Induction of ASCORBATE PEROXIDASE 2 expression in wounded Arabidopsis leaves does not involve known wound-signalling pathways but is associated with changes in photosynthesis

The phytotoxin coronatine induces light-dependent reactive oxygen species in tomato seedlings

The phytotoxin coronatine (COR), which is produced by Pseudomonas syringae pv. tomato DC3000 (DC3000), has multiple roles in virulence that lead to chlorosis and a reduction in chlorophyll content. However, the physiological significance of COR-induced chlorosis in disease development is still largely unknown. Global expression analysis demonstrated that DC3000 and COR, but not the COR-defective mutant DB29, caused reduced expression of photosynthesis-related genes and result in a 1.5- to 2-fold reduction in maximum quantum efficiency of photosystem II (F(V)/F(M)). Tomato (Solanum lycopersicum) seedlings inoculated with DC3000 and incubated in a long daily photoperiod showed more necrosis than inoculated seedlings incubated in either dark or a short daily photoperiod. The accumulation of reactive oxygen species (ROS) was detected in cotyledons inoculated with either purified COR or DC3000 but not in tissues inoculated with DB29. Interestingly, COR-induced ROS accumulated only in light and was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea and diphenylene iodonium, which function to inhibit electron transport from PSII. Furthermore, COR and DC3000 suppressed expression of the gene encoding the thylakoid Cu/Zn superoxide dismutase but not the cytosolic form of the same enzyme. In conclusion, these results demonstrate a role for COR-induced effects on photosynthetic machinery and ROS in modulating necrotic cell death during bacterial speck disease of tomato.

Agriculture and the new challenges for photosynthesis research

A rising human population and changing patterns of land use mean that world food production rates will need to be increased by at least 50% by 2050, a massive rise in harvestable yield per hectare of the major crops such as rice (Oryza sativa) and wheat (Triticum aestivum). Combinations of breeding for improved morphology-related traits such as harvest index and increased inputs of water and fertilizer, which have sustained yield increases since the 1960s, will be neither sufficient nor sustainable. An important limiting factor will be the capacity to produce sufficient biomass during favourable growing periods. Here we analyse this problem in the context of increasing the efficiency of conversion of solar energy into biomass, that is, leaf and canopy photosynthesis. Focussing on crops carrying out C3 photosynthesis, we analyse the evidence for 'losses' in the process of conversion of solar energy into crop biomass and we explore novel mechanisms of improving biomass production rates, which have arisen from recent research into the fundamental primary processes of photosynthesis and carbohydrate metabolism. We show that there are several lines of evidence that these processes are not fully optimized for maximum yield. We put forward the hypothesis that the chloroplast itself should be given greater prominence as a sensor, processor and integrator of highly variable environmental signals to allow a more efficient transduction of energy supply into biomass production.

Plastid signalling to the nucleus and beyond

Communication between the compartments or organelles of cells is essential for plant growth and development. There is an emerging understanding of signals generated within energy-transducing organelles, such as chloroplasts and mitochondria, and the nuclear genes that respond to them, a process known as retrograde signalling. A recent series of unconnected breakthroughs have given scientists a glimpse inside the 'black box' of organellar signalling thanks to the identification of some of the factors involved in generating and propagating signals to the nucleus and, in some instances, systemically throughout photosynthetic tissues. This review will focus on recent developments in our understanding of retrograde and systemic signals generated by organelles, with an emphasis on chloroplasts.

Ascorbate peroxidase 1 plays a key role in the response of Arabidopsis thaliana to stress combination

Within their natural habitat plants are subjected to a combination of different abiotic stresses, each with the potential to exacerbate the damage caused by the others. One of the most devastating stress combinations for crop productivity, which frequently occurs in the field, is drought and heat stress. In this study we conducted proteomic and metabolic analysis of Arabidopsis thaliana plants subjected to a combination of drought and heat stress. We identified 45 different proteins that specifically accumulated in Arabidopsis in response to the stress combination. These included enzymes involved in reactive oxygen detoxification, malate metabolism, and the Calvin cycle. The accumulation of malic enzyme during the combined stress corresponded with enhanced malic enzyme activity, a decrease in malic acid, and lower amounts of oxaloacetate, suggesting that malate metabolism plays an important role in the response of Arabidopsis to the stress combination. Cytosolic ascorbate peroxidase 1 (APX1) protein and mRNA accumulated during the stress combination. When exposed to heat stress combined with drought, an APX1-deficient mutant (apx1) accumulated more hydrogen peroxide and was significantly more sensitive to the stress combination than wild type. In contrast, mutants deficient in thylakoid or stromal/mitochondrial APXs were not more sensitive to the stress combination than apx1 or wild type. Our findings suggest that cytosolic APX1 plays a key role in the acclimation of plants to a combination of drought and heat stress.

Generation of hydrogen peroxide in chloroplasts of Arabidopsis overexpressing glycolate oxidase as an inducible system to study oxidative stress

Arabidopsis (Arabidopsis thaliana) overexpressing glycolate oxidase (GO) in chloroplasts accumulates both hydrogen peroxide (H(2)O(2)) and glyoxylate. GO-overexpressing lines (GO plants) grown at 75 micromol quanta m(-2) s(-1) show retarded development, yellowish rosettes, and impaired photosynthetic performance, while at 30 micromol quanta m(-2) s(-1), this phenotype virtually disappears. The GO plants develop oxidative stress lesions under photorespiratory conditions but grow like wild-type plants under nonphotorespiratory conditions. GO plants coexpressing enzymes that further metabolize glyoxylate but still accumulate H(2)O(2) show all features of the GO phenotype, indicating that H(2)O(2) is responsible for the GO phenotype. The GO plants can complete their life cycle, showing that they are able to adapt to the stress conditions imposed by the accumulation of H(2)O(2) during the light period. Moreover, the data demonstrate that a response to oxidative stress is installed, with increased expression and/or activity of known oxidative stress-responsive components. Hence, the GO plants are an ideal noninvasive model system in which to study the effects of H(2)O(2) directly in the chloroplasts, because H(2)O(2) accumulation is inducible and sustained perturbations can reproducibly be provoked by exposing the plants to different ambient conditions.

Chloroplast signaling and LESION SIMULATING DISEASE1 regulate crosstalk between light acclimation and immunity in Arabidopsis

Plants are simultaneously exposed to abiotic and biotic hazards. Here, we show that local and systemic acclimation in Arabidopsis thaliana leaves in response to excess excitation energy (EEE) is associated with cell death and is regulated by specific redox changes of the plastoquinone (PQ) pool. These redox changes cause a rapid decrease of stomatal conductance, global induction of ASCORBATE PEROXIDASE2 and PATHOGEN RESISTANCE1, and increased production of reactive oxygen species (ROS) and ethylene that signals through ETHYLENE INSENSITIVE2 (EIN2). We provide evidence that multiple hormonal/ROS signaling pathways regulate the plant's response to EEE and that EEE stimulates systemic acquired resistance and basal defenses to virulent biotrophic bacteria. In the Arabidopsis LESION SIMULATING DISEASE1 (lsd1) null mutant that is deregulated for EEE acclimation responses, propagation of EEE-induced programmed cell death depends on the plant defense regulators ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and PHYTOALEXIN DEFICIENT4 (PAD4). We find that EDS1 and PAD4 operate upstream of ethylene and ROS production in the EEE response. The data suggest that the balanced activities of LSD1, EDS1, PAD4, and EIN2 regulate signaling of programmed cell death, light acclimation, and holistic defense responses that are initiated, at least in part, by redox changes of the PQ pool.

Antioxidant response system in the short-term post-wounding effect in Mesembryanthemum crystallinum leaves

Mechanical wounding of Mesembryanthemum crystallinum leaves in planta induced a fast decrease in stomatal conductance, which was related to accumulation of hydrogen peroxide (H(2)O(2)). Higher levels of H(2)O(2) were accompanied by an increase in total activity of superoxide dismutase (SOD) and a decrease in catalase (CAT) activity. Among SOD forms, manganese SOD (MnSOD) and copper/zinc SOD (Cu/ZnSOD) seem to be especially important sources of H(2)O(2) at early stages of wounding response. Moreover, NADP-malic enzyme (NADP-ME), one of the key enzymes of primary carbon metabolism, which is also involved in stress responses, showed a strong increase in activity in wounded leaves. All these symptoms: high accumulation of H(2)O(2), high activities of Cu/ZnSOD and NADP-ME, together with the decrease of CAT activity, were also observed in the major veins of unwounded leaves. The potential role of veinal tissues as an important source of H(2)O(2) during wounding response is discussed.

Light has a specific role in modulating Arabidopsis gene expression at low temperature

BACKGROUND

Light and temperature are the key abiotic modulators of plant gene expression. In the present work the effect of light under low temperature treatment was analyzed by using microarrays. Specific attention was paid to the up and down regulated genes by using promoter analysis. This approach revealed putative regulatory networks of transcription factors behind the induction or repression of the genes.

RESULTS

Induction of a few oxidative stress related genes occurred only under the Cold/Light treatment including genes encoding iron superoxide dismutase (FeSOD) and glutathione-dependent hydrogen peroxide peroxidases (GPX). The ascorbate dependent water-water cycle genes showed no response to Cold/Light or Cold/Dark treatments. Cold/Light specifically induced genes encoding protective molecules like phenylpropanoids and photosynthesis-related carotenoids also involved in the biosynthesis of hormone abscisic acid (ABA) crucial for cold acclimation. The enhanced/repressed transcript levels were not always reflected on the respective protein levels as demonstrated by dehydrin proteins.

CONCLUSION

Cold/Light up regulated twice as many genes as the Cold/Dark treatment and only the combination of light and low temperature enhanced the expression of several genes earlier described as cold-responsive genes. Cold/Light-induced genes included both cold-responsive transcription factors and several novel ones containing zinc-finger, MYB, NAC and AP2 domains. These are likely to function in concert in enhancing gene expression. Similar response elements were found in the promoter regions of both the transcription factors and their target genes implying a possible parallel regulation or amplification of the environmental signals according to the metabolic/redox state in the cells.

Impact of chloroplastic- and extracellular-sourced ROS on high light-responsive gene expression in Arabidopsis

The expression of 28 high light (HL)-responsive genes of Arabidopsis was analysed in response to environmental and physiological factors known to influence the expression of the HL-responsive gene, ASCORBATE PEROXIDASE2 (APX2). Most (81%) of the HL-responsive genes, including APX2, required photosynthetic electron transport for their expression, and were responsive to abscisic acid (ABA; 68%), strengthening the impression that these two signals are crucial in the expression of HL-responsive genes. Further, from the use of mutants altered in reactive oxygen species (ROS) metabolism, it was shown that 61% of these genes, including APX2, may be responsive to chloroplast-sourced ROS. In contrast, apoplastic/plasma membrane-sourced H2O2, in part directed by the respiratory burst NADPH oxidases AtrbohD and AtrbohF, was shown to be important only for APX2 expression. APX2 expression in leaves is limited to bundle sheath parenchyma; however, for the other genes in this study, information on their tissue specificity of expression is sparse. An analysis of expression in petioles, enriched for bundle sheath tissue compared with distal leaf blade, in HL and control leaves showed that 25% of them had >10-fold higher expression in the petiole than in the leaf blade. However, this did not mean that these petiole expression genes followed a pattern of regulation observed for APX2.

The grateful dead: damage-associated molecular pattern molecules and reduction/oxidation regulate immunity

The response to pathogens and damage in plants and animals involves a series of carefully orchestrated, highly evolved, molecular mechanisms resulting in pathogen resistance and wound healing. In metazoans, damage- or pathogen-associated molecular pattern molecules (DAMPs, PAMPs) execute precise intracellular tasks and are also able to exert disparate functions when released into the extracellular space. The emergent consequence for both inflammation and wound healing of the abnormal extracellular persistence of these factors may underlie many clinical disorders. DAMPs/PAMPs are recognized by hereditable receptors including the Toll-like receptors, the NOD1-like receptors and retinoic-acid-inducible gene I-like receptors, as well as the receptor for advanced glycation end products. These host molecules 'sense' not only pathogens but also misfolded/glycated proteins or exposed hydrophobic portions of molecules, activating intracellular cascades that lead to an inflammatory response. Equally important are means to not only respond to these molecules but also to eradicate them. We have speculated that their destruction through oxidative mechanisms normally exerted by myeloid cells, such as neutrophils and eosinophils, or their persistence in the setting of pathologic extracellular reducing environments, maintained by exuberant necrotic cell death and/or oxidoreductases, represent important molecular means enabling chronic inflammatory states.

Lysigenous aerenchyma formation in Arabidopsis is controlled by LESION SIMULATING DISEASE1

Aerenchyma tissues form gas-conducting tubes that provide roots with oxygen under hypoxic conditions. Although aerenchyma have received considerable attention in Zea mays, the signaling events and genes controlling aerenchyma induction remain elusive. Here, we show that Arabidopsis thaliana hypocotyls form lysigenous aerenchyma in response to hypoxia and that this process involves H(2)O(2) and ethylene signaling. By studying Arabidopsis mutants that are deregulated for excess light acclimation, cell death, and defense responses, we find that the formation of lysigenous aerenchyma depends on the plant defense regulators LESION SIMULATING DISEASE1 (LSD1), ENHANCED DISEASE SUSCEPIBILITY1 (EDS1), and PHYTOALEXIN DEFICIENT4 (PAD4) that operate upstream of ethylene and reactive oxygen species production. The obtained results indicate that programmed cell death of lysigenous aerenchyma in hypocotyls occurs in a similar but independent manner from the foliar programmed cell death. Thus, the induction of aerenchyma is subject to a genetic and tissue-specific program. The data lead us to conclude that the balanced activities of LSD1, EDS1, and PAD4 regulate lysigenous aerenchyma formation in response to hypoxia.

Altered ABA, proline and hydrogen peroxide in an Arabidopsis glutamate:glyoxylate aminotransferase mutant

Plant responses to abiotic stress are determined both by the severity of the stress as well as the metabolic status of the plant. Abscisic acid (ABA) is a key component in integrating these various signals and controlling downstream stress responses. By screening for plants with decreased RD29A:LUC expression, we isolated two alleles, glutamate:glyoxylate transferase1-1 (ggt1-1) and ggt1-2, of a mutant with altered ABA sensitivity. In addition to reduced ABA induction of RD29A, ggt1-1 was altered in ABA and stress regulation of Delta1-pyrroline-5-carboxylate synthase, proline dehydrogenase and 9-cis-epoxycarotenoid dioxygenase 3, which encode enzymes involved in Pro and ABA metabolism, respectively. ggt1-1 also had altered ABA and Pro contents after stress or ABA treatments while root growth and leaf water loss were relatively unaffected. A light-dependent increase in H2O2 accumulation was observed in ggt1-1 consistent with the previously characterized role of GGT1 in photorespiration. Treatment with exogenous H2O2, as well as analysis of a mutant in nucleoside diphosphate kinase 2 which also had increased H2O2 content but is not involved in photorespiration or amino acid metabolism, demonstrated that the greater ABA stimulation of Pro accumulation in these mutants was caused by altered H2O2 content as opposed to other metabolic changes. The results suggest that metabolic changes that alter H2O2 levels can affect both ABA accumulation and ABA sensitivity.

Cytosolic NADP-isocitrate dehydrogenase of pea plants: genomic clone characterization and functional analysis under abiotic stress conditions

NADPH is an essential electron donor in numerous biosynthetic and detoxification reactions. In animal, yeast and bacteria, the NADP-dependent isocitrate dehydrogenase (NADP-ICDH), which catalyzes the production of NADPH, is being recognized as an essential component of the antioxidative defence mechanisms. In plant cells, there is little information on the antioxidant properties of NADP-ICDH. Using a pea cDNA lambdagt11 library, the full-length cDNA of a NADP-ICDH was obtained. In pea leaves, the analyses of activity, protein and transcript expression of NADP-ICDH under six different abiotic stress conditions (CL, continuous light, HLI, high light intensity, D, continuous dark, LT, low-temperature HT, high-temperature and W, mechanical wounding) revealed a differential regulation at transcriptional and post-translational level depending on the abiotic stress. The activity and protein expression of NADP-ICDH and catalase increased only under HLI but the NADP-ICDH transcripts were up-regulated by cold stress (70%) and W (40%). Under the same conditions, the transcript analysis of glutathione reductase (GR), monodehydroascorbate reductase (MDAR) and ascorbate peroxidase (APX), key components of the antioxidative ascorbate-glutathione cycle, showed similar inductions. These data indicate that in pea plants the cytosolic NADP-ICDH shows a differential response, at mRNA and activity level, depending on the type of abiotic stress and suggests that this dehydrogenase could have a protective antioxidant role against certain environmental stresses in plants.

An HPLC-based method of estimation of the total redox state of plastoquinone in chloroplasts, the size of the photochemically active plastoquinone-pool and its redox state in thylakoids of Arabidopsis

We have described a direct, high-performance liquid chromatography-based method of estimation of the total level of plastoquinone (PQ) in leaves, the redox state of total (photoactive and non-photoactive) PQ, as well as the redox state of the PQ-pool that is applicable to any illumination conditions. This method was applied to Arabidopsis thaliana leaves but it can be applied to any other plant species. The obtained results show that the level of total PQ was 25+/-3 molecules/1000 chlorophyll (Chl) molecules in relation to foliar total Chl content. The level of the photoactive PQ, i.e., the PQ-pool, was about 31% of the total PQ present in Arabidopsis leaves that corresponds to about 8 PQ molecules/1000 Chl molecules. The reduction level of the non-photoactive PQ fraction, present outside thylakoids in chloroplasts, was estimated to account for about 49%. The measurements of the redox state of the PQ-pool showed that the pool was reduced during the dark period in about 24%, and during the light period (150 micromol/m(2).s) the reduction of the PQ-pool increased to nearly 100%. The obtained results were discussed in terms of the activity of chlororespiration pathways in Arabidopsis and the regulatory role of the redox state of PQ-pool in various physiological and molecular processes in plants.

A novel wound-responsive cis-element, VWRE, of the vascular system-specific expression of a tobacco peroxidase gene, tpoxN1

The wound-induced expression of tpoxN1, encoding a tobacco peroxidase, is unique because of its vascular system-specific expression and insensitivity to known wound-signal compounds such as jasmonic acid, ethylene, and plant hormones [Sasaki et al. (2002) Plant Cell Physiol 43:108-117]. To study the mechanism of expression, the 2-kbp tpoxN1 promoter region and successive 5'-deletion of the promoter were introduced as GUS fusion genes into tobacco plants. Analysis of GUS activity in transgenic plants indicated that a vascular system-specific and wound-responsive cis-element (VWRE) is present at the -239/-200 region of the promoter. Gel mobility shift assays suggested that a nuclear factor(s) prepared from wounded tobacco stems binds a 14-bp sequence (-229/-215) in the -239/-200 region in a sequence-specific manner. A mutation in this 14-bp region of the -239 promoter fragment resulted in a considerable decrease in wound-responsive GUS activity in transgenic plants. An 11-bp sequence, which completely overlaps with the 14-bp sequence, was found in the 5' distal region (-420/-410) and is thought to contribute to the wound-induced expression together with the 14-bp. The -114-bp core promoter of the tpoxN1 gene was indispensable for wound-induced expression, indicating that the 14-bp region is a novel wound-responsive cis-element VWRE, which may work cooperatively with other factors in the promoter.

Rice ascorbate peroxidase gene family encodes functionally diverse isoforms localized in different subcellular compartments

Aerobic organisms evolved a complex antioxidant system, which protect the cells against oxidative damage caused by partially reduced oxygen intermediates, also known as reactive oxygen species. In plants, ascorbate peroxidases (EC, 1.11.1.11) catalyze the conversion of H(2)O(2) to H(2)O, using ascorbate as the specific electron donor in this enzymatic reaction. Previously, eight APx genes were identified in the rice (Oryza sativa L.) genome through in silico analysis: two cytosolic isoforms, two putative peroxisomal isoforms, and four putative chloroplastic ones. Using gene-specific probes, we confirmed the presence of the eight APx genes in the rice genome by Southern blot hybridization. Transcript accumulation analysis showed specific expression patterns for each member of the APx family according to developmental stage and in response to salt stress, revealing the complexity of the antioxidant system in plants. Finally, the subcellular localization of rice APx isoforms was determined using GFP-fusion proteins in BY-2 tobacco cells. In agreement with the initial prediction, OSAPX3 was localized in the peroxisomes. On the other hand, the OSAPX6-GFP fusion protein was found in mitochondria of the BY-2 cells, in contrast to the chloroplastic location predicted by sequence analysis. Our findings reveal the functional diversity of the rice APx genes and suggest complementation and coordination of the antioxidant defenses in different cellular compartments during development and abiotic stress.

A mutation affecting ASCORBATE PEROXIDASE 2 gene expression reveals a link between responses to high light and drought tolerance

Molecular analyses of plants have revealed a number of genes whose expression changes in response to high light (HL), including the H2O2 scavenger, ASCORBATE PEROXIDASE 2 (APX2). We carried out a screen in Arabidopsis thaliana for lesions that alter HL-induced expression of APX2 to identify components in abiotic stress signalling pathways. High light was used as it can be instantaneously applied or removed and accurately measured. We identified a number of alx mutations causing altered APX2 expression. Here we describe the gain-of-function mutant, alx8, which has constitutively higher APX2 expression and higher levels of foliar abscisic acid (ABA) than wild type. In fact, exogenous ABA increased APX2 expression and the APX2 promoter contains ABA response elements. Furthermore, we have shown that HL stress increases ABA in wild-type plants, implicating ABA in the regulation of HL-inducible genes. The alx8 mutant is drought tolerant, exhibits improved water-use efficiency and a number of drought-tolerance genes are upregulated. Additionally, alx8 demonstrates the complexity of ABA-dependent and ABA-independent transcriptional networks as some components in both pathways are upregulated in alx8. This study provides evidence for common steps in drought and HL stress response pathways.

Isolation and characterization of four ascorbate peroxidase cDNAs responsive to water deficit in cowpea leaves

BACKGROUND AND AIMS

Abiotic stresses stimulate formation of active oxygen species in plant tissues. Among antioxidant mechanisms, H2O2 detoxication by ascorbate peroxidases (APX) plays an important role. Several APX isoforms exist in plant cells, and they have rarely been studied separately. The aim of this work was to study changes in cytosolic, peroxisomal, stromatic and thylakoid APX gene expression in response to progressive drought, rapid desiccation and application of exogenous abscisic acid in the leaves of cowpea (Vigna unguiculata) plants.

METHODS

Two cowpea (V. unguiculata) cultivars, 'EPACE-1' which is drought-tolerant and '1183'which is drought-sensitive, were submitted to drought stress by withholding irrigation. Detached leaves were air-dried or treated with exogenous abscisic acid. APX cDNAs were isolated by PCR and cloned in plasmid vectors. Changes in gene expression were studied using reverse-transcription PCR.

KEY RESULTS

Four new V. unguiculata cDNAs encoding putative cytosolic, peroxisomal and chloroplastic (stromatic and thylakoidal) APX were isolated and characterized. In response to the different treatments, higher increases in steady-state transcript levels of the cytoplasmic and peroxisomal APX genes were observed in '1183' compared with 'EPACE-1'. On the other hand, the expression of the chloroplastic APX genes was stimulated earlier in the tolerant cultivar when submitted to progressive drought.

CONCLUSIONS

Water deficit induced differences in transcript accumulation of APX genes between the two cultivars that were related to their respective tolerance to drought. Chloroplastic APX genes responded early to progressive water deficit in the tolerant plant, suggesting a capacity to efficiently detoxify active oxygen species at their production site. The more sensitive '1183' was also able to respond to drought by activating its whole set of APX genes.

Imaging photosynthesis in wounded leaves of Arabidopsis thaliana

Chlorophyll fluorescence imaging provides a non-invasive and non-destructive means with which to measure photosynthesis. This technique has been used, in combination with 14CO2 feeding, to study the spatial and temporal changes in source-sink relationships which occur in mechanically wounded leaves of Arabidopsis thaliana. Twenty-four hours after wounding, cells proximal to the wound margin showed a rapid induction of PhiII upon illumination (a measure of the efficiency of photosystem II photochemistry) whilst cells more distal to the wound margin exhibited a much slower induction of PhiII and a large, transient increase in NPQ (a measure of the rate constant for non-photochemical energy dissipation within the light-harvesting antenna). These results are indicative of an increase in sink strength in the vicinity of the wound and this was confirmed by the retention of 14C photosynthate in this region. It has been hypothesized that wound-induced cell wall (apoplastic) invertase (cwINV) activity plays a central role in generating localized increases in sink strength in stressed plant tissue and that hexose sugars generated by the sucrolytic activity of cwINV may act as a signal regulating gene expression. Enzyme activity measurements, quantitative RT-PCR, and T-DNA insertional mutagenesis have been used to determine that expression of AtcwINV1 is responsible for all induced cwINV activity in mechanically wounded leaves. Whilst inactivation of this gene abolished wound-induced cwINV activity, it did not affect localized alterations in source-sink relationships of wounded leaves or wound-regulated gene expression. The signals that may regulate source-sink relationships and signalling in wounded leaves are discussed.

Seduced by the dark side: integrating molecular and ecological perspectives on the influence of light on plant defence against pests and pathogens

Plants frequently suffer attack from herbivores and microbial pathogens, and have evolved a complex array of defence mechanisms to resist defoliation and disease. These include both preformed defences, ranging from structural features to stores of toxic secondary metabolites, and inducible defences, which are activated only after an attack is detected. It is well known that plant defences against pests and pathogens are commonly affected by environmental conditions, but the mechanisms by which responses to the biotic and abiotic environments interact are only poorly understood. In this review, we consider the impact of light on plant defence, in terms of both plant life histories and rapid scale molecular responses to biotic attack. We bring together evidence that illustrates that light not only modulates defence responses via its influence on biochemistry and plant development but, in some cases, is essential for the development of resistance. We suggest that the interaction between the light environment and plant defence is multifaceted, and extends across different temporal and biological scales.

Glutathione, photosynthesis and the redox regulation of stress-responsive gene expression

The ubiquitous antioxidant thiol tripeptide glutathione is present in millimolar concentrations in plant tissues and is regarded as one of the major determinants of cellular redox homeostasis. Recent research has highlighted a regulatory role for glutathione in influencing the expression of many genes important in plants' responses to both abiotic and biotic stress. Therefore, it becomes important to consider how glutathione levels and its redox state are influenced by environmental factors, how glutathione is integrated into primary metabolism and precisely how it can influence the functioning of signal transduction pathways by modulating cellular redox state. This review draws on a number of recent important observations and papers to present a unified view of how the responsiveness of glutathione to changes in photosynthesis may be one means of linking changes in nuclear gene expression to changes in the plant's external environment.

Chloroplasts as source and target of cellular redox regulation: a discussion on chloroplast redox signals in the context of plant physiology

During the evolution of plants, chloroplasts have lost the exclusive genetic control over redox regulation and antioxidant gene expression. Together with many other genes, all genes encoding antioxidant enzymes and enzymes involved in the biosynthesis of low molecular weight antioxidants were transferred to the nucleus. On the other hand, photosynthesis bears a high risk for photo-oxidative damage. Concomitantly, an intricate network for mutual regulation by anthero- and retrograde signals has emerged to co-ordinate the activities of the different genetic and metabolic compartments. A major focus of recent research in chloroplast regulation addressed the mechanisms of redox sensing and signal transmission, the identification of regulatory targets, and the understanding of adaptation mechanisms. In addition to redox signals communicated through signalling cascades also used in pathogen and wounding responses, specific chloroplast signals control nuclear gene expression. Signalling pathways are triggered by the redox state of the plastoquinone pool, the thioredoxin system, and the acceptor availability at photosystem I, in addition to control by oxolipins, tetrapyrroles, carbohydrates, and abscisic acid. The signalling function is discussed in the context of regulatory circuitries that control the expression of antioxidant enzymes and redox modulators, demonstrating the principal role of chloroplasts as the source and target of redox regulation.

Different signaling and cell death roles of heterotrimeric G protein alpha and beta subunits in the Arabidopsis oxidative stress response to ozone

Arabidopsis thaliana plants with null mutations in the genes encoding the alpha and beta subunits of the single heterotrimeric G protein are less and more sensitive, respectively, to O3 damage than wild-type Columbia-0 plants. The first peak of the bimodal oxidative burst elicited by O3 in wild-type plants is almost entirely missing in both mutants. The late peak is normal in plants lacking the Gbeta protein but missing in plants lacking the Galpha protein. Endogenous reactive oxygen species (ROS) are first detectable in chloroplasts of leaf epidermal guard cells. ROS production in adjacent cells is triggered by extracellular ROS signals produced by guard cell membrane-associated NADPH oxidases encoded by the AtrbohD and AtrbohF genes. The late, tissue damage-associated component of the oxidative burst requires only the Galpha protein and arises from multiple cellular sources. The early component of the oxidative burst, arising primarily from chloroplasts, requires signaling through the heterotrimer (or the Gbetagamma complex) and is separable from Galpha-mediated activation of membrane-bound NADPH oxidases necessary for both intercellular signaling and cell death.

Two distinct redox signaling pathways for cytosolic APX induction under photooxidative stress

The cross-talk of two redox factors, H2O2 accumulation and redox status of photosynthetic electron transport (PET) in tobacco chloroplasts, on the expression of cytosolic ascorbate peroxidase (cAPX) was studied. Transgenic tobacco plants, which expressed respectively the Escherichia coli catalase and spinach thylakoid-membrane-bound APX in chloroplasts and showed increased tolerance to photooxidative stress, were used for evaluation of the relationship between H2O2 accumulation or change of redox status of PET and cAPX induction under photooxidative stress condition. There was no difference in the increase in the transcript level of cAPX in the first 1 h after irradiation with high-intensity light between the wild-type and either of the transgenic plants. The transcript level of cAPX in the wild-type showed a steady gradual increase during the next 5 h, while that in the transgenic plants during this period was stable. The H2O2 level of wild-type plants increased after 1 h, while those of transgenic plants remained constant. The decrease in q(p) value in all types of plants occurred earlier than the increase in H2O2 level. These results indicate that the induction of cAPX expression is caused by a redox change in PET, probably through the plastquinone pool at an early stage and thereafter by an increase in the cellular H2O2 level.

Evidence for a direct link between glutathione biosynthesis and stress defense gene expression in Arabidopsis

The mutant regulator of APX2 1-1 (rax1-1) was identified in Arabidopsis thaliana that constitutively expressed normally photooxidative stress-inducible ASCORBATE PEROXIDASE2 (APX2) and had >/=50% lowered foliar glutathione levels. Mapping revealed that rax1-1 is an allele of gamma-GLUTAMYLCYSTEINE SYNTHETASE 1 (GSH1), which encodes chloroplastic gamma-glutamylcysteine synthetase, the controlling step of glutathione biosynthesis. By comparison of rax1-1 with the GSH1 mutant cadmium hypersensitive 2, the expression of 32 stress-responsive genes was shown to be responsive to changed glutathione metabolism. Under photo-oxidative stress conditions, the expression of a wider set of defense-related genes was altered in the mutants. In wild-type plants, glutathione metabolism may play a key role in determining the degree of expression of defense genes controlled by several signaling pathways both before and during stress. This control may reflect the physiological state of the plant at the time of the onset of an environmental challenge and suggests that changes in glutathione metabolism may be one means of integrating the function of several signaling pathways.