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 heterologous system by AsO(4)(3-) ions, but not by Cu(2+), Zn(2+), Mn(2+) or Ni(2+) ions. Expressed in shoots, roots, leaves, stems and flowers. Not induced by cadmium or other heavy metal stress. Expression of PCS2 is too low to complement a PCS1-defective mutant. Belongs to the phytochelatin synthase family.