Enhanced expression of SaHMA3 plays critical roles in Cd hyperaccumulation and hypertolerance in Cd hyperaccumulator Sedum alfredii Hance

MAIN CONCLUSION : The enhanced expression of a P ₁B -type ATPase gene (SaHMA3) is essential for Cd hyperaccumulation and hypertolerance in Sedum alfredii Hance. A functional understanding of the mechanism through which hyperaccumulator plants accumulate and tolerate extremely toxic metals is a prerequisite for the development of novel strategies for improving phytoremediation using engineered plants or natural hyperaccumulators as well as biofortification and food crop safety. Most hyperaccumulator species, however, are small and slow-growing, and their potential for large-scale decontamination of polluted soils is limited. Sedum alfredii Hance, the only one metal hyperaccumulator from the Crassulaceae family, is an ideal candidate for gaining a functional understanding of the intra-family hyperaccumulation mechanisms as well as their potential applications. In the present study, we isolated and functionally characterized a P₁B-type ATPase gene (SaHMA3) from S. alfredii Hance. SaHMA3 alleles from a hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) were constitutively expressed in both shoot and root and encoded tonoplast-localized proteins, but showed differences in transport substrate specificity and expression level. SaHMA3 h from the HE plant was a Cd transporter. In contrast, SaHMA3n from NHE plants was able to transport both Zn and Cd. SaHMA3 showed a significantly higher constitutive expression level in HE plants than in NHE plants. Furthermore, the expression level of SaHMA3 in the shoots of HE plants was considerably higher than in the roots. Overexpression of SaHMA3h in tobacco plants significantly enhanced Cd tolerance and accumulation and greatly increased the root sequestration of Cd. In summary, our data suggested that SaHMA3 plays critical roles in Cd hyperaccumulation and hypertolerance in Cd hyperaccumulator S. alfredii Hance.