Confers resistance to heavy metal ions (e.g. cadmium (CdCl(2)) and copper (CuCl(2))) by chelating them at the plasma membrane of root cells, thus stopping their entry and reducing their accumulation. Expressed only in roots. Down-regulated in root tissues but up-regulated in shoot tissues in response to cadmium (CdCl(2)). Belongs to the CYSTM1 family.