Role of Phytochelatins in Heavy Metal Stress and Detoxification Mechanisms in Plants

Abstract

Being sessile in nature, plants respond to heavy metal stress in the environment in different ways. The responses include immobilization, exclusion, chelation, and compartmentalization of the metal ions. Simultaneously, plants have general stress response mechanisms within their system especially through the expression of stress molecules like metallothionein and phytochelatins (PCs). PCs are the best-characterized heavy metal chelators especially in the context of cadmium (Cd) tolerance in plants; they were first discovered as Cd-binding “Cadystins A and B” in a fission yeast and then found in plants, fungi, and all groups of algae including cyanobacteria. PCs are non-protein cysteine-rich oligopeptides having the general structure of (γ-glutamyl-cysteinyl) n-glycine (n = 2–11) and produced by the enzyme phytochelatin synthase. They are capable of binding to various metals including Cd, As Cu, or Zn via sulfhydryl and carboxyl residues, but the biosyntheses are preferentially controlled by the metal Cd or metalloid As. The fundamental roles of PCs in metal detoxification by plant cells are now well known and tolerance of Cd increases in yeast and bacteria with the overexpression of PC synthase genes. Sequestration of PC-metal complex in both plant and yeast cells occurs at the vacuole, where PCs are involved in the accumulation of the metal as complexes, particularly in response to Cd by forming high molecular weight compounds after incorporation of sulfur (S2−). The role of PCs may further be explored to improve the metal detoxification activities and tolerance characteristics of higher plants under various conditions.