Reactive Oxygen Species Generation, Hazards, and Defense Mechanisms in Plants under Environmental (Abiotic and Biotic) Stress Conditions

Reactive oxygen species (ROS) are unavoidable by-products of normal cell metabolism. ROS are generated by electron transport activities of the chloroplast, mitochondria, and plasma membrane or as a by-product of various metabolic pathways localized in different cellular compartments. Under normal growth condition, ROS production in various cell compartments is low. However, various environmental stresses such as drought, salinity, chilling, metal toxicity, and UV-B, if prolonged over to a certain extent, disrupt the cellular homeostasis and enhance the production of ROS. ROS play two divergent roles in plants; at low concentrations, they act as signaling molecules that mediate several plant responses in plant cells, including responses under stresses, whereas at high concentrations, they cause exacerbating damage to cellular components. ROS are essential part of several pathways related to physiology, hormone, and plant development, and regulate various cellular signals. Thus, they play a key role in plant defense and adaptation under abiotic and biotic stresses. Enhanced level of ROS causes oxidative damage to lipid, protein, and DNA leading to altered intrinsic membrane properties like fluidity, ion transport, loss of enzyme activity, protein cross-linking, inhibition of protein synthesis, and DNA damage, ultimately resulting in cell death. In order to avoid the oxidative damage, higher plants possess a complex antioxidative defense system comprising of nonenzymatic and enzymatic components. A high antioxidant capacity in the tissues is often correlated with enhanced tolerance of plants toward biotic and abiotic stresses. Although rapid progress has been made in recent years, there are many uncertainties and gaps in our knowledge of ROS formation and their effect on plants mainly due to short half-life and high reactivity of ROS. Study of formation and fate of ROS using advanced analytical techniques will help in developing a broader view of the role of ROS in plants. Future progress in genomics, metabolomics, and proteomics will help in the clear understanding of biochemical networks involved in cellular responses to oxidative stress. An improved understanding of these will be helpful in producing plants with inbuilt capacity of enhanced levels of tolerance to ROS using biotechnological approach.