Regulation of the Ascorbate-Glutathione Cycle in Plants Under Drought Stress

Ascorbate (Asc) and glutathione (GSH) are important molecules functioning in several vital processes in plant cells, including the ascorbate-glutathione cycle. They are involved in basic metabolic reactions normally occurring in plants as well as in those evoked by abiotic or biotic stresses. Asc and GSH are localized in most of cell compartments such as cytoplasm, mitochondria, peroxisomes and chloroplasts, the Asc being additionally also found in the apoplast. These small molecular weight compounds protect cells against oxidative stress and damage by detoxifying reactive oxygen species (ROS) and ROS-generated toxic metabolic products. They do this either directly by scavenging or indirectly through the activation of defense mechanisms. Asc and GSH are engaged in maintaining cellular redox homeostasis, being at the same time involved in redox signaling. They may interact with different molecules and signaling pathways during the resistance responses. Besides the total level of Asc and GSH in the cell the ratio between reduced and oxidized forms of these molecules play an important role in the activation of various defense mechanisms. Furthermore, glutathione is involved in specific functions such as detoxification of heavy metals, transfer and storage of sulfur, regulation of expression of defense-related genes and protein activity, while ascorbate acts as a signal-transducing molecule, cofactor of some enzymes and biosynthetic precursor of oxalic and l-tartaric acids. Moreover all kinds of xanthophylls cycle, considered as the most important photoprotective mechanism in plants, are strongly dependent on the acid form of ascorbate. The biochemical, physiological and genetic aspects of the involvement of ascorbate and glutathione, localized in different cell compartments, in controlling cellular redox state, plant stress tolerance, and defense mechanisms will be discussed.