ROS and Oxidative Stress: Origin and Implication

Abstract

Molecular oxygen (O2) is the primary cellular electron acceptor in aerobic respiration that serves fundamental roles in membrane-linked ATP formation and other fundamental cellular and metabolic functions. But, as an untoward but inescapable consequence of different metabolic events in oxygen-saturated cellular environment, reactive oxygen species (ROS) are incessantly generated by partial or incomplete reduction of molecular oxygen. In plants, ROS are continuously generated as oxidation – reduction cascades of different metabolism located in different cellular compartments and as by-product of various metabolic events. The most important ROS include superoxide (O2.−), perhydroxy radical (HO2.), hydrogen peroxide (H2O2), hydroxy radical (OH.), and singlet oxygen (∣O2). The other secondary oxidative products like alkoxy radical (RO.), peroxy radical (ROO.), organic hydroperoxide (ROOH), excited carbonyl (RO.), etc. are also produced in plant cells. Though ROS is generated under natural conditions, their productions are augmented under the exposure of unfavorable environmental cues and natural course of senescence. Major sources of ROS in plant cell encompass spilling of electrons during photosynthetic and respiratory electron transport, decompartmentalization of transition metal ions, and also various biological redox reactions. In fact, the redox cascades of chloroplast, peroxisome, and mitochondria of green cells not only determine the driving forces for metabolism but also recognized as the prime source of ROS. Lipid peroxidation, which is known to produce ROS like alkoxy, peroxy radicals as well as singlet oxygen, is also considered as bona fide source of ROS in plant cells. In plants, apoplastic enzyme respiratory burst oxidase homologs (RBOHs) or NADPH oxidases play a major role in originating ROS wave through the other network of ROS production as well. The ROS wave, which is a consequence of perception of unfavorable environmental cues should be integrated with additional metabolic/signaling pathways to enable rapid systemic acclimation of plants. However, an elaborate and efficient antioxidative defense system, comprising a variety of antioxidant molecules, quenchers, and enzymes, determines the ROS turnover and hence the steady-state level of ROS and the redox status of the cell. Plants are equipped with those defense systems not only to combat enhanced level of ROS but also to tightly regulate the endogenous concentration necessary for controlling various events of Plant Biology. However, the decontrolled level of ROS generation, if remaining unabated may cause a solemn threat to or cause oxidative deterioration and in extreme cases the death of plant cells. The present chapter describes the physicochemical basis of the production of ROS, under normal and unfavorable environmental conditions, and senescence, with an added effort to understand their implication associated with those situations.