Rice exposure to silver nanoparticles in a life cycle study: effect of dose responses on grain metabolomic profile, yield, and soil bacteria†
Here, we conducted a life-cycle study with rice grown in soil amended with different concentrations (0, 1 and 10 mg kg−1) of silver nanoparticles (AgNPs) for 120 days. A lower dose of AgNPs promoted rice tillering, resulting in a significant increase in grain yield (42.3%) and aerial biomass (19.7%) compared to an untreated control, while a higher dose of AgNPs significantly decreased grain yield (26.4%) and biomass (28.1%). Inductively coupled plasma-mass spectrometry (ICP-MS) data reveal that Ag was accumulated in roots and translocated into shoots. Importantly, Ag was detected in the rice seeds, but was primarily located in rice hull and bran, not reaching edible tissues (polished rice), suggesting a low dietary risk to human health. Gas chromatography-mass spectrometry (GC-MS) based metabolomics show that 1 mg kg−1 AgNPs altered the grain metabolome, increasing the abundance of functional metabolites (naproxen, benzyl alcohol, and metharbital) and decreasing several primary metabolites (TCA cycle intermediates). In rice grains, the content of Fe and Mo was significantly decreased by 24.7% and 31.8%, respectively, upon exposure to 1 mg kg−1 AgNPs, indicating a trade-off between grain yield and mineral nutrient content. In addition, AgNPs at 1 mg kg−1 caused a significant increase (1.2–2.3 fold) in several putative plant-beneficial taxa, inlcuding Frankiales, Rhizobiales, Chitinophagales, and Saccharimonadia. Taken together, these results show that AgNPs at 1 mg kg−1 are generally beneficial to plants and associated soil microbes, although additional work on material properties and amendment regimen is needed to optimize the agricultural benefits and minimize potential negative consequences on grain nutrient content.