Metal nanoparticles enhance ROS scavenging, nitrogen metabolism, and stress-responsive pathways to improve soybean yield and seed quality

Gold (Au)-nanoparticles (NPs), silver (Ag)-NPs, zinc oxide (ZnO)-NPs, and copper (Cu)-NPs were tested under field-relevant conditions to dissect their functional roles in soybean growth regulation, nitrogen assimilation, stress defense and seed metabolite composition. NPs accelerated vegetative development, enhanced photosynthetic performance, biomass increase and root-shoot integration. Cu-NPs produced the strongest effects, increasing shoot and root biomass by +25% and +48%, respectively, while ZnO-NPs promoted root elongation (+28%). Reinforcement of nitrogen metabolism was observed in case of Cu-NPs via increased nodulation and ureide accumulation. Cu-NPs and ZnO-NPs enhanced antioxidant enzyme activities and defense-related enzymes, coupled with transcriptional upregulation of stress-responsive genes. Yield components (grain number, 100-seed weight) improved in parallel with metabolic reprogramming, with ZnO-NPs achieving the highest enhancement (+52%). NP application also optimized protein storage, amino acid and fatty acid composition, and micronutrient allocation, establishing the possibility of a direct interaction of nanoparticles with the seed metabolism. However, the impacts of Au-NPs and Ag-NPs were not prominent as compared to Cu-NPs and ZnO-NPs. Our findings identify Cu-NPs and ZnO-NPs as biostimulants that integrate biochemical, physiological, and genetic processes to improve growth and yield in soybean. • Nanoparticle treatments modulated soybean growth and metabolism under field-relevant conditions. • Copper and zinc oxide nanoparticles enhanced photosynthesis, biomass accumulation, and root-shoot development more effectively than silver and gold nanoparticles. • Copper nanoparticles significantly boosted nodulation and nitrogen assimilation efficiency compared to other nanoparticles. • Zinc oxide and copper nanoparticles activated antioxidant and defense-related gene expression. • Nanoparticle application improved yield, seed composition, and overall nutritional quality.