General Roles of Phytochelatins and Other Peptides in Plant Defense Mechanisms Against Oxidative Stress/Primary and Secondary Damages Induced by Heavy Metals

Abstract

Phytochelatins (PCs) are nonprotein cysteine-rich oligopeptides having the general structure of (γ-glutamyl-cysteinyl)n-glycine (n = 2–11). They are synthesized from the precursor glutathione (a reduced form, GSH) by the activity of phytochelatin synthase (PCS). The biosynthesis is stimulated by several heavy metals (HMs), especially Cd and metalloid As. PCs can bind to various HMs like Cd, As, Cu, Pb, Zn, and Ag, via their sulfhydryl (–SH) and carboxyl (–COOH) groups. The complexations become more stable and massive in vacuole where acid-labile sulfides (S2−) are incorporated to make the PCs–S–HMs conjugates. Both the thiols and S2− are originated from sulfate through a partially common energy-dependent metabolism (sulfur assimilation), which is again enhanced by Cd, besides essential metals (Co, Mg). To date, fundamental roles of PCs and also related iso-peptides such as hPCs in intracellular detoxification and/or transport of HMs are well demonstrated in various plants, especially in experiments targeting genes and enzymes for PC and GSH biosynthesis. However, how they function as a defense molecule in the oxidative stresses or other biological processes are still unknown or conceiving subtle problems. Some of the possible functions are highlighted in this chapter as tentative examples for further discussion: (1) PCs–S–HMs complex as a potent pool/stock of thiols or reducing powers to be reusable for further robustious responses by the tolerant plants against various abiotic and biotic stresses including oxidative stress and (2) PCs as a possible mediator for metal translocation or redistribution via phloem rather than xylem, regardless of a trait of “hyperaccumulator” for HMs in land plants. Apart from the positive roles of PCs in HM-tolerant plants, arguments still hot arise an issue (3) the roles of PCs, GSH, and other thiols as delicate barometer or indicators in the mineral and redox balance and/or homeostasis, in addition to their well-known functions being substrates and antidotes. In the absence of HMs, the levels of PCs are too minute to account for their sufficient bindings to the essential metals. Although GSH is ubiquitous and abundant, it is a multifunctional peptide that rapidly consumed or oxidized for numerous enzymic or nonenzymic antioxidants/redox systems as well as direct substrate for PCS. Eventually, importance of preservation of thiols and sulfide (S2−) as resource for reducing powers in sensitive sessile plants against various oxidative stresses is again emphasized in return for PCs in the HM-tolerant plants in metalliferous habitats.