee, Y. Deng and A. Kolmakov, Sci. Total Environ., 2010, 408, 3053–3061.
247. D. Lin and B. Xing, Environ. Sci. Technol., 2008, 42, 5580–5585.
248. W. Du, Y. Sun, R. Ji, J. Zhu, J. Wu and H. Guo, J. Environ. Monit., 2011, 13, 822–828.
249. B. Ruttkay-Nedecky, O. Krystofova, L. Nejdl and V. Adam, J. Nanobiotechnol., 2017, 15, 1–19.
250. C. Larue, J. Laurette, N. Herlin-Boime, H. Khodja, B. Fayard, A.-M. Flank, F. Brisset and M. Carriere, Sci. Total Environ., 2012, 431, 197–208.
251. A. D. Servin, H. Castillo-Michel, J. A. Hernandez-Viezcas, B. C. Diaz, J. R. Peralta-Videa and J. L. Gardea-Torresdey, Environ. Sci. Technol., 2012, 46, 7637–7643.
252. N. Khan and A. Bano, Int. J. Phytorem., 2016, 18, 1258–1269.
253. S. R. Mousavi, M. Galavi and G. Ahmadvand, Asian J. Plant Sci., 2007, 6, 1256–1260.
254. M. R. Khan, V. Adam, T. F. Rizvi, B. Zhang, F. Ahamad, I. Jośko, Y. Zhu, M. Yang and C. Mao, Small, 2019, 15, 1901794.
255. H. Feizi, M. Kamali, L. Jafari and P. R. Moghaddam, Chemosphere, 2013, 91, 506–511.
256. D. K. Tripathi, S. Singh, V. P. Singh, S. M. Prasad, D. K. Chauhan and N. K. Dubey, Front. Environ. Sci., 2016, 4, 46.
257. S. C. C. Arruda, A. L. D. Silva, R. M. Galazzi, R. A. Azevedo and M. A. Z. Arruda, Talanta, 2015, 131, 693–705.
258. C. Ma, J. C. White, O. P. Dhankher and B. Xing, Environ. Sci. Technol., 2015, 49, 7109–7122.
259. M. H. Siddiqui and M. H. Al-Whaibi, Saudi J. Biol. Sci., 2014, 21, 13–17.
260. Z. Hossain, G. Mustafa, K. Sakata and S. Komatsu, J. Hazard. Mater., 2016, 304, 291–305.
261. T. Silva, L. R. Pokhrel, B. Dubey, T. M. Tolaymat, K. J. Maier and X. Liu, Sci. Total Environ., 2014, 468, 968–976.
262. V. K. Sharma, K. M. Siskova, R. Zboril and J. L. Gardea-Torresdey, Adv. Colloid Interface Sci., 2014, 204, 15–34.
263. X. Li, W. Liu, L. Sun, K. E. Aifantis, B. Yu, Y. Fan, Q. Feng, F. Cui and F. Watari, J. Biomed. Mater. Res., Part A, 2015, 103, 2499–2507.
264. X. Lv, H. Sha, Z. Ye, Y. Wang and B. Mao, Environ. Sci.: Nano, 2023, 10, 3232–3252.
265. R. Raliya, R. Nair, S. Chavalmane, W.-N. Wang and P. Biswas, Metallomics, 2015, 7, 1584–1594.

---