Confers resistance to heavy metal ions (e.g. aluminium) by chelating them at the plasma membrane of root cells, thus stopping their entry and reducing their accumulation (PubMed:23888867). Binds to and confers tolerance to aluminium (Al) (PubMed:23888867). Mostly expressed in roots at low levels and, to a lower extent, in shoots (PubMed:23888867). Observed in all cell types of both root tips and mature regions (at protein level) (PubMed:23888867). Accumulates in roots upon aluminium (Al) exposure in an STAR3/ART1-dependent manner (at protein level) (PubMed:23888867). Up-regulated in shoot tissues in response to cadmium (CdCl(2)) (PubMed:19017626). Decreased tolerance to aluminium (Al), but normal tolerance to cadmium (Cd) (PubMed:23888867). Lower Al accumulation in root cell walls and plasma membranes but increased Al levels in root cell sap of plants exposed to aluminium (PubMed:23888867). Belongs to the CYSTM1 family.