Phytochelatins and Metal Tolerance

Living organisms employ a range of mechanisms to resist/tolerate heavy metal stress. A tight control over the homeostasis of even essential metal ions such as copper and zinc is necessary for proper physiological functioning of cells. The regulation of intracellular concentrations of metal ions occurs by energy-dependent efflux of metal ions, changes in their redox status, binding to cell wall, extracellular or intracellular sequestration of metal ions by peptides/proteins or other biomolecules, and finally their storage within vesicular compartments.1-7 Eukaryotes regulate the concentrations of reactive free metal ions by intracellular sequestration. The chelated metal ions may also be stored within intracellular compartments such as vacuoles and tertiary lysosomes. Glutathione (GSH),8 SH-related phytochelatins (PCs), and cysteine-rich metallothioneins (MTs)6,7 are the main metal-sequestering molecules. Organic acids may also participate in metal ion sequestration in plants. GSH constitutes the primary line of defense against metal toxicity in most organisms. Inhibition of GSH synthesis by specific chemical agents sensitizes the cells to Cd and possibly other metal ions.8,9 Similarly, mutations in GSH synthesis also sensitize cells to metals.