OsNRAMP2facilitates Cd efflux from vacuoles and contributes to the difference in grain Cd accumulation between japonicaand indicarice

Cadmium (Cd) accumulation in rice grain is of health concern. Identifying genes involved in grain Cd accumulation and performing molecular breeding may reduce it. In this study, knockout of OsNRAMP2, a member of the NRAMPfamily, reduced grain Cd concentrations by more than 38%, and overexpressing OsNRAMP2increased grain Cd concentrations by more than 50%. Physiological experiments showed that OsNRAMP2facilitated Cd translocation from root to shoot by positively regulating Cd efflux from the vacuoles. At filling stage, OsNRAMP2was highly expressed in all tissues except for husk, suggesting its role in Cd remobilization. Changes in OsNRAMP2expression affected the concentrations of Fe, Mn, Zn, and Cu in grain and also affected rice growth. Phylogenetic analysis showed that the distribution of OsNRAMP2haplotypes between japonicaand indicawas different. Among the four haplotypes of OsNRAMP2, Hap 1, with a 6-bp nucleotide insertion in exon 1, had grain Cd concentration at least 45.3% lower than any of the other three haplotypes. Almost all (99.3%) japonicaaccessions but rare indicaaccessions (4.44%) from the 3K sequenced rice genomes carry Hap 1 of OsNRAMP2. Our study sheds light on the molecular mechanism of grain Cd accumulation and provides a promising target for low-Cd rice breeding.