Any of a group of sulphur-rich peptides found in plants exposed to high concentrations of heavy metals (e.g. cadmium, zinc, lead, copper, mercury) and thought to play a role in detoxification. They are unusual peptides synthesized from glutathione and with the general formula (γ‎-glutamic acid-cysteine)_n-glycine, where n is 2–11. They bind the metal ions in the cell cytoplasm and the resulting complex is transported to the cell vacuole, where they are sequestered by organic acids.