α-Fe2O3 nanomaterials strengthened the growth promoting effect of Pseudomonas aurantiaca strain JD37 on alfalfa via enhancing the nutrient interaction of the plant–rhizobacteria symbiont

Abstract

Plant growth-promoting rhizobacteria (PGPR) are an important supplement for crop production, but their application suffers from unstable efficiency in real soil environments due to the vulnerability of the plant–rhizobacteria symbiont. Notably, Fe-based nanomaterials (NMs) provide not only iron nutrition for plants but also stable micro-environments for rhizobacteria; they may positively act on the plant–rhizobacteria symbiont to strengthen crop production. To verify this hypothesis, a 65 d pot experiment was conducted, where the single and combined effects of Pseudomonas chlororaphis subsp. Aurantiaca JD37 (0, 1.5 × 10⁷, 1.5 × 10⁸, 1.5 × 10⁹ CFU per plant) and α-Fe₂O₃ NMs (0, 1000 mg kg⁻¹ dry soil) on alfalfa growth were investigated. The results showed that the inoculum of JD37 alone failed to promote alfalfa growth significantly, while the co-treatment of α-Fe₂O₃ NMs and JD37 (up to 1.5 × 10⁸ CFU per plant) increased the root dry biomass (66.2%) and shoot dry weight (26.8%). Mechanistically, JD37 amendment increased the active Fe content in the soil spiked with α-Fe₂O₃ NMs, providing more available iron nutrition for alfalfa growth. Furthermore, α-Fe₂O₃ NMs stimulated the alfalfa root metabolism, resulting in increased production of ethanolamine, D-trehalose, and low molecular acids in alfalfa roots, which may be utilized by JD37 as carbon and nitrogen sources and iron dissolving agents. These two aspects jointly strengthened the plant–rhizobacteria symbiont and promoted alfalfa growth. Overall, our findings suggest that the amendment of α-Fe₂O₃ NMs in agricultural soils can benefit crop production via enhancing the nutrient interaction of the plant–rhizobacteria symbiont.