Phytochelatins

Definition
Phytochelatins (PCs) are small, cysteine-rich peptides capable of binding heavy metal ions via thiolate coordination.

Discovery
PCs were first discovered as Cd-binding complexes produced in fission yeast, Schizosaccharomyces pombe, exposed to Cd2+ ions and named them "cadystins"1. Independently, Grill et al., in 1985 found the ubiquitous occurrence of the same peptides and those with higher degrees of polymerization (n = 2-11) in various cells of plants exposed to Cd2+ ions and termed them as "phytochelatins". The term "phyto-meaning plants" and "chelatin" the metal chelating properties2.

Structure Characteristics
PCs have the general structure (?-Glu-Cys)n-Gly. Early analyses demonstrated PCs consisted of only the three amino acids: Glu, Cys and Gly with the Glu, and Cys residues linked through a ?-carboxylamide bond. PCs form a family of structures with increasing repetitions of the ?-Glu-Cys dipeptide followed by a terminal Gly; (?-Glu-Cys)n-Gly, where n is generally in the range of 3 to 72. PCs have been identified in a wide variety of plant species and in some microorganisms. They are structurally related to glutathione (GSH; ?-Glu-Cys-Gly) and were presumed to be the products of a biosynthetic pathway. In addition, a number of structural variants, for example, (?-Glu-Cys)n-b-Ala, (?-Glu-Cys)n-Ser, and (?-Glu-Cys)n-Glu, have been identified in some plant species3.

Mode of Action
PCs and its function in heavy metal tolerance of higher plants. The toxic heavy metal accumulation in soil would deteriorates crop growth and yield components, and threaten the agro-products security. Earlier reports indicate that, there were significant differences in the accumulation and tolerance to heavy metals among plant species and genotypes. The formation of PCs in response to the stress caused by heavy metals was one of the truly adaptive responses occurred commonly in higher plants. Furthermore, the heavy metal tolerant genotypes show much higher accumulation of PCs than the non-tolerant lines. Glutathione (GSH) is the substrate for the synthesis of PCs, which chelates the metals. It has been shown that, inactive toxic metal ions of metal--PCs chelatins were subsequently transported from cytosol to vacuole before they poison the enzymes of life-supporting metabolic routes, and transiently stored in vacuole to reduce the heavy metal concentration in cytosol, thus, heavy metal detoxification was attained4.

Functions
Roles of PCs in suspension-cultured cells: Studies on heavy metal tolerance in suspension-cultured cells have been performed using common plants such as tomato. In the most culture systems, both the PCs and Cd contents in the cells increase as a function of Cd concentrations in the media. In these cells, the Cd-binding PC complexes are detected as the major fraction containing Cd2+ while free Cd ions are present in small amounts. Furthermore, after subculturing in the presence of Cd2+ ions, the cells exhibit further tolerance to Cd2+ ions at 1 mM or more in view of the over-production of PC peptides5.

PCs as biochemical indicators for heavy metal contamination: The formation PCs and PC-related peptides in plant tissues are the possible biochemical indicators for heavy metal contamination. PC formation in plant tissues occurs in function of external metal concentrations in general, irrespective of whether they are specific or non-specific to metals. For example, chickpea provides very capable indicators for heavy metal contamination6. The roots are very sensitive in producing PCs and homo-phytochelatin (hPCs) in a specific manner in response to Cd and As, and the shoots, as well as the roots of the plant, are sensitive to various metals in terms of the rapid increases in the levels of GSH, hGSH and cysteine7. These changes can be used in the biochemical evaluation of some specific or overall heavy metal contamination in various habitats. Furthermore, the signaling and transport systems between root and shoot in terms of heavy-metal response will be a matter of great importance in the future8.

References
1.Murasugi A, Wada C, Hayashi Y (1981) Cadmium-binding peptide induced in fission yeast, Schizosaccharimyces pombe. J. Biochem., 90:1561-1564.
2.Grill E, Winnacker E L, and Zenk M H (1985) Phytochelatins: the principal heavy-metal complexing peptides of higher plants. Science., 230:674-676.
3.Rauser W E (1995) Phytochelatins and related peptides: structure, biosynthesis, and function. Plant Physiol ., 109:1141–1149.
4.Wu F, Zhang G (2003). Phytochelatin and its function in heavy metal tolerance of higher plants.Ying Yong Sheng Tai Xue Bao., 14(4):632-636.
5.Inouhe M, Mitsumune M, Tohoyama H, Joho M, Murayama T (1991 b) Contributions of cell wall and metal-binding peptides in suspension-cultured cells of tomato. Bot. Mag. Tokyo 104:217-229.
6.Gupta D K, Tohoyama H, Joho M, Inouhe M (2002) Possible roles of phytochelatins and glutathione metabolism in cadmium tolerance in chickpea roots. J. Plant Res., 115:429-437.
7.Gupta D K, Tohoyama H, Joho M, Inouhe M (2004) Changes in the levels of phytochelatins and related metal-binding peptides in chickpea seedlings exposed to arsenic and different heavy metal ions. J. Plant Res., 117:253-256.
8.Masahiro I (2005). "Phytochelatins". Brazilian Journal of Plant Physiology., 17 (1): 65-78.


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