Effect of heat stress on the inhibition and recovery of the ribulose-1,5-bisphosphate carboxylase/oxygenase activation state

Endophytic Fungi for Crops Adaptation to Abiotic Stresses

Endophytic fungi (EFs) have emerged as promising modulators of plant growth and stress tolerance in agricultural ecosystems. This review synthesizes the current knowledge on the role of EFs in enhancing the adaptation of crops to abiotic stress. Abiotic stresses, such as drought, salinity, and extreme temperatures, pose significant challenges to crop productivity worldwide. EFs have shown remarkable potential in alleviating the adverse effects of these stresses. Through various mechanisms, including the synthesis of osmolytes, the production of stress-related enzymes, and the induction of plant defense mechanisms, EFs enhance plant resilience to abiotic stressors. Moreover, EFs promote nutrient uptake and modulate the hormonal balance in plants, further enhancing the stress tolerance of the plants. Recent advancements in molecular techniques have facilitated the identification and characterization of stress-tolerant EF strains, paving the way for their utilization in agricultural practices. Furthermore, the symbiotic relationship between EFs and plants offers ecological benefits, such as improved soil health and a reduced dependence on chemical inputs. However, challenges remain in understanding the complex interactions between EFs and host plants, as well as in scaling up their application in diverse agricultural systems. Future research should focus on elucidating the mechanisms underlying endophytic-fungal-mediated stress tolerance and developing sustainable strategies for harnessing their potential in crop production.

Heliotropium thermophilum adapts to high soil temperature in natural conditions due to its highly active antioxidant system protecting its photosynthetic machinery

Heliotropium thermophilum (Boraginaceae) plants have strong antioxidant properties. This study investigated the effectiveness of the antioxidant system in protecting the photosynthetic machinery of H. thermophilum . Plants were obtained from Kızıldere geothermal area in Buharkent district, Aydın, Turkey. Plants in the geothermal area that grew at 25-35°C were regarded as the low temperature group, while those that grew at 55-65°C were regarded as the high temperature group. We analysed the physiological changes of these plants at the two temperature conditions at stage pre-flowering and flowering. We meaured the effect of high soil temperature on water potential, malondialdehyde, cell membrane stability, and hydrogen peroxide analysis to determine stress levels on leaves and roots. Changes in antioxidant enzyme activities, ascorbate and chlorophyll content, chlorophyll fluorescence, photosynthetic gas exchange parameters, and photosynthetic enzymes (Rubisco and invertase) activities were also determined. Our results showed minimal changes to stress levels, indicating that plants were tolerant to high soil temperatures. In general, an increase in antioxidant enzyme activities, ascorbat levels, and all chlorophyll fluorescence parameters except for non-photochemical quenching (NPQ) and F v /F m were observed. The pre-flowering and flowering stages were both characterised by decreased NPQ, despite F v /F m not changing. Additionally, there was a rise in the levels of photosynthetic gas exchange parameters, Rubisco, and invertase activities. High temperature did not affect photosynthetic yield because H. thermophilum was found to stimulate antioxidant capacity, which reduces oxidative damage and maintains its photosynthetic machinery in high temperature conditions and therefore, it is tolerant to high soil temperature.

Transcriptome and photosynthetic analyses provide new insight into the molecular mechanisms underlying heat stress tolerance in Rhododendron × pulchrum Sweet

Rhododendron species provide excellent ornamental use worldwide, yet heat stress (HS) is one of the major threats to their cultivation. However, the intricate mechanisms underlying the photochemical and transcriptional regulations associated with the heat stress response in Rhododendron remain relatively unexplored. In this study, the analyses of morphological characteristics and chlorophyll fluorescence (ChlF) kinetics showed that HS (40 °C/35 °C) had a notable impact on both the donor's and acceptor's sides of photosystem II (PSII), resulting in reduced PSII activity and electron transfer capacity. The gradual recovery of plants observed following a 5-day period of culture under normal conditions indicates the reversible nature of the HS impact on Rhododendron × pulchrum. Analysis of transcriptome data unveiled noteworthy trends: four genes associated with photosynthesis-antenna protein synthesis (LHCb1, LHCb2 and LHCb3) and the antioxidant system (glutamate-cysteine ligase) experienced significant down-regulation in the leaves of R. × pulchrum during HS. Conversely, aseorbate peroxidase and glutathione S-transferase TAU 8 demonstrated an up-regulated pattern. Furthermore, six down-regulated genes (phos-phoenolpyruvate carboxylase 4, sedoheptulose-bisphosphatase, ribose-5-phosphate isomerase 2, high cyclic electron flow 1, beta glucosidase 32 and starch synthase 2) and two up-regulated genes (beta glucosidase 2 and UDP-glucose pyrophosphorylase 2) implicated in photosynthetic carbon fixation and starch/sucrose metabolism were identified during the recovery process. To augment these insights, a weighted gene co-expression network analysis yielded a co-expression network, pinpointing the hub genes correlated with ChlF dynamics' variation trends. The cumulative results showed that HS inhibited the synthesis of photosynthesis-antenna proteins in R. × pulchrum leaves. This disruption subsequently led to diminished photochemical activities in both PSII and PSI, albeit with PSI exhibiting heightened thermostability. Depending on the regulation of the reactive oxygen species scavenging system and heat dissipation, photoprotection sustained the recoverability of R. × pulchrum to HS.

Impact of heat priming on heat shock responses in Origanum vulgare: Enhanced foliage photosynthetic tolerance and biphasic emissions of volatiles

Climate change enhances the frequency of heatwaves that negatively affect photosynthesis and can alter constitutive volatile emissions and elicit emissions of stress volatiles, but how pre-exposure to mildly warmer temperatures affects plant physiological responses to subsequent severe heat episodes remains unclear, especially for aromatic plants with high and complex volatile defenses. We studied the impact of heat shock (45 °C/5 min) applied alone and after exposure to moderate heat stress (35 °C/1 h, priming) on foliage photosynthesis and volatile emissions in the aromatic plant Origanum vulgare through 72 h recovery period. Heat stress decreased photosynthesis rates and stomatal conductance, whereas the reductions in photosynthesis were primarily due to non-stomatal factors. In non-primed plants, heat shock-induced reductions in photosynthetic activity were the greatest, but photosynthetic activity completely recovered by the end of the experiment. In primed plants, a certain inhibition of photosynthetic activity remained, suggesting a sustained priming effect. Heat shock enhanced the emissions of volatiles including lipoxygenase pathway volatiles, long-chained fatty acid-derived compounds, mono- and sesquiterpenes, geranylgeranyl diphosphate pathway volatiles, and benzenoids, whereas different heat treatments resulted in unique emission blends. In non-primed plants, stress-elicited emissions recovered at 72 h. In primed plants, volatile emissions were multiphasic, the first phase, between 0.5 and 10 h, reflected the primary stress response, whereas the secondary rise, between 24 and 72 h, indicated activations of different defense metabolic pathways. Our results demonstrate that exposure to mild heat leads to a sustained physiological stress memory that enhances plant resistance to subsequent severe heat stress episodes.

Fresh perspectives on an established technique: Pulsed amplitude modulation chlorophyll a fluorescence

Pulsed amplitude modulation (PAM) chlorophyll a fluorescence provides information about photosynthetic energy transduction. When reliably measured, chlorophyll a fluorescence provides detailed information about critical in vivo photosynthetic processes. Such information has recently provided novel and critical insights into how the yield potential of crops can be improved and it is being used to understand remotely sensed fluorescence, which is termed solar-induced fluorescence and will be solely measured by a satellite scheduled to be launched this year. While PAM chlorophyll a fluorometers measure fluorescence intensity per se, herein we articulate the axiomatic criteria by which instrumentally detected intensities can be assumed to assess fluorescence yield, a phenomenon quite different than fluorescence intensity and one that provides critical insight about how solar energy is variably partitioned into the biosphere. An integrated mathematical, phenomenological, and practical discussion of many useful chlorophyll a fluorescence parameters is presented. We draw attention to, and provide examples of, potential uncertainties that can result from incorrect methodological practices and potentially problematic instrumental design features. Fundamentals of fluorescence measurements are discussed, including the major assumptions underlying the signals and the methodological caveats about taking measurements during both dark- and light-adapted conditions. Key fluorescence parameters are discussed in the context of recent applications under environmental stress. Nuanced information that can be gleaned from intra-comparisons of fluorescence-derived parameters and intercomparisons of fluorescence-derived parameters with those based on other techniques is elucidated.

Regulation of Calvin-Benson cycle enzymes under high temperature stress

The Calvin-Benson cycle (CBC) consists of three critical processes, including fixation of CO₂ by Rubisco, reduction of 3-phosphoglycerate (3PGA) to triose phosphate (triose-P) with NADPH and ATP generated by the light reactions, and regeneration of ribulose 1,5-bisphosphate (RuBP) from triose-P. The activities of photosynthesis-related proteins, mainly from the CBC, were found more significantly affected and regulated in plants challenged with high temperature stress, including Rubisco, Rubisco activase (RCA) and the enzymes involved in RuBP regeneration, such as sedoheptulose-1,7-bisphosphatase (SBPase). Over the past years, the regulatory mechanism of CBC, especially for redox-regulation, has attracted major interest, because balancing flux at the various enzymatic reactions and maintaining metabolite levels in a range are of critical importance for the optimal operation of CBC under high temperature stress, providing insights into the genetic manipulation of photosynthesis. Here, we summarize recent progress regarding the identification of various layers of regulation point to the key enzymes of CBC for acclimation to environmental temperature changes along with open questions are also discussed.

Short-term warming does not affect intrinsic thermotolerance but induces strong sustaining photoprotection in tropical evergreen citrus genotypes

Consequences of warming and postwarming events on photosynthetic thermotolerance (P_T ) and photoprotective responses in tropical evergreen species remain elusive. We chose Citrus to answer some of the emerging questions related to tropical evergreen species' P_T behaviour including (i) how wide is the genotypic variation in P_T ? (ii) how does P_T respond to short-term warming and (iii) how do photosynthesis and photoprotective functions respond over short-term warming and postwarming events? A study on 21 genotypes revealed significant genotypic differences in P_T , though these were not large. We selected five genotypes with divergent P_T and simulated warming events: T_max 26/20°C (day-time highest maximum/night-time lowest maximum) (Week 1) < T_max 33/30°C (Week 2) < T_max 36/32°C (Week 3) followed by T_max 26/16°C (Week 4, recovery). The P_T of all genotypes remained unaltered despite strong leaf megathermy (leaf temperature > air temperature) during warming events. Though moderate warming showed genotype-specific stimulation in photosynthesis, higher warming unequivocally led to severe loss in net photosynthesis and induced higher nonphotochemical quenching. Even after a week of postwarming, photoprotective mechanisms strongly persisted. Our study points towards a conservative P_T in evergreen citrus genotypes and their need for sustaining higher photoprotection during warming as well as postwarming recovery conditions.

Changes in photosynthesis and chlorophyll a fluorescence in relation to leaf temperature from just before to after harvest of Vitis vinifera cv. Shiraz vines grown in outdoor conditions

Harvesting fruit from horticultural species causes a down-regulation of photosynthesis but some species can recover after harvest. The objective of this study was to assess the hypothesis that the impact of fruit removal on the photosynthetic performance of Shiraz grapevines, in relation to CO2 concentration and leaf temperature, would contribute to a depreciation in photosynthetic assimilation. To assess this hypothesis, vines that were continuously vegetative were compared with vines that were harvested when fruit were ripe. These fruiting vines had higher rates of CO2 -limited photosynthesis at all leaf temperatures compared to vegetative vines before harvest but after, photosynthetic rates were highest in vegetative vines. There were few treatment differences in CO2 -saturated photosynthesis before harvest but after, below about 30°C, the harvested vines had higher photosynthesis than the vegetative vines. Maximum rates of ribulose 1,5 bisphosphate (RuBP) carboxylation and regeneration and responses to temperature were unaffected by differences in sink demand but after harvest, maximum rates increased, but markedly more in the vegetative vines, especially at higher temperatures. This conformed to higher photosynthetic rates in the vegetative vines. There were no sink demand effects on chlorophyll a fluorescence, consistent with the evidence that the fruit sink removal probably affected Rubisco activity and performance. The conclusion that sink removal caused a depreciation in photosynthesis was sustained but the temperature had a strong modulating effect through both stomatal and non-stomatal limitations driving the depreciation in assimilation. What was less clear was why assimilation of continuously vegetative vines increased during the harvest time when there were no apparent changes in sink demand.

Progress in studying heteromorphic leaves in Populus euphratica: leaf morphology, anatomical structure, development regulation and their ecological adaptation to arid environments

Populus euphratica Oliv. is a tree that is strongly resistant to drought and salt stress, which is primarily distributed in arid and semiarid lands. The leaves of the species exhibit a special feature that causes them to be designated as heterophylly. In this brief review, we primarily discuss the heteromorphic leaf development and anatomical features, such as the differentiation of spongy and palisade tissues, in heteromorphic leaves of the species. Furthermore, we also discuss the different physiological characteristics in heteromorphic leaves related to the ecological adaptation of P. euphratica to drought environments. These traits in P. euphratica may help researchers evaluate its ecological value in arid areas and evaluate its scientific merit in understanding the mechanism of development of heteromorphic leaves in plants.

ZnJ6 Is a Thylakoid Membrane DnaJ-Like Chaperone with Oxidizing Activity in Chlamydomonas reinhardtii

Assembly of photosynthetic complexes is sensitive to changing light intensities, drought and pathogens, each of which induces a redox imbalance that requires the assistance of specific chaperones to maintain protein structure. Here we report a thylakoid membrane-associated DnaJ-like protein, ZnJ6 (Cre06.g251716.t1.2), in Chlamydomonas reinhardtii. The protein has four CXXCX(G)X(G) motifs that form two zinc fingers (ZFs). Site-directed mutagenesis (Cys > Ser) eliminates the ability to bind zinc. An intact ZF is required for ZnJ6 stability at elevated temperatures. Chaperone assays with recombinant ZnJ6 indicate that it has holding and oxidative activities. ZnJ6 is unable to reduce the disulfide bonds of insulin but prevents its aggregation in a reducing environment. It also assists in the reactivation of reduced denatured RNaseA, possibly by its oxidizing activity. ZnJ6 pull-down assays revealed interactions with oxidoreductases, photosynthetic proteins and proteases. In vivo experiments with a C. reinhardtii insertional mutant (∆ZnJ6) indicate enhanced tolerance to oxidative stress but increased sensitivity to heat and reducing conditions. Moreover, ∆ZnJ6 has reduced photosynthetic efficiency shown by the Chlorophyll fluorescence transient. Taken together, we identify a role for this thylakoid-associated DnaJ-like oxidizing chaperone that assists in the prevention of protein misfolding and aggregation, thus contributing to stress endurance, redox maintenance and photosynthetic balance.

Why Is the Invasive Plant Sphagneticola trilobata More Resistant to High Temperature than Its Native Congener?

Climate change and invasive alien species threaten biodiversity. High temperature is a worrying ecological factor. Most responses of invasive plants aimed at coping with adversity are focused on the physiological level. To explore the molecular mechanisms underlying the response of an invasive plant (Sphagneticola trilobata L.) to high temperature, using a native species (Sphagneticola calendulacea L.) as the control, relevant indicators, including photosynthetic pigments, gas exchange, chlorophyll fluorescence, the antioxidant system, and related enzyme-coding genes were measured. The results showed that the leaves of S. calendulacea turned yellow, photosynthetic pigment content (Chl a, Chl b, Car, Chl) decreased, gas exchange (P_n) and chlorophyll fluorescence parameters (F_v/F_m, Φ_PSII) decreased under high temperature. It was also found that high temperature caused photoinhibition and a large amount of ROS accumulated, resulting in an increase in MDA and relative conductivity. Antioxidant enzymes (including SOD, POD, CAT, and APX) and antioxidants (including flavonoids, total phenols, and carotenoids) were decreased. The qPCR results further showed that the expression of the PsbP, PsbA, and RubiscoL, SOD, POD, CAT, and APX genes was downregulated, which was consistent with the results of physiological data. Otherwise, the resistance of S. trilobata to high temperature was better than that of S. calendulacea, which made it a superior plant in the invasion area. These results further indicated that the gradual warming of global temperature will greatly accelerate the invasion area of S. trilobata.

Metabolomics by UHPLC-HRMS reveals the impact of heat stress on pathogen-elicited immunity in maize

INTRODUCTION

Studies investigating crop resistance to abiotic and biotic stress have largely focused on plant responses to singular forms of stress and individual biochemical pathways that only partially represent stress responses. Thus, combined abiotic and biotic stress treatments and the global assessment of their elicited metabolic expression remains largely unexplored. In this study, we employed targeted and untargeted metabolomics to investigate the molecular responses of maize (Zea mays) to abiotic, biotic, and combinatorial stress.

OBJECTIVE

We compared the inducible metabolomes of heat-stressed (abiotic) and C. heterostrophus-infected (biotic) maize and examined the effects of heat stress on the ability of maize to defend itself against C. heterostrophus.

METHODS

Ultra-high-performance liquid chromatography-high-resolution mass spectrometry was performed on plants grown under control conditions (28 °C), heat stress (38 °C), Cochliobolus heterostrophus infection, or combinatorial stress [heat (38 °C) + C. heterostrophus infection].

RESULTS

Multivariate analyses revealed differential metabolite expression between heat stress, C. heterostrophus infection, and their respective controls. In combinatorial experiments, treatment with heat stress prior to fungal inoculation negatively impacted maize disease resistance against C. heterostrophus, and distinct metabolome separation between combinatorial stressed plants and the non-heat-stressed infected controls was observed. Targeted analysis revealed inducible primary and secondary metabolite responses to abiotic/biotic stress, and combinatorial experiments indicated that deficiency in the hydroxycinnamic acid, p-coumaric acid, may contribute to the heat-induced susceptibility of maize to C. heterostrophus.

CONCLUSION

These findings demonstrate that abiotic stress can predispose crops to more severe disease symptoms, underlining the increasing need to investigate defense chemistry in plants under combinatorial stress.

Photosynthetic Metabolism under Stressful Growth Conditions as a Bases for Crop Breeding and Yield Improvement

Increased periods of water shortage and higher temperatures, together with a reduction in nutrient availability, have been proposed as major factors that negatively impact plant development. Photosynthetic CO2 assimilation is the basis of crop production for animal and human food, and for this reason, it has been selected as a primary target for crop phenotyping/breeding studies. Within this context, knowledge of the mechanisms involved in the response and acclimation of photosynthetic CO2 assimilation to multiple changing environmental conditions (including nutrients, water availability, and rising temperature) is a matter of great concern for the understanding of plant behavior under stress conditions, and for the development of new strategies and tools for enhancing plant growth in the future. The current review aims to analyze, from a multi-perspective approach (ranging across breeding, gas exchange, genomics, etc.) the impact of changing environmental conditions on the performance of the photosynthetic apparatus and, consequently, plant growth.

Combined Proteome and Transcriptome Analysis of Heat-Primed Azalea Reveals New Insights Into Plant Heat Acclimation Memory

Plants can obtain superinduction of defense against unpredictable challenges based on prior acclimation, but the mechanisms involved in the acclimation memory are little known. The objective of this study was to characterize mechanisms of heat acclimation memory in Rhododendron hainanense, a thermotolerant wild species of azalea. Pretreatment of a 2-d recovery (25/18°C, day/night) after heat acclimation (37°C, 1 h) (AR-pt) did not weaken but enhanced acquired thermotolerance in R. hainanense with less damaged phenotype, net photosynthetic rate, and membrane stability than non-acclimation pretreated (NA-pt) plants. Combined transcriptome and proteome analysis revealed that a lot of heat-responsive genes still maintained high protein abundance rather than transcript level after the 2-d recovery. Photosynthesis-related genes were highly enriched and most decreased under heat stress (HS: 42°C, 1 h) with a less degree in AR-pt plants compared to NA-pt. Sustainably accumulated chloroplast-localized heat shock proteins (HSPs), Rubisco activase 1 (RCA1), beta-subunit of chaperonin-60 (CPN60β), and plastid transcriptionally active chromosome 5 (pTAC5) in the recovery period probably provided equipped protection of AR-pt plants against the subsequent HS, with less damaged photochemical efficiency and chloroplast structure. In addition, significant higher levels of RCA1 transcripts in AR-pt compared to NA-pt plants in early stage of HS showed a more important role of RCA1 than other chaperonins in heat acclimation memory. The novel heat-induced RCA1, rather than constitutively expressed RCA2 and RCA3, showed excellent thermostability after long-term HS (LHS: 42/35°C, 7 d) and maintained balanced Rubisco activation state in photosynthetic acclimation. This study provides new insights into plant heat acclimation memory and indicates candidate genes for genetic modification and molecular breeding in thermotolerance improvement.

Exploring high temperature responses of photosynthesis and respiration to improve heat tolerance in wheat

High temperatures account for major wheat yield losses annually and, as the climate continues to warm, these losses will probably increase. Both photosynthesis and respiration are the main determinants of carbon balance and growth in wheat, and both are sensitive to high temperature. Wheat is able to acclimate photosynthesis and respiration to high temperature, and thus reduce the negative affects on growth. The capacity to adjust these processes to better suit warmer conditions stands as a potential avenue toward reducing heat-induced yield losses in the future. However, much remains to be learnt about such phenomena. Here, we review what is known of high temperature tolerance in wheat, focusing predominantly on the high temperature responses of photosynthesis and respiration. We also identify the many unknowns that surround this area, particularly with respect to the high temperature response of wheat respiration and the consequences of this for growth and yield. It is concluded that further investigation into the response of photosynthesis and respiration to high temperature could present several methods of improving wheat high temperature tolerance. Extending our knowledge in this area could also lead to more immediate benefits, such as the enhancement of current crop models.

Short-term temperature dependency of the photosynthetic and PSII photochemical responses to photon flux density of leaves of Vitis vinifera cv. Shiraz vines grown in field conditions with and without fruit

Shiraz vines grown outdoors with and without a crop load were used to determine photosynthetic and chlorophyll fluorescence responses to light across a range of leaf temperatures to evaluate the impact of presence/absence of a sink on these responses. Results indicate maximum rates of photosynthesis and light saturation in fruiting vines were biased towards higher temperatures whereas these processes in vegetative vines were biased towards lower temperatures. The maximum efficiency of PSII photochemistry was similarly biased, with higher efficiency for the vegetative vines below 30°C and a higher efficiency for the fruiting vines above. The quantum efficiency of PSII electron transport was generally higher across all temperatures in the fruiting compared with vegetative vines. Photochemical quenching was not sensitive to temperature in fruiting vines but strongly so in vegetative vines, with an optimum at 30°C and marked increases in photochemical quenching at other temperatures. Non-photochemical quenching was not strongly temperature dependent, but there were marked increases in both treatments at 45°C, consistent with marked decreases in assimilation. These results suggest demand for assimilates in fruiting vines induced an acclimation response to high summer temperatures to enhance assimilate supply and this was underpinned by comparable shifts in PSII photochemistry.

Identification of genomic loci associated with 21chlorophyll fluorescence phenotypes by genome-wide association analysis in soybean

BACKGROUND

Photosynthesis is able to convert solar energy into chemical energy in the form of biomass, but the efficiency of photosynthetic solar energy conversion is low. Chlorophyll fluorescence measurements are rapid, non-destructive, and can provide a wealth of information about the efficiencies of the photosynthetic light reaction processes. Efforts aimed at assessing genetic variation and/or mapping of genetic loci associated with chlorophyll fluorescence phenotypes have been rather limited.

RESULTS

Evaluation of SoySNP50K iSelect SNP Beadchip data from the 189 genotypes phenotyped in this analysis identified 32,453 SNPs with a minor allele frequency (MAF) ≥ 5%. A total of 288 (non-unique) SNPs were significantly associated with one or more of the 21 chlorophyll fluorescence phenotypes. Of these, 155 were unique SNPs and 100 SNPs were only associated with a single fluorescence phenotype, while 28, 11, 2, and 14 SNPs, were associated with two, three, four and five or more fluorescence phenotypes, respectively. The 288 non-unique SNPs represent 155 unique SNPs that mark 53 loci. The 155 unique SNPs included 27 that were associated with three or more phenotypes, and thus were called multi-phenotype SNPs. These 27 multi-phenotype SNPs marked 13 multi-phenotype loci (MPL) identified by individual SNPs associated with multiple chlorophyll fluorescence phenotypes or by more than one SNP located within 0.5 MB of other multi-phenotype SNPs.

CONCLUSION

A search in the genomic regions highlighted by these 13 MPL identified genes with annotations indicating involvement in photosynthetic light dependent reactions. These, as well as loci associated with only one or two chlorophyll fluorescence traits, should be useful to develop a better understanding of the genetic basis of photosynthetic light dependent reactions as a whole as well as of specific components of the electron transport chain in soybean. Accordingly, additional genetic and physiological analyses are necessary to determine the relevance and effectiveness of the identified loci for crop improvement efforts.

Identification of genomic loci associated with 21chlorophyll fluorescence phenotypes by genome-wide association analysis in soybean

BACKGROUND

Photosynthesis is able to convert solar energy into chemical energy in the form of biomass, but the efficiency of photosynthetic solar energy conversion is low. Chlorophyll fluorescence measurements are rapid, non-destructive, and can provide a wealth of information about the efficiencies of the photosynthetic light reaction processes. Efforts aimed at assessing genetic variation and/or mapping of genetic loci associated with chlorophyll fluorescence phenotypes have been rather limited.

RESULTS

Evaluation of SoySNP50K iSelect SNP Beadchip data from the 189 genotypes phenotyped in this analysis identified 32,453 SNPs with a minor allele frequency (MAF) ≥ 5%. A total of 288 (non-unique) SNPs were significantly associated with one or more of the 21 chlorophyll fluorescence phenotypes. Of these, 155 were unique SNPs and 100 SNPs were only associated with a single fluorescence phenotype, while 28, 11, 2, and 14 SNPs, were associated with two, three, four and five or more fluorescence phenotypes, respectively. The 288 non-unique SNPs represent 155 unique SNPs that mark 53 loci. The 155 unique SNPs included 27 that were associated with three or more phenotypes, and thus were called multi-phenotype SNPs. These 27 multi-phenotype SNPs marked 13 multi-phenotype loci (MPL) identified by individual SNPs associated with multiple chlorophyll fluorescence phenotypes or by more than one SNP located within 0.5 MB of other multi-phenotype SNPs.

CONCLUSION

A search in the genomic regions highlighted by these 13 MPL identified genes with annotations indicating involvement in photosynthetic light dependent reactions. These, as well as loci associated with only one or two chlorophyll fluorescence traits, should be useful to develop a better understanding of the genetic basis of photosynthetic light dependent reactions as a whole as well as of specific components of the electron transport chain in soybean. Accordingly, additional genetic and physiological analyses are necessary to determine the relevance and effectiveness of the identified loci for crop improvement efforts.

Systematic biology analysis on photosynthetic carbon metabolism of maize leaf following sudden heat shock under elevated CO2

Plants would experience more complex environments, such as sudden heat shock (SHS) stress combined with elevated CO₂ in the future, and might adapt to this stressful condition by optimizing photosynthetic carbon metabolism (PCM). It is interesting to understand whether this acclimation process would be altered in different genotypes of maize under elevated CO₂, and which metabolites represent key indicators reflecting the photosynthetic rates (P_N) following SHS. Although B76 had greater reduction in P_N during SHS treatment, our results indicated that P_N in genotype B76, displayed faster recovery after SHS treatment under elevated CO₂ than in genotype B106. Furthermore, we employed a stepwise feature extraction approach by partial linear regression model. Our findings demonstrated that 9 key metabolites over the total (35 metabolites) can largely explain the variance of P_N during recovery from SHS across two maize genotypes and two CO₂ grown conditions. Of these key metabolites, malate, valine, isoleucine, glucose and starch are positively correlated with recovery pattern of P_N. Malate metabolites responses to SHS were further discussed by incorporating with the activities and gene expression of three C₄ photosynthesis-related key enzymes. We highlighted the importance of malate metabolism during photosynthesis recovery from short-term SHS, and data integration analysis to better comprehend the regulatory framework of PCM in response to abiotic stress.

Modelling seasonal changes in the temperature-dependency of CO2 photosynthetic responses in two Vitis vinifera cultivars

A study of photosynthesis of two grapevine cultivars, Vitis vinifera L. cv. Chardonnay and cv. Merlot in relation to the seasonal climate and internal CO2 (Ci) concentration at leaf temperatures from 15 to 45°C was undertaken. Average rates of photosynthesis at saturating CO2 concentrations and all leaf temperatures were higher in Merlot compared with Chardonnay leaves. This was attributable to higher rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (Vcmax) and regeneration (Jmax) in Merlot leaves. These differences in photosynthesis were extended as the season progressed, partly because rates of RuBP carboxylation and regeneration of Chardonnay leaves declined markedly whereas rates for Merlot leaves remained high. Although there was no cultivar difference in the seasonal average temperature optima for assimilation (34°C) and the underlying metabolism (40°C for Vcmax and 35°C for Jmax), for temperatures above 35°C, the Merlot leaves had 50% higher rates. Across the season, activation energies of the temperature sensitivity of Vcmax and Jmax declined in response to the seasonal climate but were consistently lower in Merlot than Chardonnay. This suggested some apparent differences in the biochemistry occurred between the two cultivars that limited assimilation in Chardonnay leaves, especially at higher temperatures, but did not limit assimilation in Merlot leaves.

An attempt to interpret a biochemical mechanism of C4 photosynthetic thermo-tolerance under sudden heat shock on detached leaf in elevated CO2 grown maize

Detached leaves at top canopy structures always experience higher solar irradiance and leaf temperature under natural conditions. The ability of tolerance to high temperature represents thermotolerance potential of whole-plants, but was less of concern. In this study, we used a heat-tolerant (B76) and a heat-susceptible (B106) maize inbred line to assess the possible mitigation of sudden heat shock (SHS) effects on photosynthesis (PN) and C4 assimilation pathway by elevated [CO2]. Two maize lines were grown in field-based open top chambers (OTCs) at ambient and elevated (+180 ppm) [CO2]. Top-expanded leaves for 30 days after emergence were suddenly exposed to a 45°C SHS for 2 hours in midday during measurements. Analysis on thermostability of cellular membrane showed there was 20% greater electrolyte leakage in response to the SHS in B106 compared to B76, in agreement with prior studies. Elevated [CO2] protected PN from SHS in B76 but not B106. The responses of PN to SHS among the two lines and grown CO2 treatments were closely correlated with measured decreases of NADP-ME enzyme activity and also to its reduced transcript abundance. The SHS treatments induced starch depletion, the accumulation of hexoses and also disrupted the TCA cycle as well as the C4 assimilation pathway in the both lines. Elevated [CO2] reversed SHS effects on citrate and related TCA cycle metabolites in B106 but the effects of elevated [CO2] were small in B76. These findings suggested that heat stress tolerance is a complex trait, and it is difficult to identify biochemical, physiological or molecular markers that accurately and consistently predict heat stress tolerance.

Comparative proteomic analysis provides novel insights into chlorophyll biosynthesis in celery under temperature stress

Chlorophyll (Chl) is essential for light harvesting and energy transduction in photosynthesis. A proper amount of Chl within plant cells is important to celery (Apium graveolens) yield and quality. Temperature stress is an influential abiotic stress affecting Chl biosynthesis and plant growth. There are limited proteomic studies regarding Chl accumulation under temperature stress in celery leaves. Here, the proteins from celery leaves under different temperature treatments (4, 25 and 38°C) were analyzed using a proteomic approach. There were 71 proteins identified through MALDI-TOF-TOF analysis. The relative abundance of proteins involved in carbohydrate and energy metabolism, protein metabolism, amino acid metabolism, antioxidant and polyamine biosynthesis were enhanced under cold stress. These temperature stress-responsive proteins may establish a new homeostasis to enhance temperature tolerance. Magnesium chelatase (Mg-chelatase) and glutamate-1-semialdehyde aminotransferase (GSAT), related to Chl biosynthesis, showed increased abundances under cold stress. Meanwhile, the Chl contents were decreased in heat- and cold-stressed celery leaves. The inhibition of Chl biosynthesis may be due to the downregulated mRNA levels of 15 genes involved in Chl biosynthesis. The study will expand our knowledge on Chl biosynthesis and the temperature tolerance mechanisms in celery leaves.

Immediate and potential long-term effects of consecutive heat waves on the photosynthetic performance and water balance in Douglas-fir

The frequency and intensity of climatic extremes, such as heat waves, are predicted to increase globally, with severe implications for terrestrial carbon and water cycling. Temperatures may rise above critical thresholds that allow trees to function optimally, with unknown long-term consequences for forest ecosystems. In this context, we investigated how photosynthetic traits and the water balance in Douglas-fir are affected by exposure to three heat waves with temperatures about 12°C above ambient. Photosynthetic carboxylation efficiency (V_cmax) was mostly unaffected, but electron transport (J_max) and photosynthetic rates under saturating light (A_sat) were strongly influenced by the heat waves, with lagging limitations on photosynthesis still being observed six weeks after the last heat wave. We also observed lingering heat-induced inhibitions on transpiration, minimum stomatal conductance, and night-time stomatal conductance (g_s-night). Results from the stomatal models used to calculate minimum stomatal conductance were similar to g_s-night and indicated changes in leaf morphology, e.g. stomatal occlusions and alterations in epicuticular wax. Our results show Douglas-fir's ability to restrict water loss following heat stress, but at the price of reduced photosynthetic performance. Such limitations indicate potential long-term restrictions that heat waves can impose on tree development and functioning under extreme climatic conditions.

Improved tolerance to post-anthesis drought stress by pre-drought priming at vegetative stages in drought-tolerant and -sensitive wheat cultivars

Wheat crop endures a considerable penalty of yield reduction to escape the drought events during post-anthesis period. Drought priming under a pre-drought stress can enhance the crop potential to tolerate the subsequent drought stress by triggering a faster and stronger defense mechanism. Towards these understandings, a set of controlled moderate drought stress at 55-60% field capacity (FC) was developed to prime the plants of two wheat cultivars namely Luhan-7 (drought tolerant) and Yangmai-16 (drought sensitive) during tillering (Feekes 2 stage) and jointing (Feekes 6 stage), respectively. The comparative response of primed and non-primed plants, cultivars and priming stages was evaluated by applying a subsequent severe drought stress at 7 days after anthesis. The results showed that primed plants of both cultivars showed higher potential to tolerate the post-anthesis drought stress through improved leaf water potential, more chlorophyll, and ribulose-1, 5-bisphosphate carboxylase/oxygenase contents, enhanced photosynthesis, better photoprotection and efficient enzymatic antioxidant system leading to less yield reductions. The primed plants of Luhan-7 showed higher capability to adapt the drought stress events than Yangmai-16. The positive effects of drought priming to sustain higher grain yield were pronounced in plants primed at tillering than those primed at jointing. In consequence, upregulated functioning of photosynthetic apparatus and efficient enzymatic antioxidant activities in primed plants indicated their superior potential to alleviate a subsequently occurring drought stress, which contributed to lower yield reductions than non-primed plants. However, genotypic and priming stages differences in response to drought stress also contributed to affect the capability of primed plants to tolerate the post-anthesis drought stress conditions in wheat.

Response and Tolerance Mechanism of Cotton Gossypium hirsutum L. to Elevated Temperature Stress: A Review

Cotton is an important multipurpose crop which is highly sensitive to both biotic and abiotic stresses. Proper management of this cash crop requires systematic understanding of various environmental conditions that are vital to yield and quality. High temperature stress can severely affect the viability of pollens and anther indehiscence, which leads to significant yield losses. Cotton can respond to withstand adverse environmental condition in several phases among which the accumulation of chemicals is extremely vital. Calcium, kinases, reactive oxygen species, carbohydrate, transcription factors, gene expression regulation, and plant hormones signaling pathways are playing a handy role in activating the major genes responsible to encounter and defend elevated temperature stress. The production of heat shock proteins is up-regulated when crops are unleashed to high temperature stress. Molecular breeding can play a functional role to identify superior genes for all the important attributes as well as provide breeder ready markers for developing ideotypes. The development of high-temperature resistant transgenic cultivars of cotton can grant a stability benefit and can also ameliorate the production capacity in response to elevated temperature.

A self-photoprotection mechanism helps Stipa baicalensis adapt to future climate change

We examined the photosynthetic responses of Stipa baicalensis to relative long-term exposure (42 days) to the predicted elevated temperature and water availability changes to determine the mechanisms through which the plant would acclimate to future climate change. Two thermal regimes (ambient and +4 °C) and three irrigation levels (partial, normal and excess) were used in environmental control chambers. The gas exchange parameters, light response curves and A/Ci curves were determined. The elevated temperature and partial irrigation reduced the net photosynthetic rate due to a limitation in the photosynthetic capacity instead of the intercellular CO2 concentration. Partial irrigation decreased Rubisco activation and limited RuBP regeneration. The reduction in Vcmax increased with increasing temperature. Excess irrigation offset the negative effect of drought and led to a partial recovery of the photosynthetic capacity. Although its light use efficiency was restricted, the use of light and dark respiration by Stipa baicalensis was unchanged. We concluded that nonstomatal limitation was the primary reason for photosynthesis regulation in Stipa baicalensis under relative long-term climate change conditions. Although climate change caused reductions in the light use efficiency and photosynthetic rate, a self-photoprotection mechanism in Stipa baicalensis resulted in its high ability to maintain normal live activities.

Plant responses to ambient temperature fluctuations and water-limiting conditions: A proteome-wide perspective

BACKGROUND

Every year, environmental stresses such as limited water and nutrient availability, salinity, and temperature fluctuations inflict significant losses on crop yields across the globe. Recently, developments in analytical techniques, e.g. mass spectrometry, have led to great advances towards understanding how plants respond to environmental stresses. These processes are mediated by many molecular pathways and, at least partially, via proteome-environment interactions.

SCOPE OF REVIEW

This review focuses on the current state of knowledge about interactions between the plant proteome and the environment, with a special focus on drought and temperature responses of plant proteome dynamics, and subcellular and organ-specific compartmentalization, in Arabidopsis thaliana and crop species.

MAJOR CONCLUSIONS

Correct plant development under non-optimal conditions requires complex self-protection mechanisms, many of them common to different abiotic stresses. Proteome analyses of plant responses to temperature and drought stresses have revealed an intriguing interplay of modifications, mainly affecting the photosynthetic machinery, carbohydrate metabolism, and ROS activation and scavenging. Imbalances between transcript-level and protein-level regulation observed during adaptation to abiotic stresses suggest that many of the regulatory processes are controlled at translational and post-translational levels; proteomics is thus essential in revealing important regulatory networks.

GENERAL SIGNIFICANCE

Because information from proteomic data extends far beyond what can be deduced from transcriptome analysis, the results of proteome studies have substantially deepened our understanding of stress adaptation in plants; this is clearly a prerequisite for designing strategies to improve the yield and quality of crops grown under unfavorable conditions brought about by ongoing climatic change. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Lipid Antioxidant and Galactolipid Remodeling under Temperature Stress in Tomato Plants

Increased temperatures are a major scenario in climate change and present a threat to plant growth and agriculture. Plant growth depends on photosynthesis. To function optimally, the photosynthetic machinery at the thylakoid membrane in chloroplasts continuously adapts to changing conditions. Here, we set out to discover the most important changes arising at the lipid level under high temperature (38°C) in comparison to mild (20°C) and moderately cold temperature (10°C) using a non-targeted lipidomics approach. To our knowledge, no comparable experiment at the level of the whole membrane system has been documented. Here, 791 molecular species were detected by mass spectrometry and ranged from membrane lipids, prenylquinones (tocopherols, phylloquinone, plastoquinone, plastochromanol), carotenoids (β-carotene, xanthophylls) to numerous unidentified compounds. At high temperatures, the most striking changes were observed for the prenylquinones (α-tocopherol and plastoquinone/-ol) and the degree of saturation of fatty acids in galactolipids and phosphatidyl ethanolamine. Photosynthetic efficiency at high temperature was not affected but at moderately cold temperature mild photoinhibition occurred. The results indicate, that the thylakoid membrane is remodeled with regard to fatty acid saturation in galactolipids and lipid antioxidant concentrations under high temperature stress. The data strongly suggest, that massively increased concentrations of α-tocopherol and plastoquinone are important for protection against high temperature stress and proper function of the photosynthetic apparatus.

Temperature-dependent responses of the photosynthetic and chlorophyll fluorescence attributes of apple (Malus domestica) leaves during a sustained high temperature event

The objective of this study was to follow changes in the temperature-dependent responses of photosynthesis and photosystem II performance in leaves of field-grown trees of Malus domestica (Borkh.) cv. 'Red Gala' before and after exposure to a long-term heat event occurring late in the growing season. Light-saturated photosynthesis was optimal at 25 °C before the heat event. The high temperatures caused a reduction in rates at low temperatures (15-20 °C) but increased rates at high temperatures (30-40 °C) and a shift in optimum to 30 °C. Rates at all temperatures increased after the heat event and the optimum shifted to 33 °C, indicative of some acclimation to the high temperatures occurring. Photosystem II attributes were all highly temperature-dependent. The operating quantum efficiency of PSII during the heat event declined, but mostly at high temperatures, partly because of decreased photochemical quenching but also from increased non-photochemical quenching. However, a further reduction in PSII operating efficiency occurred after the heat event subsided. Non-photochemical quenching had subsided, whereas photochemical quenching had increased in the post-heat event period and consistent with a greater fraction of open PSII reaction centres. What remained uncertain was why these effects on PSII performance appeared to have no effect on the process of light-saturated photosynthesis. However, the results provide an enhanced understanding of the impacts of sustained high temperatures on the photosynthetic process and its underlying reactions, notably photochemistry.

Warming and drought differentially influence the production and resorption of elemental and metabolic nitrogen pools in Quercus rubra

The process of nutrient retranslocation from plant leaves during senescence subsequently affects both plant growth and soil nutrient cycling; changes in either of these could potentially feed back to climate change. Although elemental nutrient resorption has been shown to respond modestly to temperature and precipitation, we know remarkably little about the influence of increasing intensities of drought and warming on the resorption of different classes of plant metabolites. We studied the effect of warming and altered precipitation on the production and resorption of metabolites in Quercus rubra. The combination of warming and drought produced a higher abundance of compounds that can help to mitigate climatic stress by functioning as osmoregulators and antioxidants, including important intermediaries of the tricarboxylic acid (TCA) cycle, amino acids including proline and citrulline, and polyamines such as putrescine. Resorption efficiencies (REs) of extractable metabolites surprisingly had opposite responses to drought and warming; drought treatments generally increased RE of metabolites compared to ambient and wet treatments, while warming decreased RE. However, RE of total N differed markedly from that of extractable metabolites such as amino acids; for instance, droughted plants resorbed a smaller fraction of elemental N from their leaves than plants exposed to the ambient control. In contrast, plants in drought treatment resorbed amino acids more efficiently (>90%) than those in ambient (65-77%) or wet (42-58%) treatments. Across the climate treatments, the RE of elemental N correlated negatively with tissue tannin concentration, indicating that polyphenols produced in leaves under climatic stress could interfere with N resorption. Thus, senesced leaves from drier conditions might have a lower nutritive value to soil heterotrophs during the initial stages of litter decomposition despite a higher elemental N content of these tissues. Our results suggest that N resorption may be controlled not only by plant demand, but also by climatic influences on the production and resorption of plant metabolites. As climate-carbon models incorporate increasingly sophisticated nutrient cycles, these results highlight the need to adequately understand plant physiological responses to climatic variables.

Responses of tree species to heat waves and extreme heat events

The number and intensity of heat waves has increased, and this trend is likely to continue throughout the 21st century. Often, heat waves are accompanied by drought conditions. It is projected that the global land area experiencing heat waves will double by 2020, and quadruple by 2040. Extreme heat events can impact a wide variety of tree functions. At the leaf level, photosynthesis is reduced, photooxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. In some species, stomatal conductance increases at high temperatures, which may be a mechanism for leaf cooling. At the whole plant level, heat stress can decrease growth and shift biomass allocation. When drought stress accompanies heat waves, the negative effects of heat stress are exacerbated and can lead to tree mortality. However, some species exhibit remarkable tolerance to thermal stress. Responses include changes that minimize stress on photosynthesis and reductions in dark respiration. Although there have been few studies to date, there is evidence of within-species genetic variation in thermal tolerance, which could be important to exploit in production forestry systems. Understanding the mechanisms of differing tree responses to extreme temperature events may be critically important for understanding how tree species will be affected by climate change.

Biochemical analysis of 'kerosene tree' Hymenaea courbaril L. under heat stress

Hymenaea courbaril or jatoba is a tropical tree known for its medically important secondary metabolites production. Considering climate change, the goal of this study was to investigate differential expression of proteins and lipids produced by this tree under heat stress conditions. Total lipid was extracted from heat stressed plant leaves and various sesquiterpenes produced by the tree under heat stress were identified. Gas chromatographic and mass spectrometric analysis were used to study lipid and volatile compounds produced by the plant. Several volatiles, isoprene, 2-methyl butanenitrile, β ocimene and a numbers of sesquiterpenes differentially produced by the plant under heat stress were identified. We propose these compounds were produced by the tree to cope up with heat stress. A protein gel electrophoresis (2-D DIGE) was performed to study differential expression of proteins in heat stressed plants. Several proteins were found to be expressed many folds different in heat stressed plants compared to the control. These proteins included heat shock proteins, histone proteins, oxygen evolving complex, and photosynthetic proteins, which, we believe, played key roles in imparting thermotolerance in Hymenaea tree. To the best of our knowledge, this is the first report of extensive molecular physiological study of Hymenaea trees under heat stress. This work will open avenues of further research on effects of heat stress in Hymenaea and the findings can be applied to understand how global warming can affect physiology of other plants.

High temperature stress monitoring and detection using chlorophyll a fluorescence and infrared thermography in chrysanthemum (Dendranthema grandiflora)

Modern highly insulated greenhouses are more energy efficient than conventional types. Furthermore applying dynamic greenhouse climate control regimes will increase energy efficiency relatively more in modern structures. However, this combination may result in higher air and crop temperatures. Too high temperature affects the plant photosynthetic responses, resulting in a lower rate of photosynthesis. To predict and analyse physiological responses as stress indicators, two independent experiments were conducted, to detect the effect of high temperature on photosynthesis: analysing photosystem II (PSII) and stomatal conductance (gs). A combination of chlorophyll a fluorescence, gas exchange measurements and infrared thermography was applied using Chrysanthemum (Dendranthema grandiflora Tzvelev) 'Coral Charm' as a model species. Increasing temperature had a highly significant effect on PSII when the temperature exceeded 38 °C for a period of 7 (±1.8) days. High temperature decreased the maximum photochemical efficiency of PSII (Fv/Fm), the conformation term for primary photochemistry (Fv/Fo) and performance index (PI), as well as increased minimal fluorescence (Fo). However, at elevated CO2 of 1000 μmol mol(-1) and with a photosynthetic photon flux density (PPFD) of 800 μmol m(-2) s(-1), net photosynthesis (Pn) reached its maximum at 35 °C. The thermal index (IG), calculated from the leaf temperature and the temperature of a dry and wet reference leaf, showed a strong correlation with gs. We conclude that 1) chlorophyll a fluorescence and a combination of fluorescence parameters can be used as early stress indicators as well as to detect the temperature limit of PSII damage, and 2) the strong relation between gs and IG enables gs to be estimated non-invasively, which is an important first step in modelling leaf temperature to predict unfavourable growing conditions in a (dynamic) semi closed greenhouse.

A flexible culture process for production of the green microalga Scenedesmus dimorphus rich in protein, carbohydrate or lipid

Microalgae have the ability to undergo programmatic changes in photosynthetic carbon partitioning and thus cellular biochemical composition, particularly in the relative amounts of crude protein, lipids, and carbohydrate, in response to changes in environmental and culture conditions. In this study, a novel strategy that employs a single microalgal strain Scenedesmus dimorphus grown in a single cultivation platform to produce protein-, carbohydrate- or lipid-rich biomass, as so desired, was introduced. With the combined manipulation of nitrogen availability and light intensity and cell inoculation density, it was successfully demonstrated that highest yields for protein and carbohydrate were 0.2 and 0.7 g L(-1) d(-1), respectively, which could be obtained in early stages of cultivation, while the highest yield for lipid, 0.17 g L(-1) d(-1), occured in a late stage of cultivation.

Rubisco activity is associated with photosynthetic thermotolerance in a wild rice (Oryza meridionalis)

Oryza meridionalis is a wild species of rice, endemic to tropical Australia. It shares a significant genome homology with the common domesticated rice Oryza sativa. Exploiting the fact that the two species are highly related but O. meridionalis has superior heat tolerance, experiments were undertaken to identify the impact of temperature on key events in photosynthesis. At an ambient CO(2) partial pressure of 38 Pa and irradiance of 1500 µmol quanta m(-2) s(-1), the temperature optimum of photosynthesis was 33.7 ± 0.8°C for O. meridionalis, significantly higher than the 30.6 ± 0.7°C temperature optimum of O. sativa. To understand the basis for this difference, we measured gas exchange and rubisco activation state between 20 and 42°C and modeled the response to determine the rate-limiting steps of photosynthesis. The temperature response of light respiration (R(light)) and the CO(2) compensation point in the absence of respiration (Γ(*)) were determined and found to be similar for the two species. C3 photosynthesis modeling showed that despite the difference in susceptibility to high temperature, both species had a similar temperature-dependent limitation to photosynthesis. Both rice species were limited by ribulose-1,5-bisphosphate (RuBP) regeneration at temperatures of 25 and 30°C but became RuBP carboxylation limited at 35 and 40°C. The activation state of rubisco in O. meridionalis was more stable at higher temperatures, explaining its greater heat tolerance compared with O. sativa.

Expression of AtSAP5 in cotton up-regulates putative stress-responsive genes and improves the tolerance to rapidly developing water deficit and moderate heat stress

The regulation of gene expression is a key factor in plant acclimation to stress, and it is thought that manipulation of the expression of critical stress-responsive genes should ultimately provide increased protection against abiotic stress. The aim of this study was to test the hypothesis that the ectopic expression of the AtSAP5 (AT3G12630) gene in transgenic cotton (Gossypium hirsutum, cv. Coker 312) will improve tolerance to drought and heat stress by up-regulating the expression of endogenous stress-responsive genes. The SAP5 gene is a member of the stress-associated family of genes that encode proteins containing A20/AN1 zinc finger domains. Under non-stressful conditions, cotton plants that expressed the AtSAP5 gene showed elevated expression of at least four genes normally induced during water deficit or heat stress. The rate of net CO(2) assimilation A for three of four transgenic lines tested was less sensitive to rapidly developing water deficit over 4d than untransformed wild-type plants, but the recovery of A following drought was not significantly affected. The enhanced protection of photosynthesis during drought was determined to be primarily at the biochemical level, since the extent of stomatal closure was not significantly different for all genotypes. Expression of AtSAP5 resulted in the complete protection of photosystem (PS) II complexes from photodamage at mid-day after 4 d of drought, whereas wild-type plants experienced a 20% decline in active photosystem II (PSII) complexes. In addition, enhanced protection of seedling growth and leaf viability was associated with the expression of AtSAP5. Since A for the transgenic plants was significantly more heat tolerant than A for wild-type plants, we conclude that ectopic expression of SAP genes is a potentially viable approach to improving carbon gain and productivity for cotton grown in semi-arid regions with severe drought and heat stress.

Changes at the 3'-untranslated region stabilize Rubisco activase transcript levels during heat stress in Arabidopsis

Inhibition of photosynthesis by heat stress is accompanied by functional impairment of Rubisco's chaperone, activase (RCA), resulting in deactivation of Rubisco. Since activase is extremely sensitive to thermal denaturation, changes in expression of RCA at the transcript or protein level could provide a mechanism for acclimation of photosynthesis to prolonged heat stress. Using quantitative real-time PCR (qPCR) we show steady-state RCA transcript levels in Arabidopsis thaliana are stabilized during prolonged exposure to moderate heat (35  °C). A survey of RCA transcripts indicates heat stress did not alter the relative abundance of transcripts encoding α and β-isoforms of activase that are produced by alternative splicing of the pre-mRNA. Instead, mRNA stabilization in heat-stressed plants coincided with a significant reduction in the average length of activase 3'-untranslated regions, and was associated with enrichment of an uncharacterized activase mRNA splice variant, AtRCAβ2. Transcript-specific qPCR revealed AtRCAβ2 mRNA was more stable than AtRCAα and AtRCAβ mRNA in heat-stressed plants. Using an inducible transgenic system, we found that RCA transcripts lacking their native 3'-untranslated region were significantly more stable than their full-length counterparts in vivo. Using this system, stability of the RCA protein was examined over 24 h in vivo, in the absence of RCA transcription. At both optimal and elevated temperatures, RCA protein levels remained stable in plants lacking RCA mRNA, but increased when RCA mRNA was present, particularly in heat-stressed plants. This study reveals a possible mechanism, involving post-transcriptional regulation of an important photosynthesis regulatory gene, for acclimation of photosynthesis to heat stress.

A meta-analysis of plant physiological and growth responses to temperature and elevated CO(2)

Atmospheric carbon dioxide (CO(2)) and global mean temperature are expected to be significantly higher by the end of the 21st century. Elevated CO(2) (eCO(2)) and higher temperature each affect plant physiology and growth, but their interactive effects have not been reviewed statistically with respect to higher chronic mean temperatures and abrupt heat stress. In this meta-analysis, we examined the effect of CO(2) on the physiology and growth of plants subjected to different temperature treatments. The CO(2) treatments were categorized into ambient (<400 ppm) or elevated (>560 ppm) levels, while temperature treatments were categorized into ambient temperature (AT), elevated temperature (ET; AT + 1.4-6°C), or heat stress (HS; AT + >8°C). Plant species were grouped according to photosynthetic pathways (C(3), C(4)), functional types (legumes, non-legumes), growth forms (herbaceous, woody), and economic purposes (crop, non-crop). eCO(2) enhanced net photosynthesis at AT, ET, and HS in C(3) species (especially at the HS level), but in C(4) species, it had no effect at AT, a positive effect at ET, and a negative effect at HS. The positive effect of eCO(2) on net photosynthesis was greater for legumes than for non-legumes at HS, for non-crops than crops at ET, and for woody than herbaceous species at ET and HS. Total (W (T)) and above- (W (AG)) and below-ground (W (BG)) biomass were increased by eCO(2) for most species groups at all temperatures, except for C(4) species and W (BG) of legumes at HS. Hence, eCO(2) × heat effects on growth were often not explained by effects on net photosynthesis. Overall, the results show that eCO(2) effects on plant physiology and growth vary under different temperature regimes, among functional groups and photosynthetic pathways, and among response variables. These findings have important implications for biomass accumulation and ecosystem functioning in the future when the CO(2) level is higher and climate extremes, such as heat waves, become more frequent.

Photosynthetic responses to heat treatments at different temperatures and following recovery in grapevine (Vitis amurensis L.) leaves

BACKGROUND

The electron transport chain, Rubisco and stomatal conductance are important in photosynthesis. Little is known about their combined responses to heat treatment at different temperatures and following recovery in grapevines (Vitis spp.) which are often grown in climates with high temperatures.

METHODOLOGY/FINDINGS

The electron transport function of photosystem II, the activation state of Rubisco and the influence of stomatal behavior were investigated in grapevine leaves during heat treatments and following recovery. High temperature treatments included 35, 40 and 45°C, with 25°C as the control and recovery temperature. Heat treatment at 35°C did not significantly (P>0.05) inhibit net photosynthetic rate (P(n)). However, with treatments at 40 and 45°C, P(n) was decreased, accompanied by an increase in substomatal CO(2) concentration (C(i)), decreases in stomatal conductance (g(s)) and the activation state of Rubisco, and inhibition of the donor side and the reaction center of PSII. The acceptor side of PSII was inhibited at 45°C but not at 40°C. When grape leaves recovered following heat treatment, P(n), g(s) and the activation state of Rubisco also increased, and the donor side and the reaction center of PSII recovered. The increase in P(n) during the recovery period following the second 45°C stress was slower than that following the 40°C stress, and these increases corresponded to the donor side of PSII and the activation state of Rubisco.

CONCLUSIONS

Heat treatment at 35°C did not significantly (P>0.05) influence photosynthesis. The decrease of P(n) in grape leaves exposed to more severe heat stress (40 or 45°C) was mainly attributed to three factors: the activation state of Rubisco, the donor side and the reaction center of PSII. However, the increase of P(n) in grape leaves following heat stress was also associated with a stomatal response. The acceptor side of PSII in grape leaves was responsive but less sensitive to heat stress.

Structural changes associated with the acute thermal instability of Rubisco activase

Inhibition of photosynthesis by heat has been linked to the instability of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) chaperone, Rubisco activase. Examination of the recombinant enzyme showed that ADP and ATP protected against inactivation, whereas Mg(2+) promoted inactivation. Heating caused aggregation of Rubisco activase characterized by disruption of secondary structure content and formation of insoluble protein. In contrast, incubation at room temperature without nucleotide caused the active approximately 660 kDa protein to form a soluble, but inactive aggregate of > 2 x 10(6) Da. Circular dichroism (CD) spectroscopy and fluorescence established that structural perturbations in the aggregate did not reduce alpha-helical content significantly. Differences in the thermal stability between wild type and mutant Rubisco activase were observed for the recombinant proteins and when the proteins were expressed in transgenic Arabidopsis. That the sensitivity of these plants to heat differs indicates that the thermal instability of Rubisco activase is a main determinant of the temperature-sensitivity of photosynthesis.

Physiological and molecular changes in Oryza meridionalis Ng., a heat-tolerant species of wild rice

Oryza meridionalis Ng. is a wild relative of Oryza sativa L. found throughout northern Australia where temperatures regularly exceed 35 degrees C in the monsoon growing season. Heat tolerance in O. meridionalis was established by comparing leaf elongation and photosynthetic rates at 45 degrees C with plants maintained at 27 degrees C. By comparison with O. sativa ssp. japonica cv. Amaroo, O. meridionalis was heat tolerant. Elongation rates of the third leaf of O. meridionalis declined by 47% over 24 h at 45 degrees C compared with a 91% decrease for O. sativa. Net photosynthesis was significantly higher in O. sativa at 27 degrees C whereas the two species had the same assimilation rates at 45 degrees C. The leaf proteome and expression levels of individual heat-responsive genes provided insight into the heat response of O. meridionalis. After 24 h of heat exposure, many enzymes involved in the Calvin Cycle were more abundant, while mRNA of their genes generally decreased. Ferredoxin-NADP(H) oxidoreductase, a key enzyme in photosynthetic electron transport had both reduced abundance and gene expression, suggesting light reactions were highly susceptible to heat stress. Rubisco activase was strongly up-regulated after 24 h of heat, with the large isoform having the largest relative increase in protein abundance and a significant increase in gene expression. The protective proteins Cpn60, Hsp90, and Hsp70 all increased in both protein abundance and gene expression. A thiamine biosynthesis protein (THI1), previously shown to act protectively against stress, increased in abundance during heat, even as thiamine levels fell in O. meridionalis.

The differential response of photosynthesis to high temperature for a boreal and temperate Populus species relates to differences in Rubisco activation and Rubisco activase properties

Significant inhibition of photosynthesis occurs at temperatures only a few degrees (<or= 10 degrees C) above the optimum, resulting in a considerable loss of potential productivity. Most studies of heat stress have focused on crop or weedy annual plants, whereas similar studies with trees have been limited in number. As temperature is a major factor limiting the geographic ranges of most plants, the aim of this study was to use two Populus species adapted to contrasting thermal environments for determining the factors that constrain photosynthetic assimilation (A) under moderate heat stress in tree species. Consistent with its native range in temperate regions, Populus deltoides Bartr. ex Marsh. exhibited a significantly higher temperature optimum for A than did Populus balsamifera L., a boreal species. The higher A exhibited by P. deltoides at 33-40 degrees C compared to that for P. balsamifera was associated with a higher activation state of Rubisco and correlated with a higher ATPase activity of Rubisco activase. The temperature response of minimal chlorophyll a fluorescence for darkened leaves was similar for both species and was not consistent with a thylakoid lipid phase change contributing to the decline in A in the range of 30-40 degrees C. Taken together, these data support the idea that the differences in the temperature response of A for the two Populus species could be attributed to the differences in the response of Rubisco activation and ultimately to the thermal properties of Rubisco activase. That the primary sequence of Rubisco activase differed between the species, especially in regions associated with ATPase activity and Rubisco recognition, indicates that the genotypic differences in Rubisco activase might underlie the differences in the heat sensitivity of Rubisco activase and photosynthesis at moderately high temperatures.

Effect of Rubisco activase deficiency on the temperature response of CO2 assimilation rate and Rubisco activation state: insights from transgenic tobacco with reduced amounts of Rubisco activase

The activation of Rubisco in vivo requires the presence of the regulatory protein Rubisco activase. To elucidate its role in maintaining CO(2) assimilation rate at high temperature, we examined the temperature response of CO(2) assimilation rate at 380 microL L(-1) CO(2) concentration (A(380)) and Rubisco activation state in wild-type and transgenic tobacco (Nicotiana tabacum) with reduced Rubisco activase content grown at either 20 degrees C or 30 degrees C. Analyses of gas exchange and chlorophyll fluorescence showed that in the wild type, A(380) was limited by ribulose 1,5-bisphosphate regeneration at lower temperatures, whereas at higher temperatures, A(380) was limited by ribulose 1,5-bisphosphate carboxylation irrespective of growth temperatures. Growth temperature induced modest differences in Rubisco activation state that declined with measuring temperature, from mean values of 76% at 15 degrees C to 63% at 40 degrees C in wild-type plants. At measuring temperatures of 25 degrees C and below, an 80% reduction in Rubisco activase content was required before Rubisco activation state was decreased. Above 35 degrees C, Rubisco activation state decreased slightly with more modest decreases in Rubisco activase content, but the extent of the reductions in Rubisco activation state were small, such that a 55% reduction in Rubisco activase content did not alter the temperature sensitivity of Rubisco activation and had no effect on in vivo catalytic turnover rates of Rubisco. There was a strong correlation between Rubisco activase content and Rubisco activation state once Rubisco activase content was less that 20% of wild type at all measuring temperatures. We conclude that reduction in Rubisco activase content does not lead to an increase in the temperature sensitivity of Rubisco activation state in tobacco.

A comparative proteomic analysis of rice seedlings under various high-temperature stresses

To understand the responses of rice seedlings to different high-temperature stresses, seven-day-old rice seedlings were exposed to different high temperatures for 48 h, and the maximal quantum yield of PS II photochemistry measurements, ascorbate peroxidase activity assays and proteomic analyses in leaf tissue were performed. The results showed that when rice seedlings were exposed to high temperatures at 35 degrees C, 40 degrees C and 45 degrees C, the maximal quantum yield of photosystem II photochemistry, the activity of ascorbate peroxidase and the proteome changed greater at higher temperature. The proteomics analysis showed that proteins such as lignification-related proteins were regulated by high temperature and distinct proteins related to protection were up-regulated at different high temperatures. All the results indicated that different strategies were adopted at different levels of high temperature: the higher the temperature, the more protection machineries were involved. At 35 degrees C, some protective mechanisms were activated to maintain the photosynthetic capability. At 40 degrees C, antioxidative pathways were also active. When rice seedlings encountered high-temperature stress at 45 degrees C, in addition to those induced at 35 degrees C and 40 degrees C, heat shock proteins were effectively induced.

EFFECTS OF MODERATE HEAT STRESS AND DISSOLVED INORGANIC CARBON CONCENTRATION ON PHOTOSYNTHESIS AND RESPIRATION OF SYMBIODINIUM SP. (DINOPHYCEAE) IN CULTURE AND IN SYMBIOSIS(1)

The influence of temperature and inorganic carbon (Ci ) concentration on photosynthesis was examined in whole corals and samples of cultured symbiotic dinoflagellates (Symbiodinium sp.) using combined measurements from a membrane inlet mass spectrometer and chl a fluorometer. In whole corals, O2 production at 26°C was significantly limited at Ci concentrations below ambient seawater (∼2.2 mM). Further additions of Ci up to ∼10 mM caused no further stimulation of oxygenic photosynthesis. Following exposure to 30°C (2 d), net oxygen production decreased significantly in whole corals, as a result of reduced production of photosynthetically derived oxygen rather than increased host consumption. Whole corals maintained a rate of oxygen evolution around eight times lower than cultured Symbiodinium sp. at inorganic carbon concentrations <2 mM, but cultures displayed greater levels of photoinhibition following heat treatment (30°C, 2 d). Whole corals and cultured zooxanthellae differed considerably in their responses to Ci concentration and moderate heat stress, demonstrating that cultured Symbiodinium make an incongruous model for those in hospite. Reduced net oxygen evolution, in whole corals, under conditions of low Ci (<2 mM) has been interpreted in terms of possible sink limitation leading to increased nonphotochemical energy dissipation. The advantages of combined measurement of net gas exchange and fluorometry offered by this method are discussed.