Molecular Insights into Transporters Associated with Cadmium (Cd) Uptake, Translocation and Accumulation, and Genetic Engineering for Cd Tolerance in Plants

Cadmium (Cd) is classified as the most toxic metal and is a group 1 carcinogen. Plant growth and crop yield are severely affected by the uptake of cadmium. Cd toxicity in plants causes reduced chlorophyll synthesis, declined photosynthesis, distressed plant water metabolism as well as hormonal and/or nutritional imbalances. Molecular mechanism of Cd uptake and transport has shown that there are several regulatory networks of functional genes. The defence machinery against Cd toxicity includes foraging Cd from active tissues and its sequestration. Studies in several model plants have demonstrated the role of ATP binding cassette (ABC) family, heavy metal transporting P-type ATPase (HMA) family, ZRT and IRT-like protein (ZIP) family, and natural resistance-associated macrophage proteins (NRAMP) in Cd homeostasis. These metal transporters have become as prime candidates for genetic transformation aimed to inhibit heavy metal uptake and accumulation. Further, research on the mechanism of transporters, high-throughput transcriptomics, QTL mapping, and genetic engineering is underway to elucidate the underlying molecular mechanisms of plant responses to Cd and tolerance. Herein, we have presented an account of the plant responses to Cd, molecular mechanisms, transporters involved in uptake and translocation of Cd in plants, as well as the candidate genes for gene transfer to inhibit heavy metal uptake and accumulation, and discuss the prospects of genome editing for improving Cd tolerance in crop plants.