Heavy metal detoxification in higher plants - A review

A set of heavy-metal-complexing peptides was isolated from plants and plant suspension cultures. The structure of these peptides was established as (gamma-glutamic acid-cysteine)(n)-glycine (n=2-11) [(gamma-Glu-Cys)(n)-Gly]. 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. These peptides, called phytochelatins (PC), are induced in all autotrophic plants so far analyzed, as well as in select fungi. Some species of the order Fabales and the family Poaceae synthesize aberrant PC that contain, at their C-terminal end, either (b)eta-alanine, serine or glutamic acid. For this group of peptides the name iso-PC is proposed. The biosynthesis of PC proceeds by metal activation of a constitutive enzyme that uses glutathione (GSH) as a substrate; this enzyme is a gamma-glutamylcysteine dipeptidyl transpeptidase which was given the trivial name PC synthase. It catalyzes the following reaction: gamma-Glu-Cys-Gly+(gamma-Glu-Cys)(n)-Gly-->(gamma-Glu-Cys)n+1-Gly+Gly. The plant vacuole is the transient storage compartment for these peptides. They probably dissociate, and the metal-free peptide is subsequently degraded. Sequestration of heavy metals by PC confers protection for heavy-metal-sensitive enzymes. The isolation of a Cd2+-sensitive cad1 mutant of Arabidopsis thaliana, that is deficient in PC synthase, demonstrates conclusively the importance of PC for heavy metal tolerance. In spite of the fact that nucleic acid sequences and proteins are found in higher plants that have distant homology to animal metallothioneins, there is absolutely no experimental evidence that these 'plant metallothioneins' are involved in the detoxification of heavy metals. PC synthase will be an interesting target for biotechnological modification of heavy metal tolerance in higher plants.