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Oxidative Stress in Plants Under Drought Conditions and the Role of Different Enzymes

Leonora Mansur Mattos, Celso Luiz Moretti

The effects of drought stresses on plant metabolism are either direct or secondary. Oxidative stress is induced by a wide range of biotic and abiotic stresses including UV-light, pathogen invasion (hypersensitive reaction), herbicide action, oxygen shortage, among others. Drought and salt stresses usually lead to the production of reacting oxygen species (ROS) such as hydrogen peroxide (H2O2) and superoxide (O2 ·–), both produced in a number of cellular reactions, including the iron-catalysed Fenton reaction, and by various enzymes such as lipoxygenases, peroxidases, NADPH oxidase and xanthine oxidase. To control the level of ROS under stress conditions, plant tissues contain a series of enzyme scavengers of ROS. The main cellular components susceptible to damage by free radicals are lipids (peroxidation of unsaturated fatty acids in membranes), proteins and enzymes (denaturation), carbohydrates and nucleic acids. Plant carbon balance during a period of salt/water stress and subsequent recovery may depend as much on the speed and degree of photosynthetic recovery, as it depends on the degree and speed of photosynthesis decline during water reduction. Current knowledge about physiological limitations to photosynthetic recovery after different intensities of water and salt stress is still scarce. From the large amount of data available on transcript-profiling studies in plants subjected to drought it is becoming apparent that plants perceive and respond to these stresses by quickly altering gene expression in parallel with physiological and biochemical alterations; this occurs even under mild to moderate stress conditions. From a recent comprehensive study that compared salt and drought stress it is apparent that both stresses led to down-regulation of some photosynthetic genes, with most of the changes being small possibly reflecting the mild stress imposed. Drought and salt stresses are significant challenges for mankind. The utilization of different strategies, namely genetic and enzyme engineering, can contribute to the alleviation of the associated oxidative stresses. Regulating the expression of genes encoding for specific proteins and enzymes can result into drought and salt tolerance. Different crop genotypes, such as sugarcane, soybean, and wheat have already been engineered for drought tolerance. Wheat genotypes showed alterations in antioxidant enzymes as well as in enzymes associated with carbon metabolism. These important strategies will be a vitally important tool in the quest to alleviate the earth’s future problems concerning food, energy, and the environment. The present review focus on oxidative stresses associated with drought and salt conditions addressing the metabolomics involved in such constraints.