The Cation/H⁺exchanger OsCAX2 is involved in cadmium tolerance and accumulation through vacuolar sequestration in rice

AbstractExcessive cadmium (Cd) in rice grains is a serious food safety problem. The development of Cd-safe varieties requires the identification of germplasms and genes with major effect on Cd accumulation but without negative effects on other important traits. Here, we reported that OsCAX2, a member of the rice Cation/H+exchanger (CAX) family, is an important Cd transporter.OsCAX2encodes a tonoplast-localized protein and is strongly upregulated by Cd, mainly expresses in root exodermis, parenchyma in cortex, endodermis and stele cells. Depletion ofOsCAX2resulted in enhanced Cd sensitivity and root-to-shoot translocation in rice, while overexpression ofOsCAX2significantly increased Cd tolerance and reduced Cd transport by promoting root Cd influx and vacuolar storage, which ultimately reduced Cd transport via xylem.OsCAX2also had significant effects on tissues/organs distribution of Cd but had no effects on grain yield and agronomic traits. Importantly, theOsCAX2overexpressing lines had more than 70% lower grain Cd accumulation, increased iron (Fe), zinc (Zn) and manganese (Mn) and reduced copper (Cu) accumulation. Therefore,OsCAX2is an ideal gene for developing Cd-safe rice varieties via transgenic approach.