Red light photobiomodulation rescues murine brain mitochondrial respiration after acute hypobaric hypoxia.

Low-level laser therapy, or photobiomodulation, utilizes red or near-infrared light for the treatment of pathological conditions due to the presence of intracellular photoacceptors, such as mitochondrial cytochrome c oxidase, that serve as intermediates for the therapeutic effects. We present an in-detail analysis of the effect of low-intensity LED red light irradiation on the respiratory chain of brain mitochondria. We tested whether low-level laser therapy at 650 nm could alleviate the brain mitochondrial dysfunction in the model of acute hypobaric hypoxia in mice. The irradiation of the mitochondrial fraction of the left cerebral cortex with low-intensity LED red light rescued Complex I-supported respiration during oxidative phosphorylation, normalized the initial polarization of the inner mitochondrial membrane, but has not shown any significant effect on the activity of Complex IV. In comparison, the postponed effect (in 24 h) of the similar transcranial irradiation following hypoxic exposure led to a less pronounced improvement of the mitochondrial functional state, but normalized respiration related to ATP production and membrane polarization. In contrast, the similar irradiation of the mitochondria isolated from control healthy animals exerted an inhibitory effect on CI-supported respiration. The obtained results provide significant insight that can be beneficial for the development of non-invasive phototherapy.

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.

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.

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 ß (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.

Photobiomodulation and Oxidative Stress: 980 nm Diode Laser Light Regulates Mitochondrial Activity and Reactive Oxygen Species Production.

Photobiomodulation with 808 nm laser light electively stimulates Complexes III and IV of the mitochondrial respiratory chain, while Complexes I and II are not affected. At the wavelength of 1064 nm, Complexes I, III, and IV are excited, while Complex II and some mitochondrial matrix enzymes seem to be not receptive to photons at that wavelength. Complex IV was also activated by 633 nm. The mechanism of action of wavelengths in the range 900-1000 nm on mitochondria is less understood or not described. Oxidative stress from reactive oxygen species (ROS) generated by mitochondrial activity is an inescapable consequence of aerobic metabolism. The antioxidant enzyme system for ROS scavenging can keep them under control. However, alterations in mitochondrial activity can cause an increment of ROS production. ROS and ATP can play a role in cell death, cell proliferation, and cell cycle arrest. In our work, bovine liver isolated mitochondria were irradiated for 60 sec, in continuous wave mode with 980 nm and powers from 0.1 to 1.4 W (0.1 W increment at every step) to generate energies from 6 to 84 J, fluences from 7.7 to 107.7 J/cm2, power densities from 0.13 to 1.79 W/cm2, and spot size 0.78 cm2. The control was equal to 0 W. The activity of the mitochondria's complexes, Krebs cycle enzymes, ATP production, oxygen consumption, generation of ROS, and oxidative stress were detected. Lower powers (0.1-0.2 W) showed an inhibitory effect; those that were intermediate (0.3-0.7 W) did not display an effect, and the higher powers (0.8-1.1 W) induced an increment of ATP synthesis. Increasing the power (1.2-1.4 W) recovered the ATP production to the control level. The interaction occurred on Complexes III and IV, as well as ATP production and oxygen consumption. Results showed that 0.1 W uncoupled the respiratory chain and induced higher oxidative stress and drastic inhibition of ATP production. Conversely, 0.8 W kept mitochondria coupled and induced an increase of ATP production by increments of Complex III and IV activities. An augmentation of oxidative stress was also observed, probably as a consequence of the increased oxygen consumption and mitochondrial isolation experimental conditions. No effect was observed using 0.5 W, and no effect was observed on the enzymes of the Krebs cycle.

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.

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.

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 Rdark (leaf dark respiratory CO2 release) and An (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 Rdark , An 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.

Effects of the Photosystem II Inhibitors CCCP and DCMU on Hydrogen Production by the Unicellular Halotolerant Cyanobacterium Aphanothece halophytica.

The unicellular halotolerant cyanobacterium Aphanothece halophytica is a potential dark fermentative producer of molecular hydrogen (H2) that produces very little H2 under illumination. One factor limiting the H2 photoproduction of this cyanobacterium is an inhibition of bidirectional hydrogenase activity by oxygen (O2) obtained from splitting water molecules via photosystem II activity. The present study aimed to investigate the effects of the photosystem II inhibitors carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on H2 production of A. halophytica under light and dark conditions and on photosynthetic and respiratory activities. The results showed that A. halophytica treated with CCCP and DCMU produced H2 at three to five times the rate of untreated cells, when exposed to light. The highest H2 photoproduction rates, 2.26 ±â€Š0.24 and 3.63 ±â€Š0.26 µmol H2 g-1 dry weight h-1, were found in cells treated with 0.5 µM CCCP and 50 µM DCMU, respectively. Without inhibitor treatment, A. halophytica incubated in the dark showed a significant increase in H2 production compared with cells that were incubated in the light. Only CCCP treatment increased H2 production of A. halophytica during dark incubation, because CCCP functions as an uncoupling agent of oxidative phosphorylation. The highest dark fermentative H2 production rate of 39.50 ±â€Š2.13 µmol H2 g-1 dry weight h-1 was found in cells treated with 0.5 µM CCCP after 2 h of dark incubation. Under illumination, CCCP and DCMU inhibited chlorophyll fluorescence, resulting in a low level of O2, which promoted bidirectional hydrogenase activity in A. halophytica cells. In addition, only CCCP enhanced the respiration rate, further reducing the O2 level. In contrast, DCMU reduced the respiration rate in A. halophytica.

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 Cefflux 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.

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 Rd ) 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 Rd 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 Rd . Estimates of Rd 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 Rd 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 Rd , enlightens the dependence of Rd estimates on reassimilation and clarifies (dis)advantages of existing methods.

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.

Predicting dark respiration rates of wheat leaves from hyperspectral reflectance.

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

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.

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.

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 Rlight . We investigated whether responses of Rlight to short- and long-term changes in leaf temperature depend on the technique used to estimate Rlight . We grew Eucalyptus tereticornis in whole-tree chambers under ambient temperature (AT) or AT + 3°C (elevated temperature, ET). We assessed dark respiration Rdark and light respiration with the Kok (RKok ) and Laisk (RLaisk ) 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 Rdark , RKok or RLaisk . Although the thermal sensitivities of RKok or RLaisk were similar, RKok was higher than RLaisk . We found negative values of RLaisk at the lowest measurement temperatures, indicating positive net CO2 uptake, which we propose may be related to phosphoenolpyruvate carboxylase activity. Light suppression of Rdark 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 Rlight or light suppression of Rdark between 20 and 35°C. Negative rates of RLaisk imply that this method may become less reliable at low temperatures.