Gamma-glutamylcysteine synthase plays an important role in the synthesis of glutathione (glutamyl-cysteinyl-glycine, GSH) insofar as it specifically catalyses the ligation of glutamic acid and cysteine to form glutamylcysteine under energy consumption. Other gamma-ECS substrates are not known. Glutathione is formed in a second step, in which glycine is bound to the dipeptide by glutathione synthetase.

Glutathione performs several functions in plant cells. As an antioxidant, it protects plant cells against oxidation, acts as a reserve for organic sulphur and as a precursor for phytochelatins [(gamma-glutamylcysteine)n-glycine; n=2-11] it contributes to the detoxification of xenobiotics and heavy metals. This is brought about by the formation of stable complexes with metal ions from the thiol group of the cysteine residues of phytochelatins, which are pumped under ATP consumption into the plant cell vacuoles where they are stored.