Plays a role in metal homeostasis. Confers tolerance to high zinc (Zn) and nickel (Ni). Homodimer. Expressed constitutively in roots, leaves, and stems. Up regulated by iron deficiency in roots and leaves, as well as by nickel, high zinc or high copper treatments. Repressed by heat treatment, high iron, low copper and low zinc treatments. May be due to a competing acceptor splice site.