Glutathione serves as a precursor of peptides, known as phytochelatins, which are composed of two or more repeating gamma-glutamylcysteine units with a terminal glycine residue; (gamma-glutamylcysteine)_n-gly, where n = 2 to 11 (Steffens et al, 1986; Reese and Wagner, 1987). The enzyme responsible for the synthesis of these peptides is known as phytochelatin synthase (Rauser, 1990; 1995). Phytochelatins (PCs) play an important role in the detoxification of certain heavy metals (particularly cadmium) in plants (Rauser, 1990; Howden et al, 1995ab; Rauser, 1995). These peptides appear upon induction of plants with metals of the transition and main groups (Ib-Va, Z = 29-83) of the periodic table of elements (Zenk, 1996). In Rubia tinctorum phytochelatins (class III metallothionein) are induced by many metal ions, but only a few (Ag, Cd and Cu) were bound to the PCs that they induced (Maitani et al, 1996). These peptides are induced in all autotrophic plants so far analyzed, as well as in certain fungi (Zenk, 1996). Phytochelatin synthase (PC synthase) (glutathione gamma-glutamylcysteinyltransferase or gamma-glutamylcysteine dipeptidyl transpeptidase) [EC 2.3.2.15] is a constitutive enzyme that is activated by cadmium and other metal ions (Rauser, 1995). It catalyzes the following reaction: gamma-Glu-Cys-Gly + (gamma-Glu-Cys)_n-Gly-->(gamma-Glu-Cys)_n+1-Gly + Gly.

The isolation of a Cd²⁺-sensitive cadl mutant of Arabidopsis thaliana, that is deficient in PC synthase, demonstrates conclusively the importance of PC for heavy metal tolerance (Howden et al, 1995ab; Zenk, 1996). Overexpression of the E. coli glutathione synthetase gene in Indian mustard leads to increased cadmium tolerance, in part due to increased production of PCs (Zhu et al, 1999a). Cadmium tolerance and accumulation in Indian mustard is also enhanced by overexpressing gamma-glutamylcysteine synthetase (Zhu et al, 1999b).

In certain plants (notably legumes) which can synthesize homoglutathione, in which B-alanine is substituted for glycine as the terminal amino acid, homophytochelatins are synthesized along with PCs in response to Cd (Klapheck et al, 1995).

In maize and certain other species of the Poaceae, a third family of phytochelatins has been found in which serine is the carboxy-terminal amino acid (Rauser and Meuwly, 1995). Some species of the family Poaceae synthesize PCs that contain glutamic acid at their C-terminal end (Zenk, 1996).

The plant vacuole is the transient storage compartment for these peptides (Zenk, 1996); Cd detoxification may require transport of the Cd-phytochelatin complexes into the vacuole. A transport system has been recently described for these complexes (Salt and Rauser, 1995). Glutathione-S-conjugates are also transported into the vacuole in an ATP-dependent manner. The Cd-phytochelatin complexes probably dissociate, and the metal-free peptide is subsequently degraded (Zenk, 1996).

In Arabidopsis, both Cd and Cu induce transcription of the genes for glutathione synthesis (gamma-glutamylcysteine synthetase and glutathione synthetase), as well as glutathione reductase (Xiang and Oliver, 1998). Jasmonic acid also activates the same genes, but does not elevate glutathione content (Xiang and Oliver, 1998).

Although nucleic acid sequences and proteins are found in higher plants that have distant homology to animal metallothioneins, there is little evidence that these "plant metallothioneins" are involved in the detoxification of heavy metals (Zenk, 1996).

Howden R, Goldsbrough PB, Andersen CR, Cobbett CS 1995a Cadmium-sensitive, cad1 mutants of Arabidopsis thaliana are phytochelatin deficient. Plant Physiol. 107: 1059-1066.

Howden R, Andersen CR, Goldsbrough PB, Cobbett CS 1995b A cadmium-sensitive, glutathione-deficient mutant of Arabidopsis thaliana. Plant Physiol. 107: 1067-1073.

Klapheck S, Schlunz S, Bergmann L 1995 Synthesis of phytochelatins and homo-phytochelatins in Pisum sativum L. Plant Physiol. 107: 515-521.

Maitani T, Kubota H, Sato K, Yamada T 1996 The composition of metals bound to class III metallothionein (phytochelatin and its desglycyl peptide) induced by various metals in root cultures of Rubia tinctorum. Plant Physiol. 110: 1145-1150.

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Steffens JC, Hunt DF, Williams BG 1986 Accumulation of non-protein metal-binding polypeptides (gamma-glutamyl-cysteinyl)_n-glycine in selected cadmium resistant tomato cells. J. Biol. Chem. 261: 13879-13882.

Xiang C, Oliver DJ 1998 Glutathione metabolic genes coordinately respond to heavy metals and jasmonic acid in Arabidopsis. Plant Cell 10: 1539-1550.

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Zhu YL, Pilon-Smits EAH, Jouanin L, Terry N 1999a Overexpression of glutathione synthetase in Indian mustard enhances cadmium accumulation and tolerance. Plant Physiol. 119: 73-79.

Zhu YL, Pilon-Smits EA, Tarun AS, Weber SU, Jouanin L, Terry N 1999b Cadmium tolerance and accumulation in Indian mustard is enhanced by overexpressing gamma-glutamylcysteine synthetase. Plant Physiol. 121: 1169-1178.