Chemistry, Biosynthesis, and Antioxidative Function of Glutathione in Plants

Glutathione, a tripeptide constituted by glutamate, cysteine, and glycine, is an abundant metabolite that functions as a master regulator of intracellular redox homeostasis. Under optimal conditions, glutathione is mostly present in the reduced form (GSH), with a free thiol group. The link of two molecules of GSH, via a disulfide bond, leads to the formation of glutathione disulfide (GSSG). GSH can be oxidized, directly or indirectly, by reactive oxygen species, working as a scavenger that prevents excessive oxidation of cellular environment. GSH can also react with different thiols to form mixed disulfides. These reversible redox reactions are responsible for many GSH functions. GSH biosynthesis is dependent on the activity of the two ATP-dependent enzymes ?-glutamylcysteine synthetase and glutathione synthetase, encoded, respectively, by the nuclear GSH1 and GSH2 genes. The first step of GSH biosynthesis occurs in the plastids, while the second step can take place in both plastids and cytosol. The use of different gsh1 mutants and GSH1 overexpressing plants has helped to shed light on the multiple roles of GSH in plant growth, development, and response to changing environment. The maintenance of a high GSH/GSSG ratio is crucial for many physiological functions, and a decrease in this ratio can be utilized as an indicator of oxidative stress. The GSH/GSSG ratio also acts as an important regulator of several mechanisms involved in plant development and in plant stress response. In addition to redox state, also GSH concentration and its subcellular distribution are central factors controlling redox homeostasis and signaling.