Journal of Experimental Botany, Vol. 53, No. 372, pp. 1227-1236,
May 15, 2002
© 2002 Oxford University Press
Original Papers |
Signal transduction during oxidative stress
Vakgroep Moleculaire Genetica, Departement Plantengenetica, Vlaams Interuniversitair Instituut voor Biotechnologie (VIB), Universiteit Gent, KL Ledeganckstraat 35, B-9000 Gent, Belgium
Received 10 July 2001; Accepted 7 January 2002
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Abstract |
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 Top Abstract Biochemical properties of active...AOS as signal moleculesAOS and redox signallingAOS and redox-regulated gene...ConclusionsReferences |
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As an unfortunate consequence of aerobic life, active oxygen species (AOS) are formed by partial reduction of molecular oxygen. Plants possess a complex battery of enzymatic and non-enzymatic antioxidants that can protect cells from oxidative damage by scavenging AOS. It is becoming evident that AOS, which are generated during pathogen attack and abiotic stress situations, are recognized by plants as a signal for triggering defence responses. An overview of the literature is presented on the signalling role of AOS in plant defence responses, cell death, and development. Special attention is given to AOS and redox-regulated gene expression and the role of kinases and phosphatases in redox signal transduction.
Key words: Oxidative stress, redox regulation, signal transduction, transcription.
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Biochemical properties of active oxygen species |
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TopAbstractBiochemical properties of active...AOS as signal moleculesAOS and redox signallingAOS and redox-regulated gene...ConclusionsReferences |
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Plants, as other aerobic organisms, require oxygen for the efficient production of energy. During the reduction of O2 to H2O, active oxygen species (AOS), namely superoxide radical (
), hydrogen peroxide (H2O2), and hydroxyl radical (OH·) can be formed (Fig. 1
). Initially, the reaction chain needs an input of energy, whereas subsequent steps are exothermic and can occur spontaneously, either catalysed or not. Acceptance of excess energy by O2 can additionally lead to the formation of singlet oxygen (1O2), a highly reactive molecule when compared to O2. 1O2 can last for nearly 4 µs in water and 100 µs in a non-polar environment (Foyer and Harbinson, 1994). 1O2 can either transfer its excitation energy to other biological molecules or react with them, thus forming endoperoxides or hydroperoxides (Halliwell and Gutteridge, 1989). is a moderately reactive, short-lived AOS with a half-life of approximately 2–4 µs. Therefore, cannot cross biological membranes and is dismutated readily to H2O2. Alternatively, reduces quinones and transition metal complexes of Fe3+ and Cu2+, thus affecting the activity of metal-containing enzymes. Hydroperoxyl radicals (
) that are formed from by protonation in aqueous solutions can cross biological membranes and subtract hydrogen atoms from polyunsaturated fatty acids and lipid hydroperoxides, thus initiating lipid auto-oxidation (Halliwell and Gutteridge, 1989). H2O2 is moderately reactive and is a relatively long-lived molecule (half-life of 1 ms) that can diffuse some distances from its production site. H2O2 may inactivate enzymes by oxidizing their thiol groups. For example, enzymes of the Calvin cycle, copper/zinc superoxide dismutase and iron superoxide dismutase are inactivated by H2O2 (Charles and Halliwell, 1980; Bowler et al., 1994). The most reactive of all AOS is the hydroxyl radical that is formed from H2O2 by the so-called Haber–Weiss or Fenton reactions by using metal catalysts (Halliwell and Gutteridge, 1989). OH· can potentially react with all biological molecules, and because cells have no enzymatic mechanism to eliminate this highly reactive AOS, its excess production leads ultimately to cell death.
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Sources of AOS in plants
Most cellular compartments have the potential to become a source of AOS. Environmental stress conditions that limit CO2 fixation, such as drought and salt stress, ozone and high or low temperatures, reduce the NADP+ regeneration by the Calvin cycle, consequently, the photosynthetic electron transport chain is overreduced, forming superoxide radicals and singlet oxygen (1O2) in the chloroplasts (Krause, 1994
). To prevent overreduction of the electron transport chain under conditions that limit CO2 fixation, plants have evolved the photorespiratory pathway to regenerate NADP+ (Kozaki and Takeba, 1996). As part of the photorespiratory pathway, H2O2 is formed in the peroxisomes, where it can also be produced during the catabolism of lipids as a by-product of ß-oxidation of fatty acids (Somerville et al., 2000). Catabolism of purines probably takes place in the peroxisomes as well (del Río et al., 1998). The first reaction of this catabolic chain, the oxidation of xanthine to uric acid by xanthine oxidase, generates , whereas uric acid is oxidized to allantoin, yielding H2O2 and CO2. When growth and other energy-requiring processes in plants are reduced or cease as a consequence of stress, the electron transport chain in the mitochondria may become overreduced, favouring the generation of (Purvis, 1997). Another source of AOS in plants that has received relatively little attention is the detoxification reactions catalysed by the cytochromes, in particular cytochrome P450 in the cytoplasm and the endoplasmic reticulum. During these reactions, electron leakage to oxygen and the decomposition of the intermediate oxygenate of cytochrome P450 can form (Urban et al., 1997). AOS are also generated in plants at the plasma membrane level or extracellularly in the apoplast. NADPH-dependent oxidase (NADPH oxidase) of plasma membranes has recently been regarded as a source of AOS for the oxidative burst, which is typical of plant–pathogen incompatible interactions (Lamb and Dixon, 1997). Chemical inhibitors of NADPH oxidase, such as diphenylene iodonium (DPI), have been shown to block or severely reduce AOS production upon biotic and abiotic stresses (Allan and Fluhr, 1997; Cazalé et al., 1999; Orozco-Cardenas and Ryan, 1999; Pellinen et al., 1999; Rao and Davis, 1999). In addition to NADPH oxidase, pH-dependent cell wall peroxidases, germin-like oxalate oxidases, and amine oxidases have been proposed as sources of H2O2 in the apoplast (Bolwell and Wojtaszek, 1997). pH-dependent cell wall peroxidases are activated by alkaline pH and, in the presence of a reductant, H2O2 is formed. Alkalinization of the apoplast upon elicitor recognition precedes the oxidative burst and the production of H2O2 by pH-dependent cell wall peroxidases has been proposed as an alternative way of AOS production during biotic stress (Bolwell and Wojtaszek, 1997; Wojtaszek, 1997). Oxalate oxidase catalyses the conversion of oxalate to CO2 and H2O2 and the activity of this enzyme may also be important in certain plant–pathogen interactions (Wojtaszek, 1997). Amine oxidases catalyse the oxidation of a wide variety of biogenic amines to their corresponding aldehydes with the release of NH3 and H2O2. H2O2 formed by the oxidation of amines may be directly utilized by wall-bound peroxidases in lignification and cell wall strengthening, both during normal growth and in response to external stimuli such as wounding and pathogenesis (Allan and Fluhr, 1997; Bolwell and Wojtaszek, 1997).
Because of the highly cytotoxic and reactive nature of AOS, their accumulation must be under tight control. Plants possess very efficient enzymatic and non-enzymatic antioxidant defence systems that allow scavenging of AOS and protection of plant cells from oxidative damage. The distinct subcellular localization and biochemical properties of antioxidant enzymes, their differential inducibility at the enzyme and gene expression level and the plethora of non-enzymatic scavengers render the antioxidant systems a very versatile and flexible unit that can control AOS accumulation temporally and spatially (for reviews see Alscher and Hess, 1993; Bowler et al., 1994; Scandalios, 1994; Van Breusegem et al., 1998). Such a controlled modulation of AOS levels is significant in the light of the recent evidence for a signalling capacity of AOS.
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AOS as signal molecules |
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TopAbstractBiochemical properties of active...AOS as signal moleculesAOS and redox signallingAOS and redox-regulated gene...ConclusionsReferences |
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Signalling role of AOS in plant defence responses
With the identification of catalase as a salicylic acid (SA)-binding protein, together with a set of experiments which suggest that H2O2 is downstream from SA in pathogenesis-related (PR-1) gene induction (Chen et al., 1993
a), attention was alerted to a putative signalling role of H2O2 in plant defence responses. In an elegant series of experiments, it was demonstrated that H2O2 is indeed a diffusible signal in the induction of plant defence genes, such as glutathione-S-transferase (GST) and glutathione peroxidase (GPx) (Levine et al., 1994). A catalase trap, placed between soybean cells inoculated with an avirulent pathogen and uninfected cells, blocked the diffusible signal that originated from the infected cells and was necessary for defence gene induction (Levine et al., 1994). Induction of the PR-1 protein in tobacco leaves by UV-B treatment also required the accumulation of AOS (Green and Fluhr, 1995). Other in planta experiments demonstrated that H2O2 can act as a local as well as a systemic signal in pathogen defence. Transgenic plants with elevated levels of H2O2 due to the constitutive overproduction of glucose oxidase or repression of peroxisomal catalase were more resistant to pathogens, accumulated SA, and expressed PR genes and proteins (Wu et al., 1997; Chamnongpol et al., 1998). Accumulation of H2O2 in leaves of catalase-deficient tobacco plants was sufficient to induce the production of defence proteins (GPx, PR-1) not only locally, but also systemically (Chamnongpol et al., 1998). Although H2O2 is a diffusible molecule, its half-life is only 1 ms, which essentially excludes it from being the mobile signal for the induction of defence responses in systemic tissues. This problem may be overcome by a relay of H2O2-generating microbursts, including NADPH oxidase, as a mechanism for the reiteration of these microbursts (Alvarez et al., 1998; Park et al., 1998). Such a model was proposed based on the observation of the microscopic hypersensitive response (HR) lesions that appeared throughout Arabidopsis thaliana plants upon infection with avirulent bacterial pathogens. These micro-HRs correlated with resistance against the pathogens and expression of defence genes (GST, PR-2) and could be blocked by DPI, an inhibitor of NADPH oxidase. Moreover, co-application of glucose/glucose oxidase (generating H2O2) to the plants was sufficient to induce these responses (Alvarez et al., 1998). For such a system of H2O2 spreading, signal amplification is needed and SA has been proposed to play this agonistic role (Van Camp et al., 1998). Although the field of pathogenesis certainly led the way in oxidative stress signalling in plants for many years, evidence is now also accumulating for a signalling role of AOS in defence responses to abiotic stresses. Pre-treatment of maize seedlings with H2O2 or menadione, a superoxide-generating compound, induces chilling tolerance (Prasad et al., 1994). Plants regenerated from potato nodal explants treated with H2O2 are significantly more thermotolerant than control plants (Lopez-Delgado et al., 1998). The partial exposure of Arabidopsis plants to excess light results in systemic acclimation of unexposed leaves to photo-oxidative stress. Acclimation correlates with the expression of the ascorbate peroxidase gene (APx2) and is proposed to be mediated by H2O2, because the induction of APx2 is sensitive to catalase. Arabidopsis leaves pretreated with H2O2 also become tolerant to excess light (Karpinski et al., 1999). H2O2 also accumulates systemically in wounded tomato plants, and this H2O2 accumulation can be blocked by DPI, suggesting an NADPH-dependent mechanism of H2O2 spreading (Orozco-Cardenas and Ryan, 1999). In addition, it was shown that the wounding-dependent H2O2 accumulation acts as a secondary messenger for the induction of a subset of defence genes, including proteinase inhibitors and polyphenol oxidase (Orozco-Cárdenas et al., 2001).
Although H2O2 is generally considered to be a signalling molecule in defence responses, (or a derived product) probably plays this role as well. Phytoalexin synthesis in soybean cells in response to pathogens or elicitors is blocked by DPI and SOD, but not by catalase (Jabs et al., 1997). Similarly, , but not H2O2, is necessary and sufficient to induce lesion formation and PR-1 mRNA accumulation in the ‘lesion-simulating disease resistance response’ mutant (lsd1) of Arabidopsis (Jabs et al., 1996). Furthermore, one of the members of a tomato multigene family that encodes extensin is transcriptionally induced upon treatment with the -generating compounds digitonin or xanthine oxidase, but not with H2O2 (Wisniewski et al., 1999). Conversely, paraquat treatment induces a cytosolic APx gene in rice through H2O2 (promoted by inhibition of catalase or APx) rather than (reduced by SOD inhibitors) (Morita et al., 1999). Bacteria and yeast induce distinct defence proteins in response to either or H2O2, although a considerable overlap exists between the two responses (Demple, 1991; Jamieson, 1992). Thus, probably acts as a signalling molecule in defence responses to execute its function independently of H2O2.
Signalling role of AOS in cell death
Cell death is an essential process in the plant's life cycle. Two modes of cell death have been described in plants: programmed cell death (PCD) and necrosis. PCD is controlled genetically and has characteristic features of the apoptotic cell death in animal cells, such as cell shrinkage, cytoplasmic and nuclear condensation, chromatin condensation, and DNA fragmentation. Necrosis results from severe and persistent trauma and is considered not to be orchestrated genetically (Pennell and Lamb, 1997; O'Brien et al., 1998). PCD and necrosis may be just two distinct ends of the same process that can be initiated by the same signal, AOS (Jabs, 1999).
Plant cell death is best studied during the HR, which is typical of incompatible plant–pathogen interactions (Lamb and Dixon, 1997). During the HR, the oxidative burst coincides with the induction of cell death at the site of the pathogen attack. This localized cell death limits the spread of the invading pathogen. The source of the oxidative burst is considered to be an NADPH oxidase complex. However, modulation of the activity of antioxidant enzymes probably contributes to AOS generation during the HR as well. In tobacco cells that undergo HR upon infiltration with fungal elicitors, the catalase Cat1 and Cat2 mRNA and protein levels decrease and catalase activity is suppressed, which is paralleled by a strong H2O2 accumulation (Dorey et al., 1998). Similarly, virus-induced HR-like cell death is accompanied by the suppression of cytosolic APx expression (Mittler et al., 1998). This suppression probably contributes to the accumulation of H2O2 and activation of the cell death programme.
The first evidence that AOS act as a signal that initiates a transduction pathway towards plant cell death rather than kills the cell by reaching toxic levels came from experiments in soybean cell cultures, in which a short pulse of H2O2 was sufficient to activate a hypersensitive cell death mechanism (Levine et al., 1994). Accordingly, exogenous H2O2 (>5 mM) initiated an active cell death pathway (requiring DNA and protein synthesis) in Arabidopsis suspension cultures (Desikan et al., 1998). In both cases, the concentration of H2O2 that activated the cell death pathway was higher than that inducing expression of defence genes. H2O2 (10 mM) has to be present for 60 min to initiate an irreversible cell death process in Arabidopsis cells. Within these 60 min, H2O2 initiates a cell death programme probably through an interplay with other signalling molecules, such as ethylene and SA (Rao and Davis, 1999; Overmyer et al., 2000).
The ability of H2O2 to induce cell death was also demonstrated in planta. In transgenic plants with lower H2O2-scavenging capacities or in others that overproduce H2O2-generating enzymes, cell death appeared spontaneously or could be easily induced by stress (Chamnongpol et al., 1998; Kazan et al., 1998). Transient exposure of catalase-deficient tobacco plants to conditions that perturb H2O2 homeostasis (high light) was sufficient to activate a PCD programme similar to that observed during incompatible plant–pathogen interactions (J Dat, unpublished data).
, but not H2O2, has been shown to initiate a runaway cell death phenotype in the Arabidopsis lsd1 mutant, providing genetic evidence for the role of in plant cell death (Jabs et al., 1996). Lsd1 plants grown under long days spontaneously forms necrotic lesions on leaves and cannot stop the spreading of cell death. In front of the spreading zone of cell death, accumulates dramatically. Hence, seems to be the critical signal in the cell death process that is monitored via a ‘rheostat’ LSD1. Despite rather controversial data on versus H2O2 in cell death activation, AOS are indisputably the signals that activate genetically controlled cell death programme(s) in plants. In the Arabidopsis radical-induced cell death (rcd1) mutant, in which ozone and , but not H2O2, induce cellular accumulation and transient spreading lesions, the cellular accumulation depends on ethylene. Exogenous ethylene increases -dependent cell death, whereas impairment of ethylene perception blocks accumulation and spreading lesions (Overmyer et al., 2000).
Signalling role of AOS in growth and morphogenesis
Under stress conditions, one of the strategies that plants have adopted is to slow down growth. The ability to reduce cell division under unfavourable conditions may not only allow conservation of energy for defence purposes, but also may limit the risk of heritable damage (May et al., 1998). AOS, as ubiquitous messengers of stress responses, probably play a signalling role in these adaptive processes. Low concentrations of menadione impair the G1-to-S transition, retard DNA replication, and delay entry into mitosis (Reichheld et al., 1999). Accordingly, exogenous application of micromolar concentrations of reduced glutathione (GSH) raises the number of meristematic cells undergoing mitosis, whereas depletion of GSH has the opposite effect (Sánchez-Fernández et al., 1997). While cell cycle progression is under negative control of AOS, H2O2 stimulates somatic embryogenesis (Cui et al., 1999) and is essential for root gravitropism (Joo et al., 2001). However, the role of AOS in plant growth and development is still poorly understood and requires further research.
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AOS and redox signalling |
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TopAbstractBiochemical properties of active...AOS as signal moleculesAOS and redox signallingAOS and redox-regulated gene...ConclusionsReferences |
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Redox-sensitive proteins
Plants can sense, transduce, and translate the AOS signals into appropriate cellular responses. This process requires the presence of redox-sensitive proteins that can undergo reversible oxidation/reduction and may switch ‘on’ and ‘off’ depending on the cellular redox state. AOS can oxidize the redox-sensitive proteins directly (Halliwell and Gutteridge, 1989
; Storz and Imlay, 1999) or indirectly, via the ubiquitous redox-sensitive molecules, such as GSH or thioredoxins, which control the cellular redox state (Arrigo, 1999). Redox-sensitive metabolic enzymes may directly modulate cellular metabolism, whereas redox-sensitive signalling proteins execute their function via downstream signalling components, such as kinases, phosphatases, and transcription factors. Two molecular mechanisms of redox-sensitive regulation of protein function prevail in living organisms. One mechanism employs oxidation of thiol groups of proteins. The thiol group (–SH) can gain oxygen atoms to yield sulphenic (–SOH), sulphinic (–SO2H) or sulphonic (–SO3H) moieties or may form intramolecular or intermolecular disulphide bonds. Changes in the chemistry of –SH groups modify the electronic and steric conformation of the cysteine residue, thereby affecting protein conformations and/or protein–protein interactions. All these changes alter the functionality of proteins that have cysteine residues at strategic positions (Arrigo, 1999). The other mechanism for redox control oxidizes iron–sulphur clusters (Fe–S) within these cluster-containing proteins. Oxidation of Fe–S clusters by inactivates the cluster and affects enzyme activity. Additionally, free Fe2+ released from an oxidized Fe–S cluster forms OH· in cells (Imsande, 1999). The activity of many chloroplastic enzymes, such as the key enzymes of the Calvin cycle, glycolytic glucose-6-phosphate dehydrogenase and the coupling factor that provides ATP for biosynthetic reactions, are under redox control. Reduction of their regulatory disulphide bridges activates the enzymes, with the exception of glucose-6-phosphate dehydrogenase, which is inactivated. The reducing power used to modulate the activity of these enzymes originates from the chloroplast electron transport chain, namely from ferredoxin that is oxidized by ferredoxin-thioredoxin reductases. This oxidation is coupled to the reduction of thioredoxins that ultimately reduce regulatory cysteines in metabolic enzymes (Ruelland and Miginiac-Maslow, 1999). Many prokaryotic and eukaryotic metabolic enzymes, such as aconitase, succinate dehydrogenase, fumarase, sulphite reductase, nitrite reductase, and enzymes of purine metabolism, contain Fe–S clusters, suggesting that they may also be subject to redox regulation (Imsande, 1999).
The best-studied example of a redox-sensing receptor in plants is the cytochrome bf complex of the photosynthetic electron transport chain located in chloroplasts. Because photosystem II (PSII) and I (PSI) act in sequence during linear electron flow, the amount of light energy delivered to the two reaction centres must be controlled. When light harvesting is not balanced by light energy utilization and dissipation, toxic radicals are formed, leading to oxidative damage. One of the control mechanisms regulates dissociation of the light-harvesting complex from PSII, a process controlled by phosphorylation. The kinase responsible for that phosphorylation is activated by reduction of the plastoquinone pool, a signal that is transduced to kinase activation via a structural change of the Fe–S protein associated with the cytochrome bf complex (Vener et al., 1998). Additionally, by a yet unresolved mechanism, the redox state of plastoquinone controls the rate of transcription of the chloroplast genes that encode the reaction centre apoproteins of PSII and PSI (Pfannschmidt et al., 1999), as well as their mRNA stability and translation rate (Salvador and Klein, 1999; Trebitsh et al., 2000). The redox state of plastoquinones also controls expression of the nuclear genes APx1 and APx2 (Karpinski et al., 1997).
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AOS and redox-regulated gene expression |
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TopAbstractBiochemical properties of active...AOS as signal moleculesAOS and redox signallingAOS and redox-regulated gene...ConclusionsReferences |
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In bacteria, AOS induce expression of at least 80 proteins and most of these are believed to be regulated at the transcriptional level (Demple, 1991
). In yeast, approximately 300 genes (Gasch et al., 2000) and in plants 113 genes are induced by AOS (Desikan et al., 2001a). Bacterial genes are organized in regulons that are controlled by specific transcription factors. -induced genes are controlled by the SoxR protein that has Fe–S clusters and, upon oxidation, induces the expression of a downstream transcription factor called SoxS. H2O2-induced genes are controlled by the oxidation of thiol groups present in the transcription factor OxyR (Storz and Imlay, 1999). Alternatively, two-component systems may activate the expression of bacterial genes upon redox changes. A redox sensor, a membrane-associated phosphoprotein, becomes phosphorylated on histidine when it is either oxidized or reduced by components of the electron transport chain. Its substrate, the redox response regulator, is a sequence-specific DNA-binding protein that is phosphorylated at an aspartate residue that regulates transcription (Allen, 1993). In yeast, genes induced by redox signals consist of a complex network of different regulons, so-called stimulons (Jamieson, 1998). Activity of one of the best-studied redox-sensitive transcription factors, yAP1, is controlled by redox signals at the level of nuclear localization and DNA binding (Kuge et al., 1997). Upon imposed oxidative stress, yAP1 relocalizes from the cytoplasm to the nucleus and its DNA-binding capacity increases. In mammalian systems, many studies point to the significance of two classes of transcription factors that are sensitive to redox signals: the nuclear factor 
B (NF-B) and the activator protein-1 (AP-1). The pro-oxidant state in the cytoplasm (determined by the ratio of oxidized and reduced glutathione) or AOS activates these transcription factors and induces their translocation to the nucleus, where a reducing environment is required for proper DNA binding. Thioredoxins and the redox factor Ref-1 provide the reducing power for DNA binding (Arrigo, 1999). Thus, two major steps in the transcriptional activation of eukaryotic transcription factors seem to be influenced by redox balance: nuclear relocalization and DNA binding. Although AOS and the cellular redox state are known to control expression of plant genes, neither signalling pathway(s) nor transcription factors and promoter elements specific for the redox regulation have been identified in plants to date. There are, however, several candidates for promoter elements as well as for DNA-binding factors that may act as redox response elements.
GSTs catalyse the conjugation of GSH to a variety of hydrophobic electrophilic compounds, which, in activated form, may attack cellular macromolecules. Compounds with bound GSH are then subject to cellular detoxification pathways (Daniel, 1993). A range of factors, such as growth factors, pathogens, herbicides, hormones, and cellular stress agents, induce expression of GST genes (Chen et al., 1996). The signal by which electrophilic compounds regulate GST gene expression is believed to be a pro-oxidant state in the cells probably resulting from a reduced GSH content (Chen et al., 1996). The promoter element responsible for the induction of the Ya subunit in mouse GST by electrophilic compounds consists of two adjacent AP1-like sites (Friling et al., 1992). The consensus sequence of the AP-1 site is TGACA(A/T)(A/T)GC and is called an antioxidant-responsive element or electrophile-responsive element. Two adjacent AP1-like sites are also present in the Arabidopsis GST6 gene, which constitutes the ocs-like promoter element, and, at least in part, is required for GST6 induction by auxin, H2O2, and SA (Chen et al., 1996). Because the ocs element can also be activated by inactive analogues of auxin or SA, stress induced by these compounds rather than the ‘true hormonal effect’ may be responsible for the gene activation (Ulmasov et al., 1994). A single antioxidant-responsive element has recently been identified in the promoter of a maize catalase gene (Cat1) and found to bind nuclear factors from senescing scutella that accumulate Cat1 transcripts, possibly as a result of oxidative stress (Polidoros and Scandalios, 1999).
WRKY proteins are a large family of recently discovered transcriptional regulators that are specific to plants (Eulgem et al., 2000). WRKY transcription factors are induced by several stresses and during senescence. They possess a redox-sensitive zinc-finger DNA-binding domain in which two cysteines together with two histidines interact electrostatically with a zinc atom to form a ‘zinc finger’, which makes them excellent candidates for redox regulation (Arrigo, 1999). WRKY proteins bind W boxes (consensus sequence (T)(T)TGAC(C/T)) that are present in promoters of many defence genes. Interestingly, the W box is the only common motif in promoters of Arabidopsis genes that are co-ordinately regulated with the PR-1 gene, a marker of the systemic-acquired resistance (SAR) (Eulgem et al., 2000). The W box is present also in a minimal promoter of the stilbene synthase gene (Vst1) that is required for ozone inducibility (Schubert et al., 1997). Additional promoter elements, such as the G box, H box, and ethylene-responsive GCC box are present in the ozone-responsive part of the promoter.
The G box (CACGTG) is a ubiquitous cis element present in many plant genes and is thought to mediate responses to diverse environmental stimuli, including light, elicitors, and redox changes (Menkens et al., 1995; Dröger-Laser et al., 1997). Together with the H box (CCTACC), the G box activates phenylpropanoid biosynthetic genes. Transcription of at least two of these genes, which encode phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS), are under redox control (induced by GSH) (Wingate et al., 1988). Both a G box and an adjacent H box in the bean chs15 promoter bind a bZIP protein G/HBF-1. The binding is enhanced by phosphorylation of the G/HBF-1 that was triggered by GSH (Dröger-Laser et al., 1997). Similarly, elicitation of a bean cell suspension with GSH increases specific nuclear activities of KAP-1 and KAP-2 protein factors that recognize an H box motif (Yu et al., 1993). Heat shock factors and heat shock elements can participate also in redox-regulated gene expression. Activation of a heat shock factor is characterized by conversion from a monomeric to a trimeric state, a process induced by heat shock and a large variety of conditions that generate abnormally folded proteins. Disulphide-linked aggregates of cellular proteins are formed as a consequence of disturbed intracellular redox homeostasis and are one of the signals required for heat shock factor trimerization (Arrigo, 1999). A mutation of the heat shock element in the promoter of the Arabidopsis Apx1 gene delays its inducibility by oxidative stress (for example, methyl viologen) (Storozhenko et al., 1998). Interestingly, there are also some indications for the existence of a mammalian-like redox-regulated transcription factor NF-B in plant cells. In mammals, NF-B resides normally in the cytoplasm where it is associated with a transcriptional repressor IB. AOS stimulate IB phosphorylation and its dissociation from NF-B, leading to the nuclear localization of NF-B. The Arabidopsis mutants, npr1 and nim1, that are compromized in inducing SAR, are mutated in the gene that encodes a protein highly similar to the transcriptional repressor IB (Cao et al., 1997; Ryals et al., 1997). Moreover, a sequence similar to the NF-B recognition site was identified in the promoters of several plant defence genes (Desikan et al., 1998; Lebel et al., 1998; Etienne et al., 2000).
Kinases and phosphatases in redox signal transduction
A cascade of three protein kinases (mitogen-activated protein kinase kinase kinase [MAPKKK], protein kinase kinase [MAPKK], and protein kinase [MAPK]) is a conserved functional module in different signal transduction pathways of various organisms (for review, see Hirt, 2000). Mammalian redox-sensitive transcription factor AP1 is activated by de novo transcription of its subunits c-jun and c-fos. The signal that activates c-jun expression via phosphorylation of the cognate transcription factor is transduced via the MAPK pathway (Cano and Mahadevan, 1995). Several lines of evidence show that AOS activate the MAPK pathway in plants as well (Desikan et al., 1999; Grant et al., 2000; Kovtun et al., 2000; Samuel et al., 2000; Desikan et al., 2001b). The MAPK module that senses the H2O2 signal and translates it to the expression of defence genes (GST6, HSP18.2) was partially characterized in Arabidopsis (Kovtun et al., 2000). This module consists of an upstream kinase ANP1 (MAPKKK) and the downstream kinases AtMPK3 and AtMPK6 (MAPKs). H2O2, but not auxin, cold, or abscisic acid (ABA) activate this kinase cascade (Kovtun et al., 2000; Desikan et al., 2001b). Two Arabidopsis MAPK-like activities induced by H2O2 were shown to be independent of ethylene, jasmonic acid, and one of them was also SA independent (Grant et al., 2000). However, it is still not known whether these MAPKs correspond to AtMPK3 and AtMPK6. At the amino acid level, AtMPK3 and AtMPK6 are highly similar to the wound-induced and SA-induced protein kinases (WIPK and SIPK) of tobacco MAPKs, respectively (Jonak et al., 1999). However, only SIPK, and not WIPK, is activated by ozone, H2O2, and xanthine/xanthine oxidase (generating ) in tobacco (Samuel et al., 2000). In addition to MAPKs, a receptor-like protein kinase gene that is transcriptionally activated by AOS has been identified in Arabidopsis as well (Czernic et al., 1999).
Type 2C protein phosphatase (PP2C) has been implicated in a negative feedback loop that controls the wound-induced MAPK pathway in alfalfa, in which the corresponding gene (MP2C) is expressed (Meskiene et al., 1998). Recently, a gene encoding PP2C (NtPP2C1) that is transcriptionally responsive to different oxidative stress stimuli has been isolated in tobacco (Vranová et al., 2000). However, NtPP2C1 expression is suppressed by oxidative stress, suggesting that NtPP2C1 is not part of a negative feedback loop in the AOS signalling pathway. High homology of NtPP2C1 to PP2Cs, which are implicated in the negative feedback control of the ABA signal transduction pathway, suggests a possible interaction of AOS with the ABA signalling pathway.
AOS: part of a signalling network
To affect plant growth and metabolism so strongly, AOS must utilize and/or interfere with other signalling pathways or molecules. There is evidence that plant hormones are positioned downstream of the AOS signal. H2O2 induces accumulation of stress hormones, such as SA and ethylene (León et al., 1995; Chamnongpol et al., 1998). Tobacco plants exposed to ozone accumulate ABA (Ederli et al., 1997) and induction of the PDF1.2 gene by paraquat is impaired in Arabidopsis mutants insensitive to jasmonates (coi1) and ethylene (ein2) (Penninckx et al., 1998). Plant hormones are not only located downstream of the AOS signal, but AOS themselves are also secondary messengers in many hormone signalling pathways (Chen et al., 1993b; Pei et al., 2000; Orozco-Cárdenas et al., 2001). Therefore, feedback or feedforward interactions may conceivably occur between different hormones and AOS (for an overview, see Van Breusegem et al., 2001).
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Conclusions |
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TopAbstractBiochemical properties of active...AOS as signal moleculesAOS and redox signallingAOS and redox-regulated gene...ConclusionsReferences |
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Besides exacerbating cellular damage, AOS can act as ubiquitous signal molecules in plants. AOS are a central component in stress responses and the level of AOS determines the type of response. Whereas at low concentrations AOS induce defence genes and adaptive responses, at high concentrations cell death is initiated. To allow for this dual role, cellular levels of AOS must be tightly controlled. The numerous AOS sources and a complex system of oxidant scavengers provide the flexibility necessary for these functions. How these systems are regulated to achieve the temporal and spatial control of AOS production is still poorly understood. Sublethal amounts of AOS acclimate plants to biotic and abiotic stress conditions and reduce plant growth, probably as part of an acclimatory mechanism. Although substantial genome response and the activity of many enzymes are known to be affected by AOS, molecular and biochemical mechanisms of acclimation are still not understood and the signalling pathways involved remain elusive. AOS communicate with other signal molecules and pathways, being part of the signalling network that controls responses downstream of AOS. Recently, information on the role of AOS as signal molecules in growth and morphogenesis has emerged, suggesting that AOS are not only stress signal molecules but may also be an intrinsic signal in plant growth and development. Genetic analysis in addition to physiological studies will be required to position AOS signals in the transduction pathway(s) and to understand how the signals are perceived and transduced to specific downstream responses.
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The authors wish to acknowledge James Dat for sharing unpublished data and Martine De Cock, Stijn Debruyne and Rebecca Verbanck for the excellent help in preparing the manuscript. FVB is indebted to the Vlaams Instituut voor de Bevordering van het Wetenschappelijk-Technologisch Onderzoek in de Industrie for a postdoctoral fellowship.
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1 To whom correspondence should be addressed. Fax: +32 9 264 5349. E-mail: diinz{at}gengenp.rug.ac.be
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