Engineering plants for heavy metal stress tolerance

Plants are continuously exposed to abiotic environmental pressures. One of principal abiotic stress factors is heavy metal (HM), which as edaphic contaminants is noteworthy environmental hazard posing great negative impact on overall plants' growth, metabolism and hence economic crop productivity and sustainability. During plants' exposure to elevated HM stress, plants suffer from oxidative stress leading to changes in processes at molecular, biochemical, morpho-physiological and at whole levels. In high HM-contaminated soils, it is essential for plants to generate specific, appropriate protective/defensive mechanisms to nullify the toxic effects of these pollutants, for the normal growth and development. Plants are equipped with efficient strategies which enable them to uptake and accumulate the HMs in various parts or phytoremediate them into non-toxic forms from contaminated soils. Recent advancement in different disciplines of biosciences, such as genetic engineering, plant stress physiology, plant nutrition, transgenics, have aided us in the identification and characterization of compounds, transcription factors, gene products, exogenous phytoprotectants and segments of DNA which involve signal transduction cascades and stress-inducible proteins involved in HM detoxification and tolerance, however, underpinning various strategies for engineered heavy metal plant-stress tolerance is a topic of burning issue which remain least discussed. Taking into consideration several recent literature, the present paper (a) sheds light on the responses and impacts of various HMs to an array of plants' physiological and cellular processes, (b) shows role of various underlying mechanisms behind tolerance or detoxification against specific metal/metalloid, and finally, (c) briefly highlights the possibility of obtaining transgenic improved HM stress tolerant crop plants which could clear the desks for engineering HM stress tolerance in plants for developing improved HM tolerant crop plants and challenging the heavy metal-induced threats to sustainable agricultural system and for qualitative and quantitative improvements in economic yield of crop plants.