Oxidative Stress, Antioxidants And Stress Tolerance

Review

TRENDS in Plant Science Vol.7 No.9 September 2002

405

Oxidative stress, antioxidants and stress tolerance
Ron Mittler
Traditionally, reactive oxygen intermediates (ROIs) were considered to be toxic by-products of aerobic metabolism, which were disposed of using antioxidants. However, in recent years, it has become apparent that plants actively produce ROIs as signaling moleculesto control processes such as programmed cell death, abiotic stress responses, pathogen defense and systemic signaling. Recent advances including microarray studies and the development of mutants with altered ROI-scavenging mechanisms provide new insights into how the steady-state level of ROIs are controlled in cells. In addition, key steps of the signal transduction pathway that senses ROIs inplants have been identified. These raise several intriguing questions about the relationships between ROI signaling, ROI stress and the production and scavenging of ROIs in the different cellular compartments. Published online: 12 August 2002

Reactive oxygen intermediates (ROIs) are partially reduced forms of atmospheric oxygen (O2). They typically result from the excitation of O2 to formsinglet oxygen (O21) or from the transfer of one, two or three electrons to O2 to form, respectively, a superoxide radical (O2−), hydrogen peroxide (H2O2) or a hydroxyl radical (HO−). In contrast to atmospheric oxygen, ROIs are capable of unrestricted oxidation of various cellular components and can lead to the oxidative destruction of the cell [1–4].
Production of ROIs in cells

Ron Mittler Dept ofBotany, Plant Sciences Institute, 353 Bessey Hall, Iowa State University, Ames, IA 50011, USA. e-mail: rmittler@ iastate.edu

There are many potential sources of ROIs in plants (Table 1). Some are reactions involved in normal metabolism, such as photosynthesis and respiration. These are in line with the traditional concept, considering ROIs as unavoidable byproducts of aerobic metabolism [1].Other sources of ROIs belong to pathways enhanced during abiotic stresses, such as glycolate oxidase in peroxisomes during photorespiration. However, in recent years, new sources of ROIs have been identified in plants, including NADPH oxidases, amine oxidases and cell-wall-bound peroxidases. These are tightly regulated and participate in the production of ROIs during processes such as programmedcell death (PCD) and pathogen defense [2,4,5]. Whereas, under normal growth conditions, the production of ROIs in cells is low (240 µM s−1 O2− and a steady-state level of 0.5 µM H2O2 in chloroplasts) [6], many stresses that disrupt the cellular homeostasis of cells enhance the production of ROIs (240–720 µM s−1 O2− and a steady-state level of 5–15 µM H2O2) [6]. These include drought stress anddesiccation, salt stress, chilling, heat shock, heavy metals, ultraviolet
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radiation, air pollutants such as ozone and SO2, mechanical stress, nutrient deprivation, pathogen attack and high light stress [2,7–10]. The production of ROIs during these stresses results from pathways such as photorespiration, from the photosynthetic apparatus and from mitochondrial respiration. Inaddition, pathogens and wounding or environmental stresses (e.g. drought or osmotic stress) have been shown to trigger the active production of ROIs by NADPH oxidases [4,11–13]. The enhanced production of ROIs during stress can pose a threat to cells but it is also thought that ROIs act as signals for the activation of stress-response and defense pathways [9,14]. Thus, ROIs can be viewed ascellular indicators of stress and as secondary messengers involved in the stress-response signal transduction pathway. Although the steady-state level of ROIs can be used by plants to monitor their intracellular level of stress, this level has to be kept under tight control because over-accumulation of ROIs can result in cell death [1–4]. ROI-induced cell death can result from oxidative processes...