Production and scavenging of reactive oxygen species in chloroplasts and their functions

Redox regulation and overreduction control in the photosynthesizing cell: complexity in redox regulatory networks

Regulation of the photosynthetic apparatus between efficient energy conversion at low light and avoidance of overreduction and damage development at excess light resembles dangerous navigating between Scylla and Charybdis. Photosynthesis is a high rate redox metabolic pathway that generates redox intermediates with extreme redox potentials and eventually reactive oxygen species and oxidative stress. Therefore it is not surprising that the states of defined redox reactions in the chloroplast provide the predominant information and thus directly or indirectly the decisive signals for the multilevel control of cell activities in the chloroplast, cytoplasm, mitochondrion and nucleus. This review elaborates on the diversity of photosynthesis-derived redox signals such as the plastoquinone and thiol redox state that regulate and coordinate light use efficiency, electron transport activity, metabolic reactions, gene transcription and translation not only in the chloroplast but through retrograde signaling also essentially in all other cell compartments. The synergistic and antagonistic interrelations between the redox-dependent signaling pathways and their interactions with other signals such as abscisic acid and tetrapyrol intermediates constitute a redundant and probably buffered regulatory network to optimize performance of photosynthesis on the cellular and whole leaf level.

Protein oxidation in the leaves and roots of cucumber plants (Cucumis sativus L.), mutant MSC16 and wild type

Reactive oxygen species (ROS) may cause irreversible carbonylation of proteins, resulting in structural and/or functional modifications. Carbonylated proteins were analyzed and compared in tissue extracts or purified mitochondria isolated from the leaves and roots of wild-type (WT) or MSC16 mutant cucumber plants. For analysis of the oxidized protein formation and degradation, several techniques were applied: Western blotting, quantitative, spectrophotometric assay of carbonyl concentration and protease activity measurements. Oxidized proteins were tagged with 2,4-dinitrophenylhydrazine (DNPH) and detected with anti-DNP antibodies. Western blots of 1D gels indicated that, in the leaves of both WT and MSC16 plants, certain oxidized proteins have chloroplastic origin. In MSC16 plants, protein oxidation is probably higher in chloroplasts than in mitochondria. Carbonyl concentration is similar in MSC16 and WT leaf extracts, but this may be the result of twice as high protease activity observed in MSC16 leaf extracts and indicates that chloroplastic proteases may effectively remove the oxidized proteins from chloroplasts. In mitochondria of both WT and MSC16 leaves, the levels of oxidized proteins and protease activity are similar. In MSC16 root extracts, the carbonyl concentration is lower and protease activity is similar as compared to WT plants. Nevertheless, in MSC16 root mitochondria, the 30% lower carbonyl concentration, lower band abundance for oxidized proteins and over 50% higher protease activity indicate that mitochondrial proteases are involved in degradation of the oxidatively damaged proteins. In matrix and membrane subfractions, the levels of oxidized proteins are similar in leaf mitochondria or lower in root mitochondria from MSC16 as compared to WT plants. The results show that the oxidized protein degradation network in MSC16 cucumber mutants is well developed, thus becoming a survival factor for plants with mitochondrial dysfunctions.

Drought stress and reactive oxygen species: Production, scavenging and signaling

As sessile organisms, plants have evolved mechanisms that allow them to adapt and survive periods of drought stress. One of the inevitable consequences of drought stress is enhanced ROS production in the different cellular compartments, namely in the chloroplasts, the peroxisomes and the mitochondria. This enhanced ROS production is however kept under tight control by a versatile and cooperative antioxidant system that modulates intracellular ROS concentration and sets the redox-status of the cell. Furthermore, ROS enhancement under stress functions as an alarm signal that triggers acclimatory/defense responses by specific signal transduction pathways that involve H(2)O(2) as secondary messenger. ROS signaling is linked to ABA, Ca(2+) fluxes and sugar sensing and is likely to be involved both upstream and downstream of the ABA-dependent signaling pathways under drought stress. Nevertheless, if drought stress is prolonged over to a certain extent, ROS production will overwhelm the scavenging action of the anti-oxidant system resulting in extensive cellular damage and death.

Discovery of oxidative burst in the field of plant immunity: Looking back at the early pioneering works and towards the future development

This article is introductory to the series of works presented in this special issue on the homeostasis and the signaling roles of reactive oxygen species (ROS) in plants. Upper half of this article briefly describes the history of the ROS study in the field of plant immunity research initiated by the observation that the attacks by pathogenic microorganisms possibly stimulate the burst of ROS production in the plant tissues. The topics covered in the series of works presented here include the plants' responses to abiotic oxidative stress (atmospheric ozone), regulation of seed germination, chemical interaction between parasitic and host plants and the draught tolerance, all controlled through homeostasis of ROS at biochemical and molecular biological levels. Lastly a discussion forum was proposed to further deepen our understanding of ROS behaviors in plants.

Isoprene emission rates under elevated CO2 and O3 in two field-grown aspen clones differing in their sensitivity to O3

Isoprene is the most important nonmethane hydrocarbon emitted by plants. The role of isoprene in the plant is not entirely understood but there is evidence that it might have a protective role against different oxidative stresses originating from heat shock and/or exposure to ozone (O(3)). Thus, plants under stress conditions might benefit by constitutively high or by higher stress-induced isoprene emission rates. In this study, measurements are presented of isoprene emission from aspen (Populus tremuloides) trees grown in the field for several years under elevated CO(2) and O(3). Two aspen clones were investigated: the O(3)-tolerant 271 and the O(3)-sensitive 42E. Isoprene emission decreased significantly both under elevated CO(2) and under elevated O(3) in the O(3)-sensitive clone, but only slightly in the O(3)-tolerant clone. This study demonstrates that long-term-adapted plants are not able to respond to O(3) stress by increasing their isoprene emission rates. However, O(3)-tolerant clones have the capacity to maintain higher amounts of isoprene emission. It is suggested that tolerance to O(3) is explained by a combination of different factors; while the reduction of O(3) uptake is likely to be the most important, the capacity to maintain higher amounts of isoprene is an important factor in strengthening this character.

Activities of antioxidative enzymes during Chenopodium rubrum L. Ontogenesis in vitro

For the short-day plant Chenopodium rubrum, a 14 h/10 h photoperiod is inductive for flowering, while continuous light (CL) is noninductive. Plants of one group were grown continuously under an inductive photoperiod, while in the other group flowering induction was delayed by 17 days of CL in order to separate on the time scale different developmental phases in plants of the same age. Regardless of the photoperiodic conditions the plants were exposed to, seed maturation occurred in 10 weeks. Activities of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) were determined in different phases of development (vegetative growth, flowering, seed development, and maturation). The activities of antioxidative enzymes depended on both the phase of development and the photoperiod. In plants grown continuously under an inductive photoperiod, high CAT and POD activities were detected at the time of flowering and decreased during seed development and maturation. In plants in which flowering induction was delayed by 17 days of CL, the activities of POD and SOD were lowest in the vegetative phase of development and attained maximum values in the phase of seed maturation. In both groups of plants, the highest CAT activity was measured at the time of flowering.

Programmed cell death in plants: new insights into redox regulation and the role of hydrogen peroxide

Programmed cell death (PCD), the highly regulated dismantling of cells, is essential for plant growth and survival. PCD plays key roles in embryo development, formation and maturation of many cell types and tissues, and plant reaction/adaptation to environmental conditions. Reactive oxygen species (ROS) are not only toxic by products of aerobic metabolism with strictly controlled cellular levels, but they also function as signaling agents regulating many biological processes and producing pleiotropic effects. Over the last decade, ROS have become recognized as important modulators of plant PCD. Molecular genetic approaches using plant mutants and transcriptome studies related to ROS-mediated PCD have revealed a wide array of plant-specific cell death regulators and have contributed to unraveling the elaborate redox signaling network. This review summarizes the biological processes, in which plant PCD participates and discusses the signaling functions of ROS with emphasis on hydrogen peroxide.

Are Phragmites australis enzymes involved in the degradation of the textile azo dye acid orange 7?

The role of antioxidant and detoxification enzymes of Phragmites australis, in the degradation of an azo dye, acid orange 7 (AO7), was studied. Activities of several enzymes involved in plant protection against stress were assayed through the activity characterization of superoxide dismutase (SOD), peroxidases (POD), catalase (CAT), ascorbate peroxidase (APOX), dehydroascorbate reductase (DHAR) and glutathione S-transferase (GST), obtained from P. australis crude extracts of leaves, stems and roots. A sub-surface vertical flow constructed wetland, planted with P. australis was used to test the plants response to the AO7 exposure at two different concentrations (130 and 700 mg l(-1)). An activity increase was detected for an AO7 concentration of 130 mg l(-1) for most enzymes studied (SOD, CAT and APOX), especially in leaves, suggesting a response of the reactive oxygen species scavenging enzymes to the chemical stress imposed. GST activity increase in this situation can also be interpreted as an activation of the detoxification pathway and subsequent AO7 conjugation. A totally different behaviour was observed for AO7 at 700 mg l(-1). An evident decrease in activity was observed for SOD, CAT, APOX and GST, probably due to enzymatic inhibition by AO7. Contrarily, DHAR activity augmented drastically in this situation. POD activity was not greatly affected during trial. Altogether these results suggest that P. australis effectively uses the ascorbate-glutathione pathway for the detoxification of AO7.

Photo- and antioxidant protection and salicylic acid accumulation during post-anthesis leaf senescence in Salvia lanigera grown under Mediterranean climate

Post-anthesis leaf senescence is a key developmental process in the life of plants as it is the time during which material built up by the plant during its growth phase is mobilized into reproductive tissues. Here we aimed to study the extent of photo- and antioxidant protection and salicylic acid (SA) accumulation during post-anthesis leaf senescence in a perennial plant, Salvia lanigera Poir. grown under Mediterranean field conditions. SA levels increased sharply (up to 2.7-fold) during early stages of leaf senescence until fruit and seed formation occurred (i.e. 4 weeks after anthesis). Later on, SA levels kept at constant high levels until leaf abscission occurred (i.e. 7 weeks after anthesis). Reductions in chlorophyll and carotenoid (lutein, violaxanthin and beta-carotene) levels occurred progressively during leaf senescence. In contrast, xanthophyll cycle de-epoxidation increased during early stages of leaf senescence and remained constant later, similar to SA accumulation. Indeed, xanthophyll cycle de-epoxidation strongly positively correlated with SA levels (r(2) = 0.92). The maximum efficiency of PSII (F(v)/F(m) ratio) kept around 0.80 throughout the experiment, except during the latest stage of leaf senescence (i.e. after fruit and seed formation), when this ratio decreased to 0.72, thus indicating damage to PSII. It is concluded that endogenous SA levels increase sharply during early stages of post-anthesis leaf senescence and concomitantly with activation of photoprotection mechanisms, such as xanthophyll cycle-dependent excess energy dissipation, thus avoiding damage to PSII until fruit and seed formation have been accomplished.

Genetic and genomic approaches to develop rice germplasm for problem soils

Soils that contain toxic amounts of minerals or are deficient in essential plant nutrients are widespread globally and seriously constrain rice production. New methods are necessary to incorporate the complex adaptive traits associated with tolerance of these abiotic stresses, while simultaneously retaining the high yield potential of rice varieties when conditions are favorable. Significant progress in the genetic characterization of stress response pathways and recent advances in genomics have provided powerful tools for in-depth dissection of tolerance mechanisms. Additionally, tolerance of most of these abiotic stresses in rice is controlled by a few QTLs with large effects despite the intricacy of the numerous traits involved. Genetic dissection of these QTLs and their incorporation into high-yielding varieties will significantly enhance and stabilize rice productivity in these problem soils. Current efforts at IRRI and in rice breeding programs worldwide are seeking to explore diverse germplasm collections and genetically dissect the causal mechanisms of tolerance to facilitate their use in breeding. This review focuses on salinity and P and Zn deficiency as the major problems encountered in rice soils, and examines current understanding of the mechanisms involved and efforts toward germplasm improvement.

Cool temperatures interfere with D1 synthesis in tomato by causing ribosomal pausing

Photodamage occurs when leaves are exposed to light in excess of what can be used for photosynthesis and in excess of the capacity of ancillary photoprotective as well as repair mechanisms. An important site of photodamage is the chloroplast encoded D1 protein, a component of the photosystem II (PSII) reaction center. Even under optimal growth irradiance, D1 is photodamaged necessitating rapid turnover to prevent the accumulation of photodamaged PSII reaction centers and consequent inhibition of photosynthesis. However, this on-going process of D1 turnover and replacement was impeded in the chilling-sensitive tomato (Solanum lycopersicum) plants when exposed to high-growth light at cool temperature. The decrease in D1 turnover and replacement was found not to be due to changes in the steady-state level of the psbA message. While the recruitment of ribosomes to psbA transcript, initiation of D1 translation, and the association of polysomes with the thylakoid membrane occurred normally, chilling temperatures caused ribosomal pausing during D1 peptide elongation in tomato. The pause locations were non-randomly located on the D1 transcript. The interference with translation caused by ribosomal pausing allowed photodamaged PSII centers to accumulate leading to the consequent inhibition of photosynthesis.

Inhibition of spinach chloroplast F0F1 by an Fe2+/ascorbate/H2O2 system

Plant chloroplasts are particularly threatened by free radical attack. We incubated purified soluble spinach chloroplast F(0)F(1) (CF(0)F(1), EC 3.6.3.34) with an Fe(2+)/H(2)O(2)/ascorbate system, and about 60% inactivation of the ATPase activity was reached after 60 min. Inactivation was not prevented by omission of H(2)O(2), by addition of catalase or superoxide dismutase, nor by the scavengers mannitol, DMSO, or BHT. No evidence for enzyme fragmentation or oligomerization was detected by SDS-PAGE. The chloroplast ATP synthase is resistant to attack by the reactive oxygen species commonly found at the chloroplast level. DTT in the medium completely prevented the inhibition, and its addition after the inhibition partially recovered the activity of the enzyme. CF(0)F(1) thiol residues were lost upon oxidation. The rate of thiol modification was faster than the rate of enzyme inactivation, suggesting that the thiol residues accounting for the inhibition may be hindered. Enzyme previously oxidized by iodobenzoate was not further inhibited by the oxidative system. The production of ascorbyl radical was identified by EPR and is possibly related to CF(0)F(1) inactivation. It is thus suggested that the ascorbyl radical, which accumulates under plant stress, might regulate CF(0)F(1).

Proteomics-based dissection of stress-responsive pathways in plants

Abiotic stress has an ability to alter the levels of a number of proteins, which may be soluble or structural in nature or which may exist before and after folding in the plant cell. The most crucial function of plant cell is to respond to stress by developing defence mechanisms. This defence is brought about by alteration in the pattern of gene expression. This leads to modulation of certain metabolic and defensive pathways. Owing to gene expression altered under stress, qualitative and quantitative changes in proteins are obvious. These proteins might play a role in signal transduction, antioxidative defence, antifreezing, heat shock, metal binding, antipathogenesis or osmolyte synthesis. A significant part of the literature shows the quantitative and qualitative changes in proteins, mainly employing western analysis, enzymatic kinetics, fraction isolation, one-dimensional SDS-PAGE electrophoresis, etc. Fortunately, recent developments in sensitivity and accuracy for proteome analysis have provided new dimensions to assess the changes in protein types and their expression levels under stress. The novel aim of this review is to do a side-by-side comparison of the proteins that are induced or overexpressed under abiotic stress, examining those from biochemical literature and the ones observed, sequenced and identified using the advanced proteomics and bioinformatic techniques.

Double mutants deficient in cytosolic and thylakoid ascorbate peroxidase reveal a complex mode of interaction between reactive oxygen species, plant development, and response to abiotic stresses

Reactive oxygen species (ROS) play a key signaling role in plants and are controlled in cells by a complex network of ROS metabolizing enzymes found in several different cellular compartments. To study how different ROS signals, generated in different cellular compartments, are integrated in cells, we generated a double mutant lacking thylakoid ascorbate peroxidase (tylapx) and cytosolic ascorbate peroxidase1 (apx1). Our analysis suggests that two different signals are generated in plants lacking cytosolic APX1 or tylAPX. The lack of a chloroplastic hydrogen peroxide removal enzyme triggers a specific signal in cells that results in enhanced tolerance to heat stress, whereas the lack of a cytosolic hydrogen peroxide removal enzyme triggers a different signal, which results in stunted growth and enhanced sensitivity to oxidative stress. When the two signals are coactivated in cells (i.e. tylapx/apx1), a new response is detected, suggesting that the integration of the two different signals results in a new signal that manifests in late flowering, low protein oxidation during light stress, and enhanced accumulation of anthocyanins. Our results demonstrate a high degree of plasticity in ROS signaling in Arabidopsis (Arabidopsis thaliana) and suggest the existence of redundant pathways for ROS protection that compensate for the lack of classical ROS removal enzymes such as cytosolic and chloroplastic APXs. Further investigation of the enhanced heat tolerance in plants lacking tylAPX, using mutants deficient in chloroplast-to-nuclei retrograde signaling, suggests the existence of a chloroplast-generated stress signal that enhances basal thermotolerance in plants.

High light inhibits chlorophyll biosynthesis at the level of 5-aminolevulinate synthesis during de-etiolation in cucumber (Cucumis sativus) cotyledons

Using the vascular plant Cucumis sativus (cucumber) as a model, we studied the effects of high (intense and excess) light upon chlorophyll biosynthesis during de-etiolation. When illuminated with high light (1500-1600 microE/m2/s), etiolated cucumber cotyledons failed to synthesize chlorophyll entirely. However, upon transfer to low light conditions (40-45 microE/m2/s), chlorophyll biosynthesis and subsequent accumulation resumed following an initial 2-12 h delay. Duration of high light treatment negatively correlated with chlorophyll biosynthetic activity. Specifically, we found that high light severely inhibited 5-aminolevulinic acid (ALA) synthesis. This effect partly could be because of the decrease in protein level of glutamyl-tRNA reductase (GluTR) observed. Protein level of glutamate-1-semialdehyde (GSA-AT) remained unchanged. It was also found that high light did not suppress HEMA 1 expression. Therefore, we speculated that this significant inhibition of ALA synthesis might have occurred mainly because of concomitant inactivation of GluTR and/or inhibition of complex formation between GluTR and GSA-AT. Our further observation that both methyl viologen and rose bengal similarly inhibit ALA synthesis under low light conditions suggested that reactive oxygen species (ROS) could be responsible for the inhibition of ALA synthesis in cotyledons exposed to high light conditions.

Before gene expression: early events in plant-insect interaction

Successful defense depends on the ability of the plant to recognize an attacking 'enemy' as early as possible. Early defense responses require enemy-initiated signaling cascades. Their activation ensures an induced response that is quantitative, timely and coordinated with other activities of the host cells. Damage-induced ion imbalances and modulations of channel activities are the first events occurring in the plasma membrane and result in rapid perturbations of the plasma membrane potential (V(m)) involving variations of cytosolic Ca(2+) concentrations. Interacting downstream networks of kinases and phytohormones mediate the signal and result in concerted gene activation. Here we review and discuss early events occurring before herbivore attack-related gene expression that are responsible for cascades of events and signal transductions, eventually leading to indirect and direct plant responses.

Responses of photosynthesis, chlorophyll fluorescence and ROS-scavenging systems to salt stress during seedling and reproductive stages in rice

BACKGROUND AND AIMS

Salinity is a widespread soil problem limiting productivity of cereal crops worldwide. Rice is particularly sensitive to salt stress during the seedling stage, with consequent poor crop establishment, as well as during reproduction where salinity can severely disrupt grain formation and yield. Tolerance at the seedling stage is weakly associated with tolerance during reproduction. Physiological responses to salinity were evaluated for contrasting genotypes, during the seedling and reproductive stages.

METHODS

Three rice genotypes differing in their tolerance of salinity were evaluated in a set of greenhouse experiments under salt stress during both seedling stage and reproduction.

KEY RESULTS

Photosynthetic CO2 fixation, stomatal conductance (gs) and transpiration decreased substantially because of salt stress, but with greater reduction in the sensitive cultivar IR29. The tolerant lines IR651 and IR632 had more responsive stomata that tended to close faster during the first few hours of stress, followed by partial recovery after a brief period of acclimation. However, in the sensitive line, gs continued to decrease for longer duration and with no recovery afterward. Chlorophyll fluorescence measurements revealed that non-photochemical quenching increased, whereas the electron transport rate decreased under salt stress. Salt-tolerant cultivars exhibited much lower lipid peroxidation, maintained elevated levels of reduced ascorbic acid and showed increased activities of the enzymes involved in the reactive oxygen scavenging system during both developmental stages.

CONCLUSIONS

Upregulation of the anti-oxidant system appears to play a role in salt tolerance of rice, with tolerant genotypes also maintaining relatively higher photosynthetic function; during both the vegetative and reproductive stages.

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.

Impairment of the photorespiratory pathway accelerates photoinhibition of photosystem II by suppression of repair but not acceleration of damage processes in Arabidopsis

Oxygenation of ribulose-1,5-bisphosphate catalyzed by Rubisco produces glycolate-2-P. The photorespiratory pathway, which consists of photorespiratory carbon and nitrogen cycles, metabolizes glycolate-2-P to the Calvin cycle intermediate glycerate-3-P and is proposed to be important for avoiding photoinhibition of photosystem II (PSII), especially in C3 plants. We show here that mutants of Arabidopsis (Arabidopsis thaliana) with impairment of ferredoxin-dependent glutamate synthase, serine hydroxymethyltransferase, glutamate/malate transporter, and glycerate kinase had accelerated photoinhibition of PSII by suppression of the repair of photodamaged PSII and not acceleration of the photodamage to PSII. We found that suppression of the repair process was attributable to inhibition of the synthesis of the D1 protein at the level of translation. Our results suggest that the photorespiratory pathway helps avoid inhibition of the synthesis of the D1 protein, which is important for the repair of photodamaged PSII upon interruption of the Calvin cycle.

Analysis of donors of electrons to photosystem I and cyclic electron flow by redox kinetics of P700 in chloroplasts of isolated bundle sheath strands of maize

Bundle sheath chloroplasts of NADP-malic enzyme (NADP-ME) type C4 species have a high demand for ATP, while being deficient in linear electron flow and oxidation of water by photosystem II (PSII). To evaluate electron donors to photosystem I (PSI) and possible pathways of cyclic electron flow (CEF1) in isolated bundle sheath strands of maize (Zea mays L.), an NADP-ME species, light-induced redox kinetics of the reaction center chlorophyll of PSI (P700) were followed under aerobic conditions. Donors of electrons to CEF1 are needed to compensate for electrons lost from the cycle. When stromal electron donors to CEF1 are generated during pre-illumination with actinic light (AL), they retard the subsequent rate of oxidation of P700 by far-red light. Ascorbate was more effective than malate in generating stromal electron donors by AL. The generation of stromal donors by ascorbate was inhibited by DCMU, showing ascorbate donates electrons to the oxidizing side of PSII. The inhibitors of NADPH dehydrogenase (NDH), amytal and rotenone, accelerated the oxidation rate of P700 by far-red light after AL, indicating donation of electrons to the intersystem from stromal donors via NDH. These inhibitors, however, did not affect the steady-state level of P700+ under AL, which represents a balance of input and output of electrons in P700. In contrast, antimycin A, the inhibitor of the ferredoxin-plastoquinone reductase-dependent CEF1, substantially lowered the level of P700+ under AL. Thus, the primary pathway of ATP generation by CEF1 may be through ferredoxin-plastoquinone, while function of CEF1 via NDH may be restricted by low levels of ferredoxin-NADP reductase. NDH may contribute to redox poising of CEF1, or function to generate ATP in linear electron flow to O2 via PSI, utilizing NADPH generated from malate by chloroplastic NADP-ME.

Dark production of reactive oxygen species in photosystem II membrane particles at elevated temperature: EPR spin-trapping study

In our study, EPR spin-trapping technique was employed to study dark production of two reactive oxygen species, hydroxyl radicals (OH.) and singlet oxygen ((1)O2), in spinach photosystem II (PSII) membrane particles exposed to elevated temperature (47 degrees C). Production of OH., evaluated as EMPO-OH adduct EPR signal, was suppressed by the enzymatic removal of hydrogen peroxide and by the addition of iron chelator desferal, whereas externally added hydrogen peroxide enhanced OH. production. These observations reveal that OH. is presumably produced by metal-mediated reduction of hydrogen peroxide in a Fenton-type reaction. Increase in pH above physiological values significantly stimulated the formation of OH., whereas the presence of chloride and calcium ions had the opposite effect. Based on our results it is proposed that the formation of OH. is linked to the thermal disassembly of water-splitting manganese complex on PSII donor side. Singlet oxygen production, followed as the formation of nitroxyl radical TEMPO, was not affected by OH. scavengers. This finding indicates that the production of these two species was independent and that the production of (1)O2 is not closely linked to PSII donor side.

Alpha-tocopherol may influence cellular signaling by modulating jasmonic acid levels in plants

Most studies on the function of tocopherols in plants have focused on their photo-protective and antioxidant properties, and it has been recently suggested, though not yet demonstrated, that they may also play a role in cellular signaling. By using vte1 mutants of Arabidopsis thaliana, with an insertion in the promoter region of the gene encoding tocopherol cyclase, we demonstrate here for the first time that tocopherol deficiency may alter endogenous phytohormone levels in plants, thereby reducing plant growth and triggering anthocyanin accumulation in leaves. In plants grown under a combination of high light and low temperature conditions to induce anthocyanin accumulation, we evaluated age-dependent changes in tocopherols, indicators of photo-oxidative stress, phytohormone levels, plant growth and anthocyanin levels in wild type and vte1 mutants. These mutants showed lower tocopherol levels, reduced growth and enhanced anthocyanin accumulation compared with the wild type, while both the maximum and relative efficiencies of PSII, chlorophylls, and carotenoids were not significantly altered. Analyses of phytohormone levels revealed that reduced growth and enhanced anthocyanin accumulation in tocopherol-deficient plants were preceded by increased jasmonic acid levels. This is the first study suggesting a direct effect of tocopherols on phytohormones levels in plants and will undoubtedly help us to better understand the multiple functions tocopherols play in plants, as well as the cellular signaling mechanisms responsible for the phenotypes thus far described in tocopherol-deficient plants.

The dual function of plant peroxiredoxins in antioxidant defence and redox signaling

With 8 to 10 members, the peroxiredoxin gene family of each higher plant with known genome sequence is larger than in other eukaryotes. Likewise, the complexity of reductive regenerants is very high, e.g. the chloroplast 2-Cys Prx is reduced by various thioredoxins, cyclophilin Cyp20-3, the drought induced CDSP32 and the NADPH-dependent reductant NTRC. In the light of the apparent versatility of the peroxiredoxin system in plants, its specific and important functions in antioxidant defence, photosynthesis and stress adaptation, the review attempts a survey of present day knowledge on plant peroxiredoxins, their biochemical features and transcript regulation, as well as their function in photosynthesis, development, stress response and pathogenesis. The emerging evidence for plant Prx function in cell signaling is summarized.

Activation of mechanisms of photoprotection by desiccation and by light: poikilohydric photoautotrophs

Mechanisms of protection against photo-oxidation in selected desiccation-tolerant lichens and mosses have been investigated by measuring loss of light absorption during desiccation and chlorophyll fluorescence as indicators of photoprotection. Apparent absorption (1-T) spectra measured in the reflectance mode revealed stronger absorption of photosynthetic pigments in hydrated than in desiccated organisms, but differences were pronounced only in a cyanolichen, less so in some chlorolichens, and even less in mosses. Since the amplitude of chlorophyll fluorescence is a product of (1-T) light absorption by chlorophyll and quantum yield of fluorescence, and since fluorescence is inversely related to thermal energy dissipation, when chemical fluorescence quenching is negligible, fluorescence measurements were used to measure changes in energy dissipation. Preincubation of the hydrated organisms and desiccation in darkness excluded the contribution of mechanisms of energy dissipation to photoprotection which are dependent on the presence of zeaxanthin or on the light-dependent formation of a quencher of fluorescence within the reaction centre of photosystem II. Fast drying in darkness or in very low light was less effective in decreasing chlorophyll fluorescence than slow drying. Heating the desiccated organisms increased fluorescence by inactivating the mechanism responsible for fluorescence quenching. Glutaraldehyde inhibited fluorescence quenching during desiccation. Prolonged exposure of a desiccated moss or a desiccated lichen to very strong light caused more photo-induced damage after fast drying than after slow drying. The photo-oxidative nature of damage was emphasized by the observation that irreversible loss of fluorescence was larger in air than in a nitrogen atmosphere. It is concluded from these observations that desiccation-induced conformational changes of a chlorophyll protein complex result in the fast radiationless dissipation of absorbed light energy. This mechanism of photoprotection is more effective in preventing photo-oxidative damage than other mechanisms of energy dissipation which require light for activation such as zeaxanthin-dependent energy dissipation or quencher formation within the reaction centre of photosystem II.

Oxidative modifications to cellular components in plants

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced in many places in living cells and at an increased rate during biotic or abiotic stress. ROS and RNS participate in signal transduction, but also modify cellular components and cause damage. We first look at the most common ROS and their properties. We then consider the ways in which the cell can regulate their production and removal. We critically assess current knowledge about modifications of polyunsaturated fatty acids (PUFAs), DNA, carbohydrates, and proteins and illustrate this knowledge with case stories wherever possible. Some oxidative breakdown products, e.g., from PUFA, can cause secondary damage. Other oxidation products are secondary signaling molecules. We consider the fate of the modified components, the energetic costs to the cell of replacing such components, as well as strategies to minimize transfer of oxidatively damaged components to the next generation.

α‐Tocopherol: A Multifaceted Molecule in Plants

alpha-Tocopherol, which belongs to the vitamin E group of compounds, is a lipophilic antioxidant that has a number of functions in plants. Synthesized from homogentisic acid and isopentenyl diphosphate in the chloroplast envelope, alpha-tocopherol is essential to maintain the integrity of photosynthetic membranes and plays a major role in photo- and antioxidant protection. alpha-Tocopherol scavenges lipid peroxy radicals, thereby preventing the propagation of lipid peroxidation, and protects lipids and other membrane components by physically quenching and reacting chemically with singlet oxygen. Moreover, given that alpha-tocopherol increases membrane rigidity, its concentration, together with that of the other membrane components, may be regulated to afford adequate fluidity for membrane function. Furthermore, recent studies on tocopherol-deficient plants indicate that alpha-tocopherol may affect cellular signaling in plants. Evidence thus far indicates that the effects of this compound in plant cellular signaling may be linked to the control of redox homeostasis. alpha-Tocopherol may influence cellular signaling by controlling the propagation of lipid peroxidation in chloroplasts, therefore modulating the formation of oxylipins such as the phytohormone jasmonic acid.

Calibration of simultaneous measurements of photosynthetic carbon dioxide uptake and oxygen evolution in leaves

The stoichiometric ratio of O2 evolution to CO2 uptake during photosynthesis reveals information about reductive metabolism, including the reduction of alternative electron acceptors, such as nitrite and oxaloacetate. Recently we reported that in simultaneous measurements of CO2 uptake and O2 evolution in a sunflower leaf, O2 evolution changed by 7% more than CO2 uptake when light intensity was varied. Since the O2/CO2 exchange ratio is approximately 1, small differences are important. Thus, these gas exchange measurements need precise calibration. In this work, we describe a new calibration procedure for such simultaneous measurements, based on the changes of O2 concentration caused by the addition of pure CO2 or O2 into a flow of dry air (20.95% O2) through one and the same capillary. The relative decrease in O2 concentration during the addition of CO2 and the relative increase in O2 concentration during the addition of O2 allowed us to calibrate the CO2 and O2 scales of the measurement system with an error (relative standard deviation, RSD) of <1%. Measurements on a sunflower leaf resulted in an O2/CO2 ratio between 1.0 and 1.03 under different CO2 concentrations and light intensities, in the presence of an ambient O2 concentration of 20-50 micromol mol(-1). This shows that the percentage use of reductive power from photochemistry in synthesis of inorganic or organic matter other than CO2 assimilation in the C3 cycle is very low in mature leaves and, correspondingly, the reduction of alternative acceptors is a weak source of coupled ATP synthesis.

Cyclic electron transport around photosystem I: genetic approaches

The light reactions in photosynthesis convert light energy into chemical energy in the form of ATP and drive the production of NADPH from NADP+. The reactions involve two types of electron flow in the chloroplast. While linear electron transport generates both ATP and NADPH, photosystem I cyclic electron transport is exclusively involved in ATP synthesis. The physiological significance of photosystem I cyclic electron transport has been underestimated, and our knowledge of the machineries involved remains very limited. However, recent genetic approaches using Arabidopsis thaliana have clarified the essential functions of this electron flow in both photoprotection and photosynthesis. Based on several lines of evidence presented here, it is necessary to reconsider the fundamental mechanisms of chloroplast energetics.

Cross-talk between singlet oxygen- and hydrogen peroxide-dependent signaling of stress responses in Arabidopsis thaliana

Upon a dark-to-light shift, the conditional fluorescent (flu) mutant of Arabidopsis releases singlet oxygen (1O2) within the plastid compartment. Distinct sets of nuclear genes are activated that are different from those induced by superoxide (O2*-)) and/or hydrogen peroxide (H2O2), suggesting that different types of reactive oxygen species activate distinct signaling pathways. It is not known whether the pathways operate separately or interact with each other. We have addressed this problem by modulating noninvasively the level of H2O2 in plastids by means of a transgenic line that overexpresses the thylakoid-bound ascorbate peroxidase (tAPX). The overexpression of the H2O2-specific scavenger reduced strongly the activation of nuclear genes in plants treated with the herbicide paraquat that in the light leads to the enhanced generation of O2*- and H2O2. In the flu mutant overexpressing tAPX, the intensity of 1O2-mediated cell death and growth inhibition was increased when compared with the flu parental line. Also, the expression of most of the nuclear genes that were rapidly activated after the release of 1O2 was significantly higher in flu plants overexpressing tAPX, whereas in wild-type plants, overexpression of tAPX did not lead to visible stress responses and had only a very minor impact on nuclear gene expression. The results suggest that H2O2 antagonizes the 1O2-mediated signaling of stress responses as seen in the flu mutant. This cross-talk between H2O2- and 1O2-dependent signaling pathways might contribute to the overall stability and robustness of wild-type plants exposed to adverse environmental stress conditions.

Reactive oxygen species as signals that modulate plant stress responses and programmed cell death

Reactive oxygen species (ROS) are known as toxic metabolic products in plants and other aerobic organisms. An elaborate and highly redundant plant ROS network, composed of antioxidant enzymes, antioxidants and ROS-producing enzymes, is responsible for maintaining ROS levels under tight control. This allows ROS to serve as signaling molecules that coordinate an astonishing range of diverse plant processes. The specificity of the biological response to ROS depends on the chemical identity of ROS, intensity of the signal, sites of production, plant developmental stage, previous stresses encountered and interactions with other signaling molecules such as nitric oxide, lipid messengers and plant hormones. Although many components of the ROS signaling network have recently been identified, the challenge remains to understand how ROS-derived signals are integrated to eventually regulate such biological processes as plant growth, development, stress adaptation and programmed cell death.