Sulfur Metabolism and Cadmium Stress in Higher Plants

Plant sulfur metabolism is deeply affected by cadmium (Cd) stress, mainly as a consequence of the activation of a wide range of adaptive responses involving glutathione (GSH) consuming activities. In fact, GSH not only acts as a direct or indirect antioxidant in mitigating Cd-induced oxidative stress, but also represents a key intermediate for the synthesis of phytochelatins (PCs), a class of cysteine-rich heavy metal-binding peptides involved in buffering cytosolic metal concentration. As a consequence, Cd exposure may result in a depletion of the cell GSH pools which in turn increases the plant demand for reduced sulfur compounds. In this condition the need for maintaining GSH homeostasis and an adequate PC biosynthesis rate is met by a general induction of enzyme activities directly or indirectly involved in sulfur metabolism. Transgenic plants overexpressing these enzymes exhibit a greater production of GSH and an increased Cd tolerance, confirming this pathway to be crucial for plant survival in metal polluted environments. In the present paper we analyze and discuss the biochemical and molecular mechanisms involved in the regulation of sulfate (the main sulfur source for plants) uptake and assimilation, and GSH synthesis during Cd detoxification.