Uptake and toxicity of heavy metals: The protective frontiers of metal binding proteins

In environment, several contaminants and pollutants are released, the content of which is increasing alarmingly. Out of several contaminants released into environment, one such contaminant is heavy metals. Such contaminants being released into the environment then enter into plant system via soil. Plants uptake heavy metals from soil via apoplast symplast continuum. Plant require several nutrients in minute concentrations however presence of such nutrients in excess cause toxic effects on plants. Such heavy metals caused varied toxicities such as chlorosis, impaired photosynthesis, lipid peroxidation etc. in plants resulting in overall decline in plant biomass. Excess concentration of heavy metals such as copper, chromium, nickel are known to induce morphological, physiological deformities in several plant species. In response to ROS generated due to heavy metal toxicity, plants activate several defense mechanisms. In addition to this, several metal binding proteins such as metallothioneins, phytochelatins, glutathione etc. are activated. These metal binding protein act to reduce the toxic effects of heavy metals by binfing to them and sequestering then into vacuoles. The current review will highlight the uptake mechanism of heavy metals by plants, toxicity caused by some commonly occurring heavy metals in plants and role of metal binding proteins in sequestering such heavy metals.