Ionizing Radiation Activates Mitochondrial Function in Osteoclasts and Causes Bone Loss in Young Adult Male Mice

The damaging effects of ionizing radiation (IR) on bone mass are well-documented in mice and humans and are most likely due to increased osteoclast number and function. However, the mechanisms leading to inappropriate increases in osteoclastic bone resorption are only partially understood. Here, we show that exposure to multiple fractions of low-doses (10 fractions of 0.4 Gy total body irradiation [TBI]/week, i.e., fractionated exposure) and/or a single exposure to the same total dose of 4 Gy TBI causes a decrease in trabecular, but not cortical, bone mass in young adult male mice. This damaging effect was associated with highly activated bone resorption. Both osteoclast differentiation and maturation increased in cultures of bone marrow-derived macrophages from mice exposed to either fractionated or singular TBI. IR also increased the expression and enzymatic activity of mitochondrial deacetylase Sirtuin-3 (Sirt3)-an essential protein for osteoclast mitochondrial activity and bone resorption in the development of osteoporosis. Osteoclast progenitors lacking Sirt3 exposed to IR exhibited impaired resorptive activity. Taken together, targeting impairment of osteoclast mitochondrial activity could be a novel therapeutic strategy for IR-induced bone loss, and Sirt3 is likely a major mediator of this effect.

Glycolate oxidase-dependent H2O2 production regulates IAA biosynthesis in rice

BACKGROUND

Glycolate oxidase (GLO) is not only a key enzyme in photorespiration but also a major engine for H₂O₂ production in plants. Catalase (CAT)-dependent H₂O₂ decomposition has been previously reported to be involved in the regulation of IAA biosynthesis. However, it is still not known which mechanism contributed to the H₂O₂ production in IAA regulation.

RESULTS

In this study, we found that in glo mutants of rice, as H₂O₂ levels decreased IAA contents significantly increased, whereas high CO₂ abolished the difference in H₂O₂ and IAA contents between glo mutants and WT. Further analyses showed that tryptophan (Trp, the precursor for IAA biosynthesis in the Trp-dependent biosynthetic pathway) also accumulated due to increased tryptophan synthetase β (TSB) activity. Moreover, expression of the genes involved in Trp-dependent IAA biosynthesis and IBA to IAA conversion were correspondingly up-regulated, further implicating that both pathways contribute to IAA biosynthesis as mediated by the GLO-dependent production of H₂O₂.

CONCLUSION

We investigated the function of GLO in IAA signaling in different levels from transcription, enzyme activities to metabolic levels. The results suggest that GLO-dependent H₂O₂ signaling, essentially via photorespiration, confers regulation over IAA biosynthesis in rice plants.

An illuminated respiratory activity monitoring system identifies priming-active compounds in plant seedlings

BACKGROUND

Growing large crop monocultures and heavily using pesticides enhances the evolution of pesticide-insensitive pests and pathogens. To reduce pesticide use in crop cultivation, the application of priming-active compounds (PrimACs) is a welcome alternative. PrimACs strengthen the plant immune system and could thus help to protect plants with lower amounts of pesticides. PrimACs can be identified, for example, by their capacity to enhance the respiratory activity of parsley cells in culture as determined by the oxygen transfer rate (OTR) using the respiration activity monitoring system (RAMOS) or its miniaturized version, µRAMOS. The latter was designed for with suspensions of bacteria and yeast cells in microtiter plates (MTPs). So far, RAMOS or µRAMOS have not been applied to adult plants or seedlings, which would overcome the limitation of (µ)RAMOS to plant suspension cell cultures.

RESULTS

In this work, we introduce a modified µRAMOS for analysis of plant seedlings. The novel device allows illuminating the seedlings and records the respiratory activity in each well of a 48-well MTP. To validate the suitability of the setup for identifying novel PrimAC in Arabidopsis thaliana, seedlings were grown in MTP for seven days and treated with the known PrimAC salicylic acid (SA; positive control) and the PrimAC candidate methyl 1-(3,4-dihydroxyphenyl)-2-oxocyclopentane-1-carboxylate (Tyr020). Twenty-eight h after treatment, the seedlings were elicited with flg22, a 22-amino acid peptide of bacterial flagellin. Upon elicitation, the respiratory activity was monitored. The evaluation of the OTR course reveals Tyr020 as a likely PrimAC. The priming-inducing activity of Tyr020 was confirmed using molecular biological analyses in A. thaliana seedlings.

CONCLUSION

We disclose the suitability of µRAMOS for identifying PrimACs in plant seedlings. The difference in OTR during a night period between primed and unprimed plants was distinguishable after elicitation with flg22. Thus, it has been shown that the µRAMOS device can be used for a reliable screening for PrimACs in plant seedlings.

The gene expression profiles of mitochondrial respiratory components in Arabidopsis plants with differing amounts of ALTERNATIVE OXIDASE1a under high intensity light

We compared the expression of mitochondrial alternative oxidase (AOX) and other non-phosphorylating respiratory components (NPhPs) in wild type and AOX1a transgenic Arabidopsis thaliana following short-term transfer of plants to higher irradiance conditions to gain more insight into the mechanisms of AOX functioning under light. The AOX1a overexpressing line (XX-2) showed the highest amount of AOX1a transcripts and AOX1A synthesis during the entire experiment, and many NPhPs genes were down-regulated after 6-8 h under the higher light conditions. Antisense AS-12 plants displayed a compensatory effect, typically after 8 h of exposure to higher irradiance, by up-regulating their expression of the majority of genes encoding AOX and other respiratory components. In addition, AS-12 plants displayed 'overcompensation effects' prior to their transfer to high light conditions, i.e., they showed a higher expression level of certain genes. As a result, the ROS content in AS-12, as in XX-2, was consistently lower than in the wild type. All NPhPs genes share, in common with AOX1a, light- and stress-related cis-acting regulatory elements (CAREs) in their promoters. However, the expression of respiratory genes does not always depend on the level of AOX1a expression. This suggests the presence of multiple combinations of signaling pathways in gene induction. Based on our results, we outline possible directions for future research.

A Synthetic Photorespiratory Shortcut Enhances Photosynthesis to Boost Biomass and Grain Yield in Rice

Several photorespiratory bypasses have been introduced into plants and shown to improve photosynthesis by increasing chloroplastic CO₂ concentrations or optimizing energy balance. We recently reported that an engineered GOC bypass could increase photosynthesis and productivity in rice. However, the grain yield of GOC plants was unstable, fluctuating in different cultivation seasons because of varying seed setting rates. In this study, we designed a synthetic photorespiratory shortcut (the GCGT bypass) consisting of genes encoding Oryza sativa glycolate oxidase and Escherichia coli catalase, glyoxylate carboligase, and tartronic semialdehyde reductase. The GCGT bypass was guided by an optimized chloroplast transit peptide that targeted rice chloroplasts and redirected 75% of carbon from glycolate metabolism to the Calvin cycle, identical to the native photorespiration pathway. GCGT transgenic plants exhibited significantly increased biomass production and grain yield, which were mainly attributed to enhanced photosynthesis due to increased chloroplastic CO₂ concentrations. Despite the increases in biomass production and grain yield, GCGT transgenic plants showed a reduced seed setting rate, a phenotype previously reported for the GOC plants. Integrative transcriptomic, physiological, and biochemical assays revealed that photosynthetic carbohydrates were not transported to grains in an efficient manner, thereby reducing the seed setting rate. Taken together, our results demonstrate that the GCGT photorespiratory shortcut confers higher yield by promoting photosynthesis in rice, mainly through increasing chloroplastic CO₂ concentrations.

In planta study of photosynthesis and photorespiration using NADPH and NADH/NAD+ fluorescent protein sensors

The challenge of monitoring in planta dynamic changes of NADP(H) and NAD(H) redox states at the subcellular level is considered a major obstacle in plant bioenergetics studies. Here, we introduced two circularly permuted yellow fluorescent protein sensors, iNAP and SoNar, into Arabidopsis thaliana to monitor the dynamic changes in NADPH and the NADH/NAD+ ratio. In the light, photosynthesis and photorespiration are linked to the redox states of NAD(P)H and NAD(P) pools in several subcellular compartments connected by the malate-OAA shuttles. We show that the photosynthetic increases in stromal NADPH and NADH/NAD+ ratio, but not ATP, disappear when glycine decarboxylation is inhibited. These observations highlight the complex interplay between chloroplasts and mitochondria during photosynthesis and support the suggestions that, under normal conditions, photorespiration supplies a large amount of NADH to mitochondria, exceeding its NADH-dissipating capacity, and the surplus NADH is exported from the mitochondria to the cytosol through the malate-OAA shuttle.

The role of cyanide-resistant respiration in Solanum tuberosum L. against high light stress

Cyanide-resistant respiration in potato mitochondria is an important pathway for energy dissipation. It can be activated by high light; however, it is unclear what roles cyanide-resistant respiration plays in the response to high light stress in potato. We designed a CRISPR vector for the functional gene StAOX of the potato cyanide-resistant respiratory pathway. Agrobacterium tumefaciens GV3101 was transformed into potato. Hydrogen peroxide level, MDA content, antioxidant activity and cyanide-resistant respiratory capacity of potato leaves under high light stress were determined. Photosynthetic efficiency and chlorophyll content were determined. In addition, the operation of the malate-oxaloacetate shuttle route and transcription level of photorespiration-related enzymes were also examined. The results showed that two base substitutions occurred at the sequencing target site on leaves of the transformed potato. Accumulation of ROS and increased membrane lipid peroxidation were detected in the transformed potato leaves and lower photosynthetic efficiency was observed. The transcription level of the malate-oxaloacetate shuttle route and photorespiration-related enzymes also significantly increased. These results indicate that the cyanide-resistant respiration is an important physiological pathway in potato in response to high light stress. It also suggests that plant cyanide-resistant respiration is closely related to photosynthesis. This implies the unexplored importance of plant cyanide-resistant respiration in plant photosynthesis, energy conversion and carbon skeleton formation.

Molecular and physiological responses during thermal acclimation of leaf photosynthesis and respiration in rice

To further our understanding of how sustained changes in temperature affect the carbon economy of rice (Oryza sativa), hydroponically grown plants of the IR64 cultivar were developed at 30°C/25°C (day/night) before being shifted to 25/20°C or 40/35°C. Leaf messenger RNA and protein abundance, sugar and starch concentrations, and gas-exchange and elongation rates were measured on preexisting leaves (PE) already developed at 30/25°C or leaves newly developed (ND) subsequent to temperature transfer. Following a shift in growth temperature, there was a transient adjustment in metabolic gene transcript abundance of PE leaves before homoeostasis was reached within 24 hr, aligning with R_dark (leaf dark respiratory CO₂ release) and A_n (net CO₂ assimilation) changes. With longer exposure, the central respiratory protein cytochrome c oxidase (COX) declined in abundance at 40/35°C. In contrast to R_dark , A_n was maintained across the three growth temperatures in ND leaves. Soluble sugars did not differ significantly with growth temperature, and growth was fastest with extended exposure at 40/35°C. The results highlight that acclimation of photosynthesis and respiration is asynchronous in rice, with heat-acclimated plants exhibiting a striking ability to maintain net carbon gain and growth when exposed to heat-wave temperatures, even while reducing investment in energy-conserving respiratory pathways.

Symbiosis extended: exchange of photosynthetic O2 and fungal-respired CO2 mutually power metabolism of lichen symbionts

Lichens are a symbiosis between a fungus and one or more photosynthetic microorganisms that enables the symbionts to thrive in places and conditions they could not compete independently. Exchanges of water and sugars between the symbionts are the established mechanisms that support lichen symbiosis. Herein, we present a new linkage between algal photosynthesis and fungal respiration in lichen Flavoparmelia caperata that extends the physiological nature of symbiotic co-dependent metabolisms, mutually boosting energy conversion rates in both symbionts. Measurements of electron transport by oximetry show that photosynthetic O₂ is consumed internally by fungal respiration. At low light intensity, very low levels of O₂ are released, while photosynthetic electron transport from water oxidation is normal as shown by intrinsic chlorophyll variable fluorescence yield (period-4 oscillations in flash-induced Fv/Fm). The rate of algal O₂ production increases following consecutive series of illumination periods, at low and with limited saturation at high light intensities, in contrast to light saturation in free-living algae. We attribute this effect to arise from the availability of more CO₂ produced by fungal respiration of photosynthetically generated sugars. We conclude that the lichen symbionts are metabolically coupled by energy conversion through exchange of terminal electron donors and acceptors used in both photosynthesis and fungal respiration. Algal sugars and O₂ are consumed by the fungal symbiont, while fungal delivered CO₂ is consumed by the alga.

Mechanistic understanding of photorespiration paves the way to a new green revolution

Photorespiration is frequently considered a wasteful and inefficient process. However, mutant analysis demonstrated that photorespiration is essential for recycling of 2-phosphoglycolate in C₃ and C₄ land plants, in algae, and even in cyanobacteria operating carboxysome-based carbon (C) concentrating mechanisms. Photorespiration links photosynthetic C assimilation with other metabolic processes, such as nitrogen and sulfur assimilation, as well as C₁ metabolism, and it may contribute to balancing the redox poise between chloroplasts, peroxisomes, mitochondria and cytoplasm. The high degree of metabolic interdependencies and the pleiotropic phenotypes of photorespiratory mutants impedes the distinction between core and accessory functions. Newly developed synthetic bypasses of photorespiration, beyond holding potential for significant yield increases in C₃ crops, will enable us to differentiate between essential and accessory functions of photorespiration.

What is the fate of xylem-transported CO2 in Kranz-type C4 plants?

High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C₃ plants exit leaves in the dark. In the light, C₃ species use a small portion of xylem-transported CO₂ for leaf photosynthesis. However, it is not known if xylem-transported CO₂ will exit leaves in the dark or be used for photosynthesis in the light in Kranz-type C₄ plants. Cut leaves of Amaranthus hypochondriacus were placed in one of three solutions of [NaH¹³ CO₃ ] dissolved in KCl water to measure the efflux of xylem-transported CO₂ exiting the leaf in the dark or rates of assimilation of xylem-transported CO₂ * in the light, in real-time, using a tunable diode laser absorption spectroscope. In the dark, the efflux of xylem-transported CO₂ increased with increasing rates of transpiration and [¹³ CO₂ *]; however, rates of ¹³ C_efflux in A. hypochondriacus were lower compared to C₃ species. In the light, A. hypochondriacus fixed nearly 75% of the xylem-transported CO₂ supplied to the leaf. Kranz anatomy and biochemistry likely influence the efflux of xylem-transported CO₂ out of cut leaves of A. hypochondriacus in the dark, as well as the use of xylem-transported CO₂ * for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz-type C₄ species over C₃ species.

Predicting dark respiration rates of wheat leaves from hyperspectral reflectance

Greater availability of leaf dark respiration (R_dark ) data could facilitate breeding efforts to raise crop yield and improve global carbon cycle modelling. However, the availability of R_dark data is limited because it is cumbersome, time consuming, or destructive to measure. We report a non-destructive and high-throughput method of estimating R_dark from leaf hyperspectral reflectance data that was derived from leaf R_dark measured by a destructive high-throughput oxygen consumption technique. We generated a large dataset of leaf R_dark for wheat (1380 samples) from 90 genotypes, multiple growth stages, and growth conditions to generate models for R_dark . Leaf R_dark (per unit leaf area, fresh mass, dry mass or nitrogen, N) varied 7- to 15-fold among individual plants, whereas traits known to scale with R_dark , leaf N, and leaf mass per area (LMA) only varied twofold to fivefold. Our models predicted leaf R_dark , N, and LMA with r² values of 0.50-0.63, 0.91, and 0.75, respectively, and relative bias of 17-18% for R_dark and 7-12% for N and LMA. Our results suggest that hyperspectral model prediction of wheat leaf R_dark is largely independent of leaf N and LMA. Potential drivers of hyperspectral signatures of R_dark are discussed.

Using a reaction-diffusion model to estimate day respiration and reassimilation of (photo)respired CO2 in leaves

Methods using gas exchange measurements to estimate respiration in the light (day respiration R d ) make implicit assumptions about reassimilation of (photo)respired CO2 ; however, this reassimilation depends on the positions of mitochondria. We used a reaction-diffusion model without making these assumptions to analyse datasets on gas exchange, chlorophyll fluorescence and anatomy for tomato leaves. We investigated how R d values obtained by the Kok and the Yin methods are affected by these assumptions and how those by the Laisk method are affected by the positions of mitochondria. The Kok method always underestimated R d . Estimates of R d by the Yin method and by the reaction-diffusion model agreed only for nonphotorespiratory conditions. Both the Yin and Kok methods ignore reassimilation of (photo)respired CO2 , and thus underestimated R d for photorespiratory conditions, but this was less so in the Yin than in the Kok method. Estimates by the Laisk method were affected by assumed positions of mitochondria. It did not work if mitochondria were in the cytosol between the plasmamembrane and the chloroplast envelope. However, mitochondria were found to be most likely between the tonoplast and chloroplasts. Our reaction-diffusion model effectively estimates R d , enlightens the dependence of R d estimates on reassimilation and clarifies (dis)advantages of existing methods.

Effects of moisture dynamics on bryophyte carbon fluxes in a tropical cloud forest

Bryophytes play key roles in the ecological function of a number of major world biomes but remain understudied compared with vascular plants. Little is known about bryophyte responses to different aspects of predicted changes in moisture dynamics with climate change. In this study, CO₂ fluxes and photosynthetic light responses were measured within bryophyte mesocosms, being subjected to different amounts, frequencies, and types (mist or rainfall) of water addition, both before and after different periods of complete desiccation. Bryophyte carbon fluxes and photosynthetic light response were generally affected by the magnitude and type, but not frequency, of watering events. Desiccation suppressed bryophyte carbon uptake even after rehydration, and the degree of uptake suppression progressively increased with desiccation duration. Estimated ecosystem-level bryophyte respiration and net carbon uptake were c. 58% and c. 3%, respectively, of corresponding fluxes from tree foliage at the site. Our results suggest that a simplified representation of precipitation processes may be sufficient to accurately model bryophyte carbon cycling under future climate scenarios. Further, we find that projected increases in drought could have strong negative impacts on bryophyte and ecosystem carbon storage, with major consequences for a wide range of ecosystem processes.

Cross-compartment metabolic coupling enables flexible photoprotective mechanisms in the diatom Phaeodactylum tricornutum

Photoacclimation consists of short- and long-term strategies used by photosynthetic organisms to adapt to dynamic light environments. Observable photophysiology changes resulting from these strategies have been used in coarse-grained models to predict light-dependent growth and photosynthetic rates. However, the contribution of the broader metabolic network, relevant to species-specific strategies and fitness, is not accounted for in these simple models. We incorporated photophysiology experimental data with genome-scale modeling to characterize organism-level, light-dependent metabolic changes in the model diatom Phaeodactylum tricornutum. Oxygen evolution and photon absorption rates were combined with condition-specific biomass compositions to predict metabolic pathway usage for cells acclimated to four different light intensities. Photorespiration, an ornithine-glutamine shunt, and branched-chain amino acid metabolism were hypothesized as the primary intercompartment reductant shuttles for mediating excess light energy dissipation. Additionally, simulations suggested that carbon shunted through photorespiration is recycled back to the chloroplast as pyruvate, a mechanism distinct from known strategies in photosynthetic organisms. Our results suggest a flexible metabolic network in P. tricornutum that tunes intercompartment metabolism to optimize energy transport between the organelles, consuming excess energy as needed. Characterization of these intercompartment reductant shuttles broadens our understanding of energy partitioning strategies in this clade of ecologically important primary producers.

Responses of respiration in the light to warming in field-grown trees: a comparison of the thermal sensitivity of the Kok and Laisk methods

The Kok and Laisk techniques can both be used to estimate light respiration R_light . We investigated whether responses of R_light to short- and long-term changes in leaf temperature depend on the technique used to estimate R_light . We grew Eucalyptus tereticornis in whole-tree chambers under ambient temperature (AT) or AT + 3°C (elevated temperature, ET). We assessed dark respiration R_dark and light respiration with the Kok (R_Kok ) and Laisk (R_Laisk ) methods at four temperatures to determine the degree of light suppression of respiration using both methods in AT and ET trees. The ET treatment had little impact on R_dark , R_Kok or R_Laisk . Although the thermal sensitivities of R_Kok or R_Laisk were similar, R_Kok was higher than R_Laisk . We found negative values of R_Laisk at the lowest measurement temperatures, indicating positive net CO₂ uptake, which we propose may be related to phosphoenolpyruvate carboxylase activity. Light suppression of R_dark decreased with increasing leaf temperature, but the degree of suppression depended on the method used. The Kok and Laisk methods do not generate the same estimates of R_light or light suppression of R_dark between 20 and 35°C. Negative rates of R_Laisk imply that this method may become less reliable at low temperatures.

Engineering a New Chloroplastic Photorespiratory Bypass to Increase Photosynthetic Efficiency and Productivity in Rice

Over the past few years, three photorespiratory bypasses have been introduced into plants, two of which led to observable increases in photosynthesis and biomass yield. However, most of the experiments were carried out using Arabidopsis under controlled environmental conditions, and the increases were only observed under low-light and short-day conditions. In this study, we designed a new photorespiratory bypass (called GOC bypass), characterized by no reducing equivalents being produced during a complete oxidation of glycolate into CO₂ catalyzed by three rice-self-originating enzymes, i.e., glycolate oxidase, oxalate oxidase, and catalase. We successfully established this bypass in rice chloroplasts using a multi-gene assembly and transformation system. Transgenic rice plants carrying GOC bypass (GOC plants) showed significant increases in photosynthesis efficiency, biomass yield, and nitrogen content, as well as several other CO₂-enriched phenotypes under both greenhouse and field conditions. Grain yield of GOC plants varied depending on seeding season and was increased significantly in the spring. We further demonstrated that GOC plants had significant advantages under high-light conditions and that the improvements in GOC plants resulted primarily from a photosynthetic CO₂-concentrating effect rather than from improved energy balance. Taken together, our results reveal that engineering a newly designed chloroplastic photorespiratory bypass could increase photosynthetic efficiency and yield of rice plants grown in field conditions, particularly under high light.

Losing a winner: thermal stress and local pressures outweigh the positive effects of ocean acidification for tropical seagrasses

Seagrasses are globally important coastal habitat-forming species, yet it is unknown how seagrasses respond to the combined pressures of ocean acidification and warming of sea surface temperature. We exposed three tropical species of seagrass (Cymodocea serrulata, Halodule uninervis, and Zostera muelleri) to increasing temperature (21, 25, 30, and 35°C) and pCO₂ (401, 1014, and 1949 μatm) for 7 wk in mesocosms using a controlled factorial design. Shoot density and leaf extension rates were recorded, and plant productivity and respiration were measured at increasing light levels (photosynthesis-irradiance curves) using oxygen optodes. Shoot density, growth, photosynthetic rates, and plant-scale net productivity occurred at 25°C or 30°C under saturating light levels. High pCO₂ enhanced maximum net productivity for Z. muelleri, but not in other species. Z. muelleri was the most thermally tolerant as it maintained positive net production to 35°C, yet for the other species there was a sharp decline in productivity, growth, and shoot density at 35°C, which was exacerbated by pCO₂ . These results suggest that thermal stress will not be offset by ocean acidification during future extreme heat events and challenges the current hypothesis that tropical seagrass will be a 'winner' under future climate change conditions.

More than just CO2 -recycling: corticular photosynthesis as a mechanism to reduce the risk of an energy crisis induced by low oxygen

Reassimilation of internal CO₂ via corticular photosynthesis (PS_cort ) has an important effect on the carbon economy of trees. However, little is known about its role as a source of O₂ supply to the stem parenchyma and its implications in consumption and movement of O₂ within trees. PS_cort of young Populus nigra (black poplar) trees was investigated by combining optical micro-optode measurements with monitoring of stem chlorophyll fluorescence. During times of zero sap flow in spring, stem oxygen concentrations (cO₂ ) exhibited large temporal changes. In the sapwood, over 80% of diurnal changes in cO₂ could be explained by respiration rates (R_d(mod) ). In the cortex, photosynthetic oxygen release during the day altered this relationship. With daytime illumination, oxygen levels in the cortex steadily increased from subambient and even exhibited a diel period of superoxia of up to 110% (% air sat.). By contrast, in the sapwood, cO₂ never reached ambient levels; the diurnal oxygen deficit was up to 25% of air saturation. Our results confirm that PS_cort is not only a CO₂ -recycling mechanism, it is also a mechanism to actively raise the cortical O₂ concentration and counteract temporal/spatial hypoxia inside plant stems.

Determination of leaf respiration in the light: comparison between an isotopic disequilibrium method and the Laisk method

Quantification of leaf respiration is important for understanding plant physiology and ecosystem biogeochemical processes. Leaf respiration continues in the light (R_L ) but supposedly at a lower rate than in the dark (R_Dk ). However, there is no method for direct measurement of R_L and the available methods require nonphysiological measurement conditions. A method based on isotopic disequilibrium quantified R_L (R_L13C ) and mesophyll conductance of young and old fully expanded leaves of six species. R_L13C was compared to R_L determined by the Laisk method (R_L Laisk ) on the very same leaves with a minimum time lag. R_L 13C and R_L Laisk were generally lower than R_Dk , and were not significantly affected by leaf ageing. R_L Laisk and R_L 13C were positively correlated (r²  = 0.35), and both were positively correlated with R_Dk (r²  ≥ 0.6). R_L Laisk was systematically lower than R_L 13C by 0.4 μmol m^-2  s^-1 . Using A/C_c instead of A/C_i curves, a higher photocompensation point Γ* (by 5 μmol mol^-1 ) was found but no influence on R_L Laisk estimates was observed. The results imply that the Laisk method underestimates actual R_L significantly, probably related to the measurement condition of low CO₂ and irradiance. The isotopic disequilibrium method is useful for assessing responses of R_L to irradiance and CO₂ , improving our mechanistic understanding of R_L .

The energy budget in C4 photosynthesis: insights from a cell-type-specific electron transport model

Extra ATP required in C₄ photosynthesis for the CO₂ -concentrating mechanism probably comes from cyclic electron transport (CET). As metabolic ATP : NADPH requirements in mesophyll (M) and bundle-sheath (BS) cells differ among C₄ subtypes, the subtypes may differ in the extent to which CET operates in these cells. We present an analytical model for cell-type-specific CET and linear electron transport. Modelled NADPH and ATP production were compared with requirements. For malic-enzyme (ME) subtypes, c. 50% of electron flux is CET, occurring predominantly in BS cells for standard NADP-ME species, but in a ratio of c. 6 : 4 in BS : M cells for NAD-ME species. Some C₄ acids follow a secondary decarboxylation route, which is obligatory, in the form of 'aspartate-malate', for the NADP-ME subtype, but facultative, in the form of phosphoenolpyruvate-carboxykinase (PEP-CK), for the NAD-ME subtype. The percentage for secondary decarboxylation is c. 25% and that for 3-phosphoglycerate reduction in BS cells is c. 40%; but these values vary with species. The 'pure' PEP-CK type is unrealistic because its is impossible to fulfil ATP : NADPH requirements in BS cells. The standard PEP-CK subtype requires negligible CET, and thus has the highest intrinsic quantum yields and deserves further studies in the context of improving canopy productivity.

Mechanisms of glacial-to-future atmospheric CO2 effects on plant immunity

The impacts of rising atmospheric CO2 concentrations on plant disease have received increasing attention, but with little consensus emerging on the direct mechanisms by which CO2 shapes plant immunity. Furthermore, the impact of sub-ambient CO2 concentrations, which plants have experienced repeatedly over the past 800 000 yr, has been largely overlooked. A combination of gene expression analysis, phenotypic characterisation of mutants and mass spectrometry-based metabolic profiling was used to determine development-independent effects of sub-ambient CO2 (saCO2 ) and elevated CO2 (eCO2 ) on Arabidopsis immunity. Resistance to the necrotrophic Plectosphaerella cucumerina (Pc) was repressed at saCO2 and enhanced at eCO2 . This CO2 -dependent resistance was associated with priming of jasmonic acid (JA)-dependent gene expression and required intact JA biosynthesis and signalling. Resistance to the biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa) increased at both eCO2 and saCO2 . Although eCO2 primed salicylic acid (SA)-dependent gene expression, mutations affecting SA signalling only partially suppressed Hpa resistance at eCO2 , suggesting additional mechanisms are involved. Induced production of intracellular reactive oxygen species (ROS) at saCO2 corresponded to a loss of resistance in glycolate oxidase mutants and increased transcription of the peroxisomal catalase gene CAT2, unveiling a mechanism by which photorespiration-derived ROS determined Hpa resistance at saCO2 . By separating indirect developmental impacts from direct immunological effects, we uncover distinct mechanisms by which CO2 shapes plant immunity and discuss their evolutionary significance.

Carbon allocation to major metabolites in illuminated leaves is not just proportional to photosynthesis when gaseous conditions (CO2 and O2 ) vary

In gas-exchange experiments, manipulating CO₂ and O₂ is commonly used to change the balance between carboxylation and oxygenation. Downstream metabolism (utilization of photosynthetic and photorespiratory products) may also be affected by gaseous conditions but this is not well documented. Here, we took advantage of sunflower as a model species, which accumulates chlorogenate in addition to sugars and amino acids (glutamate, alanine, glycine and serine). We performed isotopic labelling with ¹³ CO₂ under different CO₂ /O₂ conditions, and determined ¹³ C contents to compute ¹³ C-allocation patterns and build-up rates. The ¹³ C content in major metabolites was not found to be a constant proportion of net fixed carbon but, rather, changed dramatically with CO₂ and O₂ . Alanine typically accumulated at low O₂ (hypoxic response) while photorespiratory intermediates accumulated under ambient conditions and at high photorespiration, glycerate accumulation exceeding serine and glycine build-up. Chlorogenate synthesis was relatively more important under normal conditions and at high CO₂ and its synthesis was driven by phosphoenolpyruvate de novo synthesis. These findings demonstrate that carbon allocation to metabolites other than photosynthetic end products is affected by gaseous conditions and therefore the photosynthetic yield of net nitrogen assimilation varies, being minimal at high CO₂ and maximal at high O₂ .

Aldehyde dehydrogenases and the hypothesis of a glycolaldehyde shunt pathway of photorespiration

Aldehyde dehydrogenase enzymes (ALDHs) catalyse the oxidation of a broad range of aliphatic and aromatic aldehydes to their corresponding carboxylic acids using NAD⁺ or NADP⁺ as cofactors. In our article published in Scientific Reports, we demonstrated that mutations in Arabidopsis ALDH3I1 and ALDH7B4 genes altered the cellular contents of NAD(P)H, the total as well as the reduction state of glutathione; and decreased the efficiency of photosynthesis, thus placing ALDH activity as an important source of reducing power for cellular redox homeostasis. Our results also revealed that the ALDHs contribute to the reducing power required for the nitrate assimilation. Here, we discussed and elucidated the innovative hypothesis of the glycolaldehyde shunt pathway of photorespiration that would involve ALDHs generating in contrast to the known core photorespiration reactions, a net gain of two moles of NAD(P)H to support nitrate assimilation, glutathione homeostasis and ROS detoxification.

Tissue breathing and topology of rats thymocytes surface under acute total γ-irradiation

OBJECTIVE

Assessment of the effect of single total γ irradiation to the parameters of mitochondrial oxidation and the topology of the thymocyte surface.

MATERIALS AND METHODS

The study was performed in sexually mature white outbreeding male rats divided into three groups: two experimental and one control. The states of energy metabolism were determined by the rate of oxygen consumption by the thymus tissues on endogenous substrates at the presence of 2,4 dinitrophenol, uncoupler of a tissue breathing (TB) and oxidative phosphorylation (OP) after a single total γ irradiation at a dose of 1.0 Gy at 3, 10, 40 and 60 days. The topology of thymus cells was assessed using atomic force microscopy (AFM) and scanning electron microscopy (SEM).

RESULTS

On the 3rd and 10th days after total gamma irradiation at a dose of 1.0 Gy, a significant decrease in respira tory activity was determined in thymus tissues on endogenous substrates. Simultaneously, on the 3rd day, pro nounced changes in the morphological parameters of thymocytes (height, volume, area of contact with the sub strate) and the topology of their surface were also observed. On the 10th day after irradiation, most of the morpho logical parameters of thymocytes, except for their volume, were characterized by restoration to normal. In the long term (on the 30th and 60th days after exposure), a gradual but not complete recovery of the respiratory activity of thymocytes was observed, accompanied by an increase in the degree of dissociation of TD and OP.

CONCLUSIONS

The obtained data reflect and refine mechanisms of post radiation repair of lymphopoiesis, showing the presence of conjugated changes in the parameters of aerobic energy metabolism of thymocytes, morphology and topology of their surface. The synchronism of changes in the parameters under study is a reflection of the state of the cytoskeleton, the functional activity of which largely depends on the level and efficiency of mitochondrial oxidation.

Photosynthate partitioning to starch in Arabidopsis thaliana is insensitive to light intensity but sensitive to photoperiod due to a restriction on growth in the light in short photoperiods

Photoperiod duration can be predicted from previous days, but irradiance fluctuates in an unpredictable manner. To investigate how allocation to starch responds to changes in these two environmental variables, Arabidopsis Col-0 was grown in a 6 h and a 12 h photoperiod at three different irradiances. The absolute rate of starch accumulation increased when photoperiod duration was shortened and when irradiance was increased. The proportion of photosynthate allocated to starch increased strongly when photoperiod duration was decreased but only slightly when irradiance was decreased. There was a small increase in the daytime level of sucrose and twofold increases in glucose, fructose and glucose 6-phosphate at a given irradiance in short photoperiods compared to long photoperiods. The rate of starch accumulation correlated strongly with sucrose and glucose levels in the light, irrespective of whether these sugars were responding to a change in photoperiod or irradiance. Whole plant carbon budget modelling revealed a selective restriction of growth in the light period in short photoperiods. It is proposed that photoperiod sensing, possibly related to the duration of the night, restricts growth in the light period in short photoperiods, increasing allocation to starch and providing more carbon reserves to support metabolism and growth in the long night.

Does triacylglycerol (TAG) serve a photoprotective function in plant leaves? An examination of leaf lipids under shading and drought

Plant survival in many ecosystems requires tolerance of large radiation loads, unreliable water supply and suboptimal soil fertility. We hypothesized that increased production of neutral lipids (triacylglycerols, TAGs) in plant leaves is a mechanism for dissipating excess radiation energy. In a greenhouse experiment, we combined drought and shade treatments and examined responses among four species differing in life form, habitat, and drought- and shade-tolerance. We also present a lipid extraction protocol suitable for sclerophyllous leaves of native Australian trees (e.g. Acacia, Eucalyptus). Fluorescence measurements indicated that plants exposed to full sunlight experienced mild photoinhibition during our experiment. Accumulation of TAGs did not follow photosynthetic capacity, but instead, TAG concentration increased with non-photochemical quenching. This suggests that plants under oxidative stress may increase biosynthesis of TAGs. Moderate drought stress resulted in a 60% reduction in TAG concentration in wheat (Triticum aestivum). Shading had no effect on TAGs, but increased concentrations of polar lipids in leaves; for example, acclimation to shade in Austrodanthonia spp., a native Australian grass, resulted in a 60% increase in associated polar lipids and higher foliar chlorophyll concentrations. Shading also reduced the digalactosyldiacylglycerol:monogalactosyldiacylglycerol (DGDG:MGDG) ratio in leaves, with a corresponding increase in the degree of unsaturation and thus fluidity of thylakoid membranes of chloroplasts. Our results suggest that prevention of photodamage may be coordinated with accumulation of TAGs, although further research is required to determine if TAGs serve a photoprotective function in plant leaves.

Land plants drive photorespiration as higher electron-sink: comparative study of post-illumination transient O2 -uptake rates from liverworts to angiosperms through ferns and gymnosperms

In higher plants, the electron-sink capacity of photorespiration contributes to alleviation of photoinhibition by dissipating excess energy under conditions when photosynthesis is limited. We addressed the question at which point in the evolution of photosynthetic organisms photorespiration began to function as electron sink and replaced the flavodiiron proteins which catalyze the reduction of O₂ at photosystem I in cyanobacteria. Algae do not have a higher activity of photorespiration when CO₂ assimilation is limited, and it can therefore not act as an electron sink. Using land plants (liverworts, ferns, gymnosperms, and angiosperms) we compared photorespiration activity and estimated the electron flux driven by photorespiration to evaluate its electron-sink capacity at CO₂ -compensation point. In vivo photorespiration activity was estimated by the simultaneous measurement of O₂ -exchange rate and chlorophyll fluorescence yield. All C3-plants leaves showed transient O₂ -uptake after actinic light illumination (post-illumination transient O₂ -uptake), which reflects photorespiration activity. Post-illumination transient O₂ -uptake rates increased in the order from liverworts to angiosperms through ferns and gymnosperms. Furthermore, photorespiration-dependent electron flux in photosynthetic linear electron flow was estimated from post-illumination transient O₂ -uptake rate and compared with the electron flux in photosynthetic linear electron flow in order to evaluate the electron-sink capacity of photorespiration. The electron-sink capacity at the CO₂ -compensation point also increased in the above order. In gymnosperms photorespiration was determined to be the main electron-sink. C3-C4 intermediate species of Flaveria plants showed photorespiration activity, which intermediate between that of C3- and C4-flaveria species. These results indicate that in the first land plants, liverworts, photorespiration started to function as electron sink. According to our hypothesis, the dramatic increase in partial pressure of O₂ in the atmosphere about 0.4 billion years ago made it possible to drive photorespiration with higher activity in liverworts.

Persistent negative temperature response of mesophyll conductance in red raspberry (Rubus idaeus L.) leaves under both high and low vapour pressure deficits: a role for abscisic acid?

The temperature dependence of mesophyll conductance (g_m ) was measured in well-watered red raspberry (Rubus idaeus L.) plants acclimated to leaf-to-air vapour pressure deficit (VPDL) daytime differentials of contrasting amplitude, keeping a fixed diurnal leaf temperature (T_leaf ) rise from 20 to 35 °C. Contrary to the great majority of g_m temperature responses published to date, we found a pronounced reduction of g_m with increasing T_leaf irrespective of leaf chamber O₂ level and diurnal VPDL regime. Leaf hydraulic conductance was greatly enhanced during the warmer afternoon periods under both low (0.75 to 1.5 kPa) and high (0.75 to 3.5 kPa) diurnal VPDL regimes, unlike stomatal conductance (g_s ), which decreased in the afternoon. Consequently, the leaf water status remained largely isohydric throughout the day, and therefore cannot be evoked to explain the diurnal decrease of g_m . However, the concerted diurnal reductions of g_m and g_s were well correlated with increases in leaf abscisic acid (ABA) content, thus suggesting that ABA can induce a significant depression of g_m under favourable leaf water status. Our results challenge the view that the temperature dependence of g_m can be explained solely from dynamic leaf anatomical adjustments and/or from the known thermodynamic properties of aqueous solutions and lipid membranes.​.

Biochemical model of C3 photosynthesis applied to wheat at different temperatures

We examined the effects of leaf temperature on the estimation of maximal Rubisco capacity (V_cmax ) from gas exchange measurements of wheat leaves using a C₃ photosynthesis model. Cultivars of spring wheat (Triticum aestivum (L)) and triticale (X Triticosecale Wittmack) were grown in a greenhouse or in the field and measured at a range of temperatures under controlled conditions in a growth cabinet (2 and 21% O₂ ) or in the field using natural diurnal variation in temperature, respectively. Published Rubisco kinetic constants for tobacco did not describe the observed CO₂ response curves well as temperature varied. By assuming values for the Rubisco Michaelis-Menten constants for CO₂ (K_c ) and O₂ (K_o ) at 25 °C derived from tobacco and the activation energies of V_cmax from wheat and respiration in the light, R_d , from tobacco, we derived activation energies for K_c and K_o (93.7 and 33.6 kJ mol^-1 , respectively) that considerably improved the fit of the model to observed data. We confirmed that temperature dependence of dark respiration for wheat was well described by the activation energy for R_d from tobacco. The new parameters improved the estimation of V_cmax under field conditions, where temperatures increased through the day.

Impaired Mitochondrial Transcription Termination Disrupts the Stromal Redox Poise in Chlamydomonas

In photosynthetic eukaryotes, the metabolite exchange between chloroplast and mitochondria ensures efficient photosynthesis under saturating light conditions. The Chlamydomonas reinhardtii mutant stm6 is devoid of the mitochondrial transcription termination factor MOC1 and aberrantly expresses the mitochondrial genome, resulting in enhanced photosynthetic hydrogen production and diminished light tolerance. We analyzed the modulation of mitochondrial and chlororespiration during the acclimation of stm6 and the MOC1-complemented strain to excess light. Although light stress stimulated mitochondrial respiration via the energy-conserving cytochrome c pathway in both strains, the mutant was unable to fine-tune the expression and activity of oxidative phosphorylation complex I in excess light, which was accompanied by an increased mitochondrial respiration via the alternative oxidase pathway. Furthermore, stm6 failed to fully activate chlororespiration and cyclic electron flow due to a more oxidized state of the chloroplast stroma, which is caused by an increased mitochondrial electron sink capacity. Increased susceptibility to photoinhibition of PSII in stm6 demonstrates that the MOC1-dependent modulation of mitochondrial respiration helps control the stromal redox poise as a crucial part of high-light acclimation in C. reinhardtii.

The redox control of photorespiration: from biochemical and physiological aspects to biotechnological considerations

Photorespiration is a complex and tightly regulated process occurring in photosynthetic organisms. This process can alter the cellular redox balance, notably via the production and consumption of both reducing and oxidizing equivalents. Under certain circumstances, these equivalents, as well as reactive oxygen or nitrogen species, can become prominent in subcellular compartments involved in the photorespiratory process, eventually promoting oxidative post-translational modifications of proteins. Keeping these changes under tight control should therefore be of primary importance. In order to review the current state of knowledge about the redox control of photorespiration, we primarily performed a careful description of the known and potential redox-regulated or oxidation sensitive photorespiratory proteins, and examined in more details two interesting cases: the glycerate kinase and the glycine cleavage system. When possible, the potential impact and subsequent physiological regulations associated with these changes have been discussed. In the second part, we reviewed the extent to which photorespiration contributes to cellular redox homeostasis considering, in particular, the set of peripheral enzymes associated with the canonical photorespiratory pathway. Finally, some recent biotechnological strategies to circumvent photorespiration for future growth improvements are discussed in the light of these redox regulations.

NADPH oxidases differentially regulate ROS metabolism and nutrient uptake under cadmium toxicity

The role of NADPH oxidases under cadmium (Cd) toxicity was studied using Arabidopsis thaliana mutants AtrbohC, AtrbohD and AtrbohF, which were grown under hydroponic conditions with 25 and 100 μM Cd for 1 and 5 days. Cadmium reduced the growth of leaves in WT, AtrbohC and D, but not in AtrbohF. A time-dependent increase in H₂ O₂ and lipid peroxidation was observed in all genotypes, with AtrbohC showing the smallest increase. An opposite behaviour was observed with NO accumulation. Cadmium increased catalase activity in WT plants and decreased it in Atrboh mutants, while glutathione reductase and glycolate oxidase activities increased in Atrboh mutants, and superoxide dismutases were down-regulated in AtrbohC. The GSH/GSSG and ASA/DHA couples were also affected by the treatment, principally in AtrbohC and AtrbohF, respectively. Cadmium translocation to the leaves was severely reduced in Atrboh mutants after 1 day of treatment and even after 5 days in AtrbohF. Similar results were observed for S, P, Ca, Zn and Fe accumulation, while an opposite trend was observed for K accumulation, except in AtrbohF. Thus, under Cd stress, RBOHs differentially regulate ROS metabolism, redox homeostasis and nutrient balance and could be of potential interest in biotechnology for the phytoremediation of polluted soils.

Atmospheric CO2 mole fraction affects stand-scale carbon use efficiency of sunflower by stimulating respiration in light

Plant carbon-use-efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO₂ . Sunflower stands were grown at low (200 μmol mol^-1 ) or high CO₂ (1000 μmol mol^-1 ) in controlled environment mesocosms. CUE of stands was measured by dynamic stand-scale ¹³ C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO₂ (compared with low CO₂ ) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO₂ (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand-scale respiratory metabolism at high CO₂ . Two main processes contributed to the reduction of CUE at high CO₂ : a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions.

Differences in photosynthetic responses of NADP-ME type C4 species to high light

MAIN CONCLUSION

Three species chosen as representatives of NADP-ME C4 subtype exhibit different sensitivity toward photoinhibition, and great photochemical differences were found to exist between the species. These characteristics might be due to the imbalance in the excitation energy between the photosystems present in M and BS cells, and also due to that between species caused by the penetration of light inside the leaves. Such regulation in the distribution of light intensity between M and BS cells shows that co-operation between both the metabolic systems determines effective photosynthesis and reduces the harmful effects of high light on the degradation of PSII through the production of reactive oxygen species (ROS). We have investigated several physiological parameters of NADP-ME-type C4 species (e.g., Zea mays, Echinochloa crus-galli, and Digitaria sanguinalis) grown under moderate light intensity (200 µmol photons m-2 s-1) and, subsequently, exposed to excess light intensity (HL, 1600 µmol photons m-2 s-1). Our main interest was to understand why these species, grown under identical conditions, differ in their responses toward high light, and what is the physiological significance of these differences. Among the investigated species, Echinochloa crus-galli is best adapted to HL treatment. High resistance of the photosynthetic apparatus of E. crus-galli to HL was accompanied by an elevated level of phosphorylation of PSII proteins, and higher values of photochemical quenching, ATP/ADP ratio, activity of PSI and PSII complexes, as well as integrity of the thylakoid membranes. It was also shown that the non-radiative dissipation of energy in the studied plants was not dependent on carotenoid contents and, thus, other photoprotective mechanisms might have been engaged under HL stress conditions. The activity of the enzymes superoxide dismutase and ascorbate peroxidase as well as the content of malondialdehyde and H2O2 suggests that antioxidant defense is not responsible for the differences observed in the tolerance of NADP-ME species toward HL stress. We concluded that the chloroplasts of the examined NADP-ME species showed different sensitivity to short-term high light irradiance, suggesting a role of other factors excluding light factors, thus influencing the response of thylakoid proteins. We also observed that HL affects the mesophyll chloroplasts first hand and, subsequently, the bundle sheath chloroplasts.

Temperature and CO2 dependency of the photosynthetic photon flux density responses of leaves of Vitis vinifera cvs. Chardonnay and Merlot grown in a hot climate

Comparisons of the photosynthetic responses to light and temperature between related cultivars are important to understand how well matched they are to the climate where they are grown. Photosynthetic light responses at a range of leaf temperatures and two CO₂ concentrations were measured on leaves of two grapevine cultivars (Vitis vinifera L.) Chardonnay and Merlot vines growing in field conditions. The objective was to assess the interaction between photon flux density (PFD), leaf temperature and CO₂ on photosynthesis and to compare the two cultivars. Merlot leaves maintained higher light-saturated rates of photosynthesis at all leaf temperatures compared with the Chardonnay leaves. At low temperatures, a reduced photon yield offset with a high stomatal conductance accounted for the low rates of the Chardonnay leaves. At moderate to high temperatures, photon yields, PFDs at light saturation and stomatal conductances did not account for differences between Merlot and Chardonnay leaves. At elevated CO₂ (800 μmol mol^-1) concentrations, the differences in photosynthetic performance between the cultivars were enhanced, with 30% higher light saturated rates for Merlot compared with Chardonnay leaves. Merlot berries accumulated more sugar, consistent with published data. These results demonstrate Chardonnay, unlike Merlot, appeared to be poorly matched to the hot climate. However, considering the current market and political trends, low alcoholic wines (and, thus, low sugar grapes) should be preferred. Especially in hot climates, it is always hard to obtain such kind of wines and, thus, the most interesting agronomical challenge, especially for Chardonnay vines could be interpreted in an opposite way.

Impaired Cyclic Electron Flow around Photosystem I Disturbs High-Light Respiratory Metabolism

The cyclic electron flow around photosystem I (CEF-PSI) increases ATP/NADPH production in the chloroplast, acting as an energy balance mechanism. Higher export of reducing power from the chloroplast in CEF-PSI mutants has been correlated with higher mitochondrial alternative oxidase (AOX) capacity and protein amount under high-light (HL) conditions. However, in vivo measurements of AOX activity are still required to confirm the exact role of AOX in dissipating the excess of reductant power from the chloroplast. Here, CEF-PSI single and double mutants were exposed to short-term HL conditions in Arabidopsis (Arabidopsis thaliana). Chlorophyll fluorescence, in vivo activities of the cytochrome oxidase (νcyt) and AOX (νalt) pathways, levels of mitochondrial proteins, metabolite profiles, and pyridine nucleotide levels were determined under normal growth and HL conditions. νalt was not increased in CEF-PSI mutants, while AOX capacity was positively correlated with photoinhibition, probably due to a reactive oxygen species-induced increase of AOX protein. The severe metabolic impairment observed in CEF-PSI mutants, as indicated by the increase in photoinhibition and changes in the levels of stress-related metabolites, can explain their lack of νalt induction. By contrast, νcyt was positively correlated with photosynthetic performance. Correlations with metabolite changes suggest that νcyt is coordinated with sugar metabolism and stress-related amino acid synthesis. Furthermore, changes in glycine-serine and NADH-NAD+ ratios were highly correlated to νcyt Taken together, our results suggest that νcyt can act as a sink for the excess of electrons from the chloroplast, probably via photorespiratory glycine oxidation, thus improving photosynthetic performance when νalt is not induced under severe HL stress.

Unravelling the in vivo regulation and metabolic role of the alternative oxidase pathway in C3 species under photoinhibitory conditions

The mitochondrial alternative oxidase pathway (AOP) has been suggested to act as a sink for excess reducing power generated in the chloroplast under high-light (HL) stress and thus may reduce photoinhibition. The aim of this study was to compare different species to investigate the in vivo regulation and role of AOP under HL stress. The in vivo activities of AOP (νalt ) and the cytochrome oxidase pathway, chlorophyll fluorescence, metabolite profiles, alternative oxidase (AOX) capacity and protein amount were determined in leaves of five C3 species under growth light and after HL treatment. Differences in respiration and metabolite levels were observed among species under growth light conditions. The HL response of νalt was highly species dependent, correlated with the AOP capacity and independent of AOX protein content. Nevertheless, significant correlations were observed between νalt , levels of key metabolites and photosynthetic parameters. The results show that the species-specific response of νalt is caused by the differential post-translational regulation of AOX. Significant correlations between respiration, metabolites and photosynthetic performance across species suggest that AOP may permit stress-related amino acid synthesis, whilst maintaining photosynthetic activity under HL stress.

The role of photorespiration during astaxanthin accumulation in Haematococcus pluvialis (Chlorophyceae)

Most previous studies on Haematococcus pluvialis have been focused on growth and astaxanthin accumulation. However, the relationships between photorespiration and astaxanthin accumulation have not been clarified. The purpose of this study was to examine the role of photorespiration during the process of astaxanthin accumulation in H. pluvialis. During astaxanthin accumulation, the astaxanthin content was reduced significantly when photorespiration was inhibited by its specific inhibitor, carboxymethoxylamine. The inhibition of photorespiration did not change the dry weight, chlorophyll content and OJIP transients during the incubation; however, the inhibition of photorespiration significantly decreased the photochemistry of photosystem II and total photosynthetic O2 evolution capacity. Moreover, the restriction in photorespiration was synchronized with a decrease of astaxanthin accumulation. These results suggest that the photorespiratory pathway in H. pluvialis can accelerate astaxanthin accumulation. We speculate that photorespiration can enhance astaxanthin accumulation in the following ways: (i) photorespiration directly affects the glycerate-3-phosphate (PGA) level, which is intrinsically related to the accumulation of astaxanthin in H. pluvialis; (ii) the photorespiratory pathway indirectly affects the PGA level by effecting the dark reactions of photosynthesis, which then results in the enhancement of astaxanthin accumulation in H. pluvialis.

Convergent acclimation of leaf photosynthesis and respiration to prevailing ambient temperatures under current and warmer climates in Eucalyptus tereticornis

Understanding physiological acclimation of photosynthesis and respiration is important in elucidating the metabolic performance of trees in a changing climate. Does physiological acclimation to climate warming mirror acclimation to seasonal temperature changes? We grew Eucalyptus tereticornis trees in the field for 14 months inside 9-m tall whole-tree chambers tracking ambient air temperature (Tair ) or ambient Tair  + 3°C (i.e. 'warmed'). We measured light- and CO2 -saturated net photosynthesis (Amax ) and night-time dark respiration (R) each month at 25°C to quantify acclimation. Tree growth was measured, and leaf nitrogen (N) and total nonstructural carbohydrate (TNC) concentrations were determined to investigate mechanisms of acclimation. Warming reduced Amax and R measured at 25°C compared to ambient-grown trees. Both traits also declined as mean daily Tair increased, and did so in a similar way across temperature treatments. Amax and R (at 25°C) both increased as TNC concentrations increased seasonally; these relationships appeared to arise from source-sink imbalances, suggesting potential substrate regulation of thermal acclimation. We found that photosynthesis and respiration each acclimated equivalently to experimental warming and seasonal temperature change of a similar magnitude, reflecting a common, nearly homeostatic constraint on leaf carbon exchange that will be important in governing tree responses to climate warming.

Variation potential-induced photosynthetic and respiratory changes increase ATP content in pea leaves

Local damage induces a physiological response in higher plants by means of generation and propagation of variation potential (VP). The response includes changes in photosynthesis and respiration. The aim of the present study was to investigate the effect of these changes on adenosine triphosphate (ATP) content in pea leaves. VP was induced by local heating of the first mature leaf and registered using extracellular and intracellular electrodes. Photosynthesis and respiration were measured using Dual-PAM-100 and GFS-3000. ATP content was determined using a bioluminescence-based ATP determination kit. Two non-stimulated leaves (second and fourth) were investigated. We showed that heating induced VP that propagated into the second mature leaf, but only a slight electrical reaction was registered in the fourth mature leaf. VP-induced inactivation of photosynthesis developed in the second leaf and included two stages: short- and long-term inactivation. Local heating also caused a two-stage increase in ATP content in the second leaf, which was connected with the photosynthetic responses. Changes in photosynthesis and ATP content were not observed in the fourth leaf. The effect of VP on respiration was investigated under dark conditions. We found that variation potential induced short-term activation of respiration in the second leaf. Local heating induced ATP content increase which included only one stage under dark conditions. Changes in ATP and respiration were absent in the fourth leaf under dark conditions. Thus, VP-induced photosynthetic and respiratory changes are likely to increase ATP content in pea leaves.

The influence of photoperiod and light intensity on the growth and photosynthesis of Dunaliella salina (chlorophyta) CCAP 19/30

The green microalga Dunaliella salina survives in a wide range of salinities via mechanisms involving glycerol synthesis and degradation and is exploited for large amounts of nutraceutical carotenoids produced under stressed conditions. In this study, D. salina CCAP 19/30 was cultured in varying photoperiods and light intensities to study the relationship of light with different growth measurement parameters, with cellular contents of glycerol, starch and carotenoids, and with photosynthesis and respiration. Results show CCAP 19/30 regulated cell volume when growing under light/dark cycles: cell volume increased in the light and decreased in the dark, and these changes corresponded to changes in cellular glycerol content. The decrease in cell volume in the dark was independent of cell division and biological clock and was regulated by the photoperiod of the light/dark cycle. When the light intensity was increased to above 1000 μmol photons m(-2) s(-1), cells displayed evidence of photodamage. However, these cells also maintained the maximum level of photosynthesis efficiency and respiration possible, and the growth rate increased as light intensity increased. Significantly, the intracellular glycerol content also increased, >2-fold compared to the content in light intensity of 500 μmol photons m(-2) s(-1), but there was no commensurate increase in the pool size of carotenoids. These data suggest that in CCAP 19/30 glycerol stabilized the photosynthetic apparatus for maximum performance in high light intensities, a role normally attributed to carotenoids.

Mitochondrial Alternative Pathway-Associated Photoprotection of Photosystem II is Related to the Photorespiratory Pathway

Respiratory electron transport has two ubiquinol-oxidizing pathways, the cytochrome pathway (CP) and the alternative pathway (AP). The AP, which is catalyzed by the alternative oxidase (AOX), is energetically wasteful but may alleviate PSII photoinhibition under light conditions excessive for photosynthesis. However, its mechanism remains unknown. We used Arabidopsis aox1a mutants lacking AOX activity and studied the mutation's effects on photoinhibition by measuring the decrease in the maximum quantum yield of PSII (F_v/F_m) after high light exposure. Since the CP compensates for the lack of AOX, we monitored the extent of photoinhibition under conditions where CP activity is partially inhibited by antimycin A. When leaves were exposed to high light at 350 µmol m^-2 s^-1, the decline in F_v/F_m was significantly faster in the aox1a mutants than in the wild type. However, under conditions where photorespiration was suppressed by high CO₂ or low O₂ levels, the decline in F_v/F_m was suppressed in the aox1a mutants, but not in the wild type, making the difference between the wild type and mutants small. Our results demonstrate that the lack of the AP causes an acceleration of PSII photoinhibition in relation to the photorespiratory pathway, suggesting that the AP can support the activity of the photorespiratory pathway under high light conditions.

The Water to Water Cycles in Microalgae

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

Seasonality of temperate forest photosynthesis and daytime respiration

Terrestrial ecosystems currently offset one-quarter of anthropogenic carbon dioxide (CO2) emissions because of a slight imbalance between global terrestrial photosynthesis and respiration. Understanding what controls these two biological fluxes is therefore crucial to predicting climate change. Yet there is no way of directly measuring the photosynthesis or daytime respiration of a whole ecosystem of interacting organisms; instead, these fluxes are generally inferred from measurements of net ecosystem-atmosphere CO2 exchange (NEE), in a way that is based on assumed ecosystem-scale responses to the environment. The consequent view of temperate deciduous forests (an important CO2 sink) is that, first, ecosystem respiration is greater during the day than at night; and second, ecosystem photosynthetic light-use efficiency peaks after leaf expansion in spring and then declines, presumably because of leaf ageing or water stress. This view has underlain the development of terrestrial biosphere models used in climate prediction and of remote sensing indices of global biosphere productivity. Here, we use new isotopic instrumentation to determine ecosystem photosynthesis and daytime respiration in a temperate deciduous forest over a three-year period. We find that ecosystem respiration is lower during the day than at night-the first robust evidence of the inhibition of leaf respiration by light at the ecosystem scale. Because they do not capture this effect, standard approaches overestimate ecosystem photosynthesis and daytime respiration in the first half of the growing season at our site, and inaccurately portray ecosystem photosynthetic light-use efficiency. These findings revise our understanding of forest-atmosphere carbon exchange, and provide a basis for investigating how leaf-level physiological dynamics manifest at the canopy scale in other ecosystems.

Suppression of Chloroplastic Alkenal/One Oxidoreductase Represses the Carbon Catabolic Pathway in Arabidopsis Leaves during Night

Lipid-derived reactive carbonyl species (RCS) possess electrophilic moieties and cause oxidative stress by reacting with cellular components. Arabidopsis (Arabidopsis thaliana) has a chloroplast-localized alkenal/one oxidoreductase (AtAOR) for the detoxification of lipid-derived RCS, especially α,β-unsaturated carbonyls. In this study, we aimed to evaluate the physiological importance of AtAOR and analyzed AtAOR (aor) mutants, including a transfer DNA knockout, aor (T-DNA), and RNA interference knockdown, aor (RNAi), lines. We found that both aor mutants showed smaller plant sizes than wild-type plants when they were grown under day/night cycle conditions. To elucidate the cause of the aor mutant phenotype, we analyzed the photosynthetic rate and the respiration rate by gas-exchange analysis. Subsequently, we found that both wild-type and aor (RNAi) plants showed similar CO2 assimilation rates; however, the respiration rate was lower in aor (RNAi) than in wild-type plants. Furthermore, we revealed that phosphoenolpyruvate carboxylase activity decreased and starch degradation during the night was suppressed in aor (RNAi). In contrast, the phenotype of aor (RNAi) was rescued when aor (RNAi) plants were grown under constant light conditions. These results indicate that the smaller plant sizes observed in aor mutants grown under day/night cycle conditions were attributable to the decrease in carbon utilization during the night. Here, we propose that the detoxification of lipid-derived RCS by AtAOR in chloroplasts contributes to the protection of dark respiration and supports plant growth during the night.

Different leaf cost-benefit strategies of ferns distributed in contrasting light habitats of sub-tropical forests

BACKGROUND AND AIMS

Ferns are abundant in sub-tropical forests in southern China, with some species being restricted to shaded understorey of natural forests, while others are widespread in disturbed, open habitats. To explain this distribution pattern, we hypothesize that ferns that occur in disturbed forests (FDF) have a different leaf cost-benefit strategy compared with ferns that occur in natural forests (FNF), with a quicker return on carbon investment in disturbed habitats compared with old-growth forests.

METHODS

We chose 16 fern species from contrasting light habitats (eight FDF and eight FNF) and studied leaf functional traits, including leaf life span (LLS), specific leaf area (SLA), leaf nitrogen and phosphorus concentrations (N and P), maximum net photosynthetic rates (A), leaf construction cost (CC) and payback time (PBT), to conduct a leaf cost-benefit analysis for the two fern groups.

KEY RESULTS

The two groups, FDF and FNF, did not differ significantly in SLA, leaf N and P, and CC, but FDF had significantly higher A, greater photosynthetic nitrogen- and phosphorus-use efficiencies (PNUE and PPUE), and shorter PBT and LLS compared with FNF. Further, across the 16 fern species, LLS was significantly correlated with A, PNUE, PPUE and PBT, but not with SLA and CC.

CONCLUSIONS

Our results demonstrate that leaf cost-benefit analysis contributes to understanding the distribution pattern of ferns in contrasting light habitats of sub-tropical forests: FDF employing a quick-return strategy can pre-empt resources and rapidly grow in the high-resource environment of open habitats; while a slow-return strategy in FNF allows their persistence in the shaded understorey of old-growth forests.

Photorespiration participates in the assimilation of acetate in Chlorella sorokiniana under high light

The development of microalgae on an industrial scale largely depends on the economic feasibility of mass production. High light induces productive suspensions during cultivation in a tubular photobioreactor. Herein, we report that high light, which inhibited the growth of Chlorella sorokiniana under autotrophic conditions, enhanced the growth of this alga in the presence of acetate. We compared pigments, proteomics and the metabolic flux ratio in C. sorokiniana cultivated under high light (HL) and under low light (LL) in the presence of acetate. Our results showed that high light induced the synthesis of xanthophyll and suppressed the synthesis of chlorophylls. Acetate in the medium was exhausted much more rapidly in HL than in LL. The data obtained from LC-MS/MS indicated that high light enhanced photorespiration, the Calvin cycle and the glyoxylate cycle of mixotrophic C. sorokiniana. The results of metabolic flux ratio analysis showed that the majority of the assimilated carbon derived from supplemented acetate, and photorespiratory glyoxylate could enter the glyoxylate cycle. Based on these data, we conclude that photorespiration provides glyoxylate to speed up the glyoxylate cycle, and releases acetate-derived CO2 for the Calvin cycle. Thus, photorespiration connects the glyoxylate cycle and the Calvin cycle, and participates in the assimilation of supplemented acetate in C. sorokiniana under high light.

Three-dimensional microscale modelling of CO2 transport and light propagation in tomato leaves enlightens photosynthesis

We present a combined three-dimensional (3-D) model of light propagation, CO2 diffusion and photosynthesis in tomato (Solanum lycopersicum L.) leaves. The model incorporates a geometrical representation of the actual leaf microstructure that we obtained with synchrotron radiation X-ray laminography, and was evaluated using measurements of gas exchange and leaf optical properties. The combination of the 3-D microstructure of leaf tissue and chloroplast movement induced by changes in light intensity affects the simulated CO2 transport within the leaf. The model predicts extensive reassimilation of CO2 produced by respiration and photorespiration. Simulations also suggest that carbonic anhydrase could enhance photosynthesis at low CO2 levels but had little impact on photosynthesis at high CO2 levels. The model confirms that scaling of photosynthetic capacity with absorbed light would improve efficiency of CO2 fixation in the leaf, especially at low light intensity.

Improved photosynthetic performance during severe drought in Nicotiana tabacum overexpressing a nonenergy conserving respiratory electron sink

Chloroplasts have means to manage excess reducing power but these mechanisms may become restricted by rates of ATP turnover. Alternative oxidase (AOX) is a mitochondrial terminal oxidase that uncouples the consumption of reducing power from ATP synthesis. Physiological and biochemical analyses were used to compare respiration and photosynthesis of Nicotiana tabacum wild-type (WT) plants with that of transgenic lines overexpressing AOX, under both well-watered and drought stress conditions. With increasing drought severity, AOX overexpression acted to increase respiration in the light (RL ) relative to WT. CO2 and light response curves indicated that overexpression also improved photosynthetic performance relative to WT, as drought severity increased. This was not due to an effect of AOX amount on leaf water status or the development of the diffusive limitations that occur due to drought. Rather, AOX overexpression dampened photosystem stoichiometry adjustments and losses of key photosynthetic components that occurred in WT. The results indicate that AOX amount influences RL , particularly during severe drought, when cytochrome pathway respiration may become increasingly restricted. This impacts the chloroplast redox state, influencing how the photosynthetic apparatus responds to increasing drought severity. In particular, the development of biochemical limitations to photosynthesis are dampened in plants with increased nonenergy conserving RL .