Root responses to cadmium in the rhizosphere: a review

This article reviews the responses of plant roots to elevated rhizosphere cadmium (Cd) concentrations. Cadmium enters plants from the soil solution. It traverses the root through symplasmic or apoplasmic pathways before entering the xylem and being translocated to the shoot. Leaf Cd concentrations in excess of 5-10 mu g g(-1) dry matter are toxic to most plants, and plants have evolved mechanisms to limit Cd translocation to the shoot. Cadmium movement through the root symplasm is thought to be restricted by the production of phytochelatins and the sequestration of Cd-chelates in vacuoles. Apoplasmic movement of Cd to the xylem can be restricted by the development of the exodermis, endodermis, and other extracellular barriers. Increasing rhizosphere Cd concentrations increase Cd accumulation in the plant, especially in the root. The presence of Cd in the rhizosphere inhibits root elongation and influences root anatomy. Cadmium concentrations are greater in the root apoplasm than in the root symplasm, and tissue Cd concentrations decrease from peripheral to inner root tissues. This article reviews current knowledge of the proteins involved in the transport of Cd across root cell membranes and its detoxification through sequestration in root vacuoles. It describes the development of apoplastic barriers to Cd movement to the xylem and highlights recent experiments indicating that their maturation is accelerated by high Cd concentrations in their immediate locality. It concludes that accelerated maturation of the endodermis in response to local Cd availability is of functional significance in protecting the shoot from excessive Cd loads.