Involved in the synthesis of phytochelatins (PC) and homophytochelatins (hPC), the heavy-metal-binding peptides of plants. Also involved in glutathione-conjugates degradation. [Glu(-Cys)](n)-Gly + glutathione + H(+) = [Glu(-Cys)](n+1)-Gly + glycine Requires cadmium for activity. Also activated in vitro or in heterologous system by Ag(+), Hg(+), Zn(2+), Cu(2+), Fe(2+) or Fe(3+) ions, but not by Co(2+) or Ni(2+) ions. Expressed in roots and shoots. Expressed constitutively. Induced by copper, and by cadmium during the early stages of plant development. Gradual decrease of cadmium induction as plants continue to grow, and by 15 day after germination, total loss of induction. The intermediate part (284-372) is important for enzyme stabilization. The C-terminal part (373-485) is required to determine enzyme responsiveness to a broad range of heavy metals. Plants are highly sensitive to Cd (20- to 40-fold) and AsO(4)(3-) ions, sensitive (eightfold) to Ag ions, slightly sensitive (twofold) to Cu and Hg ions, and insensitive to Zn, SeO(3)(2-) and Ni ions. Homoglutathione (hGSH) is a good acceptor, but a poor donor, of gamma-glutamylcysteine units. Belongs to the phytochelatin synthase family.