Regulation of iron and cadmium uptake in rice roots by iron() oxide nanoparticles: insights from iron plaque formation, gene expression, and nanoparticle accumulation

The application of iron oxide nanoparticles (FeNPs) can alleviate cadmium (Cd) accumulation in rice. However, the effect of FeNPs on the interaction between Fe and Cd during uptake in rice roots remains poorly understood. Herein, Fe 2 O 3 NPs were applied to rice in a hydroponic experiment under Cd stress. The application of FeNPs significantly decreased the Cd concentrations in roots and shoots and reduced the Fe concentration in shoots. Fe plaque formation was significantly enhanced either by FeNPs or Cd stress; however, the ratios of Cd Fe plaque /Cd whole plant and Cd Fe plaque /Fe Fe plaque indicated that the contribution of Fe plaque to alleviating Cd uptake was limited. Gene expression quantification suggested that the presence of FeNPs inhibited the uptake of Fe 2+ and Cd 2+ via OsNRAMP5 , OsCd1 , OsIRT1 and OsIRT2 transporters, but it facilitated the uptake of Fe( iii ) via the OsYSL15 transporter. TEM-EDS evidenced the accumulation of FeNP aggregates in both the symplast and apoplast of roots, particularly in the symplast, which strongly restricted the root-to-shoot translocation of Cd and Fe, resulting in the Fe accumulation in shoots being even lower than those without FeNPs. This study provides a comprehensive understanding of the regulation mechanisms of Fe and Cd uptake in rice roots by FeNPs from the perspectives of Fe plaque, gene expression, and NP accumulation. The finding that FeNP accumulation in rice roots restricted Fe translocation to the shoot suggested that further investigation needs to optimize the distribution of Fe to rice grains during FeNP application. Fe 2 O 3 nanoparticles alleviated Cd uptake mainly via down-regulation of OsNRAMP5 , OsCd1 , OsIRT1 and OsIRT2 in roots, while the contribution of the enhanced formation of Fe plaque was minor.