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. Expressed in roots and shoots. 5- to 10-fold by 100 uM cadmium. Belongs to the phytochelatin synthase family.