Cellular Mechanisms in Higher Plants Governing Tolerance to Cadmium Toxicity

Cadmium (Cd) is an inorganic mineral in the earth's crust. Cadmium entry into the environment occurs through geogenic and anthropogenic sources. Industrial activities including mining, electroplating, iron and steel plants, and battery production employ Cd during their processes and often release Cd into the environment. When disseminated into soil, Cd can be detrimental to agro-ecosystems because it is relatively mobile and phytotoxic even at low concentrations. Cadmium's phytotoxicity is due to reductions in the rate of transpiration and photosynthesis and chlorophyll concentration resulting in retardation of plant growth, and an alteration in the nutrient concentration in roots and leaves. In response to Cd toxicity, plants have developed protective cellular mechanisms such as synthesis of phytochelatins and metallothioneins, metal compartmentalization in vacuoles, and the increased activity of antioxidant enzymes to neutralize Cd-induced toxicity. While these direct protective mechanisms can help alleviate Cd toxicity, other indirect mechanisms such as microelements (zinc, iron, manganese, and selenium) interfering with Cd uptake may decrease Cd concentration in plants. This comprehensive review encompasses the significance of Cd, portals of contamination and toxicity to plants, and implications for crop production. Various mitigation strategies with the beneficial effects of zinc, iron, manganese, and selenium in activating defence mechanisms against Cd stress are discussed. Furthermore, this review systematically identifies and summarises suitable strategies for mitigating Cd-induced toxicity in plants.