Comparison of Cd concentration in rice grain betweenOsNramp5knock-out mutants andOsHAM3-overexpressing line

Cadmium (Cd) is highly toxic and a carcinogen to humans. Rice is prone to absorbing Cd and accumulating it in the grain, which raises health concerns for rice consumers. OsNramp5 is a major transporter for Cd and manganese (Mn) uptake in rice, whereas OsHMA3 is a tonoplast-localized transporter for Cd detoxification. In this study, we compared the efficiency of OsNramp5 knockout mutation and OsHAM3 overexpression in reducing Cd content in the rice grain. The grain Cd content of the OsNramp5 knockout mutants was significantly lower than that of the wild-type rice T5105. However, the OsNramp5 knockout mutants still had much higher grain Cd content than the similar OsNramp5 mutants reported previously or the OsHAM3 overexpression line developed in our previous study. Pyramiding the OsNramp5 mutant allele and the OsHAM3 transgene in a double homozygous line could not further reduce grain Cd content. The OsNramp5 gene in T5105 has a haplotype II promoter, and its knockout mutation partially impairs Mn uptake in rice. Our results demonstrate that OsHMA3 overexpression works more efficiently in generating low-Cd rice grain than OsNramp5 knockout mutation without affecting Mn uptake in rice.