Drought Stress Responses in Plants, Oxidative Stress, and Antioxidant Defense

Abstract

Global climate change is currently viewed as the most devastating threat to the environment, and is now gaining considerable attention from farmers, researchers, and policy makers because of its major influence on agriculture. The situation is becoming more serious due to gradual increases in the complex nature of the environment, and due to the unpredictability of environmental conditions and global climate change. One of the most acute environmental stresses presently affecting agriculture is drought, which has pronounced adverse effects on the growth and development of crop plants. The effects of drought stress are expected to increase further with increases in climate change and a growing water crisis. Drought stress usually leads to reductions in crop yield, which can result from many drought-induced morphological, physiological, and metabolic changes that occur in plants. A key sign of drought stress at the molecular level is the accelerated production of reactive oxygen species (ROS) such as singlet oxygen (1O2), superoxide (O2√-), hydrogen peroxide (H2O2), and hydroxyl radicals (OH√). The excess production of ROS is common in many abiotic stresses, including drought stress, and results from impaired electron transport processes in the chloroplasts and mitochondria. One of the major causes of ROS production under drought stress is photorespiration, which accounts for more than 70% of the total H2O2 produced. Plants have endogenous mechanisms for adapting to ROS production and are thought to respond to drought stress by strengthening these defense mechanisms. Therefore, enhancement of the functions of the naturally occurring antioxidant components (enzymatic and non-enzymatic) may be one strategy for reducing or preventing oxidative damage and improving the drought resistance of plants. In this chapter, we review the most recent reports on drought-induced responses in plants, focusing on the role of oxidative stress as well as on other possible mechanisms and examining how different components of the antioxidant defense system may confer tolerance to drought-induced oxidative stress.