Hydrogen peroxide reduces root cadmium uptake but facilitates root-to-shoot cadmium translocation in rice through modulating cadmium transporters.

Cadmium (Cd) contamination in agricultural soils has become a serious worldwide environmental problem threatening crop production and human health. Hydrogen peroxide (HO) is a critical second messenger in plant response to Cd exposure. However, its role in Cd accumulation in various organs of plants and the mechanistic basis of this regulation remains to be elucidated. In this study, we used electrophysiological and molecular approaches to understand how HO regulates Cd uptake and translocation in rice plants. Our results showed that the pretreatment of HO significantly reduced Cd uptake by rice roots, which was associated with the downregulation of OsNRAMP1 and OsNRAMP5. On the other hand, HO promoted the root-to-shoot translocation of Cd, which might be attributed to the upregulation of OsHMA2 critical for Cd phloem loading and the downregulation of OsHMA3 involved in the vacuolar compartmentalization of Cd, leading to the increased Cd accumulation in rice shoots. Furthermore, such regulatory effects of HO on Cd uptake and translocation were notably amplified by the elevated level of exogenous calcium (Ca). Collectively, our results suggest that HO can inhibit Cd uptake but increase root to shoot translocation through modulating the transcriptional levels of genes encoding Cd transporters, furthermore, application of Ca can amplify this effect. These findings will broaden our understanding of the regulatory mechanisms of Cd transport in rice plants and provide theoretical foundation for breeding rice for low Cd accumulation.