Mechanisms of Cadmium Accumulation in Plants

Cadmium is a non-essential trace metal, which is highly toxic to nearly all living organisms. Soil pollution causes Cd contamination of crops, thereby rendering plant products responsible for the chronic low level Cd over-exposure of numerous populations in the world. For this reason, Cd accumulation in plants has been studied for about five decades now. The research first focused on the relationships between plant and soil Cd levels, on the factors of the metal availability in soil, as well as the root uptake processes. Cd distribution in plant organs was also investigated, first using a macroscopic and eco-physiological approach, and then with the help of molecular biology tools, at both tissue and cell scales. Cadmium has no biological function and hijacks the transport pathways of micronutrients such as Fe, Mn, or Zn, in order to enter the plant through the roots and be distributed to all its organs. The study of the genes that control the influx and efflux of the Cd²⁺ ion in the cytosol, vacuoles, and vascular tissues has significantly contributed to the understanding of the metal root uptake and of its transfer to the aerial parts. However, the mechanisms responsible for its distribution to the different above-ground tissues and specially to fruits and seeds have yet to be clarified. This review summarizes current knowledge in order to present a detailed overview of Cd transport and storage, from the rhizosphere to the different organs and tissues of the plant.