Phytochelatins and Their Roles in Heavy Metal Detoxification

Abstract

Plants respond to heavy metal toxicity in a variety of different ways. Such responses include immobilization, exclusion, chelation and compartmentalization of the metal ions, and the expression of more general stress response mechanisms such as ethylene and stress proteins. These mechanisms have been reviewed comprehensively by Sanita di Toppi and Gabbrielli (1999)for plants exposed to Cd, the heavy metal for which there have been arguably the greatest number and most wide-ranging studies over many decades. Understanding the molecular and genetic basis for these mechanisms will be an important aspect of developing plants as agents for the phytoremediation of contaminated sites (Salt et al., 1998). One recurrent general mechanism for heavy metal detoxification in plants and other organisms is the chelation of the metal by a ligand and, in some cases, the subsequent compartmentalization of the ligand-metal complex. A number of metal-binding ligands have now been recognized in plants. The roles of several ligands have been reviewed by Rauser (1999). Extracellular chelation by organic acids, such as citrate and malate, is important in mechanisms of aluminum tolerance. For example, malate efflux from root apices is stimulated by exposure to aluminum and is correlated with aluminum tolerance in wheat (Delhaize and Ryan, 1995). Some aluminum-resistant mutants of Arabidopsis also have increased organic acid efflux from roots (Larsen et al., 1998). Organic acids and some amino acids, particularly His, also have roles in the chelation of metal ions both within cells and in xylem sap (Kramer et al., 1996;Rauser, 1999). Peptide ligands include the metallothioneins (MTs), small gene-encoded, Cys-rich polypeptides. Our current understanding of the functions and expression of MTs in plants, particularly Arabidopsis, have been reviewed elsewhere (Fordham-Skelton et al., 1998; Rauser, 1999). In contrast, the phytochelatins (PCs), the subject of thisUpdate, are enzymatically synthesized Cys-rich peptides. The most recent review of PC structure, biosynthesis, and function in this journal was by Rauser (1995). Other more recent reviews are by Zenk (1996) and Rauser (1999). Recent advances in our understanding of aspects of PC biosynthesis and function are derived predominantly from molecular genetic approaches using model organisms. The historical context of the identification of MTs and PCs in plants is worth discussing. MTs were first identified as Cd-binding proteins in mammalian tissues. Similar proteins have been identified in a large number of animal species (Kägi, 1991). Early reports of metal-binding proteins in plants were generally assumed to be MTs. However, in the absence of detailed characterization 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 are distributed widely in the plant kingdom, it was proposed that PCs were the functional equivalent of MTs (Grill et al., 1987). Subsequently, numerous examples of MT-like genes, and in some cases MT proteins, have been isolated from a variety of plant species and it is now apparent that plants express both of these Cys-containing metal-binding ligands. Furthermore, it is likely that the two play relatively independent functions in metal detoxification and/or metabolism. However, the extent to which this is true is not yet clear and will not become apparent until a complete set of MT-deficient mutants have been identified in Arabidopsis. 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 in plants. However, the isolation of the PC synthase gene from plants and the consequent identification of similar genes in animal species, described below, suggests that, at least in some animal species, both of these mechanisms contribute to metal detoxification and/or metabolism. Although PCs have also been classified as class III MTs and the use of the term “phyto” to designate these compounds appears to be increasingly inaccurate, I believe “phytochelatins” has become so entrenched in the literature that a more broadly encompassing designation is likely to cause only confusion. Early analyses demonstrated PCs consisted of only the three amino acids: Glu, Cys and Gly with the Glu, and Cys residues linked through a γ-carboxylamide bond. PCs form a family of structures with increasing repetitions of the γ-Glu-Cys dipeptide followed by a terminal Gly; (γ-Glu-Cys)n-Gly, where n has been reported as being as high as 11, but is generally in the range of 2 to 5. PCs have been identified in a wide variety of plant species and in some microorganisms. They are structurally related to glutathione (GSH; γ-Glu-Cys-Gly) and were presumed to be the products of