Regulation of the ascorbate-glutathione cycle in plants under drought stress

Acclimation of plants to drought is often associated with increased levels of reactive oxygen species (ROS), such as superoxide anion (O 2 · − ), hydrogen peroxide (H 2 O 2 ), hydroxyl radical (HO·) and singlet oxygen ( 1 O 2 ), which are toxic for the cells. ROS are by-products of aerobic metabolism, and their production is enhanced during drought conditions through the disruption of electron transport system and oxidizing metabolic activities occurring in chloroplasts, mitochondria and microbodies. Under non-stressful conditions, ROS are efficiently eliminated by non-enzymatic and enzymatic antioxidants, whereas during drought conditions the production of ROS exceeds the capacity of the antioxidative systems to remove them, causing oxidative stress. The non-enzymatic antioxidant system includes ascorbate and glutathione, located both within the cell and in the apoplast. They are two constituents of the antioxidative ascorbate–glutathione cycle which detoxify H2O2 in the chloroplasts. Ascorbate (AsA) is a major primary antioxidant compound synthesized on the inner membrane of the mitochondria which reacts chemically with 1O2, O2·− , HO· and thiyl radical, and acts as the natural substrate of many plant peroxidases. Moreover, AsA is involved in other functions such as plant growth, gene regulation, modulation of some enzymes, and redox regulation of membrane-bound antioxidant compounds. Glutathione (GSH) is a tripeptide synthesized in the cytosol and chloroplasts which scavenges 1O2 and H2O2 , and it is oxidized to glutathione disulfide (GSSG) when acts as an antioxidant and redox regulator. GSH is the substrate of glutathione S-transferases, which have a protective role in the detoxification of xenobiotics, and dehydroascorbate reductase (DHAR). Finally, GSH is a precursor of phytochelatins, which regulate cellular heavy metals levels, and is involved in gene expression. This review, based on the most significant studies published in the last decade, focuses on the changes of antioxidant enzyme activities (ascorbate peroxidase, APX; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; glutathione reductase, GR), and of the levels of some compounds involved in the ascorbate–glutathione cycle (ascorbate and glutathione pools , H2O2 and a -tocopherol) in plants grown under water shortage