Molecular mechanisms and genetic basis of heavy metal toxicity and tolerance in plants

Abstract

Heavy metal pollutants are mainly derived from growing number of anthropogenic sources. As the environmental pollution with heavy metals increases, some new technologies are being developed, one of these being phytoremediation. Hyperaccumulator plant varieties can be achieved by using methods of genetic engineering. An uptake of excessive amounts of heavy metals by plants from soil solution leads to range of interactions at cellular level which produce toxic effects on cell metabolism in terms of enzyme activity, protein structure, mineral nutrition, water balance, respiration and ATP content, photosynthesis, growth and morphogenesis and formation of reactive oxygen species (ROS).On the basis of accumulation of heavy metals plants are divided into three main types; (i) the accumulator plants, (ii) the indicator plants, and (iii) the excluder plants. Generally, the accumulation of heavy metals in plant organ is in series root > leaves > stem > inflorescence > seed. Most of plants belong to excluder group and accumulate heavy metals in their underground parts. When roots absorb heavy metals, they accumulate primarily in rhizodermis and cortex. In intracellular parts, highest concentration of heavy metals is found in cell wall. Tolerance of plants against heavy metals is due to reduced uptake of heavy metals and increased plant internal sequestration. In the increased plant internal sequestration mechanism, plant is manifested by interaction between a genotype and its environment. There are biochemical machineries in plants that work for tolerance and accumulation of heavy metals. Metal transporters are involved in metal ion homeostasis and transportation. Some amino acids and organic acids are ligands for heavy metals and these amino acids and organic acids play an important role in tolerance and detoxification. Phytochelatins (PCs) are produced in plants under stress of heavy metals and play role in binding heavy metals to complexes and salts and sequestering the compounds inside the cell so that heavy metals can not disturb the cell metabolism. The genes for phytochelatin synthesis have been isolated and characterized. Another low molecular weight (6-7 KDa) cysteine-rich compounds known as metallothioneins (MTs) also play an important role in detoxification of metals. In the plants growing under unoptimal temperature, there is high expression of heat shock proteins (HSPs), which normally act as molecular chaperones in protein folding, but may also function in the protection and repair of protein under metal-stress. Genes for heavy metal resistance have been isolated, manipulated and used to produce transgenic plants. Introduction of above genes and heterologous metallothionein genes to raise novel transgenic crop plants is under progress and holds promise to develop superior metal tolerant/hyperaccumulator crop plants.