Ascorbate-glutathione cycle: Controlling the redox environment for drought tolerance

Abstract

Drought impact on plants, similar to other abiotic stresses, is accompanied by increased levels of reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), hydroxyl radical (.OH) and superoxide anion (O2-). Survival strategies of plants under stress conditions often involve antioxidant defense activation. The nonenzymatic antioxidants ascorbate (AsA) and glutathione (GSH), their respective oxidized forms-dehydroascorbate (DHA) and glutathione disulphide (GSSG) and the enzymes that continuously recycle them constitute the major ROS scavenging system in plants-the ascorbate-glutathione cycle. This system operates in different cell compartments and genes, encoding its components are differentially regulated according to the current needs. The ability to effectively regulate the ascorbate-glutathione cycle discriminates between drought-sensitive and drought-tolerant cultivars. Enzymes from the cycle are also central for the mechanisms of drought adaptation and tolerance in xerophyte plants. Overexpression or transgenesis of genes encoding the ascorbate-glutathione cycle enzymes could significantly improve the drought tolerance of economically important crop plants. Recent experiments also revealed the importance of ascorbate-glutathione cycle in resurrection plant metabolism, in successful rehydration after desiccation. The current chapter aims to summarize the recent data on the role of the ascorbate-glutathione cycle for the drought stress response and tolerance of plants. Special attention is paid to the compartmentalization and gene expression control of the enzymatic machinery of the cycle.