CELLULAR AND MOLECULAR MECHANISMS OF HEAVY METAL TOLERANCE IN PLANTS: A BRIEF OVERVIEW OF TRANSGENIC PLANTS OVER-EXPRESSING PHYTOCHELATIN SYNTHASE AND METALLOTHIONEIN GENES

Heavy metals are important environmental pollutants and many of them are toxic even at very low concentrations. Pollution of the biosphere with heavy metals has accelerated dramatically since the industrial revolution. Metal toxicity is one of the major abiotic stresses leading to hazardous effects in plants. Plants are susceptible to heavy metal toxicity and respond to avoid detrimental effects in a variety of different ways. Several studies revealed the additive and coordinative involvement of heavy metal transporters, ligands such as phytochelatins and metallothioneins, antioxidant, glyoxalase systems, heat shock proteins, proline, polyamines, salicylic acid, nitric oxide and miRNAs. They alleviate heavy metal stress by many different ways including restriction of uptake and transport of metals and through symbiotic association between plants and ectomycorrhizas or rhizobacteria. The aim of this review is to integrate a recent understanding of physiological and biochemical mechanisms of heavy metal induced plant stress responses based on the findings of current plant molecular biological research. This paper also describes recent developments of transgenic plants with special reference to phytochelatin synthases and metallothionein genes for improving plant metal tolerance and further applications in phytoremediation. Undoubtedly, isolation and overexpression of phytochelatin synthase and metallothionein genes have fostered some understanding of their role in metal stress tolerance. However, single genes are unlikely to help in the amelioration of heavy metal stress tolerance. Therefore, constructs with combinations of PC synthesis genes like γ-glutamylcysteine synthase (GCS), glutathione synthase (GS), ATP sulfurylase and serine acetyl transferase may be important besides many transcription factors associated with metal stress tolerance for developing an efficient phytoremediation technology useful for removal of heavy metals from soils.