Trends in Plant Science
ReviewOxidative stress, antioxidants and stress tolerance
Section snippets
Production of ROIs in cells
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
Scavenging of ROIs in cells
Major ROI-scavenging mechanisms of plants include superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) 1., 7., 16. (Table 1). The balance between SOD and APX or CAT activities in cells is crucial for determining the steady-state level of superoxide radicals and hydrogen peroxide [17]. This balance, together with sequestering of metal ions, is thought to be important to prevent the formation of the highly toxic hydroxyl radical via the metal-dependent Haber–Weiss or the
Avoiding ROI production
Avoiding ROI production might be as important as active scavenging of ROIs. Because many abiotic stress conditions are accompanied by an enhanced rate of ROI production, avoiding or alleviating the effects of stresses such as drought or high light on plant metabolism will reduce the risk of ROI production. Mechanisms that might reduce ROI production during stress (Table 1) include: (1) anatomical adaptations such as leaf movement and curling, development of a refracting epidermis and hiding of
Production and scavenging of ROIs in different cellular compartments
Recent manipulations of ROI-scavenging pathways in different cellular compartments suggest some intriguing possibilities. For years, the chloroplast was considered to be the main source of ROI production in cells and consequently one of the main targets for ROI damage during stress. However, it has recently been suggested that the chloroplast is not as sensitive to ROI damage as previously thought [26]. The mitochondrion is another cellular site of ROI production. However, recent studies
Redundancy in ROI-scavenging mechanisms
Some of the complex relationships between the different ROI-scavenging and ROI-producing mechanisms have been revealed in transgenic plants with suppressed production of ROI-detoxifying mechanisms. Thus, plants with suppressed APX production induce SOD, CAT and GR to compensate for the loss of APX, whereas plants with suppressed CAT production induce APX, GPX and mitochondrial AOX 16., 50.. CAT and APX are not completely redundant because they do not compensate for the lack of each other, as
ROIs at the interface between biotic and abiotic stresses
ROIs play a central role in the defense of plants against pathogen attack. During this response, ROIs are produced by plant cells via the enhanced enzymatic activity of plasma-membrane-bound NADPH oxidases, cell-wall-bound peroxidases and amine oxidases in the apoplast 4., 5.. H2O2 produced during this response (up to 15 μm; directly or as a result of superoxide dismutation) is thought to diffuse into cells and, together with salicylic acid (SA) and NO [34], to activate many of the plant
ROI signal transduction pathway
Recent studies have identified several components involved in the signal transduction pathway of plants that senses ROIs. These include the mitogen-activated protein (MAP) kinase kinase kinases AtANP1 and NtNPK1, and the MAP kinases AtMPK3/6 and Ntp46MAPK 39., 40.. In addition, calmodulin has been implicated in ROI signaling 9., 41.. A hypothetical model depicting some of the players involved in this pathway is shown in Fig. 4. H2O2 is sensed by a sensor that might be a two-component histidine
Future challenges and questions
The cause of cell death induced in plants by oxidative stress is not known. Is it simply the toxicity of ROIs that damages cells or is it the activation of a PCD pathway by ROIs? It is possible that the level of H2O2 that is currently thought to kill cells by direct cellular damage actually induces PCD 15., 27., and it might require a higher level of ROIs to kill cells by direct oxidation. Perhaps future studies applying oxidative stress to mutants deficient in different PCD pathways will
Acknowledgements
I apologize to all colleagues whose work could not be reviewed here because of space limitation. I thank Barbara A. Zilinskas and Eve Syrkin Wurtele for critical reading of the manuscript. Research at my laboratory is supported by funding from the Israeli Academy of Sciences and the Biotechnology Council of Iowa State University.
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