Reactive oxygen species and oxidative burst: Roles in stress, senescence and signal transduction in plants

The oxidative burst, during which large quantities of reactive oxygen species (ROS) like superoxide, hydrogen peroxide, hydroxyl radicals, peroxy radicals, alkoxy rad icals, singlet oxygen, etc. are generated, is one of the earliest responses of plant cells under various abio tic and biotic stresses and natural course of senescence. In fact, reactions involving ROS are an inherent fe ature of plant cells and contribute to a process of oxidative deterioration that may lead ultimately to cell death. Sources of ROS include leakage of electrons from electron transport systems, decompartmentalization of iron which facilitates generation of highly reactive h ydroxyl radicals, and also various biological reactions. The imposition of both abiotic and biotic stresses causes overproduction of ROS, which ultimately imposes a secondary oxidative stress in plant cells. Degradation of membrane lipids, resulting in free fatty acids, initiates oxidative deterioration by providing a substrate for enzyme lipoxygenase, causing membrane lipid peroxidation. Since lipid peroxidation is known to produce alkoxy, peroxy radicals as well as singlet oxygen, these reactions in the membrane are a major source of ROS in plant cells. Regulatory mechanisms function both at gene and protein level to coordinate antioxidant responses. Superimposed upon our understanding of ROS-induced oxidative damages and their protection by antioxidative system, is the newly discovered role of ROS in signalling processes. ROS like H2O2 act as a signalling molecule, second messenger, mediating the acquisition of tolerance to both biotic and abiotic stresses. ROS as ubiquitous messengers of stress responses likely play a signalling role in various ada ptive processes. Plants can sense, transduce and translate ROS signal into appropriate cellular responses with the help of some redox-sensitive proteins. Hydrogen peroxide has been implicated as a key factor mediating programmed cell death. Plants exposed to abiotic stresses can produce a systemic signal, a component of which may be H 2O2 which sets up an acclimatary response in unstressed regions of plants. ROS is also found to communicate with other signal molecules and the pathways forming part of signa lling network that controls responses downstream of ROS.