ABSTRACT
Arsenic is a well-known toxic metalloid that enters the soil through both natural processes and anthropological activities and occurs in the form of various chemical species. Absorption of arsenic by plant root is influenced by many factors, including plant species, the concentration of arsenic in the soil, soil properties such as pH and clay content, and the presence of other ions. Aquaglyceroporins mediate the uptake of As3+ and undissociated methylated arsenic species in plants, whereas being a phosphate analog, As5+ is taken up by phosphate transporter. The primary cause of As3+ toxicity is its interference with a variety of enzymes because it has a high affinity to sulfhydryl groups found in many enzymes, whereas being a phosphate analog, As5+ can substitute inorganic phosphate in a plethora of biochemical processes, thus affecting the key metabolic processes in the cell. Arsenic toxicity is also mediated by oxidative stress by producing excessive reactive oxygen species (ROS) and affecting antioxidant defenses in plants. In arsenic non-hyperaccumulating plants, reduced uptake of As5+ due to suppression of the high-affinity phosphate uptake system, As5+ reduction to As3+ by arsenate reductase and subsequently complex formation with thiols, particularly phytochelatins (PCs) and glutathione and sequestration in roots provide tolerance to arsenic. In hyperaccumulator plants, arsenic tolerance mechanisms involve enhanced As5+ uptake, decreased As3+–thiol complexation, As3+ efflux to the external medium, significant enhanced xylem translocation, and sequestration of As3+ in vacuoles of fronds/leaves.
There is a need to remediate arsenic contaminated soils and reduce the concentration of arsenic in the edible parts of the crops grown in these areas. Enhanced understanding of the mechanisms of arsenic uptake, translocation, resistance, accumulation, transport, and toxicity in plants is needed for the production of arsenic-resistant plants for safe cropping and phytoremediation. The existence of natural variation in arsenic tolerance of plants and soil-microbiota can be utilized for this purpose. Arsenic hyperaccumulator plants take up and sequester exceptionally high concentrations of arsenic in the aboveground parts and hence offer a great promise to phytoremediation of arsenic. Plant breeding techniques and genetic engineering approaches have been employed to enhance arsenic tolerance in plants. Arsenic-tolerant crop plants with less arsenic in edible parts and very high arsenic accumulation in leaves have been developed, but their performance under various soil conditions, climatic conditions, and geographical areas remains to be tested. Approaches till now are focused on the introduction of few genes associated with detoxification, sequestration, and efflux of arsenic in plants; however, to make real progress, a multifaceted transgenic approach using multiple gene targets and testing in various field conditions is required.
