Iron Oxide Nanoparticles Alleviated Cd Stress on Rice Plants: New Insights from Root Exudates and Metabolites

Iron-based nanoparticles (FeNPs) demonstrate potential in alleviating Cd phytotoxicity; however, whether the response mechanisms vary from inside to outside the roots remains poorly understood. Here, Fe2O3 NPs-mediated mitigation of Cd toxicity in rice seedlings was investigated through root-exudate and root-metabolome analyses. FeNPs application enhanced plant growth by 47.01% and alleviated Cd-induced oxidative stress via modulation of antioxidant enzymes. Notably, FeNPs reshaped root exudation patterns under Cd stress, increasing low-molecular-weight organic acids by 20.9% but decreasing amino acids by 42.7% compared with the Cd-only treatment. Metabolomics profiles revealed that FeNPs differentially regulated endogenous metabolites: suppressing stress-associated malate accumulation by 6.99% while elevating gibberellin levels by 16.8–20.6% and upregulating arginine and glutamate metabolisms. The response of amino acids was more severe than that of organic acids, and arginine tended to accumulate and facilitate metabolism inside the roots. These findings provide novel insights into different response mechanisms in plant roots to nanoparticle application under heavy metal stress.