Redox homeostasis and antioxidant signaling: a metabolic interface between stress perception and physiological responses - PubMed (original) (raw)

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Redox homeostasis and antioxidant signaling: a metabolic interface between stress perception and physiological responses

Christine H Foyer et al. Plant Cell. 2005 Jul.

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Figure 1.

Figure 1.

Reductant–Antioxidant–Oxidant Interactions in Redox Homeostasis and Signaling. Nonenzymic components are positioned on a nonlinear scale (right) at their approximate electrochemical potential in volts. In nonquiescent cells under optimal conditions, large pools of glutathione and ascorbate are maintained in a highly reduced state, and buffer ROS that are continuously produced by oxidases or by electron transport components, such as FeS centers, semiquinones, or (as depicted) ferredoxin. Other key redox signaling components are thioredoxins (TRX) and glutaredoxins (GRX), which are reduced by ferredoxin, NADPH, or glutathione. Production of the superoxide anion (OO.−) and H2O2 (HOOH) can be induced or promoted under certain conditions, leading to increased oxidative charge on the reductant-antioxidant system. Reductive cleavage of H2O2 produces the hydroxyl radical (OH·), an extremely reactive electron and hydrogen acceptor whose reduction potentially involves indiscriminate oxidation of cellular components. Excessive production of OH· is avoided by enzymatic processing of H2O2 to water by peroxidases or to water and O2 by catalases. Signaling linked to increased availability of ROS may be caused, limited, or mediated by changes in antioxidant capacity (see text).

Figure 2.

Figure 2.

Sensitive Control of Cellular Redox Responses by Protein Thiol Status. The numbered reactions are as follows. 1, Reversible dithiol-disulfide formation mediated by the thioredoxin (or glutaredoxin) system, as in light modulation of chloroplastic enzyme activities. 2, Direct H2O2-dependent oxidation of dithiol sensor proteins, such as the bacterial oxyR, reversed by GRX. 3, Peroxidase-catalyzed H2O2 sensing, as occurs with the yeast yAP-1 protein, which can be reversed by TRX. 4, Sulfenic acid formation, as occurs in the bacterial transcription factor OhrR or in peroxiredoxins. The sulfenic acid can be directly reduced by glutathione or lipoic acid (in 1-Cys peroxiredoxins) or give rise to an intramolecular or intermolecular disulfide bond that is reducible to thiols by thioredoxins or glutaredoxins in 2-Cys peroxiredoxins. 5, Glutathionylation through thiol-disulfide exchange of protein thiols with GSSG. 6, ROS-catalyzed glutathionylation via thiyl radical formation. 7, Glutaredoxin-catalyzed glutathionylation reaction. Whatever the route of glutathionylation, the process may be reversed by certain glutaredoxins and/or thioredoxins. Only some of the possible routes for changes in Cys sulfur status are shown. Other modifications, such as protein thiol nitrosylation, could also be important, as could more oxidized forms of protein Cys sulfur (sulfinic and sulfonic acids) and glutathionylation via nitrosylated glutathione. GRX, glutaredoxin; GSH, glutathione; GSSG, glutathione disulfide; POX, peroxidase; TRX, thioredoxin; XRX, thioredoxin or glutaredoxin.

Figure 3.

Figure 3.

Oxidant and Antioxidant Signaling in Cell Death and Acclimation Responses: Major Components of the Metabolic Interface between Plant Stresses and Orchestrated Responses. The hypothetical scheme draws together some of the redox-modulated elements recently identified in plants and others likely to be influential. Components that promote an oxidative signal are shown in red, and those that oppose such signals or transmit a reductive signal are shown in dark blue. Plasma membrane NADPH oxidases are activated by elicitor or hormone-mediated signaling and produce H2O2 (1). Production of H2O2 can oxidize putative membrane receptors, and this function is opposed by apoplastic ascorbate (2). ROS production activates signaling through specific MAP kinases (3), and some of these components are also important in ROS-mediated hormone and growth responses. A key factor in specification of pathogenesis and cell death responses is secondary production of ROS, either by positive feedback enhancement of the primary oxidative signal (4) or by withdrawal and inactivation of antioxidative capacity. Downregulation of the antioxidative system could occur by posttranslational modulation of heme functions by salicylic acid and nitric oxide (5) and/or programmed withdrawal of antioxidative enzyme expression (6). Oxidative inhibition of chloroplast metabolism by glutathionylation and/or inactivation of thioredoxin-modulated enzymes may also be crucial in increasing chloroplastic flux to oxygen to enhance ROS production (7). Greater availability of ROS activates glutathione synthesis (8), and the resulting increase in cytosolic glutathione is somehow linked to induction or activation of a thioredoxin or glutaredoxin that is able to reduce NPR1 (9). NPR1 reduction is associated with its accumulation in the nucleus and its interaction with TGA transcription factors to induce PR gene expression. Enhanced chloroplastic levels of ROS such as singlet oxygen may also activate the pathways that (10) set in train the cell death program. For discussion and references, see text. APX, ascorbate peroxidase; CAT, catalase; CIV, cytochrome oxidase; SOD, superoxide dismutase.

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