Enhanced photochemical light utilization and decreased chilling-induced photoinhibition of photosystem II in cotton overexpressing genes encoding chloroplast-targeted antioxidant enzymes

Achieving abiotic stress tolerance in plants through antioxidative defense mechanisms

Climate change has increased the overall impact of abiotic stress conditions such as drought, salinity, and extreme temperatures on plants. Abiotic stress adversely affects the growth, development, crop yield, and productivity of plants. When plants are subjected to various environmental stress conditions, the balance between the production of reactive oxygen species and its detoxification through antioxidant mechanisms is disturbed. The extent of disturbance depends on the severity, intensity, and duration of abiotic stress. The equilibrium between the production and elimination of reactive oxygen species is maintained due to both enzymatic and non-enzymatic antioxidative defense mechanisms. Non-enzymatic antioxidants include both lipid-soluble (α-tocopherol and β-carotene) and water-soluble (glutathione, ascorbate, etc.) antioxidants. Ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR) are major enzymatic antioxidants that are essential for ROS homeostasis. In this review, we intend to discuss various antioxidative defense approaches used to improve abiotic stress tolerance in plants and the mechanism of action of the genes or enzymes involved.

Effect of light quality on the growth and main chemical composition of Bletilla striata

This study aimed to investigate the effects of light quality on the morphological traits, photosynthetic pigment content, protective enzyme (superoxide dismutase, peroxidase, and catalase) activity, and bioactive compound (BSP, total phenol, and militarine) content in Bletilla striata. Plants of B. striata were grown under light filtered through three differently colored films. The treatments were red film (RF), yellow film (YF), and blue film (BF), and an uncovered treatment was included as a control (CK). Compared with the B. striata plants in the RF, YF, and CK treatment groups, those receiving BF treatment showed significantly promoted growth of the aerial parts. Meanwhile, the total phenol and militarine contents in B. striata tubers were increased without affecting the accumulation of B. striata polysaccharides. These results show that growing B. striata plants under blue film could be a useful technique to improve quality and production. This technique is conducive to achieving large-scale sustainable production of high-quality plant materials.

CfAPX, a cytosolic ascorbate peroxidase gene from Cryptomeria fortunei, confers tolerance to abiotic stress in transgenic Arabidopsis

Plants subjected to biotic or abiotic stresses produce a large amount of reactive oxygen species (ROS). If ROS cannot be cleared in time, they cause a series of harmful reactions in plants. Ascorbate peroxidase (APX) is a key enzyme that removes ROS from plant cells and plays a vital role in plant stress resistance. However, to date, no studies on APX homologs in Cryptomeria fortunei have been reported. In this study, we isolated complementary DNA (cDNA) encoding APXfrom C. fortunei needles, which is referred to as CfAPX, by rapid amplification of cDNA ends (RACE). The full-length CfAPX sequence was 1226 bp in length and included a 750-bp open reading frame (ORF) encoding a protein of 249 amino acids. Phylogenetic analysis showed that APXs of different plant species have been highly evolutionarily conserved. CfAPX was shown to belong to the cytoplasmic subgroup and was more closely related to GbAPX of the gymnosperm Ginkgo biloba. CfAPX showed no transcriptional activity in yeast cells but was highly expressed in cones. To better handle abiotic stresses, compared with wild-type (WT) Arabidopsis thaliana, 35S::CfAPX transgenic Arabidopsis strongly expressed CfAPX, presented increased antioxidant enzyme activities, ascorbic acid (AsA) contents, chlorophyll levels and fluorescence parameter and reduced malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) contents. In addition, CfAPX expression in C. fortunei was mostly upregulated under stress. In summary, CfAPX confers abiotic stress responses to plants, which provides a scientific basis for subsequent breeding for increased stress resistance in C. fortunei.

CBF transcription factors involved in the cold response of Camellia japonica (Naidong)

CBFs belong to the ERF subfamily of the AP2 supergene family and often play an important role in the cold acclimation of temperate plants. However, the role of CBFs in Camellia japonica (Naidong), the only Camellia japonica population found in the temperate zones of China, remains unclear. It is very important to study the genetic composition of C. japonica (Naidong) to adapt to low temperature for Camellia species. Using full-length transcriptome data, we identified four CjCBF genes that respond to cold stress and analyzed their evolutionary relationships, domains, and expression patterns. The phylogeny of CBFs of 19 angiosperms divided the genes into three categories, and the four CjCBFs belong to a small subcluster. The strong response of CjCBF1 to cold treatment and its sustained high level of expression indicated that it plays an important role in the process of cold acclimation. A yeast two-hybrid assay revealed an interaction between CjCBF1, CjCBF2, and CjCBF5, and subcellular localization confirmed this finding. The expression of CjCBFs was tissue-specific: CBF1 was mainly expressed in leaves, and CBF3 was mainly expressed in stem. The responses of the four CjCBFs to drought and high temperature and the effect of light were also characterized. Our study provides new insight into the role of CBFs in the cold response in C. japonica (Naidong).

Evolutionary history of the C-repeat binding factor/dehydration-responsive element-binding 1 (CBF/DREB1) protein family in 43 plant species and characterization of CBF/DREB1 proteins in Solanum tuberosum

Background

Plants are easily affected by temperature variations, and high temperature (heat stress) and low temperature (cold stress) will lead to poor plant development and reduce crop yields. Therefore, it is very important to identify resistance genes for improving the ability of plants to resist heat stress or cold stress by using modern biotechnology. Members of the C-repeat binding factor/Dehydration responsive element-binding 1 (CBF/DREB1) protein family are related to the stress resistance of many plant species. These proteins affect the growth and development of plants and play vital roles during environmental stress (cold, heat, drought, salt, etc.). In this study, we identified CBF/DREB1 genes from 43 plant species (including algae, moss, ferns, gymnosperms, angiosperms) by using bioinformatic methods to clarify the characteristics of the CBF/DREB1 protein family members and their functions in potato under heat and cold stresses.

Results

In this study, we identified 292 CBF/DREB1 proteins from 43 plant species. However, no CBF/DREB1 protein was found in algae, moss, ferns, or gymnosperms; members of this protein family exist only in angiosperms. Phylogenetic analysis of all the CBF/DREB1 proteins revealed five independent groups. Among them, the genes of group I do not exist in eudicots and are found only in monocots, indicating that these genes have a special effect on monocots. The analysis of motifs, gene duplication events, and the expression data from the PGSC website revealed the gene structures, evolutionary relationships, and expression patterns of the CBF/DREB1 proteins. In addition, analysis of the transcript levels of the 8 CBF/DREB1 genes in potato (Solanum tuberosum) under low-temperature and high-temperature stresses showed that these genes were related to temperature stresses. In particular, the expression levels of StCBF3 and StCBF4 in the leaves, stems, and roots significantly increased under high-temperature conditions, which suggested that StCBF3 and StCBF4 may be closely related to heat tolerance in potato.

Conclusion

Overall, members of the CBF/DREB1 protein family exist only in angiosperms and plays an important role in the growth and development of plants. In addition, the CBF/DREB1 protein family is related to the heat and cold resistance of potato. Our research revealed the evolution of the CBF/DREB1 family, and is useful for studying the precise functions of the CBF/DREB1 proteins when the plants are developing and are under temperature stress.

Selection and Validation of Reference Genes for Quantitative Real-Time PCR in White Clover (Trifolium repens L.) Involved in Five Abiotic Stresses

White clover (Trifolium repens L.) is a widely cultivated cool-season perennial forage legume in temperate grassland systems. Many studies have analyzed the gene expression in this grass species using quantitative real-time reverse transcription PCR (qRT-PCR). The selection of stable reference genes for qRT-PCR is crucial. However, there was no detailed study on reference genes in different tissues of white clover under various abiotic stress conditions. Herein, 14 candidate reference genes (ACT7, ACT101, TUA1109, TUB, CYP, 60SrRNA, UBQ, E3, GAPDH1, GAPDH2, PP2A, BAM3, SAMDC, and ABC) were selected and analyzed by four programs (GeNorm, NormFinder, BestKeeper, and RefFinder). Samples were taken from two tissues (leaves and roots) under five different abiotic stresses (drought, salt, heat, cold, and heavy metal stress). Our results showed that 60SrRNA and ACT101 were the two top-ranked genes for all samples. Under various experimental conditions, the most stable gene was different; however, SAMDC, UBQ, 60SrRNA, and ACT101 were always top ranked. The most suitable reference genes should be selected according to different plant tissues and growth conditions. Validation of these reference genes by expression analysis of Cyt-Cu/Zn SOD and CAT confirmed their reliability. Our study will benefit the subsequent research of gene function in this species.

How do vascular plants perform photosynthesis in extreme environments? An integrative ecophysiological and biochemical story

In this work, we review the physiological and molecular mechanisms that allow vascular plants to perform photosynthesis in extreme environments, such as deserts, polar and alpine ecosystems. Specifically, we discuss the morpho/anatomical, photochemical and metabolic adaptive processes that enable a positive carbon balance in photosynthetic tissues under extreme temperatures and/or severe water-limiting conditions in C₃ species. Nevertheless, only a few studies have described the in situ functioning of photoprotection in plants from extreme environments, given the intrinsic difficulties of fieldwork in remote places. However, they cover a substantial geographical and functional range, which allowed us to describe some general trends. In general, photoprotection relies on the same mechanisms as those operating in the remaining plant species, ranging from enhanced morphological photoprotection to increased scavenging of oxidative products such as reactive oxygen species. Much less information is available about the main physiological and biochemical drivers of photosynthesis: stomatal conductance (g_s ), mesophyll conductance (g_m ) and carbon fixation, mostly driven by RuBisCO carboxylation. Extreme environments shape adaptations in structures, such as cell wall and membrane composition, the concentration and activation state of Calvin-Benson cycle enzymes, and RuBisCO evolution, optimizing kinetic traits to ensure functionality. Altogether, these species display a combination of rearrangements, from the whole-plant level to the molecular scale, to sustain a positive carbon balance in some of the most hostile environments on Earth.

Is leaf age a predictor for cold tolerance in winter oilseed rape plants?

In the present study, low temperature-dependent physiological changes were investigated through photosynthetic activity and some endogenous mechanisms in two winter oilseed rape cultivars (Brassica napus L. ssp. oleifera cvv. Eurol and Hansen) on the basis of leaf age. Chlorophyll fluorescence measurements demonstrated that low temperature caused decreased photosynthetic activity in both cultivars. However, photosynthetic apparatus in the young leaves of Hansen is more tolerant to low temperature as demonstrated by lower F0 (minimum fluorescence yield) and 1-qp (excitation pressure of photosystem II), higher Fm (maximum fluorescence yield), Fv/Fm and non-photochemical quenching (NPQ) compared with Eurol. In addition, young leaves of Hansen represented marked increase in some antioxidant enzyme activities (superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR)) during cold exposure. In the young leaves of Eurol, however, APX and GR activity was decreased by low temperature, indicating lower efficiency of ascorbate-glutathione cycle. Lower antioxidant activity in the young leaves of Eurol may be responsible for increased malondialdehyde (MDA), H2O2 and membrane damage and decreased chlorophyll content as a result of oxidative damage during cold exposure. In the cold-stressed mature leaves, both cultivars represented similar antioxidant capacities and photosynthetic efficiency. As a consequence, coordinated increase in SOD, APX and GR activities, increased capacity to keep quinone A (QA) in an oxidised state (as indicated by lower 1-qp) and accumulation of soluble sugar and proline could be mainly attributed to higher level of tolerance of the young leaves of Hansen to low temperature when compared with Eurol.

Overexpression of transketolase gene promotes chilling tolerance by increasing the activities of photosynthetic enzymes, alleviating oxidative damage and stabilizing cell structure in Cucumis sativus L

Despite being a key enzyme of Cavin cycle, transketolase (TK) is believed to be related to abiotic resistance in higher plants. However, how TK affects chilling tolerance still remains largely unknown. Here, we describe the effect of overexpression of the Cucumis sativa TK gene (CsTK) on growth, photosynthesis, ROS metabolism and cell ultrastructure under chilling stress. Low temperature led to a decrease of the photosynthetic rate (Pn), the stomatal conductance (Gs), the actual photochemical efficiency (ΦPSII) and the sucrose content, whereas there was an increase of the intercellular CO₂ concentration (Ci) and MDA content. These changes were alleviated in the CsTK plants after 5 days of chilling stress, however, inhibition of CsTK showed the opposite results. Furthermore, transgenic plants with overexpression of CsTK showed higher increase in leaf area and dry matter, higher activity of the enzymes and higher increase in the contents of metabolism substance involved in Calvin cycle and reactive oxygen scavenging system as well as lower ^• OH and H₂ O₂ content, superoxide anion production rate compared with the control cucumber plants under chilling stress. At the end of the chilling stress, compared to wild-type (WT) which exhibited dramatically destroyed cell ultrastructure, expanded chloroplast, broken cell and chloroplast membranes as well as the disappeared grana lamella, the CsTK sense plants showed a more complete cell ultrastructure, whereas, the damage of the cell ultrastructure was aggravated in CsTK antisense plants. Taken together, these results imply that CsTK promoted chilling tolerance in cucumber plants mainly through increasing the capacity to assimilate carbon, alleviating oxidative damage and stabilizing cell structure.

Chlorophyll fluorescence parameters to assess utilization of excitation energy in photosystem II independently of changes in leaf absorption

Measurement of Pulse-Amplitude-Modulated (PAM) chlorophyll a fluorescence is widely used method for obtaining information on the functional state of photosystem II (PSII). Recently, it has been shown that some of long-established fluorescence parameters must be interpreted with caution, when the light-induced chloroplast movements occur. In our work we have analyzed the effect of chloroplast movements on these parameters. We have derived new parameters that are independent of the change in PSII absorption occurring during measurement. To verify whether there is a need for new parameters or the difference between the parameters commonly used and the newly derived ones is insignificant, we conducted an experiment with Arabidopsis thaliana wild type plants and its phot1 phot2 mutant defective in chloroplast movement. Plants were exposed to light of different qualities (450, 470, 550 or 660 nm) and quantities (100, 400 or 1200 μmol m^-2 s^-1) for up to 40 min. Since the blue light-induced chloroplast avoidance reaction is a photoprotective mechanism, we expected that phot1 phot2 mutant will compensate the lack of this mechanism by increasing non-photochemical quenching. However, using the light at both 450 and 470 nm, the calculation of commonly used parameter, Φ_NPQ (quantum yield of regulated light-induced thermal energy dissipation in PSII) based on Hendrickson et al. [L. Hendrickson, R.T. Furbank, W.S. Chow, Photosynth. Res. 82 (2004) 73-81] showed the opposite. On the other hand, the results obtained using our newly proposed formulae to determine quantum yield of PSII thermal energy dissipation were in line with our assumption. Thus, the experimental data showed that some formulae of fluorescence parameters are dependent on the change in PSII absorption and need to be interpreted carefully. On the contrary, the formulae introduced by us can remove the effect of changes in PSII absorption that occur during measurement, without additional measurements, and give the real estimate of light-induced non-photochemical quenching.

OJIP-fluorescence parameters as rapid indicators of cotton (Gossypium hirsutum L.) seedling vigor under contrasting growth temperature regimes

Vigorous seedling growth in cotton is desirable because it minimizes the negative impact of multiple early season stresses, and seedling vigor can be impacted by early season growth temperature or cultivar. OJIP fluorescence provides rapid information on a broad range of photosynthetic component processes and may be a useful surrogate for seeding vigor, but this possibility has not been evaluated previously in cotton. To this end, a controlled environment study was conducted with six cultivars selected based on seed characteristics that are widely indicative of vigor and under two growth temperature regimes (sub-optimal = 20/15 °C day/night temperature; optimal = 30/20 °C) for the first two weeks after seed germination. Thereafter multiple whole-plant vigor assessments were conducted along with extensive OJIP-fluorescence characterization in cotyledons. Growth temperature was the primary factor influencing multiple plant responses. Specifically, all whole-plant indicators of seedling vigor were negatively impacted by sub-optimal temperature as were all photosynthetic performance indices and quantum efficiencies. By comparison, most photosynthetic structural indicators or reaction center-specific fluxes were either unaffected or positively impacted by low growth temperature, largely because PSII antenna size increased. The performance index, PI_ABS, and the quantum efficiency, φ_Eo, were the most sensitive to low growth temperature and exhibited the strongest relationships with whole-plant seedling vigor. Thus, OJIP parameters incorporating intersystem electron transport beyond PSII but not additional downstream processes may represent the most useful surrogates for whole-plant seedling vigor in cotton.

A Novel L-ascorbate Peroxidase 6 Gene, ScAPX6, Plays an Important Role in the Regulation of Response to Biotic and Abiotic Stresses in Sugarcane

The L-ascorbate peroxidase 6 gene (APX6) is one of the most important genes for scavenging H₂O₂ and plays a vital role in plant resistance to environmental stresses. In this study, a novel ScAPX6 gene (GenBank Accession No. KT907352) was obtained from a sugarcane variety (ROC22). Bioinformatics analysis showed that ScAPX6 has a cDNA length of 1,086 bp and encoded 333 amino acid residues. Subcellular localization confirmed that ScAPX6 was located in the chloroplast. Enhanced growth of Escherichia coli BL21 cells that expressed ScAPX6 showed high tolerance under copper (Cu) stress. Real-time quantitative PCR analysis revealed that ScAPX6 was constitutively expressed wherein with the highest expression levels in sugarcane pith and leaf and the lowest in the root. ScAPX6 was down-regulated by salicylic acid (SA), hydrogen peroxide (H₂O₂), polyethylene glycol (PEG) and sodium chloride (NaCl) stimuli. Interestingly, it was significantly up-regulated under the stresses of abscisic acid (ABA) and methyl jasmonate (MeJA) wherein with the highest inducible expression levels at 6 h at 6.0- and 70.0-times higher, respectively than that of control. Overexpression of ScAPX6 in Nicotiana benthamiana leaves enhanced the resistance to the infection of tobacco pathogens Pseudomonas solanacearum and Fusarium solani var. coeruleum. These results implied that ScAPX6 might positively respond to ABA, MeJA, and Cu, but might negatively respond to the stresses of SA, H₂O₂, PEG, and NaCl. Keeping in view the current investigation, ScAPX6 could be associated with the hypersensitive response (HR) or immunity of sugarcane, which will provide a baseline for the function identification of sugarcane ScAPX6.

Overexpression of CaAPX Induces Orchestrated Reactive Oxygen Scavenging and Enhances Cold and Heat Tolerances in Tobacco

Ascorbate peroxidase (APX) acts indispensably in synthesizing L-ascorbate (AsA) which is pivotal to plant stress tolerance by detoxifying reactive oxygen species (ROS). Enhanced activity of APX has been shown to be a key step for genetic engineering of improving plant tolerance. However it needs a deeper understanding on the maintenance of cellular ROS homeostasis in response to stress. In this study, we identified and characterized an APX (CaAPX) gene from Camellia azalea. Quantitative real-time PCR (qRT-PCR) analysis showed that CaAPX was expressed in all tissues and peaked in immature green fruits; the expression levels were significantly upregulated upon cold and hot stresses. Transgenic plants displayed marked enhancements of tolerance under both cold and heat treatments, and plant growth was correlated with CaAPX expression levels. Furthermore, we monitored the activities of several ROS-scavenging enzymes including Cu/Zn-SOD, CAT, DHAR, and MDHAR, and we showed that stress tolerance was synchronized with elevated activities of ROS-scavenging. Moreover, gene expression analysis of ROS-scavenging enzymes revealed a role of CaAPX to orchestrate ROS signaling in response to temperature stresses. Overall, this study presents a comprehensive characterization of cellular response related to CaAPX expression and provides insights to breed crops with high temperature tolerances.

Antioxidant Enzyme Responses Induced by Whiteflies in Tobacco Plants in Defense against Aphids: Catalase May Play a Dominant Role

BACKGROUND

Bemisia tabaci MEAM1 (Middle East-Asia Minor 1) feeding alters antioxidative enzyme activity in some plant species. Infestation of B. tabaci nymphs decreases Myzus persicae performance on systemic, but not local leaves of tobacco plants. However, it is unclear if B. tabaci nymphs induced antioxidant activities contributing to the aphid resistance.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the relationship between antioxidants induced by nymphs of B. tabaci feeding on tobacco and aphid resistance. The activities of catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) and the concentration of hydrogen peroxide (H2O2) were assayed in tobacco leaves at different feeding times following infestation of B. tabaci nymphs. The infestation altered the activities of CAT and POD, but had no significant effect on SOD activity. The highest CAT activity was observed at 15 d after infestation. This was 98.2% greater than control systemic leaves, but 32.6% lower than the control in local leaves. Higher POD activity was recorded in local vs. systemic leaves after 15 d of infestation. POD activity was 71.0% and 112.9% higher in local and systemic leaves, respectively, than in the controls. The changes of CAT, but not POD or SOD activity were correlated to levels of aphid resistance. H2O2 levels were higher in local than in systemic leaves in contrast to CAT activity. Tobacco curly shoot virus mediated virus-induced gene silencing was employed to determine if CAT activation was involved in the aphid resistance induced by B. tabaci nymphs. B. tabaci induced CAT activity decreased when the Cat1 expression was silenced. The performance assay indicated that Cat1 silencing made B. tabaci infested plants a more suitable host for aphids than infested control plants. The aphid survival rate was reduced by 40.4% in infested control plants, but reduced by only 26.1% in Cat1-silenced plants compared to uninfested controls. Also, qPCR results showed that silencing of Cat1 led to the suppression of the B. tabaci mediated PR-2a expression.

CONCLUSIONS/SIGNIFICANCE

Aphid resistance in plants infested with B. tabaci nymphs is associated with enhanced antioxidant activities in which CAT may play a dominant role. This resistance probably acted via interactions with SA-mediated defense responses.

Parameters of photosynthetic energy partitioning

Almost every laboratory dealing with plant physiology, photosynthesis research, remote sensing, and plant phenotyping possesses a fluorometer to measure a kind of chlorophyll (Chl) fluorescence induction (FLI). When the slow Chl FLI is measured with addition of saturating pulses and far-red illumination, the so-called quenching analysis followed by the so-called relaxation analysis in darkness can be realized. These measurements then serve for evaluation of the so-called energy partitioning, that is, calculation of quantum yields of photochemical and of different types of non-photochemical processes. Several theories have been suggested for photosynthetic energy partitioning. The current work aims to summarize all the existing theories, namely their equations for the quantum yields, their meaning and their assumptions. In the framework of these theories it is also found here that the well-known NPQ parameter ( [Formula: see text] ; Bilger and Björkman, 1990) equals the ratio of the quantum yield of regulatory light-induced non-photochemical quenching to the quantum yield of constitutive non-regulatory non-photochemical quenching (ΦNPQ/Φf,D). A similar relationship is also found here for the PQ parameter (ΦP/Φf,D).

Ectopic expression of Arabidopsis RCI2A gene contributes to cold tolerance in tomato

Cold is a major stress that limits the quality and productivity of economically important crops such as tomato (Solanum lycopersicum L.). Generating a cold-stress-tolerant tomato by expressing cold-inducible genes would increase agricultural strategies. Rare cold-inducible 2a (RCI2A) is expressed in Arabidopsis, but its molecular function during cold stress is not fully understood. Here we ectopically expressed Arabidopsis RCI2A in transgenic tomato to evaluate tolerance to cold stress without altering agronomic traits. Biochemical and physiological study demonstrated that expression of RCI2A in transgenic tomato enhanced the activity of peroxidase and ascorbate peroxidase (APX) and reduced the accumulation of H2O2, alleviated lipid peroxidation, increased the accumulation of chlorophyll, reduced chilling-induced membrane damage, retained relative water content and enhanced cold tolerance. A motif search revealed that the motifs of photosystem II (PSII) phosphoproteins PsbJ and PsbH and reaction-center proteins PsbL and PsbK were common to cold-inducible RCI2A and peroxidase proteins RCI3A, tomato peroxidase (TPX1), TPX2, tomato ascorbate peroxidase (APX1), and horseradish peroxidase (HRP-c). In addition to membrane protection, RCI2A may cross talk with PSII-associated proteins or peroxidase family enzymes in response to cold stress. Our findings may strengthen the understanding of the molecular function of RCI2A in cold-stress tolerance. RCI2A could be used to improve abiotic stress tolerance in agronomic crops.

Revised scheme for the mechanism of photoinhibition and its application to enhance the abiotic stress tolerance of the photosynthetic machinery

When photosynthetic organisms are exposed to abiotic stress, their photosynthetic activity is significantly depressed. In particular, photosystem II (PSII) in the photosynthetic machinery is readily inactivated under strong light and this phenomenon is referred to as photoinhibition of PSII. Other types of abiotic stress act synergistically with light stress to accelerate photoinhibition. Recent studies of photoinhibition have revealed that light stress damages PSII directly, whereas other abiotic stresses act exclusively to inhibit the repair of PSII after light-induced damage (photodamage). Such inhibition of repair is associated with suppression, by reactive oxygen species (ROS), of the synthesis of proteins de novo and, in particular, of the D1 protein, and also with the reduced efficiency of repair under stress conditions. Gene-technological improvements in the tolerance of photosynthetic organisms to various abiotic stresses have been achieved via protection of the repair system from ROS and, also, by enhancing the efficiency of repair via facilitation of the turnover of the D1 protein in PSII. In this review, we summarize the current status of research on photoinhibition as it relates to the effects of abiotic stress and we discuss successful strategies that enhance the activity of the repair machinery. In addition, we propose several potential methods for activating the repair system by gene-technological methods.

Cloning and functional analysis of a novel ascorbate peroxidase (APX) gene from Anthurium andraeanum

An 888-bp full-length ascorbate peroxidase (APX) complementary DNA (cDNA) gene was cloned from Anthurium andraeanum, and designated as AnAPX. It contains a 110-bp 5'-noncoding region, a 28-bp 3'-noncoding region, and a 750-bp open reading frame (ORF). This protein is hydrophilic with an aliphatic index of 81.64 and its structure consisting of α-helixes, β-turns, and random coils. The AnAPX protein showed 93%, 87%, 87%, 87%, and 86% similarities to the APX homologs from Zantedeschia aethiopica, Vitis pseudoreticulata, Gossypium hirsutum, Elaeis guineensis, and Zea mays, respectively. AnAPX gene transcript was measured non-significantly in roots, stems, leaves, spathes, and spadices by real-time polymerase chain reaction (RT-PCR) analysis. Interestingly, this gene expression was remarkably up-regulated in response to a cold stress under 6 °C, implying that AnAPX might play an important role in A. andraeanum tolerance to cold stress. To confirm this function we overexpressed AnAPX in tobacco plants by transformation with an AnAPX expression construct driven by CaMV 35S promoter. The transformed tobacco seedlings under 4 °C showed less electrolyte leakage (EL) and malondialdehyde (MDA) content than the control. The content of MDA was correlated with chilling tolerance in these transgenic plants. These results show that AnAPX can prevent the chilling challenged plant from cell membrane damage and ultimately enhance the plant cold tolerance.

Enhanced sensitivity and characterization of photosystem II in transgenic tobacco plants with decreased chloroplast glutathione reductase under chilling stress

Chloroplast glutathione reductase (GR) plays an important role in protecting photosynthesis against oxidative stress. We used transgenic tobacco (Nicotiana tabacum) plants with severely decreased GR activities by using a gene encoding tobacco chloroplast GR for the RNAi construct to investigate the possible mechanisms of chloroplast GR in protecting photosynthesis against chilling stress. Transgenic plants were highly sensitive to chilling stress and accumulated high levels of H₂O₂ in chloroplasts. Spectroscopic analysis and electron transport measurements show that PSII activity was significantly reduced in transgenic plants. Flash-induced fluorescence relaxation and thermoluminescence measurements demonstrate that there was a slow electron transfer between Q(A) and Q(B) and decreased redox potential of Q(B) in transgenic plants, whereas the donor side function of PSII was not affected. Immunoblot and blue native gel analyses illustrate that PSII protein accumulation was decreased greatly in transgenic plants. Our results suggest that chloroplast GR plays an important role in protecting PSII function by maintaining the electron transport in PSII acceptor side and stabilizing PSII complexes under chilling stress. Our results also suggest that the recycling of ascorbate from dehydroascorbate in the ascorbate-glutathione cycle in the chloroplast plays an essential role in protecting PSII against chilling stress.

Heat stress results in loss of chloroplast Cu/Zn superoxide dismutase and increased damage to Photosystem II in combined drought-heat stressed Lotus japonicus

Drought and heat stress have been studied extensively in plants, but most reports involve analysis of response to only one of these stresses. Studies in which both stresses were studied in combination have less commonly been reported. We report the combined effect of drought and heat stress on Photosystem II (PSII) of Lotus japonicus cv. Gifu plants. Photochemistry of PSII was not affected by drought or heat stress alone, but the two stresses together decreased PSII activity as determined by fluorescence emission. Heat stress alone resulted in degradation of D1 and CP47 proteins, and D2 protein was also degraded by combined drought-heat stress. None of these proteins were degraded by drought stress alone. Drought alone induced accumulation of hydrogen peroxide but the drought-heat combination led to an increase in superoxide levels and a decrease in hydrogen peroxide levels. Furthermore, combined drought-heat stress was correlated with an increase in oxidative damage as determined by increased levels of thiobarbituric acid reactive substances. Heat also induced degradation of chloroplast Cu/Zn superoxide dismutase (SOD: EC 1.15.1.1) as shown by reduced protein levels and isozyme-specific SOD activity. Loss of Cu/Zn SOD and induction of catalase (CAT: EC 1.11.1.6) activity would explain the altered balance between hydrogen peroxide and superoxide in response to drought vs combined drought-heat stress. Degradation of PSII could thus be caused by the loss of components of chloroplast antioxidant defence systems and subsequent decreased function of PSII. A possible explanation for energy dissipation by L. japonicus under stress conditions is discussed.

Redox regulation in photosynthetic organisms: signaling, acclimation, and practical implications

Reactive oxygen species (ROS) have multifaceted roles in the orchestration of plant gene expression and gene-product regulation. Cellular redox homeostasis is considered to be an "integrator" of information from metabolism and the environment controlling plant growth and acclimation responses, as well as cell suicide events. The different ROS forms influence gene expression in specific and sometimes antagonistic ways. Low molecular antioxidants (e.g., ascorbate, glutathione) serve not only to limit the lifetime of the ROS signals but also to participate in an extensive range of other redox signaling and regulatory functions. In contrast to the low molecular weight antioxidants, the "redox" states of components involved in photosynthesis such as plastoquinone show rapid and often transient shifts in response to changes in light and other environmental signals. Whereas both types of "redox regulation" are intimately linked through the thioredoxin, peroxiredoxin, and pyridine nucleotide pools, they also act independently of each other to achieve overall energy balance between energy-producing and energy-utilizing pathways. This review focuses on current knowledge of the pathways of redox regulation, with discussion of the somewhat juxtaposed hypotheses of "oxidative damage" versus "oxidative signaling," within the wider context of physiological function, from plant cell biology to potential applications.

Triple mutation of Cys26, Trp35, and Cys126 in stromal ascorbate peroxidase confers H2O2 tolerance comparable to that of the cytosolic isoform

Ascorbate peroxidase (APX) isoforms localized in the stroma and thylakoid of the chloroplast play a principle role in detoxifying hydrogen peroxide (H(2)O(2)) generated in photosystem I; however, once the ascorbate is depleted, the enzyme is attacked by H(2)O(2) and rapidly loses its activity. Here, we report that radical transfer across the porphyrin moiety and amino acid residues in the reaction intermediate and H(2)O(2)-mediated enzyme inactivation involve cooperative interactions of the Cys26, Trp35, and Cys126 residues of stromal APX. The wild-type enzyme had a half-time of inactivation of <10s, while the triple mutant of the three residues retained 50% of the initial activity after H(2)O(2) treatment for 3 min. The H(2)O(2) tolerance of this mutant was comparable to that of the H(2)O(2)-tolerant APX isoform localized in the cytosol.

Research note: Energy partitioning in photosystem II complexes subjected to photoinhibitory treatment

Chlorophyll a fluorescence measured in vivo is frequently used to study the role of different processes influencing the distribution of excitation energy in PSII complexes. Such studies are important for understanding the regulation of photosynthetic electron transport. However, at the present time, there is no unified methodology to analyse the energy partitioning in PSII. In this article, we critically assess several approaches recently developed in this area of research and propose new simple equations, which can be used for de-convolution of non-photochemical energy quenching in PSII complexes.

The role of antioxidant enzymes in photoprotection

The enzymatic component of the antioxidant system is discussed as one of the defensive mechanisms providing protection against excessive light absorption in plants. We present an analysis of attempts to improve stress tolerance by means of the creation of transgenic plants with elevated antioxidant enzyme activities and conclude that the effect of such transgenic manipulation strongly depends on the manner in which the stress is imposed. The following factors may diminish the differences in photosynthetic performance between transgenic plants and wild type under field conditions: effective functioning of the thermal dissipation mechanisms providing a primary line of defense against excessive light, long-term adjustments of the antioxidant system and other photoprotective mechanisms, the relatively low level of control over electron transport exerted by the Water-Water cycle, especially under warm conditions, and a decrease in the content of the transgenic product during leaf aging.

Compensation for PSII Photoinactivation by Regulated Non-photochemical Dissipation Influences the Impact of Photoinactivation on Electron Transport and CO2 Assimilation

The extent to which PSII photoinactivation affects electron transport (PhiPSII) and CO2 assimilation remains controversial, in part because it frequently occurs alongside inactivation of other components of photosynthesis, such as PSI. By manipulating conditions (darkness versus low light) after a high light/low temperature treatment, we examined the influence of different levels of PSII inactivation at the same level of PSI inactivation on PhiPSII and CO2 assimilation for Arabidopsis. Furthermore, we compared PhiPSII at high light and optimum temperature for wild-type Arabidopsis and a mutant (npq4-1) with impaired capacities for energy dissipation. Levels of PSII inactivation typical of natural conditions (< 50%) were not associated with decreases in PhiPSII and CO2 assimilation at photon flux densities (PFDs) above 150 micromol m(-2) s(-1). At higher PFDs, the light energy being absorbed was in excess of the energy that could be utilized by downstream processes. Arabidopsis plants downregulate PSII activity to dissipate such excess in accordance with the level of PSII photoinactivation that also serves to dissipate absorbed energy. Therefore, the overall levels of non-photochemical dissipation and the efficiency of photochemistry were not affected by PSII inactivation at high PFD. Under low PFD conditions, such compensation is not necessary, because the amount of light energy absorbed is not in excess of that needed for photochemistry, and inactive PSII complexes are dissipating energy. We conclude that moderate photoinactivation of PSII complexes will only affect plant performance when periods of high PFD are followed by periods of low PFD.

Impacts of drought and temperature stress on coffee physiology and production: a review

Overall, drought and unfavourable temperatures are the major climatic limitations for coffee production. These limitations are expected to become increasingly important in several coffee growing regions due to the recognized changes in global climate, and also because coffee cultivation has spread towards marginal lands, where water shortage and unfavourable temperatures constitute major constraints to coffee yield. In this review, we examine the impacts of such limitations on the physiology, and consequently on the production of mainly Coffea arabica and C. canephora, which account for about 99 % of the world coffee bean production. The first section deals with climatic factors and the coffee plant’s requirements. The importance of controlling oxidative stress for the expression of drought and cold tolerance abilities is emphasized in the second section. In the third section, we examine the impacts of drought on cell-water relations, stomatal behaviour and water use, photosynthesis and crop yield, carbon and nitrogen metabolism, root growth and characteristics, and on drought tolerance. In the fourth section, the impacts of low positive and high temperatures on coffee physiology are discussed; some insights about effects of negative temperatures are also presented. Finally, the last section deals with shading in harsh environments as a mean of buffering climatic fluctuations, as well as of increasing environmental sustainability in coffee exploitation.

Predicting the Extent of Photosystem II Photoinactivation Using Chlorophyll a Fluorescence Parameters Measured during Illumination

The temperature dependence of the relationship between the decline in activity of photosystem II (PSII) and a chlorophyll a fluorescence parameter combining the excitation pressure (1-qP) and efficiency of excitation energy capture by open PSII reaction centers in the light-acclimated state (Fv'/Fm') was investigated in cotton leaves. A formula for the prediction of PSII inactivation is proposed on the basis of the results obtained. By comparison of the predicted and actual levels of PSII photoinactivation, the rate of PSII recovery was estimated from chlorophyll a fluorescence parameters measured during the day for attached cotton leaves exposed to suboptimal morning temperatures in a greenhouse.

Transgenic overproduction of glutathione reductase does not protect cotton, Gossypium hirsutum (Malvaceae), from photoinhibition during growth under chilling conditions

In some studies, tissues from plants that have been genetically transformed to overproduce antioxidant enzymes sustain less damage when abruptly exposed to short-term chilling in the laboratory. However, few studies have examined the performance of transgenic plants during longer-term growth under chilling conditions. We compared growth of transgenic cotton that overproduces glutathione reductase (GR+; ∼40-fold overproduction) to growth of the wild type in a controlled environment chamber as leaf temperature was lowered from 28° to 14°C over 9 d and for a subsequent 9-d period at 14°C. In wild-type and GR+ cotton, chilling temperatures resulted in decreased dark-adapted F(v)/F(m) (the ratio of variable to maximal fluorescence; a measure of maximum photosystem II quantum yield) and mid-light period photosystem II quantum yield, coupled with increased 1 - q(P) (a nonlinear estimate of the reduction state of the primary quinone acceptor of photosystem II). The capacity for photosynthetic oxygen evolution decreased during the first portion of the chilling exposure, but recovered slightly during the second half. At no point during the chilling exposure did the performance of GR+ plants differ significantly from that of wild-type plants in any of the above parameters. The absence of an effect of GR overproduction under longer-term chilling may be explained, in part, by the fact that wild-type cotton acclimated to chilling by upregulating native GR activity.