Sulfur assimilation and glutathione metabolism in plants

Sulfur is an essential element for all organisms. Plants utilize soil sulfate to synthesize an amino acid, cysteine, which is used for a variety of sulfur-containing compounds such as glutathione (GSH), methionine, proteins, lipids, coenzymes, and various secondary metabolites. Since animals cannot synthesize organic sulfur compounds from inorganic ones, sulfate assimilation in plants is important for the global sulfur cycle. GSH is a tripeptide synthesized from the amino acids cysteine, glutamic acid, and glycine. By controlling the redox states of proteins and chemicals, GSH functions in many biological processes including enzymatic activity, detoxification of toxic agents, and eventually influences plant growth, development, and stress management in response to both abiotic and biotic factors. Maintaining an appropriate redox environment, for which GSH levels are crucial, is thus important for plant life. GSH levels in plant cells are controlled by both synthesis and degradation processes. GSH is synthesized from cysteine by two-step reactions in plastids and cytosol. Since cysteine levels are relatively low in the cells, the sulfate assimilation pathway composed of sulfate uptake, sulfate reduction, and assimilation into cysteine, is a rate-limiting step in GSH synthesis. In this chapter, we review the molecular machineries and regulatory aspects of the sulfur assimilation pathway and GSH metabolism in plants.