Mehler Reaction Research Articles

Photoprotective and antioxidative mechanisms against oxidative damage in Fargesia rufa subjected to drought and salinity.

Drought and salinity are the two most common and frequently co-occurring abiotic stresses limiting plant productivity worldwide, yet it remains unclear whether bamboo species possess effective mechanisms to protect against oxidative damage caused by drought and salinity, either alone or in combination. In this study, we utilised Fargesia rufa Yi, a species important to forest carbon sequestration and endangered giant pandas, to evaluate physiological, biochemical and ultrastructural responses to drought, salinity and their combination. Under drought alone, F. rufa exhibited reduced water loss from leaves, photochemistry inhibition, pigment degradation, reactive oxygen species accumulation, lipid peroxidation, and damage to organelles compared with salinity and combined stress treatments. The superior performance under drought alone was attributed to greater thermal dissipation and the water-water cycle capacities, increased SOD/AsA-GSH cycle enzymes activities, and a favourable redox balance of antioxidants. Therefore, relative to salinity alone and drought+salinity, F. rufa plants under drought exhibit highly efficient mechanisms to protect against oxidative damage, which most likely allow accelerated recovery of photosynthetic plasticity once the stress is removed.

Seasonal variations in photosystem I compared with photosystem II of three alpine evergreen broad-leaf tree species

Low temperature associated with high light can induce photoinhibition of photosystem I (PSI) and photosystem II (PSII). However, the photosynthetic electron flow and specific photoprotective responses in alpine evergreen broad-leaf plants in winter is unclear. We analyzed seasonal changes in PSI and PSII activities, and energy quenching in PSI and PSII in three alpine broad-leaf tree species, Quercus guyavifolia (Fagaceae), Rhododendron decorum (Ericaceae), Euonymus tingens (Celastraceae). In winter, PSII activity remained stable in Q. guyavifolia but decreased significantly in R. decorum and E. tingens. Q. guyavifolia showed much higher capacities of cyclic electron flow (CEF), water-water cycle (WWC), non-photochemical quenching (NPQ) than R. decorum and E. tingens in winter. These results indicated that in alpine evergreen broad-leaf tree species the PSII activity in winter was closely related to these photoprotective mechanisms. Interestingly, unlike PSII, PSI activity was maintained stable in winter in the three species. Meanwhile, photosynthetic electron flow from PSII to PSI (ETRII) was much higher in Q. guyavifolia, suggesting that the mechanisms protecting PSI activity against photoinhibition in winter differed among the three species. A high level of CEF contributed the stability of PSI activity in Q. guyavifolia. By comparison, R. decorum and E. tingens prevented PSI photoinhibition through depression of electron transport to PSI. Taking together, CEF, WWC and NPQ played important roles in coping with excess light energy in winter for alpine evergreen broad-leaf tree species.

Differences in photosynthetic syndromes of four halophytic marsh grasses in Pakistan.

Salt-tolerant grasses of warm sub-tropical ecosystems differ in their distribution patterns with respect to salinity and moisture regimes. Experiments were conducted on CO2 fixation and light harvesting processes of four halophytic C4 grasses grown under different levels of salinity (0, 200 and 400mM NaCl) under ambient environmental conditions. Two species were from a high saline coastal marsh (Aeluropus lagopoides and Sporobolus tremulus) and two were from a moderate saline sub-coastal draw-down tidal marsh (Paspalum paspalodes and Paspalidium geminatum). Analyses of the carbon isotope ratios of leaf biomass in plants indicated that carbon assimilation was occurring by C4 photosynthesis in all species during growth under varying levels of salinity. In the coastal species, with increasing salinity, there was a parallel decrease in rates of CO2 fixation (A), transpiration (E) and stomatal conductance (g s), with no effect on water use efficiency (WUE). These species were adapted for photoprotection by an increase in the Mehler reaction with an increase in activity of PSII/CO2 fixed accompanied by high levels of antioxidant enzymes, superoxide dismutase and ascorbate peroxidase. The sub-coastal species P. paspalodes and P. geminatum had high levels of carotenoid pigments and non-photochemical quenching by the xanthophyll cycle.

Microoxic Niches within the Thylakoid Stroma of Air-Grown Chlamydomonas reinhardtii Protect [FeFe]-Hydrogenase and Support Hydrogen Production under Fully Aerobic Environment.

Photosynthetic hydrogen production in the microalga Chlamydomonas reinhardtii is catalyzed by two [FeFe]-hydrogenase isoforms, HydA1 and HydA2, both irreversibly inactivated upon a few seconds exposure to atmospheric oxygen. Until recently, it was thought that hydrogenase is not active in air-grown microalgal cells. In contrast, we show that the entire pool of cellular [FeFe]-hydrogenase remains active in air-grown cells due to efficient scavenging of oxygen. Using membrane inlet mass spectrometry, (18)O2 isotope, and various inhibitors, we were able to dissect the various oxygen uptake mechanisms. We found that both chlororespiration, catalyzed by plastid terminal oxidase, and Mehler reactions, catalyzed by photosystem I and Flavodiiron proteins, significantly contribute to oxygen uptake rate. This rate is considerably enhanced with increasing light, thus forming local anaerobic niches at the proximity of the stromal face of the thylakoid membrane. Furthermore, we found that in transition to high light, the hydrogen production rate is significantly enhanced for a short duration (100 s), thus indicating that [FeFe]-hydrogenase functions as an immediate sink for surplus electrons in aerobic as well as in anaerobic environments. In summary, we show that an anaerobic locality in the chloroplast preserves [FeFe]-hydrogenase activity and supports continuous hydrogen production in air-grown microalgal cells.

The role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L.

This study investigates the role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L. Electron transport rates (ETRs) and non-photochemical quenching (NPQ) of Z. marina were measured under saturating irradiance in synthetic seawater containing 2.2mM DIC and no DIC with different O2 levels (air-equilibrated levels, 3% of air equilibrium and restored air-equilibrated levels). Lowering O2 did not affect ETR when DIC was provided, while it caused a decrease in ETR and an increase in NPQ in DIC-free media, indicating that O2 acted as an alternative electron acceptor under low DIC. The ETR and NPQ as a function of irradiance were subsequently assessed in synthetic seawater containing (1) 2.2mM DIC, air-equilibrated O2; (2) saturating CO2, no O2; and (3) no DIC, air-equilibrated O2. These treatments were combined with glycolaldehyde pre-incubation. Glycolaldehyde caused a marked decrease in ETR in DIC-free medium, indicating significant electron flow supported by photorespiration. Combining glycolaldehyde with O2 depletion completely suppressed ETR suggesting the operation of the Mehler reaction, a possibility supported by the photosynthesis-dependent superoxide production. However, no notable effect of suppressing the Mehler reaction on NPQ was observed. It is concluded that during DIC-limiting conditions, such as those frequently occurring in the habitats of Z. marina, captured light energy exceeds what is utilised for the assimilation of available carbon, and photorespiration is a major alternative electron acceptor, while the contribution of the Mehler reaction is minor.

The Water to Water Cycles in Microalgae.

In oxygenic photosynthesis, light produces ATP plus NADPH via linear electron transfer, i.e. the in-series activity of the two photosystems: PSI and PSII. This process, however, is thought not to be sufficient to provide enough ATP per NADPH for carbon assimilation in the Calvin-Benson-Bassham cycle. Thus, it is assumed that additional ATP can be generated by alternative electron pathways. These circuits produce an electrochemical proton gradient without NADPH synthesis, and, although they often represent a small proportion of the linear electron flow, they could have a huge importance in optimizing CO2 assimilation. In Viridiplantae, there is a consensus that alternative electron flow comprises cyclic electron flow around PSI and the water to water cycles. The latter processes include photosynthetic O2 reduction via the Mehler reaction at PSI, the plastoquinone terminal oxidase downstream of PSII, photorespiration (the oxygenase activity of Rubisco) and the export of reducing equivalents towards the mitochondrial oxidases, through the malate shuttle. In this review, we summarize current knowledge about the role of the water to water cycles in photosynthesis, with a special focus on their occurrence and physiological roles in microalgae.

Redox-Dependent Conformational Dynamics of Decameric 2-Cysteine Peroxiredoxin and its Interaction with Cyclophilin 20-3.

2-Cysteine peroxiredoxins (2-CysPrxs) switch between functions as a thiol peroxidase, chaperone, an interaction partner and possibly a proximity-based oxidase in a redox-dependent manner. In photosynthetic eukaryotes, 2-CysPrx localizes to the plastid, functions in the context of photosynthesis and enables an ascorbate peroxidase-independent water-water cycle for detoxifying H2O2 The high degree of evolutionary conservation of 2-CysPrx suggests that the switching is an essential characteristic and needed to transduce redox information to downstream pathways and regulation. The study aimed at exploring the dissociation behavior of 2-CysPrx and its interactions with cyclophilin depending on bulk phase conditions. Isothermal titration microcalorimetry (ITC), dynamic light scattering and size exclusion chromatography (SEC) proved the previously suggested model that reduced 2-CysPrx below a critical transition concentration (CTC) exists in its dimeric state, and above the CTC adopts the decameric state. The presence of cyclophilin 20-3 (Cyp20-3) affected the CTC of a 2-CysPrx decamer suggesting interaction which was further quantified by direct titration of 2-CysPrx with Cyp20-3, and in overlays. Finally catalytic inactivation assays showed the higher catalytic efficiency of 2-CysPrx at pH 8 compared with pH 7.2, but also revealed increased inactivation by hyperoxidation at pH 8. Interestingly, calculation of the average turnover number until inactivation gave rather similar values of 243 and 268 catalytic cycles at pH 8 and pH 7.2, respectively. These quantitative data support a model where 2-CysPrx and Cyp20-3, by interaction, form a redox-sensitive regulatory module in the chloroplast which is under control of the photosynthesis-linked stromal pH value, the redox state and additional stromal protein factor(s).

Response of the water–water cycle to the change in photorespiration in tobacco

Photosynthetic electron transport produces ATP and NADPH, which are used by the primary metabolism. The production and consumption of ATP and NADPH must be balanced to maintain steady-state rates of CO2 assimilation and photorespiration. It has been indicated that the water–water cycle (WWC) is indispensable for driving photosynthesis via increasing ATP/NADPH production. However, the relationship between the WWC and photorespiration is little known. We tested the hypothesis that the WWC responds to change in photorespiration by balancing ATP/NADPH ratio. Measurements of gas exchange and chlorophyll fluorescence were conducted in tobacco plants supplied with high (HN-plants) or low nitrogen concentration (LN-plants). The WWC was activated under high light but not low light in both HN-plants and LN-plants. HN-plants had significantly higher capacities of the WWC and photorespiration than LN-plants. Under high light, the relative high WWC activation in HN-plants was accompanied with relative low levels of NPQ compared LN-plants, suggesting that the main role of the WWC under high light was to favor ATP synthesis but not to activate NPQ. Interestingly, the activation of WWC was positively correlated to the electron flow devoted to RuBP oxygenation, indicating that the WWC plays an important role in energy balancing when photorespiration is high. We conclude that the WWC is an important flexible mechanism to optimize the stoichiometry of the ATP/NADPH ratio responding to change in photorespiration. Furthermore, HN-plants enhance the WWC activity to maintain higher rates of CO2 assimilation and photorespiration.

Interactions between 2-Cys peroxiredoxins and ascorbate in autophagosome formation during the heat stress response in Solanum lycopersicum.

2-Cys peroxiredoxins (2-CPs) function in the removal of hydrogen peroxide and lipid peroxides but their precise roles in the induction of autophagy have not been characterized. Here we show that heat stress, which is known to induce oxidative stress, leads to the simultaneous accumulation of transcripts encoding 2-CPs and autophagy proteins, as well as autophagosomes, in tomato (Solanum lycopersicum) plants. Virus-induced gene silencing of the tomato peroxiredoxin genes 2-CP1, 2-CP2, and 2-CP1/2 resulted in an increased sensitivity of tomato plants to heat stress. Silencing 2-CP2 or 2-CP1/2 increased the levels of transcripts associated with ascorbate biosynthesis but had no effect on the glutathione pool in the absence of stress. However, the heat-induced accumulation of transcripts associated with the water-water cycle was compromised by the loss of 2-CP1/2 functions. The transcript levels of autophagy-related genes ATG5 and ATG7 were higher in plants with impaired 2-CP1/2 functions, and the formation of autophagosomes increased, together with an accumulation of oxidized and insoluble proteins. Silencing of ATG5 or ATG7 increased the levels of 2-CP transcripts and protein but decreased heat stress tolerance. These results demonstrate that 2-CPs fulfil a pivotal role in heat stress tolerance in tomato, via interactions with ascorbate-dependent pathways and autophagy.

Diversity and Evolution of Ascorbate Peroxidase Functions in Chloroplasts: More Than Just a Classical Antioxidant Enzyme?

Reactive oxygen species (ROS) have dual functions in plant cells as cytotoxic molecules and emergency signals. The balance between the production and scavenging of these molecules in chloroplasts, major sites for the production of ROS, is one of the key determinants for plant acclimation to stress conditions. The water-water cycle is a crucial regulator of ROS levels in chloroplasts. In this cycle, the stromal and thylakoid membrane-attached isoforms of ascorbate peroxidase (sAPX and tAPX, respectively) are involved in the metabolism of H2O2 Current genome and phylogenetic analyses suggest that the first monofunctional APX was generated as sAPX in unicellular green algae, and that tAPX occurred in multicellular charophytes during plant evolution. Chloroplastic APXs, especially tAPX, have been considered to be the source of a bottleneck in the water-water cycle, at least in higher plants, because of their high susceptibility to H2O2 A number of studies have succeeded in improving plant stress resistance by reinforcing the fragile characteristics of the enzymes. However, researchers have unexpectedly failed to find a 'stress-sensitive phenotype' among loss-of-function mutants, at least in laboratory conditions. Interestingly, the susceptibility of enzymes to H2O2 may have been acquired during plant evolution, thereby allowing for the flexible use of H2O2 as a signaling molecule in plants, and this is supported by growing lines of evidence for the physiological significance of chloroplastic H2O2 as a retrograde signal in plant stress responses. By overviewing historical, biochemical, physiological and genetic studies, we herein discuss the diverse functions of chloroplastic APXs as antioxidant enzymes and signaling modulators.

Different strategies of acclimation of photosynthesis, electron transport and antioxidative activity in leaves of two cotton species to water deficit.

To better understand the adaptation mechanisms of the photosynthetic apparatus of cotton plants to water deficit conditions, the influence of water deficit on photosynthesis, chlorophyll a fluorescence and the activities of antioxidant systems were determined simultaneously in Gossypium hirsutum L. cv. Xinluzao 45 (upland cotton) and Gossypium barbadense L. cv. Xinhai 21 (pima cotton). Water deficit decreased photosynthesis in both cotton species, but did not decrease chlorophyll content or induce any sustained photoinhibition in either cotton species. Water deficit increased ETR/4-AG, where ETR/4 estimates the linear photosynthetic electron flux and AG is the gross rate of carbon assimilation. The increase in ETR/4-AG, which represents an increase in photorespiration and alternative electron fluxes, was particularly pronounced in Xinluzao 45. In Xinluzao 45, water deficit increased the activities of antioxidative enzymes, as well as the contents of reactive oxygen species (ROS), which are related to the Mehler reaction. In contrast, moderate water deficit particularly increased non-photochemical quenching (NPQ) in Xinhai 21. Our results suggest that Xinluzao 45 relied on enhanced electron transport such as photorespiration and the Mehler reaction to dissipate excess light energy under mild and moderate water deficit. Xinhai 21 used enhanced photorespiration for light energy utilisation under mild water deficit but, when subjected to moderate water deficit, possessed a high capacity for dissipating excess light energy via heat dissipation.

Involvement of cyclic electron flow in irradiance stress responding and its potential regulation of the mechanisms in Pyropia yezoensis

Pyropia yezoensis, belongs to the genus of Porphyra before 2011, inhabit on intertidal zone rocks where irradiation changes dramatically, implying that the seaweed has gained certain mechanisms to survive a harsh environment. Based on the photosynthetic parameters with or without the inhibitors determined by a Dual-PAM-100 apparatus, we investigated the photosynthetic performance and the changes in electron flow that occurred during the algae were stressed with different light intensities previously. When the irradiation saturation was approaching, the CEF around PS I became crucial since the addition of inhibitors usually led to an increase in non-photochemical quenching. The inhibitor experiments showed that there were at least three different CEF pathways in Py. yezoensis and these pathways compensated each other. In addition to maintaining a proper ratio of ATP/NAD(P)H to support efficient photosynthesis, the potential roles of CEF might also include the regulation of different photoprotective mechanisms in Py. yezoensis. Under the regulation of CEF, chlororespiration is thought to transport electrons from the reduced plastoquinone (PQ) pool to oxygen in order to mitigate the reduction in the electron transfer chain. When irradiation was up to the high-grade stress conditions, the relative value of CEF began to decrease, which implied that the NADP+ pool or PQ+ pool was very small and that the electrons were transferred from reduced PS I to oxygen. The scavenging enzymes might be activated and the water-water cycle probably became an effective means of removing the active oxygen produced by the irradiation stressed Py. yezoensis. We believe that the different mechanisms could make up the photoprotective network to allow Py. yezoensis for survival in a highly variable light stress habitat, which may enlighten scientists in future studies on irradiance stress in other algae species.

ケニアの樹木は九州の樹木よりも葉の活性酸素消去能が高い―光合成の環境ストレス耐性（メーラー反応の能力）の数値化―

ケニアの樹木は九州の樹木よりも葉の活性酸素消去能が高い―光合成の環境ストレス耐性（メーラー反応の能力）の数値化―

Imbalance between oxygen photoreduction and antioxidant capacities in Symbiodinium cells exposed to combined heat and high light stress

During the last decades, coral reefs have been affected by several large-scale bleaching events, and such phenomena are expected to increase in frequency and severity in the future, thus compromising their survival. High sea surface temperature accompanied by high levels of solar irradiance has been found to be responsible for the induction of oxidative stress ultimately ending with the disruption of the symbiosis between cnidarians and Symbiodinium. For two decades, many studies have pointed to the water–water cycle (WWC) as being one of the primary mediators of this phenomenon, but the impacts of environmental stress on the O2 reduction by PSI and the associated reactive oxygen species (ROS)-detoxifying enzymes remain to be determined. In this study, we analyzed the impacts of acute thermal and light stress on the WWC in the model Symbiodinium strain A1. We observed that the high light treatment at 26 °C resulted in the up-regulation of superoxide dismutase, ascorbate peroxidase, and glutathione reductase activities and an increased production of ROS with no significant change in O2-dependent electron transport. Under high light and at 33 °C, O2-dependent electron transport was significantly increased relative to total electron transport. This increase was concomitant with a twofold increase in ROS generation compared with the treatment at 26 °C, while enzymes involved in the WWC were largely inactivated. These data show for the first time that combined heat and light stress inactivate antioxidant capacities of the WWC and suggests that its photoprotective functions are overwhelmed under these conditions. This study also indicates that cnidarians may be more prone to bleach if they harbor Symbiodinium cells having a highly active Mehler-type electron transport, unless they are able to quickly up-regulate their antioxidant capacities.

Photosynthetic responses and photoprotection strategies of Phacelia secunda plants exposed to experimental warming at different elevations in the central Chilean Andes

The possible effects and implications of climate change on plant growth have not yet been studied in the central Chilean Andes, where a dry growing season differentiates this area from other mountainous areas around the world. At lower elevations, plants exposed to high temperatures and dry soil may experience severe water limitations, causing lower photosynthetic performance. At higher elevations, where water is not a limiting factor but air temperature is lower, an increase in temperature could have a positive effect on photosynthesis. We assessed whether experimental warming affects the photosynthetic performance and photoprotective mechanisms as photorespiration, Mehler reaction and cyclic electron flow in Phacelia secunda plants growing at 1600, 2800 and 3600 m elevation. Plants were exposed to a passive increase in the air temperature using open top chambers (OTC). Xylematic leaf water potential increased with elevation, and the OTC’s decreased xylematic water potential at the lower elevations but not at 3600 m. While warming was associated with a decrease in the maximal carbon gain assimilation at lower elevations, it was associate with an increase at 3600 m. Photorespiration differed among elevations and was increased by warming only at 1600 m. The Mehler reaction was higher at 1600, but not affected by warming. Warming increased the cyclic electron flow only in plants at 1600 m. Plants growing inside OTCs at all three elevations exhibited lower xanthophyll pools than control plants, but similar de-epoxidation states (DEPS). Our results show that alpine plant responses to warming will most likely depend on elevation, soil moisture availability and the combination of these two factors in relation to climate change.

Effects of progressive drought on photosynthesis and partitioning of absorbed light in apple trees

To understand how drought stress affects CO2 assimilation and energy partitioning in apple (Malus domestica Borkh.), we investigated photosynthesis and photo-protective mechanisms when irrigation was withheld from potted Fuji trees. As the drought progressing, soil relative water content (SRWC) decreased from 87 to 24% in 15 d; this combined the decreasing in leaf relative water content (LRWC), net photosynthesis rate (Pn) and stomatal conductance (Gs). However, the concentrations of chlorophylls (Chl) remained unchanged while Pn values were declining. Photochemistry reactions were slightly down-regulated only under severe drought. Rubisco activity was significantly decreased as drought conditions became more severe. The actual efficiency of photosystem II (ϕPSII) was diminished as drought became more intense. Consequently, xanthophyll-regulated dissipation of thermal energy was greatly enhanced. Simultaneously, the ratio of ϕPSII to the quantum yield of carbon metabolism, which is measured under non-photorespiratory conditions, increased in parallel with drought severity. Our results indicate that, under progressive drought stress, the reduction in photosynthesis in apple leaves can be attributed primarily to stomatal limitations and the inhibited capacity for CO2 fixation. Xanthophyll cycle-dependent thermal dissipation and the Mehler reaction are the most important pathways for dispersing excess energy from apple leaves during periods of drought stress.

High light-induced hydrogen peroxide production in Chlamydomonas reinhardtii is increased by high CO2 availability.

The production of reactive oxygen species (ROS) is an unavoidable part of photosynthesis. Stress that accompanies high light levels and low CO2 availability putatively includes enhanced ROS production in the so-called Mehler reaction. Such conditions are thought to encourage O2 to become an electron acceptor at photosystemI, producing the ROS superoxide anion radical (O2·-) and hydrogen peroxide (H2 O2 ). In contrast, here it is shown in Chlamydomonas reinhardtii that CO2 depletion under high light levels lowered cellular H2 O2 production, and that elevated CO2 levels increased H2 O2 production. Using various photosynthetic and mitochondrial mutants of C.reinhardtii, the chloroplast was identified as the main source of elevated H2 O2 production under high CO2 availability. High light levels under low CO2 availability induced photoprotective mechanisms called non-photochemical quenching, or NPQ, including state transitions (qT) and high energy state quenching (qE). The qE-deficient mutant npq4 produced more H2 O2 than wild-type cells under high light levels, although less so under high CO2 availability, whereas it demonstrated equal or greater enzymatic H2 O2 -degrading capacity. The qT-deficient mutant stt7-9 produced the same H2 O2 as wild-type cells under high CO2 availability. Physiological levels of H2 O2 were able to hinder qT and the induction of state2, providing an explanation for why under high light levels and high CO2 availability wild-type cells behaved like stt7-9 cells stuck in state1.

2-cysteine peroxiredoxins and thylakoid ascorbate peroxidase create a water-water cycle that is essential to protect the photosynthetic apparatus under high light stress conditions.

Different peroxidases, including 2-cysteine (2-Cys) peroxiredoxins (PRXs) and thylakoid ascorbate peroxidase (tAPX), have been proposed to be involved in the water-water cycle (WWC) and hydrogen peroxide (H2O2)-mediated signaling in plastids. We generated an Arabidopsis (Arabidopsis thaliana) double-mutant line deficient in the two plastid 2-Cys PRXs (2-Cys PRX A and B, 2cpa 2cpb) and a triple mutant deficient in 2-Cys PRXs and tAPX (2cpa 2cpb tapx). In contrast to wild-type and tapx single-knockout plants, 2cpa 2cpb double-knockout plants showed an impairment of photosynthetic efficiency and became photobleached under high light (HL) growth conditions. In addition, double-mutant plants also generated elevated levels of superoxide anion radicals, H2O2, and carbonylated proteins but lacked anthocyanin accumulation under HL stress conditions. Under HL conditions, 2-Cys PRXs seem to be essential in maintaining the WWC, whereas tAPX is dispensable. By comparison, this HL-sensitive phenotype was more severe in 2cpa 2cpb tapx triple-mutant plants, indicating that tAPX partially compensates for the loss of functional 2-Cys PRXs by mutation or inactivation by overoxidation. In response to HL, H2O2- and photooxidative stress-responsive marker genes were found to be dramatically up-regulated in 2cpa 2cpb tapx but not 2cpa 2cpb mutant plants, suggesting that HL-induced plastid to nucleus retrograde photooxidative stress signaling takes place after loss or inactivation of the WWC enzymes 2-Cys PRX A, 2-Cys PRX B, and tAPX.

Reduced photosynthetic dark reaction triggered by ABA application increases intercellular CO2 concentration, generates H2O2 and promotes closure of stomata in ginger leaves

Exogenous abscisic acid application decreased stomatal conductance (Gs), net photosynthetic rate (Pn), photorespiration rate (Pr), carboxylation efficiency (CE), and actual photochemical efficiency (ΦPSII) but increased the Mehler reaction, antioxidant enzyme activities and H2O2 content in ginger leaves. The observed reopening of the stomata triggered by varying concentrations of external CO2 indicates that increased intercellular CO2 concentration (caused by decreased photosynthetic dark reaction activity) promotes stomatal closure under ABA content regulation. By comparing the relationships between H2O2 content, stomatal conductance and ABA content under well-watered conditions compared to osmotic drought stress conditions, we concluded that increased H2O2 content caused by exogenous ABA application also promoted stomata closure in ginger leaves. Thus, CO2 and H2O2 signalling promote more complete stomatal closure under the influence of ABA in ginger leaves.

Expression of plant ferredoxin-like protein (PFLP) enhances tolerance to heat stress in Arabidopsis thaliana

Under adverse environments, plants produce reactive oxygen species (ROS), which can trigger cell death when their accumulation surpasses the antioxidant capacity of ROS scavenging systems. These systems function in chloroplasts mainly through the ascorbate-mediated water-water cycle, in which ascorbate is photoreduced by ferredoxin in the photosynthetic system. Our previous study showed that the fraction of the reduced form of ascorbate was increased in ferredoxin-transgenic Arabidopsis (CPF) plants which overexpressed plant ferredoxin-like protein (PFLP) in their chloroplasts. Thus, we hypothesized that expression of PFLP could alter the tolerance of plants to abiotic stresses through increasing reduced form of ascorbate. In this study, we found that two CPF lines exhibited lower mortality rates at five days, following two days of heat treatment. Compared to non-transgenic wild type (Col-0) plants, CPF plants exhibited decreased H2O2 content, MDA accumulation, and ion leakage after heat treatment. To confirm the efficacy of ferredoxin against heat stress in chloroplasts, we evaluated two RNA interference (RNAi) lines on two endogenous ferredoxin isoforms, Atfd1 or Atfd2, of Arabidopsis plants. Both lines not only decreased their amounts of ascorbate, but also exhibited adverse reactions following heat treatment. Based on these results, we conclude that expression of PFLP in chloroplasts can confer tolerance to heat stress. This tolerance might be associated with the increasing of ascorbate in plants.