HYDROXYMETHYL-PHYTOCHELATINS [(GAMMA-GLUTAMYLCYSTEINE)(N)-SERINE] ARE METAL-INDUCED PEPTIDES OF THE POACEAE

Exposure of several species of the family Poaceae to cadmium results in the formation of metal-induced peptides of the general structure (gamma-Glu-Cys)(n)-Ser (n = 2-4). They are assumed to be formed from hydroxymethyl-glutathione (gamma-Glu-Cys-Ser) and are termed hydroxymethyl-phytochelatins (hm-PCs) in analogy to the homo-phytochelatins [(gamma-Glu-Cys)(n)-beta-Ala], discovered in legumes, and the phytochelatins [PCs, (gamma-Glu-Cys)(n)-Gly] found in most other plants and many fungi. The hm-PCs were isolated from the roots of cadmium-exposed rice (Oryza sativa L. cv Strella), and their structure was confirmed by amino acid analysis after total and enzymic hydrolysis and by tandem mass spectrometry. The hm-PCs probably play a significant role in heavy metal detoxication in rice. In addition to this new form of gamma-Glu-Cys (gamma EC) peptide, PCs and gamma EC peptides without C-terminal Ser or Cry are found. All gamma EC peptides are synthesized without delay after incubation of rice plants in 100 mu M CdCl2 in the roots as well as in the shoots. Incubation times exceeding 24 h or higher concentrations of cadmium result in a selective enrichment of gamma EC peptides with higher chain length and an increased ratio of PCs to hm-PCs. gamma EC peptide synthesis is accompanied by a decrease of the glutathione content and an increase of the hydroxymethyl-glutathione content in roots and shoots of rice plants.