Abstract

Reactive oxygen species (ROS) is a generic name given to a variety of molecules and free radicals derived from molecular oxygen. The reduction of oxygen produces relatively stable intermediates. One electron-reduction produces superoxide anion, which is the precursor of most ROS. As most commonly used, ROS in this chapter refer to superoxide, hydrogen peroxide and their derivatives such as the hydroxyl radical. Reactive oxygen species (ROS) were initially recognized as toxic by-products of aerobic metabolism. In recent years, it has become apparent that ROS plays an important signaling role in plants, controlling processes such as growth, development and especially response to biotic and abiotic environmental stimuli. ROS include free radicals such as superoxide anion (O₂ ^●—), hydroxyl radical (^●OH), as well as nonradical molecules like hydrogen peroxide (H₂O₂), singlet oxygen (¹O₂), and so forth. Stepwise reduction of molecular oxygen (O₂) by high-energy exposure or electron-transfer reactions leads to production of the highly reactive ROS. Reactive oxygen species (ROS) are produced by living organisms as a result of normal cellular metabolism and environmental factors, such as air pollutants or cigarette smoke. ROS are highly reactive molecules and can damage cell structures such as carbohydrates, nucleic acids, lipids, and proteins and alter their functions.