Hydroxymethyl-Phytochelatins [([gamma]-Glutamylcysteine)n-Serine] Are Metal-Induced Peptides of the Poaceae

Abstract

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 Gly 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.