Heavy Metal Detoxification in Plants: Phytochelatin Biosynthesis and Function

Plants have evolved an extensive suite of adaptive responses to heavy metal toxicity. These include the immobilisation, exclusion, chelation and compartmentalisation of metal ions and often involve metal-binding ligands. A number of such ligands have been identified in plants and include organic acids, amino acids, peptides and polypeptides. Polypeptides include the metallothioneins (MTs), small, gene-encoded, cysteine-rich polypeptides, and the phytochelatins (PCs), which, in contrast, are enzymatically-synthesised, cysteine-rich peptides. MTs were first identified as Cd-binding proteins in mammalian tissues and appear to be ubiquitous in animal species. Thus, early reports of metal-binding proteins in plants generally assumed them to be MTs. However, in the absence of detailed characterisation or primary amino acid sequences, many of these metal-binding complexes may have been comprised, at least in part, of PCs, particularly where they were identified in studies of plant responses to Cd. After the structures of PCs had been elucidated and it was found that these peptides appear to be ubiquitous in the plant kingdom, it was proposed that PCs were the functional equivalent of MTs (1). However, numerous examples of MT-like genes, and in some cases MT proteins, have now been isolated from a variety of plant species and it is apparent that plants express both of these metal-binding ligands. It is likely that the two play relatively independent functions in metal detoxification and/or metabolism. PCs have not been reported in an animal species, supporting the notion that in animals, MTs may well perform some of the functions normally contributed by PCs Heavy Metal Detoxification in Plants: Phytochelatin Biosynthesis and Function