The Sahma2h Transporter Promotes Zn/Cd Transport From Root to Stem In Zn/Cd Hyperaccumulator Sedum Alfredii Hance

Abstract

The Zn/Cd hyperaccumulating ecotype (HE) of Sedum alfredii Hance exhibited significantly higher Zn/Cd root-to-shoot translocation than the non-hyperaccumulating ecotype (NHE) of S. alfredii.This study isolated two genes encoding heavy metal ATPases,named SaHMA2h and SaHMA2n,from both ecotypes.Subcellular localization analysis revealed that both transporters were located on the plasma membrane.Both ecotypes exhibited significantly higher mRNA levels of HMA2 in the roots compared to the shoots, with SaHMA2h transcript level being over 20 times higher than that of SaHMA2n in both roots and shoots.Treatment with 500 µM Zn or 50 µM Cd significantly up-regulated the expression of SaHMA2h in both roots and shoots.Complementary assays in yeast mutants demonstrated that SaHMA2h and SaHMA2n could export Zn and Cd from yeast cells. The amino acids at positions 617 and 150 of SaHMA2h were crucial for Zn/Cd transport.Heterologous expression of 35S:SaHMA2h in tobacco and S. alfredii significantly increased Zn/Cd tolerance.The μ-XRF analysis showed that more Zn/Cd accumulate in the stems and less in root of transgenic plant.These results suggest that SaHMA2h is a Zn/Cd transporter that regulates Zn/Cd efflux from root cell into shoot via xylem vessel.

Keywords: Sedum alfredii Hance, zinc； cadmium, P1B-type ATPase, HMA2, hyperaccumulator