NRAMP family in plants: Contribution to cadmium accumulation

The Natural Resistance Associated Macrophage Proteins (NRAMPs) are membrane-targeted transporters with low substrate specificity, that mediate the import (translocation to the cytoplasm) of metals, mainly essential nutrients, e.g. iron (Fe), manganese (Mn), zinc (Zn), cobalt (Co), copper (Cu) or nickel (Ni). Depending on the subcellular localisation, they contribute to either uptake or redistribution. Numerous studies have shown that NRAMP proteins also mediate the transport of non-essential toxic metals, and constitute the major pathway for the uptake of cadmium (Cd) in plants. Given the threat to living organisms from exposure to this toxic metal, which enters the food chain through edible plant parts, this review focuses on how different NRAMP proteins are involved in Cd uptake and accumulation. The issue was discussed concerning three categories of plants: (i) the model plant Arabidopsis thaliana; (ii) crop plants that are mainly used for food; (iii) plant species used for the phytoremediation of Cd-polluted environments, including hyperaccumulators. For the sake of clarity, this paper updates the structure, function and regulation of NRAMP genes and proteins, their phylogenetic relationships, and their substrates, filling a knowledge gap, and discussing these topics in light of new data. Furthermore, this review discusses the potential applications of NRAMP genes in plant biotechnology, such as generating low-Cd food and phytoremediation.