Ascorbate and glutathione: the heart of the redox hub

Redox signals as a language of interorganellar communication in plant cells

Plants are redox systems and redox-active compounds control and regulate all aspects of their life. Recent studies have shown that changes in reactive oxygen species (ROS) concentration mediated by enzymatic and non-enzymatic antioxidants are transferred into redox signals used by plants to activate various physiological responses. An overview of the main antioxidants and redox signaling in plant cells is presented. In this review, the biological effects of ROS and related redox signals are discussed in the context of acclimation to changing environmental conditions. Special attention is paid to the role of thiol/disulfide exchange via thioredoxins (Trxs), glutaredoxins (Grxs) and peroxiredoxins (Prxs) in the redox regulatory network. In plants, chloroplasts and mitochondria occupying a chloroplasts and mitochondria play key roles in cellular metabolism as well as in redox regulation and signaling. The integrated redox functions of these organelles are discussed with emphasis on the importance of the chloroplast and mitochondrion to the nucleus retrograde signaling in acclimatory and stress response.

Abscisic acid-responsive guard cell metabolomes of Arabidopsis wild-type and gpa1 G-protein mutants

Individual metabolites have been implicated in abscisic acid (ABA) signaling in guard cells, but a metabolite profile of this specialized cell type is lacking. We used liquid chromatography-multiple reaction monitoring mass spectrometry for targeted analysis of 85 signaling-related metabolites in Arabidopsis thaliana guard cell protoplasts over a time course of ABA treatment. The analysis utilized ∼ 350 million guard cell protoplasts from ∼ 30,000 plants of the Arabidopsis Columbia accession (Col) wild type and the heterotrimeric G-protein α subunit mutant, gpa1, which has ABA-hyposensitive stomata. These metabolomes revealed coordinated regulation of signaling metabolites in unrelated biochemical pathways. Metabolites clustered into different temporal modules in Col versus gpa1, with fewer metabolites showing ABA-altered profiles in gpa1. Ca(2+)-mobilizing agents sphingosine-1-phosphate and cyclic adenosine diphosphate ribose exhibited weaker ABA-stimulated increases in gpa1. Hormone metabolites were responsive to ABA, with generally greater responsiveness in Col than in gpa1. Most hormones also showed different ABA responses in guard cell versus mesophyll cell metabolomes. These findings suggest that ABA functions upstream to regulate other hormones, and are also consistent with G proteins modulating multiple hormonal signaling pathways. In particular, indole-3-acetic acid levels declined after ABA treatment in Col but not gpa1 guard cells. Consistent with this observation, the auxin antagonist α-(phenyl ethyl-2-one)-indole-3-acetic acid enhanced ABA-regulated stomatal movement and restored partial ABA sensitivity to gpa1.

Dissecting the integrative antioxidant and redox systems in plant mitochondria. Effect of stress and S-nitrosylation

Mitochondrial respiration provides the energy needed to drive metabolic and transport processes in cells. Mitochondria are a significant site of reactive oxygen species (ROS) production in plant cells, and redox-system components obey fine regulation mechanisms that are essential in protecting the mitochondrial integrity. In addition to ROS, there are compelling indications that nitric oxide can be generated in this organelle by both reductive and oxidative pathways. ROS and reactive nitrogen species play a key role in signaling but they can also be deleterious via oxidation of macromolecules. The high production of ROS obligates mitochondria to be provided with a set of ROS scavenging mechanisms. The first line of mitochondrial antioxidants is composed of superoxide dismutase and the enzymes of the ascorbate-glutathione cycle, which are not only able to scavenge ROS but also to repair cell damage and possibly serve as redox sensors. The dithiol-disulfide exchanges form independent signaling nodes and act as antioxidant defense mechanisms as well as sensor proteins modulating redox signaling during development and stress adaptation. The presence of thioredoxin (Trx), peroxiredoxin (Prx) and sulfiredoxin (Srx) in the mitochondria has been recently reported. Cumulative results obtained from studies in salt stress models have demonstrated that these redox proteins play a significant role in the establishment of salt tolerance. The Trx/Prx/Srx system may be subjected to a fine regulated mechanism involving post-translational modifications, among which S-glutathionylation and S-nitrosylation seem to exhibit a critical role that is just beginning to be understood. This review summarizes our current knowledge in antioxidative systems in plant mitochondria, their interrelationships, mechanisms of compensation and some unresolved questions, with special focus on their response to abiotic stress.

Missing links in understanding redox signaling via thiol/disulfide modulation: how is glutathione oxidized in plants?

Glutathione is a small redox-active molecule existing in two main stable forms: the thiol (GSH) and the disulphide (GSSG). In plants growing in optimal conditions, the GSH:GSSG ratio is high in most cell compartments. Challenging environmental conditions are known to alter this ratio, notably by inducing the accumulation of GSSG, an effect that may be influential in the perception or transduction of stress signals. Despite the potential importance of glutathione status in redox signaling, the reactions responsible for the oxidation of GSH to GSSG have not been clearly identified. Most attention has focused on the ascorbate-glutathione pathway, but several other candidate pathways may couple the availability of oxidants such as H2O2 to changes in glutathione and thus impact on signaling pathways through regulation of protein thiol-disulfide status. We provide an overview of the main candidate pathways and discuss the available biochemical, transcriptomic, and genetic evidence relating to each. Our analysis emphasizes how much is still to be elucidated on this question, which is likely important for a full understanding of how stress-related redox regulation might impinge on phytohormone-related and other signaling pathways in plants.

Light intensity-dependent retrograde signalling in higher plants

Plants are able to acclimate to highly fluctuating light environment and evolved a short- and long-term light acclimatory responses, that are dependent on chloroplasts retrograde signalling. In this review we summarise recent evidences suggesting that the chloroplasts act as key sensors of light intensity changes in a wide range (low, high and excess light conditions) as well as sensors of darkness. They also participate in transduction and synchronisation of systemic retrograde signalling in response to differential light exposure of distinct leaves. Regulation of intra- and inter-cellular chloroplast retrograde signalling is dependent on the developmental and functional stage of the plastids. Therefore, it is discussed in following subsections: firstly, chloroplast biogenic control of nuclear genes, for example, signals related to photosystems and pigment biogenesis during early plastid development; secondly, signals in the mature chloroplast induced by changes in photosynthetic electron transport, reactive oxygen species, hormones and metabolite biosynthesis; thirdly, chloroplast signalling during leaf senescence. Moreover, with a help of meta-analysis of multiple microarray experiments, we showed that the expression of the same set of genes is regulated specifically in particular types of signals and types of light conditions. Furthermore, we also highlight the alternative scenarios of the chloroplast retrograde signals transduction and coordination linked to the role of photo-electrochemical signalling.

NADPH oxidase inhibitor diphenyleneiodonium and reduced glutathione mitigate ethephon-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression in sweet potato (Ipomoea batatas)

Ethephon, an ethylene releasing compound, promoted leaf senescence, H2O2 elevation, and senescence-associated gene expression in sweet potato. It also affected the glutathione and ascorbate levels, which in turn perturbed H2O2 homeostasis. The decrease of reduced glutathione and the accumulation of dehydroascorbate correlated with leaf senescence and H2O2 elevation at 72h in ethephon-treated leaves. Exogenous application of reduced glutathione caused quicker and significant increase of its intracellular level and resulted in the attenuation of leaf senescence and H2O2 elevation. A small H2O2 peak produced within the first 4h after ethephon application was also eliminated by reduced glutathione. Diphenyleneiodonium (DPI), an NADPH oxidase inhibitor, delayed leaf senescence and H2O2 elevation at 72h, and its influence was effective only within the first 4h after ethephon treatment. Ethephon-induced senescence-associated gene expression was repressed by DPI and reduced glutathione at 72h in pretreated leaves. Leaves treated with l-buthionine sulfoximine, an endogenous glutathione synthetase inhibitor, did enhance senescence-associated gene expression, and the activation was strongly repressed by reduced glutathione. In conclusion, ethephon-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression are all alleviated by reduced glutathione and NADPH oxidase inhibitor DPI. The speed and the amount of intracellular reduced glutathione accumulation influence its effectiveness of protection against ethephon-mediated effects. Reactive oxygen species generated from NADPH oxidase likely serves as an oxidative stress signal and participates in ethephon signaling. The possible roles of NADPH oxidase and reduced glutathione in the regulation of oxidative stress signal in ethephon are discussed.

Plastid-localized glutathione reductase2-regulated glutathione redox status is essential for Arabidopsis root apical meristem maintenance

Glutathione is involved in thiol redox signaling and acts as a major redox buffer against reactive oxygen species, helping to maintain a reducing environment in vivo. Glutathione reductase (GR) catalyzes the reduction of glutathione disulfide (GSSG) into reduced glutathione (GSH). The Arabidopsis thaliana genome encodes two GRs: GR1 and GR2. Whereas the cytosolic/peroxisomal GR1 is not crucial for plant development, we show here that the plastid-localized GR2 is essential for root growth and root apical meristem (RAM) maintenance. We identify a GR2 mutant, miao, that displays strong inhibition of root growth and severe defects in the RAM, with GR activity being reduced to ∼50%. miao accumulates high levels of GSSG and exhibits increased glutathione oxidation. The exogenous application of GSH or the thiol-reducing agent DTT can rescue the root phenotype of miao, demonstrating that the RAM defects in miao are triggered by glutathione oxidation. Our in silico analysis of public microarray data shows that auxin and glutathione redox signaling generally act independently at the transcriptional level. We propose that glutathione redox status is essential for RAM maintenance through both auxin/PLETHORA (PLT)-dependent and auxin/PLT-independent redox signaling pathways.

Analysis of cytosolic isocitrate dehydrogenase and glutathione reductase 1 in photoperiod-influenced responses to ozone using Arabidopsis knockout mutants

Oxidative stress caused by ozone (O3 ) affects plant development, but the roles of specific redox-homeostatic enzymes in O3 responses are still unclear. While growth day length may affect oxidative stress outcomes, the potential influence of day length context on equal-time exposures to O3 is not known. In Arabidopsis Col-0, day length affected the outcome of O3 exposure. In short-days (SD), few lesions were elicited by treatments that caused extensive lesions in long days (LD). Lesion formation was not associated with significant perturbation of glutathione, ascorbate, NADP(H) or NAD(H). To investigate roles of two genes potentially underpinning this redox stability, O3 responses of mutants for cytosolic NADP-isocitrate dehydrogenase (icdh) and glutathione reductase 1 (gr1) were analysed. Loss of ICDH function did not affect O3 -induced lesions, but slightly increased glutathione oxidation, induction of other cytosolic NADPH-producing enzymes and pathogenesis-related gene 1 (PR1). In gr1, O3 -triggered lesions, salicylic acid accumulation, and induction of PR1 were all decreased relative to Col-0 despite enhanced accumulation of glutathione. Thus, even at identical irradiance and equal-time exposures, day length strongly influences phenotypes triggered by oxidants of atmospheric origin, while in addition to its antioxidant function, the GR-glutathione system seems to play novel signalling roles during O3 exposure.

Transcriptomics of induced defense responses to greenbug aphid feeding in near isogenic wheat lines

The greenbug aphid, Schizaphis graminum (Rondani) is an important cereal pest, periodically threatening wheat yields in the United States and around the world. The single dominant gene, Gb3-based resistance is highly durable against prevailing greenbug biotypes under field conditions; however, the molecular mechanisms of Gb3-mediated defense responses remain unknown. We used Affymetrix GeneChip Wheat Genome Arrays to investigate the transcriptomics of host defense responses upon greenbug feeding on resistant and susceptible bulks (RB and SB, respectively) derived from two near-isogenic lines. The study identified 692 differentially expressed transcripts and further functional classification recognized 122 transcripts that are putatively associated to mediate biotic stress responses. In RB, Gb3-mediated resistance resulted in activation of transmembrane receptor kinases and signaling-related transcripts involved in early signal transduction cascades. While in SB, transcripts mediating final steps in jasmonic acid biosynthesis, redox homeostasis, peroxidases, glutathione S-transferases, and notable defense-related secondary metabolites were induced. Also transcripts involved in callose and cell wall decomposition were elevated SB, plausibly to facilitate uninterrupted feeding operations. These results suggest that Gb3-mediated resistance is less vulnerable to cell wall modification and the data provides ample tools for further investigations concerning R gene based model of resistance.

Redox control of plant growth and development

Redox changes determined by genetic and environmental factors display well-organized interactions in the control of plant growth and development. Diurnal and seasonal changes in the environmental conditions are important for the normal course of these physiological processes and, similarly to their mild irregular alterations, for stress adaptation. However, fast or large-scale environmental changes may lead to damage or death of sensitive plants. The spatial and temporal redox changes influence growth and development due to the reprogramming of metabolism. In this process reactive oxygen and nitrogen species and antioxidants are involved as components of signalling networks. The control of growth, development and flowering by reactive oxygen and nitrogen species and antioxidants in interaction with hormones at organ, tissue, cellular and subcellular level will be discussed in the present review. Unsolved problems of the field, among others the need for identification of new components and interactions in the redox regulatory network at various organization levels using systems biology approaches will be also indicated.

Responses of foliar antioxidative and photoprotective defence systems of trees to drought: a meta-analysis

Current climate change predictions hint to more frequent extreme weather events, including extended droughts, making better understanding of the impacts of water stress on trees even more important. At the individual plant level, stomatal closure as a result of water deficit leads to reduced CO2 availability in the leaf, which can lead to photo-oxidative stress. Photorespiration and the Mehler reaction can maintain electron transport rates under low internal CO2, but result in production of reactive oxygen species (ROS). If electron consumption is decreased, upstream photochemical processes can be affected and light energy is absorbed in excess of photochemical requirements. Trees evolved to cope with excess energy and elevated concentration of ROS by activating photoprotective and antioxidative defence systems. The meta-analysis we present here assessed responses of these defence systems reported in 50 studies. We found responses to vary depending on stress intensity, foliage type and habitat, and on whether experiments were done in the field or in controlled environments. In general, drought increased concentrations of antioxidants and photoprotective pigments. However, severe stress caused degradation of antioxidant concentrations and oxidation of antioxidant pools. Evergreen trees seemed to preferentially reinforce membrane-bound protection systems zeaxanthin and tocopherol, whereas deciduous species showed greater responses in water-soluble antioxidants ascorbic acid and glutathione. Trees and shrubs from arid versus humid habitats vary in their antioxidative and photoprotective defence responses. In field experiments, drought had greater effects on some defence compounds than under controlled conditions.

Growth temperature-induced changes in biomass accumulation, photosynthesis and glutathione redox homeostasis as influenced by hydrogen peroxide in cucumber

Hydrogen peroxide (H2O2) and glutathione (GSH) are involved in the stress response in plants. To elucidate the role of H2O2 in the acclimation of CO2 assimilation under sub- or supra-optimal growth temperatures, we examined the effect and interaction of H2O2 manipulation on the photosynthetic metabolism of cucumber plants (Cucumis sativus L.) grown under five temperature regimes spanning above and below the optimal growth temperature; 11/9, 18/15, 25/20, 32/27, and 39/33 °C (day/night), with or without dimethylthiourea (DMTU) or H2O2 treatment. As expected, exposure to sub- or supra-optimal growth temperatures resulted in decreased plant growth, associated with a decline in CO2 assimilation (Asat), Rubisco content, and activities of enzymes involved in the CO2 assimilation, as well as a decrease in the ratio of reduced (GSH) to oxidized (GSSG) glutathione (GSH/GSSG). Foliar application of H2O2 promoted, whilst DMTU retarded the capacity of plants to acclimate to non-optimal growth temperatures; this was consistently shown in altered activity of redox-sensitive enzymes involved in CO2 assimilation. These results strongly suggest that the influence of growth temperature on CO2 assimilation was primarily targeted at the activities of the redox-sensitive enzymes of CO2 assimilation. Meanwhile, the data suggest that the cellular H2O2 level is an important signal for the glutathione-dependent regulation of redox-sensitive enzymes of CO2 assimilation in cucumber plants.

ROS signaling loops - production, perception, regulation

Reactive oxygen species are recognized as important signaling components in a wide range of processes in plants and most other organisms. Reactive oxygen species are produced in different subcellular compartments in response to specific stimuli and the production is under tight control in order to avoid detrimental side-effects. Calcium signaling, protein phosphorylation and other signaling pathways are intimately involved in the control and coordination of reactive oxygen production. Any signal that should result in a specific response must eventually be perceived. Direct redox modification of transcription factors and other proteins are critical for the perception of intracellular reactive oxygen species; however, sensing of their extracellular counterparts awaits elucidation.

Thiol-based redox signaling in the nitrogen-fixing symbiosis

In nitrogen poor soils legumes establish a symbiotic interaction with rhizobia that results in the formation of root nodules. These are unique plant organs where bacteria differentiate into bacteroids, which express the nitrogenase enzyme complex that reduces atmospheric N 2 to ammonia. Nodule metabolism requires a tight control of the concentrations of reactive oxygen and nitrogen species (RONS) so that they can perform useful signaling roles while avoiding nitro-oxidative damage. In nodules a thiol-dependent regulatory network that senses, transmits and responds to redox changes is starting to be elucidated. A combination of enzymatic, immunological, pharmacological and molecular analyses has allowed us to conclude that glutathione and its legume-specific homolog, homoglutathione, are abundant in meristematic and infected cells, that their spatio-temporally distribution is correlated with the corresponding (homo)glutathione synthetase activities, and that they are crucial for nodule development and function. Glutathione is at high concentrations in the bacteroids and at moderate amounts in the mitochondria, cytosol and nuclei. Less information is available on other components of the network. The expression of multiple isoforms of glutathione peroxidases, peroxiredoxins, thioredoxins, glutaredoxins and NADPH-thioredoxin reductases has been detected in nodule cells using antibodies and proteomics. Peroxiredoxins and thioredoxins are essential to regulate and in some cases to detoxify RONS in nodules. Further research is necessary to clarify the regulation of the expression and activity of thiol redox-active proteins in response to abiotic, biotic and developmental cues, their interactions with downstream targets by disulfide-exchange reactions, and their participation in signaling cascades. The availability of mutants and transgenic lines will be crucial to facilitate systematic investigations into the function of the various proteins in the legume-rhizobial symbiosis.

Cytochromes b561: ascorbate-mediated trans-membrane electron transport

SIGNIFICANCE

Cytochromes b561 (CYB561s) constitute a family of trans-membrane (TM), di-heme proteins, occurring in a variety of organs and cell types, in plants and animals, and using ascorbate (ASC) as an electron donor. CYB561s function as monodehydroascorbate reductase, regenerating ASC, and as Fe³⁺-reductases, providing reduced iron for TM transport. A CYB561-core domain is also associated with dopamine β-monooxygenase redox domains (DOMON) in ubiquitous CYBDOM proteins. In plants, CYBDOMs form large protein families. Physiological functions supported by CYB561s and CYBDOMs include stress defense, cell wall modifications, iron metabolism, tumor suppression, and various neurological processes, including memory retention. CYB561s, therefore, significantly broaden our view on the physiological roles of ASC.

RECENT ADVANCES

The ubiquitous nature of CYB561s is only recently being recognized. Significant advances have been made through the study of recombinant CYB561s, revealing structural and functional properties of a unique "two-heme four-helix" protein configuration. In addition, the DOMON domains of CYBDOMs are suggested to contain another heme b.

CRITICAL ISSUES

New CYB561 proteins are still being identified, and there is a need to provide an insight and overview on the various roles of these proteins and their structural properties.

FUTURE DIRECTIONS

Mutant studies will reveal in greater detail the mechanisms by which CYB561s and CYBDOMs participate in cell metabolism in plants and animals. Moreover, the availability of efficient heterologous expression systems should allow protein crystallization, more detailed (atomic-level) structural information, and insights into the intra-molecular mechanism of electron transport.

Alleviation of cadmium toxicity in Medicago sativa by hydrogen-rich water

Hydrogen gas (H₂) induces plant tolerance to several abiotic stresses, including salinity and paraquat exposure. However, the role of H₂ in cadmium (Cd)-induced stress amelioration is largely unknown. Here, pretreatment with hydrogen-rich water (HRW) was used to characterize physiological roles and molecular mechanisms of H₂ in the alleviation of Cd toxicity in alfalfa plants. Our results showed that the addition of HRW at 10% saturation significantly decreased contents of thiobarbituric acid reactive substances (TBARS) caused by Cd, and inhibited the appearance of Cd toxicity symptoms, including the improvement of root elongation and seedling growth. These responses were related to a significant increase in the total or isozymatic activities of representative antioxidant enzymes, or their corresponding transcripts. In vivo imaging of reactive oxygen species (ROS), and the detection of lipid peroxidation and the loss of plasma membrane integrity provided further evidence for the ability of HRW to improve Cd tolerance significantly, which was consistent with a significant enhancement of the ratio of reduced/oxidized (homo)glutathione ((h)GSH). Additionally, plants pretreated with HRW accumulated less amounts of Cd. Together, this study suggested that the usage of HRW could be an effective approach for Cd detoxification and could be explored in agricultural production systems.

Modelling metabolic CO₂ evolution--a fresh perspective on respiration

Respiration is a major contributor to net exchange of CO₂ between plants and the atmosphere and thus an important aspect of the vegetation component of global climate change models. However, a mechanistic model of respiration is lacking, and so here we explore the potential for flux balance analysis (FBA) to predict cellular CO₂ evolution rates. Metabolic flux analysis reveals that respiration is not always the dominant source of CO₂, and that metabolic processes such as the oxidative pentose phosphate pathway (OPPP) and lipid synthesis can be quantitatively important. Moreover, there is considerable variation in the metabolic origin of evolved CO₂ between tissues, species and conditions. Comparison of FBA-predicted CO₂ evolution profiles with those determined from flux measurements reveals that FBA is able to predict the metabolic origin of evolved CO₂ in different tissues/species and under different conditions. However, FBA is poor at predicting flux through certain metabolic processes such as the OPPP and we identify the way in which maintenance costs are accounted for as a major area of improvement for future FBA studies. We conclude that FBA, in its standard form, can be used to predict CO₂ evolution in a range of plant tissues and in response to environment.

Potential link between biotic defense activation and recalcitrance to induction of somatic embryogenesis in shoot primordia from adult trees of white spruce (Picea glauca)

BACKGROUND

Among the many commercial opportunities afforded by somatic embryogenesis (SE), it is the ability to clonally propagate individual plants with rare or elite traits that has some of the most significant implications. This is particularly true for many long-lived species, such as conifers, but whose long generation times pose substantive challenges, including increased recalcitrance for SE as plants age. Identification of a clonal line of somatic embryo-derived trees whose shoot primordia have remained responsive to SE induction for over a decade, provided a unique opportunity to examine the molecular aspects underpinning SE within shoot tissues of adult white spruce trees.

RESULTS

Microarray analysis was used to conduct transcriptome-wide expression profiling of shoot explants taken from this responsive genotype following one week of SE induction, which when compared with that of a nonresponsive genotype, led to the identification of four of the most differentially expressed genes within each genotype. Using absolute qPCR to expand the analysis to three weeks of induction revealed that differential expression of all eight candidate genes was maintained to the end of the induction treatment, albeit to differing degrees. Most striking was that both the magnitude and duration of candidate gene expression within the nonresponsive genotype was indicative of an intense physiological response. Examining their putative identities further revealed that all four encoded for proteins with similarity to angiosperm proteins known to play prominent roles in biotic defense, and that their high-level induction over an extended period is consistent with activation of a biotic defense response. In contrast, the more temperate response within the responsive genotype, including induction of a conifer-specific dehydrin, is more consistent with elicitation of an adaptive stress response.

CONCLUSIONS

While additional evidence is required to definitively establish an association between SE responsiveness and a specific physiological response, these results suggest that biotic defense activation may be antagonistic, likely related to the massive transcriptional and metabolic reprogramming that it elicits. A major issue for future work will be to determine how and if suppressing biotic defense activation could be used to promote a physiological state more conducive to SE induction.

Modulation of cellular redox status by thiamine-activated NADPH oxidase confers Arabidopsis resistance to Sclerotinia sclerotiorum

Sclerotinia sclerotiorum can initially suppress host oxidative burst to aid infection establishment, but later promotes reactive oxygen species (ROS) generation as proliferation advances. Here, it was shown that the cellular redox status can be modulated by thiamine to protect Arabidopsis thaliana against Sclerotinia at the early stages of infection. The initial inhibition of host ROS generation by Sclerotinia-secreted oxalate could effectively be alleviated by thiamine. Thiamine pre-treatment and subsequent wild-type Sclerotinia invasion induced an increase of ascorbate peroxidase activity concomitant with decreased ascorbate/dehydroascorbate ratios, which led to the cellular transition towards oxidative status in infected tissues. Particularly, it was observed that wild-type Sclerotinia, but not oxalate-deficient A2 mutant, could suppress the activity of NADPH oxidase (NOX), which might be an important mechanism underlying the early inhibition of ROS burst. Nevertheless, thiamine pre-treatment followed by wild-type Sclerotinia infection promoted NOX-derived ROS accumulation. Further studies showed that cytosolic Ca(2+) and staurosporine-sensitive protein kinase(s) participated in thiamine-induced activation of NOX. Moreover, thiamine-induced tissue defence responses including callose/lignin deposition and stomatal closure were closely correlated with NOX-derived ROS generation. Additionally, studies with Brassica species indicated that the regulation of thiamine is largely conserved upon Sclerotinia infection. Collectively, it was concluded that thiamine reverses the initial reducing status through activating NOX-dependent ROS signalling to perturb the disease progress of Sclerotinia.

Bioluminescent detection of peroxynitrite with a boronic acid-caged luciferin

Peroxynitrite, a highly reactive biological oxidant, is formed under pathophysiologic conditions from the diffusion-limited reaction of nitric oxide and superoxide radical anion. Peroxynitrite has been implicated as the mediator of nitric oxide toxicity in many diseases and as an important signaling disrupting molecule (L. Liaudet et al., Front. Biosci.14, 4809-4814, 2009) [1]. Biosensors effective at capturing peroxynitrite in a specific and fast enough manner for detection, along with readouts compatible with in vivo studies, are lacking. Here we report that the boronic acid-based bioluminescent system PCL-1 (peroxy-caged luciferin-1), previously reported as a chemoselective sensor for hydrogen peroxide (G.C. Van de Bittner et al., Proc. Natl. Acad. Sci. USA107, 21316-21321, 2010) [2], reacts with peroxynitrite stoichiometrically with a rate constant of 9.8±0.3×10(5)M(-1)s(-1) and a bioluminescence detection limit of 16nM, compared to values of 1.2±0.3M(-1)s(-1) and 231nM for hydrogen peroxide. Further, we demonstrate bioluminescent detection of peroxynitrite in the presence of physiological competitors: carbon dioxide, glutathione, albumin, and catalase. We also demonstrate the utility of this method to assess peroxynitrite formation in mammalian cells by measuring peroxynitrite generated under normal culture conditions after stimulation of macrophages with bacterial endotoxin lipopolysaccharide. Thus, the PCL-1 method for measuring peroxynitrite generation shows superior selectivity over other oxidants under in vivo conditions.

Organic acids on the growth, anatomical structure, biochemical parameters and heavy metal accumulation of Iris lactea var. chinensis seedling growing in Pb mine tailings

The effect of citric acid (CA) and ethylene diamine tetraacetic acid (EDTA) on the growth, anatomical structure, physiological responses and lead (Pb) accumulation of Iris lactea var. chinensis seedling growing in Pb mine tailings for 30 days were studied. Results showed that the dry weights (DW) of roots decreased significantly under both levels of CA. The DWs of leaves and roots treated with 2 mmol/kg EDTA decreased significantly and were 23 and 54 %, respectively, lower than those of the control. The tolerant indexes of I. lactea var. chinensis under all treatments of organic acids were lower than control. The root tip anatomical structure was little affected under the treatments of 2 mmol/kg CA and 2 mmol/kg EDTA compared with control. However, the formation of photosynthesizing cells was inhibited by the treatment of 2 mmol/kg EDTA. The concentrations of chlorophyll a, chlorophyll b and total carotenoids in the leaves treated with 2 mmol/kg EDTA significantly decreased. Higher CA level and lower EDTA level could trigger the synthesis of ascorbic acid and higher level of EDTA could trigger the synthesis of glutathione. CA and EDTA could promote Pb accumulation of I. lactea var. chinensis and Pb concentration in the leaves and roots at 2 mmol/kg EDTA treatment increased significantly and reached to 160.44 and 936.08 μg/g DW, respectively, and 1.8 and 1.6 times higher than those of the control. The results indicated that I. lactea var. chinensis could be used to remediate Pb tailing and the role of EDTA in promoting Pb accumulation was better than CA did.

Glutathione and ascorbic acid protect Arabidopsis plants against detrimental effects of iron deficiency

Iron is an essential micronutrient required for a wide variety of cellular functions in plant growth and development. Chlorosis is the first visible symptom in iron-deficient plants. Glutathione (GSH) and ascorbic acid (ASC) are multifunctional metabolites playing important roles in redox balancing. In this work, it was shown that GSH and ASC treatment prevented chlorosis and the accumulation of reactive oxygen species induced by iron deficiency in Arabidopsis leaves. In iron deficiency, GSH and ASC increased the activity of the heme protein ascorbate peroxidase at a similar level to that found in iron-sufficient seedlings. GSH was also able to preserve the levels of the iron-sulfur protein ferredoxin 2. GSH content decreased 25% in iron-deficient Arabidopsis seedlings, whereas the ASC levels were not affected. Taken together, these results showed that GSH and ASC supplementation protects Arabidopsis seedlings from iron deficiency, preserving cell redox homeostasis and improving internal iron availability.

Photo-oxidative stress in emerging and senescing leaves: a mirror image?

The life cycle of a leaf can be characterized as consisting of different stages: from primordial leaf initiation in the shoot apical meristem (SAM) to leaf senescence. Leaf development, from early leaf growth to senescence, is tightly controlled by plant development and the environment. Here, we primarily focus on summarizing current evidence indicating that photo-oxidative stress occurs at the two extremes of a leaf's lifespan. Some recent studies clearly indicate that--as happens in senescing leaves--emerging new leaves suffer from photo-oxidative stress, which suggests that oxidative stress plays a key role at both ends of the leaf life cycle. We discuss the causes and consequences of suffering from photo-oxidative stress during leaf development, paying attention to the particularities of this process at the two extremes of leaf development. Of particular importance is the current evidence showing mechanisms that maintain an adequate cellular reactive oxygen species/antioxidant (redox) balance that allows growth and prevents oxidative damage in young emerging leaves, while later on photo-oxidative stress induces cell death in senescing leaves. Also of interest is the fact that reductions in the efficiency of photosystem II photochemistry may not necessarily indicate photo-oxidative stress in emerging leaves. In this review, we summarize current knowledge of photoinhibition, photoprotection, and photo-oxidative stress at the two ends of the leaf life cycle: early leaf growth and leaf senescence.

Salicylic acid alleviates copper toxicity in rice (Oryza sativa L.) seedlings by up-regulating antioxidative and glyoxalase systems

The present study investigated the effect of salicylic acid (SA) on toxic symptoms, lipid peroxidation, reactive oxygen species generation and responses of antioxidative and glyoxalase systems in rice seedlings grown hydroponically under copper (Cu) stress for 48 h. Exposures of 75 and 150 μM Cu(2+) caused toxicity symptoms (chlorosis, necrosis and rolling in leaves), sharp increases in malondialdehyde (MDA), hydrogen peroxide (H2O2) contents and lipoxygenase (LOX) activity with concomitant reductions of chlorophyll (Chl) and relative water content (RWC). Both levels of Cu decreased ascorbic acid (AsA), glutathione (GSH), non-protein thiol (NPT) and proline contents in roots but rather increased in leaves except that AsA decreased in leaves too. These results together with overaccumulation of superoxide (O 2 (•-) ) and H2O2 in leaves revealed that Cu exposures induced oxidative stress. Contrary, SA-pretreatment (100 μM for 24 h) reduced toxicity symptoms and diminished Cu-induced increases in LOX activity, H2O2, MDA and proline contents while the levels of RWC, Chl, AsA and redox ratios were elevated. Higher levels of GSH and NPT were also observed in roots of SA-pretreated Cu-exposed seedlings. SA-pretreatment also exerted its beneficial role by inhibiting the Cu upward process. Studies on antioxidant enzymes showed that SA further enhanced the activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase and glutathione peroxidase, and also elevated the depressed activities of catalase, dehydroascorbate reductase and glutathione S-transferase particularly at 150 μM Cu(2+) stress. In addition, the activity of glyoxalase system (glyoxalase I and II) was further elevated by SA pretreatment in the Cu-exposed seedlings. These results concluded that SA-mediated retention of Cu in roots and enhanced capacity of both antioxidative and glyoxalase systems might be associated with the alleviation of Cu-toxicity in rice seedlings.

Modified method for determination of sulfur metabolites in plant tissues by stable isotope dilution-based liquid chromatography-electrospray ionization-tandem mass spectrometry

A wide variety of sulfur metabolites play important roles in plant functions. We have developed a precise and sensitive method for the simultaneous measurement of several sulfur metabolites based on liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) and (34)S metabolic labeling of sulfur-containing metabolites in Arabidopsis thaliana seedlings. However, some sulfur metabolites were unstable during the extraction procedure. Our proposed method does not allow for the detection of the important sulfur metabolite homocysteine because of its instability during sample extraction. Stable isotope-labeled sulfur metabolites of A. thaliana shoot were extracted and utilized as internal standards for quantification of sulfur metabolites with LC-MS/MS using S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), methionine (Met), glutathione (GSH), and glutathione disulfide (GSSG) as example metabolites. These metabolites were detected using electrospray ionization in positive mode. Standard curves were linear (r(2)>0.99) over a range of concentrations (SAM 0.01-2.0μM, SAH 0.002-0.10μM, Met 0.05-4.0μM, GSH 0.17-20.0μM, GSSG 0.07-20.0μM), with limits of detection for SAM, SAH, Met, GSH, and GSSG of 0.83, 0.67, 10, 0.56, and 1.1nM, respectively; and the within-run and between-run coefficients of variation based on quality control samples were less than 8%.

Coping with abiotic stress: proteome changes for crop improvement

Plant breeders need new and more precise tools to accelerate breeding programs that address the increasing needs for food, feed, energy and raw materials, while facing a changing environment in which high salinity and drought have major impacts on crop losses worldwide. This review covers the achievements and bottlenecks in the identification and validation of proteins with relevance in abiotic stress tolerance, also mentioning the unexpected consequences of the stress in allergen expression. While addressing the key pathways regulating abiotic stress plant adaptation, comprehensive data is presented on the proteins confirmed as relevant to confer tolerance. Promising candidates still to be confirmed are also highlighted, as well as the specific protein families and protein modifications for which detection and characterization is still a challenge. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Dynamic compartment specific changes in glutathione and ascorbate levels in Arabidopsis plants exposed to different light intensities

BACKGROUND

Excess light conditions induce the generation of reactive oxygen species (ROS) directly in the chloroplasts but also cause an accumulation and production of ROS in peroxisomes, cytosol and vacuoles. Antioxidants such as ascorbate and glutathione occur in all cell compartments where they detoxify ROS. In this study compartment specific changes in antioxidant levels and related enzymes were monitored among Arabidopsis wildtype plants and ascorbate and glutathione deficient mutants (vtc2-1 and pad2-1, respectively) exposed to different light intensities (50, 150 which was considered as control condition, 300, 700 and 1,500 μmol m(-2) s(-1)) for 4 h and 14 d.

RESULTS

The results revealed that wildtype plants reacted to short term exposure to excess light conditions with the accumulation of ascorbate and glutathione in chloroplasts, peroxisomes and the cytosol and an increased activity of catalase in the leaves. Long term exposure led to an accumulation of ascorbate and glutathione mainly in chloroplasts. In wildtype plants an accumulation of ascorbate and hydrogen peroxide (H2O2) could be observed in vacuoles when exposed to high light conditions. The pad2-1 mutant reacted to long term excess light exposure with an accumulation of ascorbate in peroxisomes whereas the vtc2-1 mutant reacted with an accumulation of glutathione in the chloroplasts (relative to the wildtype) and nuclei during long term high light conditions indicating an important role of these antioxidants in these cell compartments for the protection of the mutants against high light stress.

CONCLUSION

The results obtained in this study demonstrate that the accumulation of ascorbate and glutathione in chloroplasts, peroxisomes and the cytosol is an important reaction of plants to short term high light stress. The accumulation of ascorbate and H2O2 along the tonoplast and in vacuoles during these conditions indicates an important route for H2O2 detoxification under these conditions.

Nitric oxide affects salt-induced changes in free amino acid levels in maize

It was assumed that salt-induced redox changes affect amino acid metabolism in maize (Zea mays L.), and this influence may be modified by NO. The applied NaCl treatment reduced the fresh weight of shoots and roots. This decrease was smaller after the combined application of NaCl and an NO-donor ((Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate, DETA/NO) in the shoots, while it was greater after simultaneous treatment with NaCl and nitro-L-arginine (L-NNA, inhibitor of NO synthesis) in the roots. The quantum yield efficiency of photosystem II was not influenced by the treatments. NaCl had a significant effect on the redox environment in the leaves as it was shown by the increase in the amount of glutathione disulphide and in the redox potential of the glutathione/glutathione disulphide redox pair. This influence of NaCl was modified by DETA/NO and L-NNA. Pharmacological modification of NO levels affected salt-induced changes in both the total free amino acid content and in the free amino acid composition. NaCl alone increased the concentration of almost all amino acids which effect was strengthened by DETA/NO in the case of Pro. L-NNA treatment resulted in a significant increase in the Ala, Val, Gly and Tyr contents. The Ile, Lys and Val concentrations rose considerably after the combined application of NaCl and DETA/NO compared to NaCl treatment alone in the recovery phase. NaCl also increased the expression of several genes related to the amino acid and antioxidant metabolism, and this effect was modified by DETA/NO. In conclusion, modification of NO levels affected salt-induced, glutathione-dependent redox changes and simultaneously the free amino acid composition and the level of several free amino acids. The observed much higher Pro content in plants treated with both NaCl and DETA/NO during recovery may contribute to the protective effect of NO against salt stress.

Low environmentally relevant levels of bioactive xenobiotics and associated degradation products cause cryptic perturbations of metabolism and molecular stress responses in Arabidopsis thaliana

Anthropic changes and chemical pollution confront wild plant communities with xenobiotic combinations of bioactive molecules, degradation products, and adjuvants that constitute chemical challenges potentially affecting plant growth and fitness. Such complex challenges involving residual contamination and mixtures of pollutants are difficult to assess. The model plant Arabidopsis thaliana was confronted by combinations consisting of the herbicide glyphosate, the fungicide tebuconazole, the glyphosate degradation product aminomethylphosphonic acid (AMPA), and the atrazine degradation product hydroxyatrazine, which had been detected and quantified in soils of field margins in an agriculturally intensive region. Integrative analysis of physiological, metabolic, and gene expression responses was carried out in dose-response experiments and in comparative experiments of varying pesticide combinations. Field margin contamination levels had significant effects on plant growth and metabolism despite low levels of individual components and the presence of pesticide degradation products. Biochemical and molecular analysis demonstrated that these less toxic degradation products, AMPA and hydroxyatrazine, by themselves elicited significant plant responses, thus indicating underlying mechanisms of perception and transduction into metabolic and gene expression changes. These mechanisms may explain observed interactions, whether positive or negative, between the effects of pesticide products (AMPA and hydroxyatrazine) and the effects of bioactive xenobiotics (glyphosate and tebuconazole). Finally, the metabolic and molecular perturbations induced by low levels of xenobiotics and associated degradation products were shown to affect processes (carbon balance, hormone balance, antioxidant defence, and detoxification) that are likely to determine environmental stress sensitivity.

The emerging role of photorespiration and non-photorespiratory peroxisomal metabolism in pathogen defence

Photorespiration represents one of the major highways of primary plant metabolism and is the most prominent example of metabolic cell organelle integration, since the pathway requires the concerted action of plastidial, peroxisomal, mitochondrial and cytosolic enzymes and organellar transport proteins. Oxygenation of ribulose-1,5-bisphosphate by Rubisco leads to the formation of large amounts of 2-phosphoglycolate, which are recycled to 3-phosphoglycerate by the photorespiratory C2 cycle, concomitant with stoichiometric production rates of H2 O2 in peroxisomes. Apart from its significance for agricultural productivity, a secondary function of photorespiration in pathogen defence has emerged only recently. Here, we summarise literature data supporting the crosstalk between photorespiration and pathogen defence and perform a meta-expression analysis of photorespiratory genes during pathogen attack. Moreover, we screened Arabidopsis proteins newly predicted using machine learning methods to be targeted to peroxisomes, the central H2 O2 -producing organelle of photorespiration, for homologues of known pathogen defence proteins and analysed their expression during pathogen infection. The analyses further support the idea that photorespiration and non-photorespiratory peroxisomal metabolism play multi-faceted roles in pathogen defence beyond metabolism of reactive oxygen species.

Glutamate receptor-like channel3.3 is involved in mediating glutathione-triggered cytosolic calcium transients, transcriptional changes, and innate immunity responses in Arabidopsis

The tripeptide reduced glutathione (GSH; γ-glutamate [Glu]-cysteine [Cys]-glycine) is a major endogenous antioxidant in both animal and plant cells. It also functions as a neurotransmitter mediating communication among neurons in the central nervous system of animals through modulating specific ionotropic Glu receptors (GLRs) in the membrane. Little is known about such signaling roles in plant cells. Here, we report that transient rises in cytosolic calcium triggered by exogenous GSH in Arabidopsis (Arabidopsis thaliana) leaves were sensitive to GLR antagonists and abolished in loss-of-function atglr3.3 mutants. Like the GSH biosynthesis-defective mutant PHYTOALEXIN DEFICIENT2, atglr3.3 showed enhanced susceptibility to the bacterial pathogen Pseudomonas syringae pv tomato DC3000. Pathogen-induced defense marker gene expression was also decreased in atglr3.3 mutants. Twenty-seven percent of genes that were rapidly responsive to GSH treatment of seedlings were defense genes, most of which were dependent on functional AtGLR3.3, while GSH suppressed pathogen propagation through the AtGLR3.3-dependent pathway. Eight previously identified putative AtGLR3.3 ligands, GSH, oxidized glutathione, alanine, asparagine, Cys, Glu, glycine, and serine, all elicited the AtGLR3.3-dependent cytosolic calcium transients, but only GSH and Cys induced the defense response, with the Glu-induced AtGLR3.3-dependent transcription response being much less apparent than that triggered by GSH. Together, these observations suggest that AtGLR3.3 is required for several signaling effects mediated by extracellular GSH, even though these effects may not be causally related.

New insights into photorespiration obtained from metabolomics

Photorespiration, one of the cornerstone pathways of primary metabolism, allows plant growth in a high oxygen-containing environment. While the oxygenase reaction of Rubisco directly influences photosynthesis per se, several other processes are also affected by photorespiration, including nitrogen assimilation, respiration, amino acid metabolism, 1-C metabolism and redox metabolism, cumulating to impose a severe impact across multiple signalling pathways. Accordingly, although the plant photorespiratory cycle is complex and highly compartmentalised, little is currently known about the participating transport proteins, and relatively few of them have been properly identified. Despite its centrality, uniqueness, and mystery, the biochemistry of photorespiration has historically been quite poorly understood, in part because at least some of its enzymes and intermediates tend to be labile and of low abundance. Fortunately, the integration of molecular and genetic approaches with biochemical ones, such as metabolite profiling, is now driving rapid advances in knowledge of the key metabolic roles and connections of the enzymes and genes of the photorespiratory pathway. While these experiments have revealed a surprising complexity in the response and established connections between photorespiration and other metabolic pathways, the mechanisms behind the observed responses have still to be fully elucidated. Here we review recent progress into photorespiration and its interaction with other metabolic processes, paying particular attention to data emanating from metabolic profiling studies.

Assessing the regulation of leaf redox status under water stress conditions in Arabidopsis thaliana: Col-0 ecotype (wild-type and vtc-2), expressing mitochondrial and cytosolic roGFP1

Using Arabidopsis plants Col-0 and vtc2 transformed with a redox sensitive green fluorescent protein, (c-roGFP) and (m-roGFP), we investigated the effects of a progressive water stress and re-watering on the redox status of the cytosol and the mitochondria. Our results establish that water stress affects redox status differently in these two compartments, depending on phenotype and leaf age, furthermore we conclude that ascorbate plays a pivotal role in mediating redox status homeostasis and that Col-0 Arabidopsis subjected to water stress increase the synthesis of ascorbate suggesting that ascorbate may play a role in buffering changes in redox status in the mitochondria and the cytosol, with the presumed buffering capacity of ascorbate being more noticeable in young compared with mature leaves. Re-watering of water-stressed plants was paralleled by a return of both the redox status and ascorbate to the levels of well-watered plants. In contrast to the effects of water stress on ascorbate levels, there were no significant changes in the levels of glutathione, thereby suggesting that the regeneration and increase in ascorbate in water-stressed plants may occur by other processes in addition to the regeneration of ascorbate via the glutathione. Under water stress in vtc2 lines it was observed stronger differences in redox status in relation to leaf age, than due to water stress conditions compared with Col-0 plants. In the vtc2 an increase in DHA was observed in water-stressed plants. Furthermore, this work confirms the accuracy and sensitivity of the roGFP1 biosensor as a reporter for variations in water stress-associated changes in redox potentials.

Seasonal characterization of antioxidant responses in plants of Ipomoea nil cv. Scarlet O'Hara

Reactive oxygen species can be produced in leaf cells during normal aerobic metabolism or in a variety of exogenous factors, which may cause oxidative damage to plants, unless they have an efficient antioxidant defense system, consisting of enzymatic and non-enzymatic substances. This work raised the hypothesis that plants of Ipomoea nil cv. Scarlet O'Hara, a native species and ornamental vine of the tropics, might tolerate oxidative stress factors imposed by natural fluctuations in weather conditions through changes in the antioxidant profile.The objective of this study was to determine the variations in three leaf antioxidants in plants growing inside a greenhouse without air pollutants and exposed to varying meteorological conditions throughout the four seasons of the year and to observe if such variations are related to the oscillations in meteorological factors. Four experimental campaigns were carried out, one in each season of 2006. Each campaign lasted 28 days and started with 45 plants. Ascorbic acid (AA) concentrations and superoxide dismutase (SOD) and peroxidase (POD) activities were determined in leaves of five plants in nine sampling days of each campaign. The antioxidant responses oscillated throughout the year. The highest values were found during the spring. This seasonal antioxidant profile was associated to variations in temperature, relative humidity and global radiation. Plants of this cultivar may then tolerate oxidative stress naturally imposed by meteorological conditions.

High resolution imaging of temporal and spatial changes of subcellular ascorbate, glutathione and H₂O₂ distribution during Botrytis cinerea infection in Arabidopsis

In order to study the mechanisms behind the infection process of the necrotrophic fungus Botrytis cinerea, the subcellular distribution of hydrogen peroxide (H₂O₂) was monitored over a time frame of 96 h post inoculation (hpi) in Arabidopsis thaliana Col-0 leaves at the inoculation site (IS) and the area around the IS which was defined as area adjacent to the inoculation site (AIS). H₂O₂ accumulation was correlated with changes in the compartment-specific distribution of ascorbate and glutathione and chloroplast fine structure. This study revealed that the severe breakdown of the antioxidative system, indicated by a drop in ascorbate and glutathione contents at the IS at later stages of infection correlated with an accumulation of H₂O₂ in chloroplasts, mitochondria, cell walls, nuclei and the cytosol which resulted in the development of chlorosis and cell death, eventually visible as tissue necrosis. A steady increase of glutathione contents in most cell compartments within infected tissues (up to 600% in chloroplasts at 96 hpi) correlated with an accumulation of H₂O₂ in chloroplasts, mitochondria and cell walls at the AIS indicating that high glutathione levels could not prevent the accumulation of reactive oxygen species (ROS) which resulted in chlorosis. Summing up, this study reveals the intracellular sequence of events during Botrytis cinerea infection and shows that the breakdown of the antioxidative system correlated with the accumulation of H₂O₂ in the host cells. This resulted in the degeneration of the leaf indicated by severe changes in the number and ultrastructure of chloroplasts (e.g. decrease of chloroplast number, decrease of starch and thylakoid contents, increase of plastoglobuli size), chlorosis and necrosis of the leaves.

Down-regulation of catalase activity allows transient accumulation of a hydrogen peroxide signal in Chlamydomonas reinhardtii

In photosynthetic organisms, excess light is a stress that induces production of reactive oxygen species inside the chloroplasts. As a response, the capacity of antioxidative defence mechanisms increases. However, when cells of Chlamydomonas reinhardtii were shifted from dark to high light, a reversible partial inactivation of catalase activity was observed, which correlated with a transient increase in the level of H2 O2 in the 10 μm range. This concentration range seems to be necessary to activate H2 O2 -dependent signalling pathways stimulating the expression of H2 O2 responsive genes, such as the heat shock protein HSP22C. Catalase knock-down mutants had lost the transient accumulation of H2 O2 , suggesting that a decrease in catalase activity was the key element for establishing a transient H2 O2 burst. Catalase was inactivated by a one-electron event consistent with the reduction of a single cysteine. We propose that under high light intensity, the redox state of the photosynthetic electron transport chain is sensed and transmitted to the cytosol to regulate the catalase activity. This allows a transient accumulation of H2 O2 , inducing a signalling event that is transmitted to the nucleus to modulate the expression of chloroplast-directed protection enzymes.

Apoplastic and chloroplastic redox signaling networks in plant stress responses

SIGNIFICANCE

Interplay among apoplastic and chloroplastic redox signaling networks is emerging as a key mechanism in plant stress responses.

RECENT ADVANCES

Recent research has revealed components involved in apoplastic and chloroplastic redox signaling. Also, the sequence of events from stress perception, activation of apoplastic reactive oxygen species (ROS) burst through NADPH oxidases, cytoplasmic and chloroplastic Ca(2+)-transients, and organellar redox signals to physiological responses is starting to emerge. Moreover, a functional overlap between light acclimation and plant immunity in photosynthetically active tissues has been demonstrated.

CRITICAL ISSUES

Any deviations from the basal cellular redox balance may induce acclimation responses that continuously readjust cellular functions. However, diversion of resources to stress responses may lead to attenuation of growth, and exaggeration of defensive reactions may thus be detrimental to the plant. The ultimate outcome of acclimation responses must therefore be tightly controlled by the redox signaling networks between organellar and apoplastic signaling systems.

FUTURE DIRECTIONS

Two major questions still remain to be solved: the sensory mechanism for ROS and the components involved in relaying the signals from the apoplast to the chloroplast. A comprehensive view of regulatory networks will facilitate the understanding on how environmental factors affect the production of phytonutrients and biomass in plants. Translation of such information from model plants to crop species will be at the cutting edge of research in the near future. These challenges give a frame for future studies on ROS and redox regulation of stress acclimation in photosynthetic organisms.

Plasma membrane electron pathways and oxidative stress

SIGNIFICANCE

Several redox compounds, including respiratory burst oxidase homologs (Rboh) and iron chelate reductases have been identified in animal and plant plasma membrane (PM). Studies using molecular biological, biochemical, and proteomic approaches suggest that PM redox systems of plants are involved in signal transduction, nutrient uptake, transport, and cell wall-related processes. Function of PM-bound redox systems in oxidative stress will be discussed.

RECENT ADVANCES

Present knowledge about the properties, structures, and functions of these systems are summarized. Judging from the currently available data, it is likely that electrons are transferred from cytosolic NAD(P)H to the apoplast via quinone reductases, vitamin K, and a cytochrome b561. In tandem with these electrons, protons might be transported to the apoplastic space.

CRITICAL ISSUES

Recent studies suggest localization of PM-bound redox systems in microdomains (so-called lipid or membrane rafts), but also organization of these compounds in putative and high molecular mass protein complexes. Although the plant flavocytochrome b family is well characterized with respect to its function, the molecular mechanism of an electron transfer reaction by these compounds has to be verified. Localization of Rboh in other compartments needs elucidation.

FUTURE DIRECTIONS

Plant members of the flavodoxin and flavodoxin-like protein family and the cytochrome b561 protein family have been characterized on the biochemical level, postulated localization, and functions of these redox compounds need verification. Compositions of single microdomains and interaction partners of PM redox systems have to be elucidated. Finally, the hypothesis of an electron transfer chain in the PM needs further proof.

Apoplastic and chloroplastic redox signaling networks in plant stress responses

SIGNIFICANCE

Interplay among apoplastic and chloroplastic redox signaling networks is emerging as a key mechanism in plant stress responses.

RECENT ADVANCES

Recent research has revealed components involved in apoplastic and chloroplastic redox signaling. Also, the sequence of events from stress perception, activation of apoplastic reactive oxygen species (ROS) burst through NADPH oxidases, cytoplasmic and chloroplastic Ca(2+)-transients, and organellar redox signals to physiological responses is starting to emerge. Moreover, a functional overlap between light acclimation and plant immunity in photosynthetically active tissues has been demonstrated.

CRITICAL ISSUES

Any deviations from the basal cellular redox balance may induce acclimation responses that continuously readjust cellular functions. However, diversion of resources to stress responses may lead to attenuation of growth, and exaggeration of defensive reactions may thus be detrimental to the plant. The ultimate outcome of acclimation responses must therefore be tightly controlled by the redox signaling networks between organellar and apoplastic signaling systems.

FUTURE DIRECTIONS

Two major questions still remain to be solved: the sensory mechanism for ROS and the components involved in relaying the signals from the apoplast to the chloroplast. A comprehensive view of regulatory networks will facilitate the understanding on how environmental factors affect the production of phytonutrients and biomass in plants. Translation of such information from model plants to crop species will be at the cutting edge of research in the near future. These challenges give a frame for future studies on ROS and redox regulation of stress acclimation in photosynthetic organisms.

Regulation of basal and oxidative stress-triggered jasmonic acid-related gene expression by glutathione

Glutathione is a determinant of cellular redox state with roles in defence and detoxification. Emerging concepts suggest that this compound also has functions in cellular signalling. Here, we report evidence that glutathione plays potentially important roles in setting signalling strength through the jasmonic acid (JA) pathway. Firstly, we show that basal expression of JA-related genes is correlated with leaf glutathione content when the latter is manipulated either genetically or pharmacologically. Secondly, analyses of an oxidative stress signalling mutant, cat2, reveal that up-regulation of the JA pathway triggered by intracellular oxidation requires accompanying glutathione accumulation. Genetically blocking this accumulation in a cat2 cad2 line largely annuls H2 O2 -induced expression of JA-linked genes, and this effect can be rescued by exogenously supplying glutathione. While most attention on glutathione functions in biotic stress responses has been focused on the thiol-regulated protein NPR1, a comparison of JA-linked gene expression in cat2 cad2 and cat2 npr1 double mutants provides evidence that glutathione acts through other components to regulate the response of this pathway to oxidative stress. Our study provides new information implicating glutathione as a factor determining basal JA gene expression and suggests novel glutathione-dependent control points that regulate JA signalling in response to intracellular oxidation.

The physiological roles and metabolism of ascorbate in chloroplasts

Ascorbate is a multifunctional metabolite in plants. It is essential for growth control, involving cell division and cell wall synthesis and also involved in redox signaling, in the modulation of gene expression and regulation of enzymatic activities. Ascorbate also fulfills crucial roles in scavenging reactive oxygen species, both enzymatically and nonenzymatically, a well-established phenomenon in the chloroplasts stroma. We give an overview on these important physiological functions and would like to give emphasis to less well-known roles of ascorbate, in the thylakoid lumen, where it also plays multiple roles. It is essential for photoprotection as a cofactor for violaxanthin de-epoxidase, a key enzyme in the formation of nonphotochemical quenching. Lumenal ascorbate has recently also been shown to act as an alternative electron donor of photosystem II once the oxygen-evolving complex is inactivated and to protect the photosynthetic machinery by slowing down donor-side induced photoinactivation; it is yet to be established if ascorbate has a similar role in the case of other stress effects, such as high light and UV-B stress. In bundle sheath cells, deficient in oxygen evolution, ascorbate provides electrons to photosystem II, thereby poising cyclic electron transport around photosystem I. It has also been shown that, by supporting linear electron transport through photosystem II in sulfur-deprived Chlamydomonas reinhardtii cells, in which oxygen evolution is largely inhibited, externally added ascorbate enhances hydrogen production. For fulfilling its multiple roles, Asc has to be transported into the thylakoid lumen and efficiently regenerated; however, very little is known yet about these processes.

Mitochondrial energy and redox signaling in plants

SIGNIFICANCE

For a plant to grow and develop, energy and appropriate building blocks are a fundamental requirement. Mitochondrial respiration is a vital source for both. The delicate redox processes that make up respiration are affected by the plant's changing environment. Therefore, mitochondrial regulation is critically important to maintain cellular homeostasis. This involves sensing signals from changes in mitochondrial physiology, transducing this information, and mounting tailored responses, by either adjusting mitochondrial and cellular functions directly or reprogramming gene expression.

RECENT ADVANCES

Retrograde (RTG) signaling, by which mitochondrial signals control nuclear gene expression, has been a field of very active research in recent years. Nevertheless, no mitochondrial RTG-signaling pathway is yet understood in plants. This review summarizes recent advances toward elucidating redox processes and other bioenergetic factors as a part of RTG signaling of plant mitochondria.

CRITICAL ISSUES

Novel insights into mitochondrial physiology and redox-regulation provide a framework of upstream signaling. On the other end, downstream responses to modified mitochondrial function have become available, including transcriptomic data and mitochondrial phenotypes, revealing processes in the plant that are under mitochondrial control.

FUTURE DIRECTIONS

Drawing parallels to chloroplast signaling and mitochondrial signaling in animal systems allows to bridge gaps in the current understanding and to deduce promising directions for future research. It is proposed that targeted usage of new technical approaches, such as quantitative in vivo imaging, will provide novel leverage to the dissection of plant mitochondrial signaling.

Structural insights into the N-terminal GIY-YIG endonuclease activity of Arabidopsis glutaredoxin AtGRXS16 in chloroplasts

Glutaredoxins (Grxs) have been identified across taxa as important mediators in various physiological functions. A chloroplastic monothiol glutaredoxin, AtGRXS16 from Arabidopsis thaliana, comprises two distinct functional domains, an N-terminal domain (NTD) with GlyIleTyr-TyrIleGly (GIY-YIG) endonuclease motif and a C-terminal Grx module, to coordinate redox regulation and DNA cleavage in chloroplasts. Structural determination of AtGRXS16-NTD showed that it possesses a GIY-YIG endonuclease fold, but the critical residues for the nuclease activity are different from typical GIY-YIG endonucleases. AtGRXS16-NTD was able to cleave λDNA and chloroplast genomic DNA, and the nuclease activity was significantly reduced in AtGRXS16. Functional analysis indicated that AtGRXS16-NTD could inhibit the ability of AtGRXS16 to suppress the sensitivity of yeast grx5 cells to oxidative stress; however, the C-terminal Grx domain itself and AtGRXS16 with a Cys123Ser mutation were active in these cells and able to functionally complement a Grx5 deficiency in yeast. Furthermore, the two functional domains were shown to be negatively regulated through the formation of an intramolecular disulfide bond. These findings unravel a manner of regulation for Grxs and provide insights into the mechanistic link between redox regulation and DNA metabolism in chloroplasts.

Antioxidant systems from Pepper (Capsicum annuum L.): involvement in the response to temperature changes in ripe fruits

Sweet pepper is susceptible to changes in the environmental conditions, especially temperatures below 15 °C. In this work, two sets of pepper fruits (Capsicum annuum L.) which underwent distinct temperature profiles in planta were investigated. Accordingly, two harvesting times corresponding to each set were established: Harvest 1, whose fruits developed and ripened at 14.9 °C as average temperature; and Harvest 2, with average temperature of 12.4 °C. The oxidative metabolism was analyzed in all fruits. Although total ascorbate content did not vary between Harvests, a shift from the reduced to the oxidized form (dehydroascorbate), accompanied by a higher ascorbate peroxidase activity, was observed in Harvest 2 with respect to Harvest 1. Moreover, a decrease of the ascorbate-generating enzymatic system, the γ-galactono-lactone dehydrogenase, was found at Harvest 2. The activity values of the NADP-dependent dehydrogenases analyzed seem to indicate that a lower NADPH synthesis may occur in fruits which underwent lower temperature conditions. In spite of the important changes observed in the oxidative metabolism in fruits subjected to lower temperature, no oxidative stress appears to occur, as indicated by the lipid peroxidation and protein oxidation profiles. Thus, the antioxidative systems of pepper fruits seem to be involved in the response against temperature changes.

Engineered resistance and hypersusceptibility through functional metabolic studies of 100 genes in soybean to its major pathogen, the soybean cyst nematode

During pathogen attack, the host plant induces genes to ward off the pathogen while the pathogen often produces effector proteins to increase susceptibility of the host. Gene expression studies of syncytia formed in soybean root by soybean cyst nematode (Heterodera glycines) identified many genes altered in expression in resistant and susceptible roots. However, it is difficult to assess the role and impact of these genes on resistance using gene expression patterns alone. We selected 100 soybean genes from published microarray studies and individually overexpressed them in soybean roots to determine their impact on cyst nematode development. Nine genes reduced the number of mature females by more than 50 % when overexpressed, including genes encoding ascorbate peroxidase, β-1,4-endoglucanase, short chain dehydrogenase, lipase, DREPP membrane protein, calmodulin, and three proteins of unknown function. One gene encoding a serine hydroxymethyltransferase decreased the number of mature cyst nematode females by 45 % and is located at the Rhg4 locus. Four genes increased the number of mature cyst nematode females by more than 200 %, while thirteen others increased the number of mature cyst nematode females by more than 150 %. Our data support a role for auxin and ethylene in susceptibility of soybean to cyst nematodes. These studies highlight the contrasting gene sets induced by host and nematode during infection and provide new insights into the interactions between host and pathogen at the molecular level. Overexpression of some of these genes result in a greater decrease in the number of cysts formed than recognized soybean cyst nematode resistance loci.

Nuclear accumulation of cytosolic glyceraldehyde-3-phosphate dehydrogenase in cadmium-stressed Arabidopsis roots

NAD-dependent glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a ubiquitous enzyme involved in the glycolytic pathway. It has been widely demonstrated that mammalian GAPDH, in addition to its role in glycolysis, fulfills alternative functions mainly linked to its susceptibility to oxidative posttranslational modifications. Here, we investigated the responses of Arabidopsis (Arabidopsis thaliana) cytosolic GAPDH isoenzymes GAPC1 and GAPC2 to cadmium-induced stress in seedlings roots. GAPC1 was more responsive to cadmium than GAPC2 at the transcriptional level. In vivo, cadmium treatments induced different concomitant effects, including (1) nitric oxide accumulation, (2) cytosolic oxidation (e.g. oxidation of the redox-sensitive Green fluorescent protein2 probe), (3) activation of the GAPC1 promoter, (4) GAPC1 protein accumulation in enzymatically inactive form, and (5) strong relocalization of GAPC1 to the nucleus. All these effects were detected in the same zone of the root tip. In vitro, GAPC1 was inactivated by either nitric oxide donors or hydrogen peroxide, but no inhibition was directly provided by cadmium. Interestingly, nuclear relocalization of GAPC1 under cadmium-induced oxidative stress was stimulated, rather than inhibited, by mutating into serine the catalytic cysteine of GAPC1 (C155S), excluding an essential role of GAPC1 nitrosylation in the mechanism of nuclear relocalization, as found in mammalian cells. Although the function of GAPC1 in the nucleus is unknown, our results suggest that glycolytic GAPC1, through its high sensitivity to the cellular redox state, may play a role in oxidative stress signaling or protection in plants.

Class III peroxidases are activated in proanthocyanidin-deficient Arabidopsis thaliana seeds

BACKGROUND AND AIMS

It has previously been shown that proanthocyanidins (PAs) in the seed coat of Arabidopsis thaliana have the ability to scavenge superoxide radicals (O2(-)). However, the physiological processess in PA-deficit seeds are not clear. It is hypothesized that there exist alternative ways in PA-deficient seeds to cope with oxidative stress.

METHODS

The content of hydrogen peroxide (H2O2) and its relevance to the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidases was investigated in both wild-type and PA-deficit mutant seeds. A biochemical staining approach was used to detect tissue localizations of peroxidase activities in PA-deficit mutant seeds.

KEY RESULTS

PA-deficient mutants possess significantly lower levels of H2O2 than the wild-type, despite their higher accumulation of superoxide radicals. Screening of the key antioxidant enzymes revealed that peroxidase activity was significantly over-activated in mutant seeds. This high peroxidase activity was mainly confined to the seed coat zone. Interestingly, neither ascorbate peroxidase nor glutathione peroxidase, just the guaiacol peroxidases (class III peroxidases), was specifically activated in the seed coat. However, no significant difference in peroxidase activity was observed in embryos of either mutants or the wild-type, although gene expressions of several candidate peroxidases were down-regulated in the embryos of PA-deficient seeds.

CONCLUSIONS

The results suggest that enhanced class III peroxidase activity in the seed coat of PA-deficient mutants is an adaptive strategy for seed development and survival.

Loss of plastoglobule kinases ABC1K1 and ABC1K3 causes conditional degreening, modified prenyl-lipids, and recruitment of the jasmonic acid pathway

Plastoglobules (PGs) are plastid lipid-protein particles. This study examines the function of PG-localized kinases ABC1K1 and ABC1K3 in Arabidopsis thaliana. Several lines of evidence suggested that ABC1K1 and ABC1K3 form a protein complex. Null mutants for both genes (abc1k1 and abc1k3) and the double mutant (k1 k3) displayed rapid chlorosis upon high light stress. Also, k1 k3 showed a slower, but irreversible, senescence-like phenotype during moderate light stress that was phenocopied by drought and nitrogen limitation, but not cold stress. This senescence-like phenotype involved degradation of the photosystem II core and upregulation of chlorophyll degradation. The senescence-like phenotype was independent of the EXECUTER pathway that mediates genetically controlled cell death from the chloroplast and correlated with increased levels of the singlet oxygen-derived carotenoid β-cyclocitral, a retrograde plastid signal. Total PG volume increased during light stress in wild type and k1 k3 plants, but with different size distributions. Isolated PGs from k1 k3 showed a modified prenyl-lipid composition, suggesting reduced activity of PG-localized tocopherol cyclase (VTE1), and was consistent with loss of carotenoid cleavage dioxygenase 4. Plastid jasmonate biosynthesis enzymes were recruited to the k1 k3 PGs but not wild-type PGs, while pheophytinase, which is involved in chlorophyll degradation, was induced in k1 k3 and not wild-type plants and was localized to PGs. Thus, the ABC1K1/3 complex contributes to PG function in prenyl-lipid metabolism, stress response, and thylakoid remodeling.

Capacity for NADPH regeneration in the leaves of two poplar genotypes differing in ozone sensitivity

Cell capacity for cytosolic NADPH regeneration by NADP-dehydrogenases was investigated in the leaves of two hybrid poplar (Populus deltoides × Populus nigra) genotypes in response to ozone (O3 ) treatment (120 ppb for 17 days). Two genotypes with differential O3 sensitivity were selected, based on visual symptoms and fallen leaves: Robusta (sensitive) and Carpaccio (tolerant). The estimated O3 flux (POD0 ), that entered the leaves, was similar for the two genotypes throughout the treatment. In response to that foliar O3 flux, CO2 assimilation was inhibited to the same extent for the two genotypes, which could be explained by a decrease in Rubisco (EC 4.1.1.39) activity. Conversely, an increase in PEPC (EC 4.1.1.31) activity was observed, together with the activation of certain cytosolic NADP-dehydrogenases above their constitutive level, i.e. NADP-G6PDH (EC 1.1.1.49), NADP-ME (malic enzyme) (EC 1.1.1.40) and NADP-ICDH (NADP-isocitrate dehydrogenase) (EC1.1.1.42). However, the activity of non-phosphorylating NADP-GAPDH (EC 1.2.1.9) remained unchanged. From the 11th fumigation day, NADP-G6PDH and NADP-ME profiles made it possible to differentiate between the two genotypes, with a higher activity in Carpaccio than in Robusta. At the same time, Carpaccio was able to maintain high levels of NADPH in the cells, while NADPH levels decreased in Robusta O3 -treated leaves. All these results support the hypothesis that the capacity for cells to regenerate the reducing power, especially the cytosolic NADPH pool, contributes to improve tolerance to high ozone exposure.