Physiological role of AOX1a in photosynthesis and maintenance of cellular redox homeostasis under high light in Arabidopsis thaliana

Acclimation of intertidal macroalgae Ulva prolifera to UVB radiation: the important role of alternative oxidase

BACKGROUND

Solar radiation is primarily composed of ultraviolet radiation (UVR, 200 - 400 nm) and photosynthetically active radiation (PAR, 400 - 700 nm). Ultraviolet-B (UVB) radiation accounts for only a small proportion of sunlight, and it is the primary cause of plant photodamage. The use of chlorofluorocarbons (CFCs) as refrigerants caused serious ozone depletion in the 1980s, and this had led to an increase in UVB. Although CFC emissions have significantly decreased in recent years, UVB radiation still remains at a high intensity. UVB radiation increase is an important factor that influences plant physiological processes. Ulva prolifera, a type of macroalga found in the intertidal zone, is intermittently exposed to UVB. Alternative oxidase (AOX) plays an important role in plants under stresses. This research examines the changes in AOX activity and the relationships among AOX, photosynthesis, and reactive oxygen species (ROS) homeostasis in U. prolifera under changes in UVB and photosynthetically active radiation (PAR).

RESULTS

UVB was the main component of solar radiation impacting the typical intertidal green macroalgae U. prolifera. AOX was found to be important during the process of photosynthesis optimization of U. prolifera due to a synergistic effect with non-photochemical quenching (NPQ) under UVB radiation. AOX and glycolate oxidase (GO) worked together to achieve NADPH homeostasis to achieve photosynthesis optimization under changes in PAR + UVB. The synergism of AOX with superoxide dismutase (SOD) and catalase (CAT) was important during the process of ROS homeostasis under PAR + UVB.

CONCLUSIONS

AOX plays an important role in the process of photosynthesis optimization and ROS homeostasis in U. prolifera under UVB radiation. This study provides further insights into the response of intertidal macroalgae to solar light changes.

Genotype-dependent Strategies to "Overcome" Excessive Light: Insights into Non-Photochemical Quenching under High Light Intensity

High light (HL) intensities have a significant impact on energy flux and distribution within photosynthetic apparatus. To understand the effect of high light intensity (HL) on the HL tolerance mechanisms in tomatoes, we examined the response of the photosynthesis apparatus of 12 tomato genotypes to HL. A reduced electron transfer per reaction center (ET0 /RC), an increased energy dissipation (DI0 /RC) and non-photochemical quenching (NPQ), along with a reduced maximum quantum yield of photosystem II (FV /FM ), and performance index per absorbed photon (PIABS ) were common HL-induced responses among genotypes; however, the magnitude of those responses was highly genotype-dependent. Tolerant and sensitive genotypes were distinguished based on chlorophyll fluorescence and energy-quenching responses to HL. Tolerant genotypes alleviated excess light through energy-dependent quenching (qE ), resulting in smaller photoinhibitory quenching (qI ) compared to sensitive genotypes. Quantum yield components also shifted under HL, favoring the quantum yield of NPQ (ՓNPQ ) and the quantum yield of basal energy loss (ՓN0 ), while reducing the efficient quantum yield of PSII (ՓPSII ). The impact of HL on tolerant genotypes was less pronounced. While the energy partitioning ratio did not differ significantly between sensitive and tolerant genotypes, the ratio of NPQ components, especially qI , affected plant resilience against HL. These findings provide insights into different patterns of HL-induced NPQ components in tolerant and sensitive genotypes, aiding the development of resilient crops for heterogeneous light conditions.

Acclimation strategies of the green alga Chlorella vulgaris to different light regimes revealed by physiological and comparative proteomic analyses

Acclimation to different light regimes is at the basis of survival for photosynthetic organisms, regardless of their evolutionary origin. Previous research efforts largely focused on acclimation events occurring at the level of the photosynthetic apparatus and often highlighted species-specific mechanisms. Here, we investigated the consequences of acclimation to different irradiances in Chlorella vulgaris, a green alga that is one of the most promising species for industrial application, focusing on both photosynthetic and mitochondrial activities. Moreover, proteomic analysis of cells acclimated to high light (HL) or low light (LL) allowed identification of the main targets of acclimation in terms of differentially expressed proteins. The results obtained demonstrate photosynthetic adaptation to HL versus LL that was only partially consistent with previous findings in Chlamydomonas reinhardtii, a model organism for green algae, but in many cases similar to vascular plant acclimation events. Increased mitochondrial respiration measured in HL-acclimated cells mainly relied on alternative oxidative pathway dissipating the excessive reducing power produced due to enhanced carbon flow. Finally, proteins involved in cell metabolism, intracellular transport, gene expression, and signaling-including a heliorhodopsin homolog-were identified as strongly differentially expressed in HL versus LL, suggesting their key roles in acclimation to different light regimes.

Brassinolide promotes interaction between chloroplasts and mitochondria during the optimization of photosynthesis by the mitochondrial electron transport chain in mesophyll cell protoplasts of Arabidopsis thaliana

The current experimental data unveils the role of brassinolide (BL), a phytohormone of class brassinosteroids (BRs), in augmenting the cross-talk between the mitochondrial electron transport chain (mETC) and chloroplasts to strengthen the efficiency of the Calvin-Benson cycle (CBC) for higher assimilation of carbon dioxide in the mesophyll cell protoplasts (MCP) of Arabidopsis thaliana. The outcome of total respiration (TR) and photosynthetic carbon assimilation (PCA) was monitored as O₂ uptake under dark and NaHCO₃-dependent O₂ evolution under light, respectively, after pre-incubation of MCP at a broad spectrum of BL concentration from 0.05 pM to 5 pM at 25 °C and optimum light intensity of 1000 μmol m^-2 s^-1. The addition of optimal concentration (0.5 pM) of BL to MCP stimulated the (i) TR, (ii) PCA, and (iii) para-benzoquinone-dependent O₂ evolution (PSII activity). Further, in response to BL, the enzyme activity or transcript levels of redox-regulated CBC enzymes and glucose-6-phosphate raised considerably. Also, the addition of BL to MCP remarkably accelerated the capacity of the cytochrome oxidase (COX) and alternative oxidase (AOX) pathways concurrently with an increase in total cellular pyruvate and reactive oxygen species (ROS) levels. Besides, malate valve components (Malate, Chl-MDH, M-MDH) increased in response to BL. At the same time, the cellular redox ratios of pyridine nucleotides (NADPH and NADH) were kept low in the presence of BL. However, BL could not keep up the CBC activity of photosynthesis along with its associated light-activated enzymes/transcripts when mETC through COX or AOX pathway is restricted by antimycin A (AA) or salicylhydroxamic acid (SHAM), respectively. In contrast, adding BL to MCP under restricted mETC showed aggravation in total cellular ROS, pyruvate, malate, and redox ratio of pyridine nucleotides with a concomitant increase in transcripts associated with malate valve and antioxidant systems. These results suggest that BL enhances the PCA by coordinating in cross-talk of chloroplasts and mitochondria to regulate the cellular redox ratio or ROS through the involvement of COX and AOX pathways along with the malate valve and antioxidant systems.

Limiting etioplast gene expression induces apical hook twisting during skotomorphogenesis of Arabidopsis seedlings

When covered by a layer of soil, seedling development follows a dark-specific program (skotomorphogenesis). In the dark, seedlings consist of small, non-green cotyledons, a long hypocotyl, and an apical hook to protect meristematic cells. We recently highlighted the role played by mitochondria in the high energy-consuming reprogramming of Arabidopsis skotomorphogenesis. Here, the role played by plastids, another energy-supplying organelle, in skotomorphogenesis is investigated. This study was conducted in dark conditions to exclude light signals so as to better focus on those produced by plastids. It was found that limitation of plastid gene expression (PGE) induced an exaggerated apical hook bending. Inhibition of PGE was obtained at the levels of transcription and translation using the antibiotics rifampicin (RIF) and spectinomycin, respectively, as well as plastid RPOTp RNA polymerase mutants. RIF-treated seedlings also showed expression induction of marker nuclear genes for mitochondrial stress, perturbation of mitochondrial metabolism, increased ROS levels, and an augmented capacity of oxygen consumption by mitochondrial alternative oxidases (AOXs). AOXs act to prevent overreduction of the mitochondrial electron transport chain. Previously, we reported that AOX1A, the main AOX isoform, is a key component in the developmental response to mitochondrial respiration deficiency. In this work, we suggest the involvement of AOX1A in the response to PGE dysfunction and propose the importance of signaling between plastids and mitochondria. Finally, it was found that seedling architecture reprogramming in response to RIF was independent of canonical organelle retrograde pathways and the ethylene signaling pathway.

Ascorbate synthesis as an alternative electron source for mitochondrial respiration: Possible implications for the plant performance

The molecule vitamin C, in the chemical form of ascorbic acid (AsA), is known to be essential for the metabolism of humans and animals. Humans do not produce AsA, so they depend on plants as a source of vitamin C for their food. The AsA synthesis pathway occurs partially in the cytosol, but the last oxidation step is physically linked to the respiratory chain of plant mitochondria. This oxidation step is catalyzed by l-galactono-1,4-lactone dehydrogenase (l-GalLDH). This enzyme is not considered a limiting step for AsA production; however, it presents a distinguishing characteristic: the l-GalLDH can introduce electrons directly into the respiratory chain through cytochrome c (Cytc) and therefore can be considered an extramitochondrial electron source that bypasses the phosphorylating Complex III. The use of Cytc as electron acceptor has been debated in terms of its need for AsA synthesis, but little has been said in relation to its impact on the functioning of the respiratory chain. This work seeks to offer a new view about the possible changes that result of the link between AsA synthesis and the mitochondrial respiration. We hypothesized that some physiological alterations related to low AsA may be not only explained by the deficiency of this molecule but also by the changes in the respiratory function. We discussed some findings showing that respiratory mutants contained changes in AsA synthesis. Besides, recent works that also indicate that the excessive electron transport via l-GalLDH enzyme may affect other respiratory pathways. We proposed that Cytc reduction by l-GalLDH may be part of an alternative respiratory pathway that is active during AsA synthesis. Also, it is proposed that possible links of this pathway with other pathways of alternative electron transport in plant mitochondria may exist. The review suggests potential implications of this relationship, particularly for situations of stress. We hypothesized that this pathway of alternative electron input would serve as a strategy for adaptation of plant respiration to changing conditions.

Metabolism and Signaling of Plant Mitochondria in Adaptation to Environmental Stresses

The interaction of mitochondria with cellular components evolved differently in plants and mammals; in plants, the organelle contains proteins such as ALTERNATIVE OXIDASES (AOXs), which, in conjunction with internal and external ALTERNATIVE NAD(P)H DEHYDROGENASES, allow canonical oxidative phosphorylation (OXPHOS) to be bypassed. Plant mitochondria also contain UNCOUPLING PROTEINS (UCPs) that bypass OXPHOS. Recent work revealed that OXPHOS bypass performed by AOXs and UCPs is linked with new mechanisms of mitochondrial retrograde signaling. AOX is functionally associated with the NO APICAL MERISTEM transcription factors, which mediate mitochondrial retrograde signaling, while UCP1 can regulate the plant oxygen-sensing mechanism via the PRT6 N-Degron. Here, we discuss the crosstalk or the independent action of AOXs and UCPs on mitochondrial retrograde signaling associated with abiotic stress responses. We also discuss how mitochondrial function and retrograde signaling mechanisms affect chloroplast function. Additionally, we discuss how mitochondrial inner membrane transporters can mediate mitochondrial communication with other organelles. Lastly, we review how mitochondrial metabolism can be used to improve crop resilience to environmental stresses. In this respect, we particularly focus on the contribution of Brazilian research groups to advances in the topic of mitochondrial metabolism and signaling.

Alternative oxidase (AOX) 1a and 1d limit proline-induced oxidative stress and aid salinity recovery in Arabidopsis

Proline (Pro) catabolism and reactive oxygen species production have been linked in mammals and Caenorhabditis elegans, while increases in leaf respiration rate follow Pro exposure in plants. Here, we investigated how alternative oxidases (AOXs) of the mitochondrial electron transport chain accommodate the large, atypical flux resulting from Pro catabolism and limit oxidative stress during Pro breakdown in mature Arabidopsis (Arabidopsis thaliana) leaves. Following Pro treatment, AOX1a and AOX1d accumulate at transcript and protein levels, with AOX1d approaching the level of the typically dominant AOX1a isoform. We therefore sought to determine the function of both AOX isoforms under Pro respiring conditions. Oxygen consumption rate measurements in aox1a and aox1d leaves suggested these AOXs can functionally compensate for each other to establish enhanced AOX catalytic capacity in response to Pro. Generation of aox1a.aox1d lines showed complete loss of AOX proteins and activity upon Pro treatment, yet full respiratory induction in response to Pro remained possible via the cytochrome pathway. However, aox1a.aox1d leaves displayed symptoms of elevated oxidative stress and suffered increased oxidative damage during Pro metabolism compared to the wild-type (WT) or the single mutants. During recovery from salt stress, when relatively high rates of Pro catabolism occur naturally, photosynthetic rates in aox1a.aox1d recovered slower than in the WT or the single aox lines, showing that both AOX1a and AOX1d are beneficial for cellular metabolism during Pro drawdown following osmotic stress. This work provides physiological evidence of a beneficial role for AOX1a but also the less studied AOX1d isoform in allowing safe catabolism of alternative respiratory substrates like Pro.

Growth and Nutritional Quality of Lemnaceae Viewed Comparatively in an Ecological and Evolutionary Context

This review focuses on recently characterized traits of the aquatic floating plant Lemna with an emphasis on its capacity to combine rapid growth with the accumulation of high levels of the essential human micronutrient zeaxanthin due to an unusual pigment composition not seen in other fast-growing plants. In addition, Lemna's response to elevated CO₂ was evaluated in the context of the source-sink balance between plant sugar production and consumption. These and other traits of Lemnaceae are compared with those of other floating aquatic plants as well as terrestrial plants adapted to different environments. It was concluded that the unique features of aquatic plants reflect adaptations to the freshwater environment, including rapid growth, high productivity, and exceptionally strong accumulation of high-quality vegetative storage protein and human antioxidant micronutrients. It was further concluded that the insensitivity of growth rate to environmental conditions and plant source-sink imbalance may allow duckweeds to take advantage of elevated atmospheric CO₂ levels via particularly strong stimulation of biomass production and only minor declines in the growth of new tissue. It is proposed that declines in nutritional quality under elevated CO₂ (due to regulatory adjustments in photosynthetic metabolism) may be mitigated by plant-microbe interaction, for which duckweeds have a high propensity.

Structural and Biophysical Characterization of Purified Recombinant Arabidopsis thaliana's Alternative Oxidase 1A (rAtAOX1A): Interaction With Inhibitor(s) and Activator

In higher plants, alternative oxidase (AOX) participates in a cyanide resistant and non-proton motive electron transport pathway of mitochondria, diverging from the ubiquinone pool. The physiological significance of AOX in biotic/abiotic stress tolerance is well-documented. However, its structural and biophysical properties are poorly understood as its crystal structure is not yet revealed in plants. Also, most of the AOX purification processes resulted in a low yield/inactive/unstable form of native AOX protein. The present study aims to characterize the purified rAtAOX1A protein and its interaction with inhibitors, such as salicylhydroxamic acid (SHAM) and n-propyl gallate (n-PG), as well as pyruvate (activator), using biophysical/in silico studies. The rAtAOX1A expressed in E. coli BL21(DE3) cells was functionally characterized by monitoring the respiratory and growth sensitivity of E. coli/pAtAOX1A and E. coli/pET28a to classical mitochondrial electron transport chain (mETC) inhibitors. The rAtAOX1A, which is purified through affinity chromatography and confirmed by western blotting and MALDI-TOF-TOF studies, showed an oxygen uptake activity of 3.86 μmol min^-1 mg^-1 protein, which is acceptable in non-thermogenic plants. Circular dichroism (CD) studies of purified rAtAOX1A revealed that >50% of the protein content was α-helical and retained its helical absorbance signal (ellipticity) at a wide range of temperature and pH conditions. Further, interaction with SHAM, n-PG, or pyruvate caused significant changes in its secondary structural elements while retaining its ellipticity. Surface plasmon resonance (SPR) studies revealed that both SHAM and n-PG bind reversibly to rAtAOX1A, while docking studies revealed that they bind to the same hydrophobic groove (Met191, Val192, Met195, Leu196, Phe251, and Phe255), to which Duroquinone (DQ) bind in the AtAOX1A. In contrast, pyruvate binds to a pocket consisting of Cys II (Arg174, Tyr175, Gly176, Cys177, Val232, Ala233, Asn294, and Leu313). Further, the mutational docking studies suggest that (i) the Met195 and Phe255 of AtAOX1A are the potential candidates to bind the inhibitor. Hence, this binding pocket could be a 'potential gateway' for the oxidation-reduction process in AtAOX1A, and (ii) Arg174, Gly176, and Cys177 play an important role in binding to the organic acids like pyruvate.

Machine learning-enabled phenotyping for GWAS and TWAS of WUE traits in 869 field-grown sorghum accessions

Sorghum (Sorghum bicolor) is a model C4 crop made experimentally tractable by extensive genomic and genetic resources. Biomass sorghum is studied as a feedstock for biofuel and forage. Mechanistic modeling suggests that reducing stomatal conductance (gs) could improve sorghum intrinsic water use efficiency (iWUE) and biomass production. Phenotyping to discover genotype-to-phenotype associations remains a bottleneck in understanding the mechanistic basis for natural variation in gs and iWUE. This study addressed multiple methodological limitations. Optical tomography and a machine learning tool were combined to measure stomatal density (SD). This was combined with rapid measurements of leaf photosynthetic gas exchange and specific leaf area (SLA). These traits were the subject of genome-wide association study and transcriptome-wide association study across 869 field-grown biomass sorghum accessions. The ratio of intracellular to ambient CO2 was genetically correlated with SD, SLA, gs, and biomass production. Plasticity in SD and SLA was interrelated with each other and with productivity across wet and dry growing seasons. Moderate-to-high heritability of traits studied across the large mapping population validated associations between DNA sequence variation or RNA transcript abundance and trait variation. A total of 394 unique genes underpinning variation in WUE-related traits are described with higher confidence because they were identified in multiple independent tests. This list was enriched in genes whose Arabidopsis (Arabidopsis thaliana) putative orthologs have functions related to stomatal or leaf development and leaf gas exchange, as well as genes with nonsynonymous/missense variants. These advances in methodology and knowledge will facilitate improving C4 crop WUE.

The Receptor-Like Kinase ERECTA Confers Improved Water Use Efficiency and Drought Tolerance to Poplar via Modulating Stomatal Density

Poplar is one of the most important tree species in the north temperate zone, but poplar plantations are quite water intensive. We report here that CaMV 35S promoter-driven overexpression of the PdERECTA gene, which is a member of the LRR-RLKs family from Populus nigra × (Populus deltoides × Populus nigra), improves water use efficiency and enhances drought tolerance in triploid white poplar. PdERECTA localizes to the plasma membrane. Overexpression plants showed lower stomatal density and larger stomatal size. The abaxial stomatal density was 24-34% lower and the stomatal size was 12-14% larger in overexpression lines. Reduced stomatal density led to a sharp restriction of transpiration, which was about 18-35% lower than the control line, and instantaneous water use efficiency was around 14-63% higher in overexpression lines under different conditions. These phenotypic changes led to increased drought tolerance. PdERECTA overexpression plants not only survived longer after stopping watering but also performed better when supplied with limited water, as they had better physical and photosynthesis conditions, faster growth rate, and higher biomass accumulation. Taken together, our data suggest that PdERECTA can alter the development pattern of stomata to reduce stomatal density, which then restricts water consumption, conferring enhanced drought tolerance to poplar. This makes PdERECTA trees promising candidates for establishing more water use efficient plantations.

Photoprotection by mitochondrial alternative pathway is enhanced at heat but disabled at chilling

The mitochondrial alternative pathway (AP) represents an important photoprotective mechanism for the chloroplast, but the temperature sensitivity of its photoprotective role is unknown. In this study, using the aox1a Arabidopsis mutant, the photoprotective role of the AP was verified under various temperatures, and the mechanism underlying the temperature sensitivity of the AP's photoprotective role was clarified. It was observed that the photoprotective role of the AP increased with rising temperature but was absent at low temperature. The photoprotective role of the AP was severely reduced under non-photorespiratory conditions. Disturbance of the AP inhibited the conversion of glycine to serine in mitochondria, which may restrain upstream photorespiratory metabolism and aggravate photoinhibition. With rising temperatures, photorespiration accelerated and the restraint of photorespiration caused by disturbance of the AP also increased, determining the temperature sensitivity of the AP's photoprotective role. We also verified that not only the AP but also the cytochrome pathway in mitochondria contributes to photoprotection by maintaining photorespiration.

Cytochrome oxidase and alternative oxidase pathways of mitochondrial electron transport chain are important for the photosynthetic performance of pea plants under salinity stress conditions

The flexible plant mitochondrial electron transport chain with cytochrome c oxidase (COX) and alternative oxidase (AOX) pathways is known to be modulated by abiotic stress conditions. The effect of salinity stress on the mitochondrial electron transport chain and the importance of COX and AOX pathways for optimization of photosynthesis under salinity stress conditions is not clearly understood. In the current study, importance of COX and AOX pathways for photosynthetic performance of pea plants (Pisum sativum L. Pea Arkel cv) was analysed by using the mitochondrial electron transport chain inhibitors Antimycin A (AA) and salicylhydroxamic acid (SHAM) which restrict the electron flow through COX and AOX pathways respectively. Salinity stress resulted in decreased CO2 assimilation rates, leaf stomatal conductance, transpiration and leaf intercellular CO2 concentration in a stress dependent manner. Superimposition of leaves of salt stressed plants with AA and SHAM caused cellular H2O2 and O2- accumulation along with cell death. Additionally, aggravation in decrease of CO2 assimilation rates, leaf stomatal conductance, transpiration and leaf intercellular CO2 concentration upon superimposition with AA and SHAM during salinity stress suggests the importance of mitochondrial oxidative electron transport for photosynthesis. Increased expression of AOX1a and AOX2 transcripts along with AOX protein levels indicated up regulation of AOX pathway in leaves during salinity stress. Chlorophyll fluorescence measurements revealed enhanced damage to Photosystem (PS) II in the presence of AA and SHAM during salinity stress. Results suggested the beneficial role of COX and AOX pathways for optimal photosynthetic performance in pea leaves during salinity stress conditions.

Distinct roles of alternative oxidase pathway during the greening process of etiolated algae

The vital function of mitochondrial alternative oxidase (AOX) pathway in optimizing photosynthesis during plant de-etiolation has been well recognized. However, whether and how AOX impacts the chloroplast biogenesis in algal cells remains unclear. In the present study, the role of AOX in regulating the reassembly of chloroplast in algal cells was investigated by treating Auxenochlorella protothecoides with salicylhydroxamic acid (SHAM), the specific inhibitor to AOX, in the heterotrophy to autotrophy transition process. Several lines of evidences including delayed chlorophyll accumulation, lagged reorganization of chloroplast structure, altered PSI/PSII stoichiometry, and declined photosynthetic activities in SHAM treated cells indicated that the impairment in AOX activity dramatically hindered the development of functioning chloroplast in algal cells. Besides, the cellular ROS levels and activities of antioxidant enzymes were increased by SHAM treatment, and the perturbation on the balance of NAD⁺/NADH and NADP⁺/NADPH ratios was also observed in A. protothecoides lacking AOX activity, indicating that AOX was essential in promoting ROS scavenging and keeping the redox homeostasis for algal chloroplast development during greening. Overall, our study revealed the essentiality of mitochondrial AOX pathway in sustaining algal photosynthetic performance and provided novel insights into the physiological roles of AOX on the biogenesis of photosynthetic organelle in algae.

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.

Photosynthesis, respiration and growth: A carbon and energy balancing act for alternative oxidase

This review summarizes knowledge of alternative oxidase, a mitochondrial electron transport chain component that lowers the ATP yield of plant respiration. Analysis of mutant and transgenic plants has established that alternative oxidase activity supports leaf photosynthesis. The interaction of alternative oxidase respiration with chloroplast metabolism is important under conditions that challenge energy and/or carbon balance in the photosynthetic cell. Under such conditions, alternative oxidase provides an extra-chloroplastic means to optimize the status of chloroplast energy pools (ATP, NADPH) and to manage cellular carbohydrate pools in response to changing rates of carbon fixation and carbon demand for growth and maintenance. Transcriptional and post-translational mechanisms ensure that alternative oxidase can respond effectively when carbon and energy balance are being challenged. This function appears particularly significant under abiotic stress conditions such as water deficit, high salinity, or temperature extremes. Under such conditions, alternative oxidase respiration positively affects growth and stress tolerance, despite it lowering the energy yield and carbon use efficiency of respiration. In part, this beneficial effect relates to the ability of alternative oxidase respiration to prevent excessive reactive oxygen species generation in both mitochondria and chloroplasts. Recent evidence suggests that alternative oxidase respiration is an interesting target for crop improvement.

The Mitochondrial Respiratory Chain Maintains the Photosynthetic Electron Flow in Arabidopsis thaliana Leaves under High-Light Stress

The plant respiratory chain includes the ATP-coupling cytochrome pathway (CP) and ATP-uncoupling alternative oxidase (AOX). Under high-light (HL) conditions, plants experience photoinhibition, leading to a damaged photosystem II (PSII). The respiratory chain is considered to affect PSII maintenance and photosynthetic electron transport under HL conditions. However, the underlying details remain unclear. In this study, we investigated the respiratory chain functions related to PSII maintenance and photosynthetic electron transport in plants exposed to HL stress. We measured the HL-induced decrease in the maximum quantum yield of PSII in the leaves of wild-type and AOX1a-knockout (aox1a) Arabidopsis thaliana plants in which CP was partially inhibited by a complex-III inhibitor. We also calculated PSII photodamage and repair rate constants. Both rate constants changed when CP was partially inhibited in aox1a plants, suggesting that the respiratory chain is related to both processes. Before HL stress, photosynthetic linear electron flow (LEF) decreased when CP was partially inhibited. After HL stress, aox1a in the presence of the CP inhibitor showed significantly decreased rates of LEF. The electron flow downstream from PSII and on the donor side of photosystem I may have been suppressed. The function of respiratory chain is required to maintain the optimal LEF as well as PSII maintenance especially under the HL stress.

Improved in vivo measurement of alternative oxidase respiration in field-collected pine roots

Cellular respiration via the alternative oxidase pathway (AOP) leads to a considerable loss in efficiency. Compared to the cytochrome pathway (COP), AOP produces 0-50% as much ATP per carbon (C) respired. Relative partitioning between the pathways can be measured in vivo based on their differing isotopic discriminations against ¹⁸ O in O₂ . Starting from published methods, we have refined and tested a new protocol to improve measurement precision and efficiency. The refinements detect an effect of tissue water content (P < 0.0001), which we have removed, and yield precise discrimination endpoints in the presence of pathway-specific respiratory inhibitors [CN^- and salicylhydroxamic acid (SHAM)], which improves estimates of AOP/COP partitioning. Fresh roots of Pinus sylvestris were sealed in vials with a CO₂ trap. The air was replaced to ensure identical starting conditions. Headspace air was repeatedly sampled and isotopically analyzed using isotope-ratio mass spectrometry. The method allows high-precision measurement of the discrimination against ¹⁸ O in O₂ because of repeated measurements of the same incubation vial. COP and AOP respiration discriminated against ¹⁸ O by 15.1 ± 0.3‰ and 23.8 ± 0.4‰, respectively. AOP contributed to root respiration by 23 ± 0.2% of the total in an unfertilized stand. In a second, nitrogen-fertilized, stand AOP contribution was only 14 ± 0.2% of the total. These results suggest the improved method can be used to assess the relative importance of COP and AOP activities in ecosystems, potentially yielding information on the role of each pathway for the carbon use efficiency of organisms.

A Functional Alternative Oxidase Modulates Plant Salt Tolerance in Physcomitrella patens

Alternative oxidase (AOX) has been reported to be involved in mitochondrial function and redox homeostasis, thus playing an essential role in plant growth as well as stress responses. However, its biological functions in nonseed plants have not been well characterized. Here, we report that AOX participates in plant salt tolerance regulation in moss Physcomitrella patens (P. patens). AOX is highly conserved and localizes to mitochondria in P. patens. We observed that PpAOX rescued the impaired cyanide (CN)-resistant alternative (Alt) respiratory pathway in Arabidopsis thaliana (Arabidopsis) aox1a mutant. PpAOX transcription and Alt respiration were induced upon salt stress in P. patens. Using homologous recombination, we generated PpAOX-overexpressing lines (PpAOX OX). PpAOX OX plants exhibited higher Alt respiration and lower total reactive oxygen species accumulation under salt stress condition. Strikingly, we observed that PpAOX OX plants displayed decreased salt tolerance. Overexpression of PpAOX disturbed redox homeostasis in chloroplasts. Meanwhile, chloroplast structure was adversely affected in PpAOX OX plants in contrast to wild-type (WT) P. patens. We found that photosynthetic activity in PpAOX OX plants was also lower compared with that in WT. Together, our work revealed that AOX participates in plant salt tolerance in P. patens and there is a functional link between mitochondria and chloroplast under challenging conditions.

Dynamics of Peroxisome Homeostasis and Its Role in Stress Response and Signaling in Plants

Peroxisomes play vital roles in plant growth, development, and environmental stress response. During plant development and in response to environmental stresses, the number and morphology of peroxisomes are dynamically regulated to maintain peroxisome homeostasis in cells. To execute their various functions in the cell, peroxisomes associate and communicate with other organelles. Under stress conditions, reactive oxygen species (ROS) produced in peroxisomes and other organelles activate signal transduction pathways, in a process known as retrograde signaling, to synergistically regulate defense systems. In this review, we focus on the recent advances in the plant peroxisome field to provide an overview of peroxisome biogenesis, degradation, crosstalk with other organelles, and their role in response to environmental stresses.

The mitochondrial alternative oxidase from Chlamydomonas reinhardtii enables survival in high light

Photosynthetic organisms often experience extreme light conditions that can cause hyper-reduction of the chloroplast electron transport chain, resulting in oxidative damage. Accumulating evidence suggests that mitochondrial respiration and chloroplast photosynthesis are coupled when cells are absorbing high levels of excitation energy. This coupling helps protect the cells from hyper-reduction of photosynthetic electron carriers and diminishes the production of reactive oxygen species (ROS). To examine this cooperative protection, here we characterized Chlamydomonas reinhardtii mutants lacking the mitochondrial alternative terminal respiratory oxidases, CrAOX1 and CrAOX2. Using fluorescent fusion proteins, we experimentally demonstrated that both enzymes localize to mitochondria. We also observed that the mutant strains were more sensitive than WT cells to high light under mixotrophic and photoautotrophic conditions, with the aox1 strain being more sensitive than aox2 Additionally, the lack of CrAOX1 increased ROS accumulation, especially in very high light, and damaged the photosynthetic machinery, ultimately resulting in cell death. These findings indicate that the Chlamydomonas AOX proteins can participate in acclimation of C. reinhardtii cells to excess absorbed light energy. They suggest that when photosynthetic electron carriers are highly reduced, a chloroplast-mitochondria coupling allows safe dissipation of photosynthetically derived electrons via the reduction of O₂ through AOX (especially AOX1)-dependent mitochondrial respiration.

Implications of terminal oxidase function in regulation of salicylic acid on soybean seedling photosynthetic performance under water stress

The aim of this study is to investigate whether exogenous application of salicylic acid (SA) could modulate the photosynthetic capacity of soybean seedlings in water stress tolerance, and to clarify the potential functions of terminal oxidase (plastid terminal oxidase (PTOX) and alternative oxidase (AOX)) in SA' s regulation on photosynthesis. The effects of SA and water stress on gas exchange, pigment contents, chlorophyll fluorescence, enzymes (guaiacol peroxidase (POD; EC 1.11.1.7), superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX; EC 1.11.1.11) and NADP-malate dehydrogenase (NADP-MDH; EC1.1.1.82)) activity and transcript levels of PTOX, AOX1, AOX2a, AOX2b were examined in a hydroponic cultivation system. Results indicate that water stress significantly decreased the photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (E), pigment contents (Chla + b, Chla/b, Car), maximum quantum yield of PSⅡphotochemistry (Fv/Fm), efficiency of excitation capture of open PSⅡcenter (Fv'/Fm'), quantum efficiency of PSⅡphotochemistry (ΦPSⅡ), photochemical quenching (qP), and increased malondialdehyde (MDA) content and the activity of all the enzymes. SA pretreatment led to significant decreases in Ci and MDA content, and increases in Pn, Gs, E, pigment contents, Fv/Fm, Fv'/Fm', ΦPSⅡ, qP, and the activity of all the enzymes. SA treatment and water stress alone significantly up-regulated the expression of PTOX, AOX1 and AOX2b. SA pretreatment further increased the transcript levels of PTOX and AOX2b of soybean seedling under water stress. These results indicate that SA application alleviates the water stress-induced decrease in photosynthesis may mainly through maintaining a lower reactive oxygen species (ROS) level, a greater PSⅡefficiency, and an enhanced alternative respiration and chlororespiration. PTOX and AOX may play important roles in SA-mediated resistance to water stress.

Contribution of the Alternative Respiratory Pathway to PSII Photoprotection in C3 and C4 Plants

The mechanism by which the mitochondrial alternative oxidase (AOX) pathway contributes to photosystem II (PSII) photoprotection is in dispute. It was generally thought that the AOX pathway protects photosystems by dissipating excess reducing equivalents exported from chloroplasts through the malate/oxaloacetate (Mal/OAA) shuttle and thus preventing the over-reduction of chloroplasts. In this study, using the aox1a Arabidopsis mutant and nine other C3 and C4 plant species, we revealed an additional action model of the AOX pathway in PSII photoprotection. Although the AOX pathway contributes to PSII photoprotection in C3 leaves treated with high light, this contribution was observed to disappear when photorespiration was suppressed. Disruption or inhibition of the AOX pathway significantly decreased the photorespiration in C3 leaves. Moreover, the AOX pathway did not respond to high light and contributed little to PSII photoprotection in C4 leaves possessing a highly active Mal/OAA shuttle but with little photorespiration. These results demonstrate that the AOX pathway contributes to PSII photoprotection in C3 plants by maintaining photorespiration to detoxify glycolate and via the indirect export of excess reducing equivalents from chloroplasts by the Mal/OAA shuttle. This new action model explains why the AOX pathway does not contribute to PSII photoprotection in C4 plants.

Coordinated regulation of photosynthetic and respiratory components is necessary to maintain chloroplast energy balance in varied growth conditions

Mitochondria have a non-energy-conserving alternative oxidase (AOX) proposed to support photosynthesis, perhaps by promoting energy balance under varying growth conditions. To investigate this, wild-type (WT) Nicotiana tabacum were compared with AOX knockdown and overexpression lines. In addition, the amount of AOX protein in WT plants was compared with that of chloroplast light-harvesting complex II (LHCB2), whose amount is known to respond to chloroplast energy status. With increased growth irradiance, WT leaves maintained higher rates of respiration in the light (RL), but no differences in RL or photosynthesis were seen between the WT and transgenic lines, suggesting that, under non-stress conditions, AOX was not critical for leaf metabolism, regardless of growth irradiance. However, under drought, the AOX amount became an important determinant of RL, which in turn was an important determinant of chloroplast energy balance (measured as photosystem II excitation pressure, EP), and photosynthetic performance. In the WT, the AOX amount increased and the LHCB2 amount decreased with increased growth irradiance or drought severity. These changes in protein amounts correlated strongly, in opposing ways, with growth EP. This suggests that a signal deriving from the photosynthetic electron transport chain status coordinately controls the amounts of AOX and LHCB2, which then both contribute to maintaining chloroplast energy balance, particularly under stress conditions.

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.

Mitochondrial GPX1 silencing triggers differential photosynthesis impairment in response to salinity in rice plants

The physiological role of plant mitochondrial glutathione peroxidases is scarcely known. This study attempted to elucidate the role of a rice mitochondrial isoform (GPX1) in photosynthesis under normal growth and salinity conditions. GPX1 knockdown rice lines (GPX1s) were tested in absence and presence of 100 mM NaCl for 6 d. Growth reduction of GPX1s line under non-stressful conditions, compared with non-transformed (NT) plants occurred in parallel to increased H2 O2 and decreased GSH contents. These changes occurred concurrently with photosynthesis impairment, particularly in Calvin cycle's reactions, since photochemical efficiency did not change. Thus, GPX1 silencing and downstream molecular/metabolic changes modulated photosynthesis differentially. In contrast, salinity induced reduction in both phases of photosynthesis, which were more impaired in silenced plants. These changes were associated with root morphology alterations but not shoot growth. Both studied lines displayed increased GPX activity but H2 O2 content did not change in response to salinity. Transformed plants exhibited lower photorespiration, water use efficiency and root growth, indicating that GPX1 could be important to salt tolerance. Growth reduction of GPX1s line might be related to photosynthesis impairment, which in turn could have involved a cross talk mechanism between mitochondria and chloroplast originated from redox changes due to GPX1 deficiency.

Alternative oxidase: a respiratory electron transport chain pathway essential for maintaining photosynthetic performance during drought stress

Photosynthesis and respiration are the hubs of energy metabolism in plants. Drought strongly perturbs photosynthesis as a result of both diffusive limitations resulting from stomatal closure, and in some cases biochemical limitations that are associated with a reduced abundance of key photosynthetic components. The effects of drought on respiration, particularly respiration in the light (RL ), are less understood. The plant mitochondrial electron transport chain includes a non-energy conserving terminal oxidase called alternative oxidase (AOX). Several studies have shown that drought increases AOX transcript, protein and maximum capacity. Here we review recent studies comparing wild-type (WT) tobacco to transgenic lines with altered AOX protein amount. Specifically during drought, RL was compromised in AOX knockdown plants and enhanced in AOX overexpression plants, compared with WT. Significantly, these differences in RL were accompanied by dramatic differences in photosynthetic performance. Knockdown of AOX increased the susceptibility of photosynthesis to drought-induced biochemical limitations, while overexpression of AOX delayed the development of such biochemical limitations, compared with WT. Overall, the results indicate that AOX is essential to maintaining RL during drought, and that this non-energy conserving respiration maintains photosynthesis during drought by promoting energy balance in the chloroplast. This review also outlines several areas for future research, including the possibility that enhancement of non-energy conserving respiratory electron sinks may be a useful biotechnological approach to increase plant performance during stress.

Expression of the alternative oxidase mitigates beta-amyloid production and toxicity in model systems

Mitochondrial dysfunction has been widely associated with the pathology of Alzheimer's disease, but there is no consensus on whether it is a cause or consequence of disease, nor on the precise mechanism(s). We addressed these issues by testing the effects of expressing the alternative oxidase AOX from Ciona intestinalis, in different models of AD pathology. AOX can restore respiratory electron flow when the cytochrome segment of the mitochondrial respiratory chain is inhibited, supporting ATP synthesis, maintaining cellular redox homeostasis and mitigating excess superoxide production at respiratory complexes I and III. In human HEK293-derived cells, AOX expression decreased the production of beta-amyloid peptide resulting from antimycin inhibition of respiratory complex III. Because hydrogen peroxide was neither a direct product nor substrate of AOX, the ability of AOX to mimic antioxidants in this assay must be indirect. In addition, AOX expression was able to partially alleviate the short lifespan of Drosophila models neuronally expressing human beta-amyloid peptides, whilst abrogating the induction of markers of oxidative stress. Our findings support the idea of respiratory chain dysfunction and excess ROS production as both an early step and as a pathologically meaningful target in Alzheimer's disease pathogenesis, supporting the concept of a mitochondrial vicious cycle underlying the disease.

Molecular Evolution of Alternative Oxidase Proteins: A Phylogenetic and Structure Modeling Approach

Alternative oxidases (AOXs) are mitochondrial cyanide-resistant membrane-bound metallo-proteins catalyzing the oxidation of ubiquinol and the reduction of oxygen to water bypassing two sites of proton pumping, thus dissipating a major part of redox energy into heat. Here, the structure of Arabidopsis thaliana AOX 1A has been modeled using the crystal structure of Trypanosoma brucei AOX as a template. Analysis of this model and multiple sequence alignment of members of the AOX family from all kingdoms of Life indicate that AOXs display a high degree of conservation of the catalytic core, which is formed by a four-α-helix bundle, hosting the di-iron catalytic site, and is flanked by two additional α-helices anchoring the protein to the membrane. Plant AOXs display a peculiar covalent dimerization mode due to the conservation in the N-terminal region of a Cys residue forming the inter-monomer disulfide bond. The multiple sequence alignment has also been used to infer a phylogenetic tree of AOXs whose analysis shows a polyphyletic origin for the AOXs found in Fungi and a monophyletic origin of the AOXs of Eubacteria, Mycetozoa, Euglenozoa, Metazoa, and Land Plants. This suggests that AOXs evolved from a common ancestral protein in each of these kingdoms. Within the Plant AOX clade, the AOXs of monocotyledon plants form two distinct clades which have unresolved relationships relative to the monophyletic clade of the AOXs of dicotyledonous plants. This reflects the sequence divergence of the N-terminal region, probably due to a low selective pressure for sequence conservation linked to the covalent homo-dimerization mode.

Alternative pathway is involved in the tolerance of highland barley to the low-nitrogen stress by maintaining the cellular redox homeostasis

Alternative pathway (AP) is involved in the tolerance of highland barley seedlings to the low-nitrogen stress by dissipating excessive reducing equivalents generated by photosynthesis and maintaining the cellular redox homeostasis. Low nitrogen (N) is a major limiting factor for plant growth and crop productivity. In this study, we investigated the roles of the alternative pathway (AP) in the tolerance of two barley seedlings, highland barley (Kunlun12) and barley (Ganpi6), to low-N stress. The results showed that the chlorophyll content and the fresh weight decreased more in Ganpi6 than those in Kunlun12 under low-N stress, suggesting that Kunlun12 has higher tolerance to low-N stress than Ganpi6. AP capacity was markedly induced by low-N stress; and it was higher in Kunlun12 than in Ganpi6. Comparatively, the cytochrome pathway capacity was not affected under all conditions. Western-blot analysis showed that the protein level of the alternative oxidase (AOX) increased under low-N stress in Kunlun12 but not in Ganpi6. Under low-N stress, the NAD(P)H content and the NAD(P)H to NAD(P)(+)+NAD(P)H ratio in Ganpi6 increased more than those in Kunlun12. Furthermore, photosynthetic parameters (Fv/Fm, qP, ETR and Yield) decreased markedly and qN increased, indicating photoinhibition occurred in both barley seedlings, especially in Ganpi6. When AP was inhibited by salicylhydroxamic acid (SHAM), the NAD(P)H content and the NAD(P)H to NAD(P)(+)+NAD(P)H ratio dramatically increased under all conditions, resulting in the marked accumulation of H(2)O(2) and malondialdehyde in leaves of both barley seedlings. Meanwhile, the malate-oxaloacetate shuttle activity and the photosynthetic efficiency were further inhibited. Taken together, AP is involved in the tolerance of highland barley seedlings to low-N stress by dissipating excess reducing equivalents and maintaining the cellular redox homeostasis.

Genetic engineering of AtAOX1a in Saccharomyces cerevisiae prevents oxidative damage and maintains redox homeostasis

This study aimed to validate the physiological importance of Arabidopsis thaliana alternative oxidase 1a (AtAOX1a) in alleviating oxidative stress using Saccharomyces cerevisiae as a model organism. The AOX1a transformant (pYES2AtAOX1a) showed cyanide resistant and salicylhydroxamic acid (SHAM)‐sensitive respiration, indicating functional expression of AtAOX1a in S. cerevisiae. After exposure to oxidative stress, pYES2AtAOX1a showed better survival and a decrease in reactive oxygen species (ROS) when compared to S. cerevisiae with empty vector (pYES2). Furthermore, pYES2AtAOX1a sustained growth by regulating GPX2 and/or TSA2, and cellular NAD⁺/NADH ratio. Thus, the expression of AtAOX1a in S. cerevisiae enhances its respiratory tolerance which, in turn, maintains cellular redox homeostasis and protects from oxidative damage.

Alternative Oxidase Pathway Optimizes Photosynthesis During Osmotic and Temperature Stress by Regulating Cellular ROS, Malate Valve and Antioxidative Systems

The present study reveals the importance of alternative oxidase (AOX) pathway in optimizing photosynthesis under osmotic and temperature stress conditions in the mesophyll protoplasts of Pisum sativum. The responses of photosynthesis and respiration were monitored at saturating light intensity of 1000 μmoles m(-2) s(-1) at 25°C under a range of sorbitol concentrations from 0.4 to 1.0 M to induce hyper-osmotic stress and by varying the temperature of the thermo-jacketed pre-incubation chamber from 25 to 10°C to impose sub-optimal temperature stress. Compared to controls (0.4 M sorbitol and 25°C), the mesophyll protoplasts showed remarkable decrease in NaHCO3-dependent O2 evolution (indicator of photosynthetic carbon assimilation), under both hyper-osmotic (1.0 M sorbitol) and sub-optimal temperature stress conditions (10°C), while the decrease in rates of respiratory O2 uptake were marginal. The capacity of AOX pathway increased significantly in parallel to increase in intracellular pyruvate and reactive oxygen species (ROS) levels under both hyper-osmotic stress and sub-optimal temperature stress under the background of saturating light. The ratio of redox couple (Malate/OAA) related to malate valve increased in contrast to the ratio of redox couple (GSH/GSSG) related to antioxidative system during hyper-osmotic stress. Further, the ratio of GSH/GSSG decreased in the presence of sub-optimal temperature, while the ratio of Malate/OAA showed no visible changes. Also, the redox ratios of pyridine nucleotides increased under hyper-osmotic (NADH/NAD) and sub-optimal temperature (NADPH/NADP) stresses, respectively. However, upon restriction of AOX pathway by using salicylhydroxamic acid (SHAM), the observed changes in NaHCO3-dependent O2 evolution, cellular ROS, redox ratios of Malate/OAA, NAD(P)H/NAD(P) and GSH/GSSG were further aggravated under stress conditions with concomitant modulations in NADP-MDH and antioxidant enzymes. Taken together, the results indicated the importance of AOX pathway in optimizing photosynthesis under both hyper-osmotic stress and sub-optimal temperatures. Regulation of ROS through redox couples related to malate valve and antioxidant system by AOX pathway to optimize photosynthesis under these stresses are discussed.

Importance of the alternative oxidase (AOX) pathway in regulating cellular redox and ROS homeostasis to optimize photosynthesis during restriction of the cytochrome oxidase pathway in Arabidopsis thaliana

BACKGROUND AND AIMS

The importance of the alternative oxidase (AOX) pathway, particularly AOX1A, in optimizing photosynthesis during de-etiolation, under elevated CO2, low temperature, high light or combined light and drought stress is well documented. In the present study, the role of AOX1A in optimizing photosynthesis was investigated when electron transport through the cytochrome c oxidase (COX) pathway was restricted at complex III.

METHODS

Leaf discs of wild-type (WT) and aox1a knock-out mutants of Arabidopsis thaliana were treated with antimycin A (AA) under growth-light conditions. To identify the impact of AOX1A deficiency in optimizing photosynthesis, respiratory O2 uptake and photosynthesis-related parameters were measured along with changes in redox couples, reactive oxygen species (ROS), lipid peroxidation and expression levels of genes related to respiration, the malate valve and the antioxidative system.

KEY RESULTS

In the absence of AA, aox1a knock-out mutants did not show any difference in physiological, biochemical or molecular parameters compared with WT. However, after AA treatment, aox1a plants showed a significant reduction in both respiratory O2 uptake and NaHCO3-dependent O2 evolution. Chlorophyll fluorescence and P700 studies revealed that in contrast to WT, aox1a knock-out plants were incapable of maintaining electron flow in the chloroplastic electron transport chain, and thereby inefficient heat dissipation (low non-photochemical quenching) was observed. Furthermore, aox1a mutants exhibited significant disturbances in cellular redox couples of NAD(P)H and ascorbate (Asc) and consequently accumulation of ROS and malondialdehyde (MDA) content. By contrast, WT plants showed a significant increase in transcript levels of CSD1, CAT1, sAPX, COX15 and AOX1A in contrast to aox1a mutants.

CONCLUSIONS

These results suggest that AOX1A plays a significant role in sustaining the chloroplastic redox state and energization to optimize photosynthesis by regulating cellular redox homeostasis and ROS generation when electron transport through the COX pathway is disturbed at complex III.

Glucose-6-phosphate dehydrogenase and alternative oxidase are involved in the cross tolerance of highland barley to salt stress and UV-B radiation

In this study, a new mechanism involving glucose-6-phosphate dehydrogenase (G6PDH) and alternative pathways (AP) in salt pretreatment-induced tolerance of highland barley to UV-B radiation was investigated. When highland barley was exposed to UV-B radiation, the G6PDH activity decreased but the AP capacity increased. In contrast, under UV-B+NaCl treatment, the G6PDH activity was restored to the control level and the maximal AP capacity and antioxidant enzyme activities were reached. Glucosamine (Glucm, an inhibitor of G6PDH) obviously inhibited the G6PDH activity in highland barley under UV-B + NaCl treatment and a similar pattern was observed in reduced glutathione (GSH) and ascorbic acid (Asc) contents. Similarly, salicylhydroxamic acid (SHAM, an inhibitor of AOX) significantly reduced the AP capacity in highland barley under UV-B + NaCl treatment. The UV-B-induced hydrogen peroxide (H2O2) accumulation was also followed. Further studies indicated that non-functioning of G6PDH or AP under UV-B+NaCl + Glucm or UV-B + NaCl + SHAM treatment also caused damages in photosynthesis and stomatal movement. Western blot analysis confirmed that the alternative oxidase (AOX) and G6PDH were dependent each other in cross tolerance to UV-B and salt. The inhibition of AP or G6PDH activity resulted in a significant accumulation or reduction of NADPH content, respectively, under UV-B+NaCl treatment in highland barley leaves. Taken together, our results indicate that AP and G6PDH mutually regulate and maintain photosynthesis and stomata movement in the cross adaptation of highland barley seedlings to UV-B and salt by modulating redox homeostasis and NADPH content.

Mitochondrial alternative oxidase maintains respiration and preserves photosynthetic capacity during moderate drought in Nicotiana tabacum

The mitochondrial electron transport chain includes an alternative oxidase (AOX) that is hypothesized to aid photosynthetic metabolism, perhaps by acting as an additional electron sink for photogenerated reductant or by dampening the generation of reactive oxygen species. Gas exchange, chlorophyll fluorescence, photosystem I (PSI) absorbance, and biochemical and protein analyses were used to compare respiration and photosynthesis of Nicotiana tabacum 'Petit Havana SR1' wild-type plants with that of transgenic AOX knockdown (RNA interference) and overexpression lines, under both well-watered and moderate drought-stressed conditions. During drought, AOX knockdown lines displayed a lower rate of respiration in the light than the wild type, as confirmed by two independent methods. Furthermore, CO2 and light response curves indicated a nonstomatal limitation of photosynthesis in the knockdowns during drought, relative to the wild type. Also relative to the wild type, the knockdowns under drought maintained PSI and PSII in a more reduced redox state, showed greater regulated nonphotochemical energy quenching by PSII, and displayed a higher relative rate of cyclic electron transport around PSI. The origin of these differences may lie in the chloroplast ATP synthase amount, which declined dramatically in the knockdowns in response to drought. None of these effects were seen in plants overexpressing AOX. The results show that AOX is necessary to maintain mitochondrial respiration during moderate drought. In its absence, respiration rate slows and the lack of this electron sink feeds back on the photosynthetic apparatus, resulting in a loss of chloroplast ATP synthase that then limits photosynthetic capacity.

Improving the sunlight-to-biomass conversion efficiency in microalgal biofactories

Microalgae represent promising organisms for the sustainable production of commodities, chemicals or fuels. Future use of such systems, however, requires increased productivity of microalgal mass cultures in order to reach an economic viability for microalgae-based production schemes. The efficiency of sunlight-to-biomass conversion that can be observed in bulk cultures is generally far lower (35-80%) than the theoretical maximum, because energy losses occur at multiple steps during the light-driven conversion of carbon dioxide to organic carbon. The light-harvesting system is a major source of energy losses and thus a prime target for strain engineering. Truncation of the light-harvesting antenna in the algal model organism Chlamydomonas reinhardtii was shown to be an effective way of increasing culture productivity at least under saturating light conditions. Furthermore engineering of the Calvin-Benson cycle or the creation of photorespiratory bypasses in A. thaliana proved to be successful in terms of achieving higher biomass productivities. An efficient generation of novel microalgal strains with improved sunlight conversion efficiencies by targeted engineering in the future will require an expanded molecular toolkit. In the meantime random mutagenesis coupled to high-throughput screening for desired phenotypes can be used to provide engineered microalgae.