Remediation of Heavy Metal Stress in Plants

Heavy metals (HM) are metals of high density. Regardless of debate on their classification, the term HM particularly in biological sense is more often used for those metals and semimetals with potential human or environmental toxicity. Although soils are natural source of HM, geologic and anthropogenic activities increase their concentration to levels that are harmful to both plants and animals. HM can be transported over long distances in gaseous as well as particulate phase which leads to their rapid accumulation in soil, water, and living systems. Although certain HM are essential for optimum plant growth but excessive amounts are harmful to the plants and other organisms in the food chain. Activities such as unpreceded use of agrochemicals and long-term application of urban sewage sludge, industrial waste disposal, waste incineration, and vehicle exhausts are the main sources of HM in agricultural soils. Soil with high concentrations of HM lead to their absorption and accumulation by plant, which ultimately pass into humans via food chain. Both underground and aboveground surfaces of plants can absorb HM which directly or indirectly affect plant health. Direct consequences are inhibition of cytoplasmic enzymes and damage to cell structures due to oxidative stress. Oxidative stress is related to formation of reactive oxygen species (ROS) and cytotoxic compounds like methylglyoxal (MG) and perturbs the equilibrium of ionic homeostasis within the plant cells. Some HM indirectly impose oxidative stress via multiple mechanisms including glutathione depletion, binding to sulfhydryl groups of proteins, inhibiting antioxidative enzymes, or inducing ROS-producing enzymes like NADPH oxidases. Whether direct or indirect, plants exposed to high levels of HM result in reduction or even complete cessation of all metabolic activities. Although it has been known that plants possess several defense strategies to avoid or tolerate HM intoxication but beyond certain limits these mechanisms fail and survival of plant is jeopardized. Hence, it becomes very essential to remove the accumulated HM for normal functioning of plant and also protect organisms dependent on them. The techniques being used for HM cleanup from contaminated sites include excavation (physical removal from contaminated sites), stabilization or in situ fixation (stabilization by adding chemicals to alter metal to a state that is not absorbed by plants), and soil washing. However, these physical processes are neither efficient nor cost effective. Therefore, the quest for cost effective, durable, and environmental friendly solutions to cleanup HM should be on priority. In this context phytoremediation (the use of growing plants reduces the concentration of HM in the soil) and use of rhizospheric microbes have emerged as important alternatives to ensure high efficiency and better performance. Rhizospheric microbes in particular show abilities to protect the plant from HM stress as well as help in their accumulation from soil. Microbes have metabolic capabilities supported by molecular machinery to adapt and perform even in presence of high concentration of HM. The review also discusses the utilization of rhizospheric microbes in fighting the HM stress in plants. In terrestrial ecosystems, soils are the major sink for metal contamination. Soil microbes especially the rhizospheric population play important role in HM detoxification in contaminated soils. This input of the rhizomicrobial population is also referred to as rhizoremediation. In fact nodulation and nitrogenase activities can be very sensitive to HM stress but HMT rhizobial strains have also been reported from contaminated sites effectively carrying out symbiotic nitrogen fixation.

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Managing Editor
Journal of Heavy Metal Toxicity and Diseases