MOLECULAR MECHANISMS OF PLANT TOLERANCE TO HEAVY METALS

Abstract

Plants have evolved many adaptive mechanisms to cope with heavy metal stress, including governing uptake of heavy metal ions, detoxification by chelation, intracellular sequestration and cellular homeostasis to minimize the damage from exposure to nonessential metal ions. Mycorrhizae, root exudates and cellular membranes are three key factors that regulate heavy metal uptake. Ectomycorrhizae can significantly enhance the heavy metal tolerance of the host plant, and root exudates reduce the absorption of heavy metal ions by changing the pH and redox state of rhizsphere as well as chelation. However, the molecular mechanisms of these processes are not yet clear. The trans-membrane transporters have been fully researched and have been either identified or isolated. Many genes are involved, such as the ZIP (ZRT IRT related proteins) family, natural resistance associated macrophage proteins (Nramp) and P 1B -type ATPase family were discovered and cloned in recent years. The primary chelators of heavy metals in plants are metallothioneins (MTs), phytochelatins (PCs), organic acids and amino acids. They alleviate the toxicity of metal ions in plants by chelation. MT-like genes encoding MTs and genes encoding γ-glutamyl-cysteine synthetase have been cloned. Gene expression in tested plants has demonstrated the role of genes in tolerance to heavy metals. Genes of phytochelatins synthetase (PCs) helping synthesis of PCs, i.e. γ-Glu-Cys, also have been cloned. The nicotianamine gene, an amino acid involved in biosynthesis of muginetic acids, has been shown to contribute to tolerance and hyperacumulation to heavy metals. ABC-type (ATP-binding cassette) transporters and CDF (cation diffusion facilitators) as well as other trans-membranes transporters help to sequester the heavy metals in vacuoles. The genes governing these transporters have been cloned and expressed in plants, which show to contribute to heavy metal adjustment. Plants with heavy metal tolerance have developed pertinent genes or functional proteins in these aspects. In recent years, we have increased our understanding of the molecular mechanisms of plant heavy metal tolerance and have used plants in the remediation of heavy metal contamination. Understanding molecular mechanisms of plant heavy metal tolerance might be a key step for heavy metal remediation.