Involved in the synthesis of phytochelatins (PC) and homophytochelatins (hPC), the heavy-metal-binding peptides of plants. [Glu(-Cys)](n)-Gly + glutathione + H(+) = [Glu(-Cys)](n+1)-Gly + glycine Requires cadmium for activity. Also activated in vitro by Zn(2+), Cu(2+), Fe(2+) or Fe(3+) ions, but not by Co(2+) or Ni(2+) ions. Expressed in roots, nodules and leaves. By cadmium. In roots, Cd(2+) and Zn(2+) induce preferentially homophytochelatin (hPC) synthesis whereas Cu(2+) has a major effect on phytochelatin (PC) synthesis. Homoglutathione (hGSH) is a good acceptor, but a poor donor, of gamma-glutamylcysteine units. Belongs to the phytochelatin synthase family.