Growth at Elevated CO2 Requires Acclimation of the Respiratory Chain to Support Photosynthesis

Physiological and Proteomic Responses of the Tetraploid Robinia pseudoacacia L. to High CO2 Levels

The increase in atmospheric CO2 concentration is a significant factor in triggering global warming. CO2 is essential for plant photosynthesis, but excessive CO2 can negatively impact photosynthesis and its associated physiological and biochemical processes. The tetraploid Robinia pseudoacacia L., a superior and improved variety, exhibits high tolerance to abiotic stress. In this study, we investigated the physiological and proteomic response mechanisms of the tetraploid R. pseudoacacia under high CO2 treatment. The results of our physiological and biochemical analyses revealed that a 5% high concentration of CO2 hindered the growth and development of the tetraploid R. pseudoacacia and caused severe damage to the leaves. Additionally, it significantly reduced photosynthetic parameters such as Pn, Gs, Tr, and Ci, as well as respiration. The levels of chlorophyll (Chl a and b) and the fluorescent parameters of chlorophyll (Fm, Fv/Fm, qP, and ETR) also significantly decreased. Conversely, the levels of ROS (H2O2 and O2·-) were significantly increased, while the activities of antioxidant enzymes (SOD, CAT, GR, and APX) were significantly decreased. Furthermore, high CO2 induced stomatal closure by promoting the accumulation of ROS and NO in guard cells. Through a proteomic analysis, we identified a total of 1652 DAPs after high CO2 treatment. GO functional annotation revealed that these DAPs were mainly associated with redox activity, catalytic activity, and ion binding. KEGG analysis showed an enrichment of DAPs in metabolic pathways, secondary metabolite biosynthesis, amino acid biosynthesis, and photosynthetic pathways. Overall, our study provides valuable insights into the adaptation mechanisms of the tetraploid R. pseudoacacia to high CO2.

Revisiting Changes in Growth, Physiology and Stress Responses of Plants under the Effect of Enhanced CO2 and Temperature

Climate change has universally affected the whole ecosystem in a unified manner and is known to have improbable effects on agricultural productivity and food security. Carbon dioxide (CO2) and temperature are the major environmental factors that have been shown to increase sharply during the last century and are directly responsible for affecting plant growth and development. A number of previous investigations have deliberated the positive effects of elevated CO2 on plant growth and development of various C3 crops, while detrimental effects of enhanced temperature on different crop plants like rice, wheat, maize and legumes are generally observed. A combined effect of elevated CO2 and temperature has yet to be studied in great detail; therefore, this review attempts to delineate the interactive effects of enhanced CO2 and temperature on plant growth, development, physiological and molecular responses. Elevated CO2 maintains leaf photosynthesis rate, respiration, transpiration and stomatal conductance in the presence of elevated temperature and sustains plant growth and productivity in the presence of both these environmental factors. Concomitantly, their interaction also affects the nutritional quality of seeds and leads to alterations in the composition of secondary metabolites. Elevated CO2 and temperature modulate phytohormone concentration in plants, and due to this fact, both environmental factors have substantial effects on abiotic and biotic stresses. Elevated CO2 and temperature have been shown to have mitigating effects on plants in the presence of other abiotic stress agents like drought and salinity, while no such pattern has been observed in the presence of biotic stress agents. This review focuses on the interactive effects of enhanced CO2 and temperature on different plants and is the first of its kind to deliver their combined responses in such detail.

The enhancement of photosynthetic performance, water use efficiency and potato yield under elevated CO2 is cultivar dependent

As a fourth major food crop, potato could fulfill the nutritional demand of the growing population. Understanding how potato plants respond to predicted increase in atmospheric CO2 at the physiological, biochemical and molecular level is therefore important to improve potato productivity. Thus, the main objectives of the present study are to investigate the effects of elevated CO2 on the photosynthetic performance, water use efficiency and tuber yield of various commercial potato cultivars combined with biochemical and molecular analyses. We grew five potato cultivars (AC Novachip, Atlantic, Kennebec, Russet Burbank and Shepody) at either ambient CO2 (400 μmol CO2 mol-1) or elevated (750 μmol CO2 mol-1) CO2. Compared to ambient CO2-grown counterparts, elevated CO2-grown Russet Burbank and Shepody exhibited a significant increase in tuber yield of 107% and 49% respectively, whereas AC Novachip, Atlantic and Kennebec exhibited a 16%, 6% and 44% increment respectively. These differences in CO2-enhancement of tuber yield across the cultivars were mainly associated with the differences in CO2-stimulation of rates of photosynthesis. For instance, elevated CO2 significantly stimulated the rates of gross photosynthesis for AC Novachip (30%), Russet Burbank (41%) and Shepody (28%) but had minimal effects for Atlantic and Kennebec when measured at growth light. Elevated CO2 significantly increased the total tuber number for Atlantic (40%) and Shepody (83%) but had insignificant effects for other cultivars. Average tuber size increased for AC Novachip (16%), Kennebec (30%) and Russet Burbank (80%), but decreased for Atlantic (25%) and Shepody (19%) under elevated versus ambient CO2 conditions. Although elevated CO2 minimally decreased stomatal conductance (6-22%) and transpiration rates (2-36%), instantaneous water use efficiency increased by up to 79% in all cultivars suggesting that enhanced water use efficiency was mainly associated with increased photosynthesis at elevated CO2. The effects of elevated CO2 on electron transport rates, non-photochemical quenching, excitation pressure, and leaf chlorophyll and protein content varied across the cultivars. We did not observe any significant differences in plant growth and morphology in elevated versus ambient CO2-grown plants. Taken all together, we conclude that the CO2-stimulation of photosynthetic performance, water use efficiency and tuber yield of potatoes is cultivar dependent.

A high efficiency CO2 concentration interval optimization method for lettuce growth

The decline in carbon fertilization effects has shifted scientific focus toward the efficient and suitable regulation of CO2 concentration ([CO2]) for plant growth. In this study, the rapid A/CO2 response curve (RAC) data of lettuce were analyzed statistically under nine photosynthetic photon flux densities (PPFDs) and four temperatures. An efficient CO2 supplementation interval acquisition method based on the frequency distribution characteristics of RACs was proposed. The characteristic subsections of jumping were obtained depending on the frequency distribution of RACs. The cumulative contribution rate (CCR) of the characteristic subsections were >97 %, which showed the efficiency of the method. Additionally, U-chord curvature theory was used to simultaneously obtain the optimal regulated [CO2] for the same RAC curves, and the results showed that the [CO2] obtained by U-chord length were all within the interval obtained by the method, which proved the rationality of the method. The [CO2] interval supplement improved the daily CO2 exchange rate by 20.27 % and 21.64 % at 150 and 200 μmol·m-2·s-1, and increased the lettuce fresh biomass by 26.78 % at 150 μmol·m-2·s-1. Based on the interval of [CO2] efficient utilization regulation at various temperatures and PPFDs, a genetic algorithm-support vector regression model was built with R2 of the model was >0.84 and the root mean square error was <35.2256 μmol·mol-1. In conclusion, the [CO2] interval obtained by this method has a positive effect on lettuce growth. This work provides a new method for obtaining high-efficiency supplementary concentration of CO2 during the growth of lettuce.

Persulfidation protects from oxidative stress under nonphotorespiratory conditions in Arabidopsis

Hydrogen sulfide is a signaling molecule in plants that regulates essential biological processes through protein persulfidation. However, little is known about sulfide-mediated regulation in relation to photorespiration. Here, we performed label-free quantitative proteomic analysis and observed a high impact on protein persulfidation levels when plants grown under nonphotorespiratory conditions were transferred to air, with 98.7% of the identified proteins being more persulfidated under suppressed photorespiration. Interestingly, a higher level of reactive oxygen species (ROS) was detected under nonphotorespiratory conditions. Analysis of the effect of sulfide on aspects associated with non- or photorespiratory growth conditions has demonstrated that it protects plants grown under suppressed photorespiration. Thus, sulfide amends the imbalance of carbon/nitrogen and restores ATP levels to concentrations like those of air-grown plants; balances the high level of ROS in plants under nonphotorespiratory conditions to reach a cellular redox state similar to that in air-grown plants; and regulates stomatal closure, to decrease the high guard cell ROS levels and induce stomatal aperture. In this way, sulfide signals the CO₂ -dependent stomata movement, in the opposite direction of the established abscisic acid-dependent movement. Our findings suggest that the high persulfidation level under suppressed photorespiration reveals an essential role of sulfide signaling under these conditions.

Bulb growth potential is independent of leaf longevity for the spring ephemeral Erythronium americanum Ker-Gawl

Growth in most spring ephemerals is decreased under warmer temperatures. Although photosynthetic activities are improved at warmer temperatures, leaves senesce earlier, which prevents the bulb from reaching a larger size. A longer leaf life duration during a warm spring, therefore, may improve bulb mass. We tested this hypothesis by modulating leaf life span of Erythronium americanum through the application of Promalin® (PRO; cytokinins and gibberellins) that prolonged or silver thiosulfate (STS) that reduced leaf duration. Gas exchange and chlorophyll fluorescence were measured along with leaf and bulb carbohydrate concentrations. Plants were also pulse labelled with 13CO2 to monitor sugar transport to the bulb. Lower photosynthetic rates and shorter leaf life span of STS plants reduced the amount of carbon that they assimilated during the season, resulting in a smaller bulb compared with control plants. PRO plants maintained their photosynthetic rates for a longer period than control plants, yet final bulb biomass did not differ between them. We conclude that seasonal growth for E. americanum is not limited by leaf life duration under warm growing conditions, but rather by limited sink growth capacity. Under global warming, spring geophytes might be at risk of being reduced in size and, eventually, reproducing less frequently.

Evidence that mitochondrial alternative oxidase respiration supports carbon balance in source leaves of Nicotiana tabacum

Alternative oxidase (AOX) represents a non-energy conserving pathway within the mitochondrial electron transport chain. One potential physiological role of AOX could be to manage leaf carbohydrate amounts by supporting respiratory carbon oxidation reactions. In this study, several approaches tested the hypothesis that AOX1a gene expression in Nicotiana tabacum leaf is enhanced in conditions expected to promote an increased leaf carbohydrate status. These approaches included supplying leaves with exogenous carbohydrates, comparing plants grown at different atmospheric CO₂ concentrations, comparing sink leaves with source leaves, comparing plants with different ratios of source to sink activity, and examining gene expression over the diel cycle. In each case, the pattern of AOX1a gene expression was compared with that of other genes known to respond to carbohydrates and/or other factors related to source:sink activity. These included GPT1 and GPT3 (that encode chloroplast glucose 6-phosphate/phosphate translocators), SPS (that encodes sucrose phosphate synthase), SUT1 (that encodes a sucrose/H⁺ symporter involved in phloem loading) and UCP1 (that encodes a mitochondrial uncoupling protein). The AOX1a transcript amount was higher following the leaf sink-to-source transition, and in plants with higher source relative to sink activity due to increasing plant age. Further, these effects were amplified in plants grown at elevated CO₂ to stimulate source activity, particularly at end-of-day time periods. The AOX1a transcript amount was also higher following treatment of leaves with carbohydrate, in particular sucrose. Overall, the results provide evidence that, while source leaf sucrose accumulation may signal for a down-regulation of sucrose synthesis and transport, it also signals for means to manage the excess cytosolic carbohydrate pools. This includes increased AOX respiration to support carbon oxidation pathways even if energy charge is high, in combination perhaps with some return flux of carbohydrate from cytosol to stroma through the GPT3 translocator. As discussed, these activities could contribute to maintaining plant source:sink balance, as well as photosynthetic and phloem loading capacity.

Respiratory and Photosynthetic Responses of Antarctic Vascular Plants Are Differentially Affected by CO2 Enrichment and Nocturnal Warming

Projected rises in atmospheric CO2 concentration and minimum night-time temperatures may have important effects on plant carbon metabolism altering the carbon balance of the only two vascular plant species in the Antarctic Peninsula. We assessed the effect of nocturnal warming (8/5 °C vs. 8/8 °C day/night) and CO2 concentrations (400 ppm and 750 ppm) on gas exchange, non-structural carbohydrates, two respiratory-related enzymes, and mitochondrial size and number in two species of vascular plants. In Colobanthus quitensis, light-saturated photosynthesis measured at 400 ppm was reduced when plants were grown in the elevated CO2 or in the nocturnal warming treatments. Growth in elevated CO2 reduced stomatal conductance but nocturnal warming did not. The short-term sensitivity of respiration, relative protein abundance, and mitochondrial traits were not responsive to either treatment in this species. Moreover, some acclimation to nocturnal warming at ambient CO2 was observed. Altogether, these responses in C. quitensis led to an increase in the respiration-assimilation ratio in plants grown in elevated CO2. The response of Deschampsia antarctica to the experimental treatments was quite distinct. Photosynthesis was not affected by either treatment; however, respiration acclimated to temperature in the elevated CO2 treatment. The observed short-term changes in thermal sensitivity indicate type I acclimation of respiration. Growth in elevated CO2 and nocturnal warming resulted in a reduction in mitochondrial numbers and an increase in mitochondrial size in D. antarctica. Overall, our results suggest that with climate change D. antarctica could be more successful than C. quitensis, due to its ability to make metabolic adjustments to maintain its carbon balance.

Reductive stress triggers ANAC017-mediated retrograde signaling to safeguard the endoplasmic reticulum by boosting mitochondrial respiratory capacity

Redox processes are at the heart of universal life processes, such as metabolism, signaling, or folding of secreted proteins. Redox landscapes differ between cell compartments and are strictly controlled to tolerate changing conditions and to avoid cell dysfunction. While a sophisticated antioxidant network counteracts oxidative stress, our understanding of reductive stress responses remains fragmentary. Here, we observed root growth impairment in Arabidopsis thaliana mutants of mitochondrial alternative oxidase 1a (aox1a) in response to the model thiol reductant dithiothreitol (DTT). Mutants of mitochondrial uncoupling protein 1 (ucp1) displayed a similar phenotype indicating that impaired respiratory flexibility led to hypersensitivity. Endoplasmic reticulum (ER) stress was enhanced in the mitochondrial mutants and limiting ER oxidoreductin capacity in the aox1a background led to synergistic root growth impairment by DTT, indicating that mitochondrial respiration alleviates reductive ER stress. The observations that DTT triggered nicotinamide adenine dinucleotide (NAD) reduction in vivo and that the presence of thiols led to electron transport chain activity in isolated mitochondria offer a biochemical framework of mitochondrion-mediated alleviation of thiol-mediated reductive stress. Ablation of transcription factor Arabidopsis NAC domain-containing protein17 (ANAC017) impaired the induction of AOX1a expression by DTT and led to DTT hypersensitivity, revealing that reductive stress tolerance is achieved by adjusting mitochondrial respiratory capacity via retrograde signaling. Our data reveal an unexpected role for mitochondrial respiratory flexibility and retrograde signaling in reductive stress tolerance involving inter-organelle redox crosstalk.

A hybrid kinetic and constraint-based model of leaf metabolism allows predictions of metabolic fluxes in different environments

While flux balance analysis (FBA) provides a framework for predicting steady-state leaf metabolic network fluxes, it does not readily capture the response to environmental variables without being coupled to other modelling formulations. To address this, we coupled an FBA model of 903 reactions of soybean (Glycine max) leaf metabolism with e-photosynthesis, a dynamic model that captures the kinetics of 126 reactions of photosynthesis and associated chloroplast carbon metabolism. Successful coupling was achieved in an iterative formulation in which fluxes from e-photosynthesis were used to constrain the FBA model and then, in turn, fluxes computed from the FBA model used to update parameters in e-photosynthesis. This process was repeated until common fluxes in the two models converged. Coupling did not hamper the ability of the kinetic module to accurately predict the carbon assimilation rate, photosystem II electron flux, and starch accumulation of field-grown soybean at two CO2 concentrations. The coupled model also allowed accurate predictions of additional parameters such as nocturnal respiration, as well as analysis of the effect of light intensity and elevated CO2 on leaf metabolism. Predictions included an unexpected decrease in the rate of export of sucrose from the leaf at high light, due to altered starch-sucrose partitioning, and altered daytime flux modes in the tricarboxylic acid cycle at elevated CO2 . Mitochondrial fluxes were notably different between growing and mature leaves, with greater anaplerotic, tricarboxylic acid cycle and mitochondrial ATP synthase fluxes predicted in the former, primarily to provide carbon skeletons and energy for protein synthesis.

Establishing an efficient protoplast transient expression system for investigation of floral thermogenesis in aroids

Floral thermogenesis is an important reproductive strategy for attracting pollinators. We developed essential biological tools for studying floral thermogenesis using two species of thermogenic aroids, Symplocarpus renifolius and Alocasia odora. Aroids contain many species with intense heat-producing abilities in their inflorescences. Several genes have been proposed to be involved in thermogenesis of these species, but biological tools for gene functional analyses are lacking. In this study, we aimed to develop a protoplast-based transient expression (PTE) system for the study of thermogenic aroids. Initially, we focused on skunk cabbage (Symplocarpus renifolius) because of its ability to produce intense as well as durable heat. In this plant, leaf protoplasts were isolated from potted and shoot tip-cultured plants with high efficiency (ca. 1.0 × 10⁵/g fresh weight), and more than half of these protoplasts were successfully transfected. Using this PTE system, we determined the protein localization of three mitochondrial energy-dissipating proteins, SrAOX, SrUCPA, and SrNDA1, fused to green fluorescent protein (GFP). These three GFP-fused proteins were localized in MitoTracker-stained mitochondria in leaf protoplasts, although the green fluorescent particles in protoplasts expressing SrUCPA-GFP were significantly enlarged. Finally, to assess whether the PTE system established in the leaves of S. renifolius is applicable for floral tissues of thermogenic aroids, inflorescences of S. renifolius and another thermogenic aroid (Alocasia odora) were used. Although protoplasts were successfully isolated from several tissues of the inflorescences, PTE systems worked well only for the protoplasts isolated from the female parts (slightly thermogenic or nonthermogenic) of A. odora inflorescences. Our developed system has a potential to be widely used in inflorescences as well as leaves in thermogenic aroids and therefore may be a useful biological tool for investigating floral thermogenesis.

The Complementary Roles of Chloroplast Cyclic Electron Transport and Mitochondrial Alternative Oxidase to Ensure Photosynthetic Performance

Chloroplasts use light energy and a linear electron transport (LET) pathway for the coupled generation of NADPH and ATP. It is widely accepted that the production ratio of ATP to NADPH is usually less than required to fulfill the energetic needs of the chloroplast. Left uncorrected, this would quickly result in an over-reduction of the stromal pyridine nucleotide pool (i.e., high NADPH/NADP⁺ ratio) and under-energization of the stromal adenine nucleotide pool (i.e., low ATP/ADP ratio). These imbalances could cause metabolic bottlenecks, as well as increased generation of damaging reactive oxygen species. Chloroplast cyclic electron transport (CET) and the chloroplast malate valve could each act to prevent stromal over-reduction, albeit in distinct ways. CET avoids the NADPH production associated with LET, while the malate valve consumes the NADPH associated with LET. CET could operate by one of two different pathways, depending upon the chloroplast ATP demand. The NADH dehydrogenase-like pathway yields a higher ATP return per electron flux than the pathway involving PROTON GRADIENT REGULATION5 (PGR5) and PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1). Similarly, the malate valve could couple with one of two different mitochondrial electron transport pathways, depending upon the cytosolic ATP demand. The cytochrome pathway yields a higher ATP return per electron flux than the alternative oxidase (AOX) pathway. In both Arabidopsis thaliana and Chlamydomonas reinhardtii, PGR5/PGRL1 pathway mutants have increased amounts of AOX, suggesting complementary roles for these two lesser-ATP yielding mechanisms of preventing stromal over-reduction. These two pathways may become most relevant under environmental stress conditions that lower the ATP demands for carbon fixation and carbohydrate export.

The flexibility of metabolic interactions between chloroplasts and mitochondria in Nicotiana tabacum leaf

To examine the effect of mitochondrial function on photosynthesis, wild-type and transgenic Nicotiana tabacum with varying amounts of alternative oxidase (AOX) were treated with different respiratory inhibitors. Initially, each inhibitor increased the reduction state of the chloroplast electron transport chain, most severely in AOX knockdowns and least severely in AOX overexpressors. This indicated that the mitochondrion was a necessary sink for photo-generated reductant, contributing to the 'P700 oxidation capacity' of photosystem I. Initially, the Complex III inhibitor myxothiazol and the mitochondrial ATP synthase inhibitor oligomycin caused an increase in photosystem II regulated non-photochemical quenching not evident with the Complex III inhibitor antimycin A (AA). This indicated that the increased quenching depended upon AA-sensitive cyclic electron transport (CET). Following 12 h with oligomycin, the reduction state of the chloroplast electron transport chain recovered in all plant lines. Recovery was associated with large increases in the protein amount of chloroplast ATP synthase and mitochondrial uncoupling protein. This increased the capacity for photophosphorylation in the absence of oxidative phosphorylation and enabled the mitochondrion to act again as a sink for photo-generated reductant. Comparing the AA and myxothiazol treatments at 12 h showed that CET optimized photosystem I quantum yield, depending upon the P700 oxidation capacity. When this capacity was too high, CET drew electrons away from other sinks, moderating the P700+ amount. When P700 oxidation capacity was too low, CET acted as an electron overflow, moderating the amount of reduced P700. This study reveals flexible chloroplast-mitochondrion interactions able to overcome lesions in energy metabolism.

Mitochondrial redox systems as central hubs in plant metabolism and signaling

Plant mitochondria are indispensable for plant metabolism and are tightly integrated into cellular homeostasis. This review provides an update on the latest research concerning the organization and operation of plant mitochondrial redox systems, and how they affect cellular metabolism and signaling, plant development, and stress responses. New insights into the organization and operation of mitochondrial energy systems such as the tricarboxylic acid cycle and mitochondrial electron transport chain (mtETC) are discussed. The mtETC produces reactive oxygen and nitrogen species, which can act as signals or lead to cellular damage, and are thus efficiently removed by mitochondrial antioxidant systems, including Mn-superoxide dismutase, ascorbate-glutathione cycle, and thioredoxin-dependent peroxidases. Plant mitochondria are tightly connected with photosynthesis, photorespiration, and cytosolic metabolism, thereby providing redox-balancing. Mitochondrial proteins are targets of extensive post-translational modifications, but their functional significance and how they are added or removed remains unclear. To operate in sync with the whole cell, mitochondria can communicate their functional status via mitochondrial retrograde signaling to change nuclear gene expression, and several recent breakthroughs here are discussed. At a whole organism level, plant mitochondria thus play crucial roles from the first minutes after seed imbibition, supporting meristem activity, growth, and fertility, until senescence of darkened and aged tissue. Finally, plant mitochondria are tightly integrated with cellular and organismal responses to environmental challenges such as drought, salinity, heat, and submergence, but also threats posed by pathogens. Both the major recent advances and outstanding questions are reviewed, which may help future research efforts on plant mitochondria.

Distinctive mitochondrial and chloroplast components contributing to the maintenance of carbon balance during plant growth at elevated CO2

Plant carbon balance depends upon the difference between photosynthetic carbon gain and respiratory carbon loss. In C₃ plants, growth at an elevated atmospheric concentration of CO₂ (ECO₂) stimulates photosynthesis and raises the leaf carbohydrate status, but how respiration responds is less understood. In this study, growth of Nicotiana tabacum at ECO₂ increased the protein amount of the non-energy conserving mitochondrial alternative oxidase (AOX). Growth at ECO₂ increased AOX1a transcript amount, and the transcript amount of a putative sugar-responsive gene encoding a chloroplast glucose-6-phosphate/phosphate translocator (GPT3). We suggest that the elevated amounts of AOX and GPT3 represent distinctive mitochondrial and chloroplast mechanisms to manage an excessive cytosolic pool of sugar phosphates. AOX respiration could consume cytosolic sugar phosphates, without this activity being restricted by rates of ATP turnover. GPT3 could allow accumulating cytosolic glucose-6-phosphate to return to the chloroplast. This could feed starch synthesis or a glucose-6-phosphate shunt in the Calvin cycle. AOX and GPT3 activities could buffer against P_i depletions that might otherwise disrupt mitochondrial and chloroplast electron transport chain activities. AOX and GPT3 activities could also buffer against a down-regulation of photosynthetic capacity by preventing a persistent imbalance between photosynthetic carbon gain and the activity of carbon utilizing sinks.

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.

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.

Photosynthesis in a Changing Global Climate: Scaling Up and Scaling Down in Crops

Photosynthesis is the major process leading to primary production in the Biosphere. There is a total of 7000bn tons of CO2 in the atmosphere and photosynthesis fixes more than 100bn tons annually. The CO2 assimilated by the photosynthetic apparatus is the basis of crop production and, therefore, of animal and human food. This has led to a renewed interest in photosynthesis as a target to increase plant production and there is now increasing evidence showing that the strategy of improving photosynthetic traits can increase plant yield. However, photosynthesis and the photosynthetic apparatus are both conditioned by environmental variables such as water availability, temperature, [CO2], salinity, and ozone. The "omics" revolution has allowed a better understanding of the genetic mechanisms regulating stress responses including the identification of genes and proteins involved in the regulation, acclimation, and adaptation of processes that impact photosynthesis. The development of novel non-destructive high-throughput phenotyping techniques has been important to monitor crop photosynthetic responses to changing environmental conditions. This wealth of data is being incorporated into new modeling algorithms to predict plant growth and development under specific environmental constraints. This review gives a multi-perspective description of the impact of changing environmental conditions on photosynthetic performance and consequently plant growth by briefly highlighting how major technological advances including omics, high-throughput photosynthetic measurements, metabolic engineering, and whole plant photosynthetic modeling have helped to improve our understanding of how the photosynthetic machinery can be modified by different abiotic stresses and thus impact crop production.

Atmospheric CO2 Concentration and N Availability Affect the Balance of the Two Photosystems in Mature Leaves of Rice Plants Grown at a Free-Air CO2 Enrichment Site

Atmospheric CO₂ concentration ([CO₂]) has been substantially increasing. Responses of leaf photosynthesis to elevated [CO₂] have been intensively investigated because leaf photosynthesis is one of the most important determinants of crop yield. The responses of photosynthesis to elevated [CO₂] can depend on nitrogen (N) availability. Here, we aimed to investigate the significance of the appropriate balance between two photosystems [photosystem I (PSI) and photosystem II (PSII)] under various [CO₂] and N levels, and thus to clarify if responses of photosynthetic electron transport rates (ETRs) of the two photosystems to elevated [CO₂] are altered by N availability. Thus, we examined parameters of the two photosystems in mature leaves of rice plants grown under two [CO₂] levels (ambient and 200 μmol mol^-1 above ambient) and three N fertilization levels at the Tsukuba free-air CO₂ enrichment experimental facility in Japan. Responses of ETR of PSII (ETRII) and ETR of PSI (ETRI) to [CO₂] levels differed among N levels. When moderate levels of N were applied (MN), ETRI was higher under elevated [CO₂], whereas at high levels of N were applied (HN), both ETRII and ETRI were lower under elevated [CO₂] compared with ambient [CO₂]. Under HN, the decreases in ETRII and ETRI under elevated [CO₂] were due to increases in the non-photochemical quenching of PSII [Y(NPQ)] and the donor side limitation of PSI [Y(ND)], respectively. The relationship between the effective quantum yields of PSI [Y(I)] and PSII [Y(II)] changed under elevated [CO₂] and low levels of N (LN). Under both conditions, the ratio of Y(I) to Y(II) was higher than under other conditions. The elevated [CO₂] and low N changed the balance of the two photosystems. This change may be important because it can induce the cyclic electron flow around PSI, leading to induction of non-photochemical quenching to avoid photoinhibition.

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.

Does the alternative respiratory pathway offer protection against the adverse effects resulting from climate change?

Elevated greenhouse gases (GHGs) induce adverse conditions directly and indirectly, causing decreases in plant productivity. To deal with climate change effects, plants have developed various mechanisms including the fine-tuning of metabolism. Plant respiratory metabolism is highly flexible due to the presence of various alternative pathways. The mitochondrial alternative oxidase (AOX) respiratory pathway is responsive to these changes, and several lines of evidence suggest it plays a role in reducing excesses of reactive oxygen species (ROS) and reactive nitrogen species (RNS) while providing metabolic flexibility under stress. Here we discuss the importance of the AOX pathway in dealing with elevated carbon dioxide (CO2), nitrogen oxides (NOx), ozone (O3), and the main abiotic stresses induced by climate change.

Roles for Plant Mitochondrial Alternative Oxidase Under Normoxia, Hypoxia, and Reoxygenation Conditions

Alternative oxidase (AOX) is a non-energy conserving terminal oxidase in the plant mitochondrial electron transport chain (ETC) that has a lower affinity for oxygen than does cytochrome (cyt) oxidase. To investigate the role(s) of AOX under different oxygen conditions, wild-type (WT) Nicotiana tabacum plants were compared with AOX knockdown and overexpression plants under normoxia, hypoxia (near-anoxia), and during a reoxygenation period following hypoxia. Paradoxically, under all the conditions tested, the AOX amount across plant lines correlated positively with leaf energy status (ATP/ADP ratio). Under normoxia, AOX was important to maintain respiratory carbon flow, to prevent the mitochondrial generation of superoxide and nitric oxide (NO), to control lipid peroxidation and protein S-nitrosylation, and possibly to reduce the inhibition of cyt oxidase by NO. Under hypoxia, AOX was again important in preventing superoxide generation and lipid peroxidation, but now contributed positively to NO amount. This may indicate an ability of AOX to generate NO under hypoxia, similar to the nitrite reductase activity of cyt oxidase under hypoxia. Alternatively, it may indicate that AOX activity simply reduces the amount of superoxide scavenging of NO, by reducing the availability of superoxide. The amount of inactivation of mitochondrial aconitase during hypoxia was also dependent upon AOX amount, perhaps through its effects on NO amount, and this influenced carbon flow under hypoxia. Finally, AOX was particularly important in preventing nitro-oxidative stress during the reoxygenation period, thereby contributing positively to the recovery of energy status following hypoxia. Overall, the results suggest that AOX plays a beneficial role in low oxygen metabolism, despite its lower affinity for oxygen than cytochrome oxidase.

Does the stromal concentration of Pi control chloroplast ATP synthase protein amount in contrasting growth environments?

Changes in the growth environment can generate imbalances in chloroplast photosynthetic metabolism. Under water deficit, stomatal closure limits CO2 availability such that the production of ATP and NADPH by the thylakoid membrane-localized electron transport chain may not match the consumption of these energy intermediates by the stroma-localized Calvin-Benson cycle, thus challenging energy balance. Alternatively, in an elevated CO2 atmosphere, carbon fixation by the Calvin-Benson cycle may outpace the activity of downstream carbohydrate-utilizing processes, thus challenging carbon balance. Our previous studies have shown that, in both of the above scenarios, a mitochondrial alternative oxidase contributes to maintaining energy or carbon balance, highlighting the importance of photosynthesis-respiration interactions in optimizing photosynthesis in different growth environments. In these previous studies, we observed aberrant amounts of chloroplast ATP synthase protein across the different transgenic plant lines and growth conditions, compared to wild-type. Based on these observations, we develop here the hypothesis that an important determinant of chloroplast ATP synthase protein amount is the stromal concentration of inorganic phosphate. ATP synthase is a master regulator of photosynthesis. Coarse control of ATP synthase protein amount by the stromal inorganic phosphate status could provide a means to coordinate the electron transport and carbon fixation reactions of photosynthesis.

Signaling interactions between mitochondria and chloroplasts in Nicotiana tabacum leaf

Research has begun to elucidate the signal transduction pathway(s) that control cellular responses to changes in mitochondrial status. Important tools in such studies are chemical inhibitors used to initiate mitochondrial dysfunction. This study compares the effect of different inhibitors and treatment conditions on the transcript amount of nuclear genes specifically responsive to mitochondrial dysfunction in leaf of Nicotiana tabacum L. cv. Petit Havana. The Complex III inhibitors antimycin A (AA) and myxothiazol (MYXO), and the Complex V inhibitor oligomycin (OLIGO), each increased the transcript amount of the mitochondrial dysfunction genes. Transcript responses to OLIGO were greater during treatment in the dark than in the light, and the dark treatment resulted in cell death. In the dark, transcript responses to AA and MYXO were similar to one another, despite MYXO leading to cell death. In the light, transcript responses to AA and MYXO diverged, despite cell viability remaining high with either inhibitor. This divergent response may be due to differential signaling from the chloroplast because only AA also inhibited cyclic electron transport, resulting in a strong acceptor-side limitation in photosystem I. In the light, chemical inhibition of chloroplast electron transport reduced transcript responses to AA, while having no effect on the response to MYXO, and increasing the response to OLIGO. Hence, when studying mitochondrial dysfunction signaling, different inhibitor and treatment combinations differentially affect linked processes (e.g. chloroplast function and cell fate) that then contribute to measured responses. Therefore, inhibitor and treatment conditions should be chosen to align with specific study goals.

Regulation of mitochondrial NAD pool via NAD+ transporter 2 is essential for matrix NADH homeostasis and ROS production in Arabidopsis

Reactive oxygen species (ROS) play a crucial role in numerous biological processes in plants, including development, responses to environmental stimuli, and programmed cell death (PCD). Deficiency in MOSAIC DEATH 1 (MOD1), a plastid-localized enoyl-ACP reductase essential for de novo fatty acid biosynthesis in Arabidopsis thaliana, leads to the increased malate export from chloroplasts to mitochondria, and the subsequent accumulation of mitochondria-generated ROS and PCD. In this study, we report the identification and characterization of a mod1 suppressor, som592. SOM592 encodes mitochondrion-localized NAD+ transporter 2 (NDT2). We show that the mitochondrial NAD pool is elevated in the mod1 mutant. The som592 mutation fully suppressed mitochondrial NADH hyper-accumulation, ROS production, and PCD in the mod1 mutant, indicating a causal relationship between mitochondrial NAD accumulation and ROS/PCD phenotypes. We also show that in wild-type plants, the mitochondrial NAD⁺ uptake is involved in the regulation of ROS production in response to continuous photoperiod. Elevation of the alternative respiration pathway can suppress ROS accumulation and PCD in mod1, but leads to growth restriction. These findings uncover a regulatory mechanism for mitochondrial ROS production via NADH homeostasis in Arabidopsis thaliana that is likely important for growth regulation in response to altered photoperiod.

Improving Potato Stress Tolerance and Tuber Yield Under a Climate Change Scenario - A Current Overview

Global climate change in the form of extreme heat and drought poses a major challenge to sustainable crop production by negatively affecting plant performance and crop yield. Such negative impact on crop yield is likely to be aggravated in future because continued greenhouse gas emissions will cause further rise in temperature leading to increased evapo-transpiration and drought severity, soil salinity as well as insect and disease threats. This has raised a major challenge for plant scientists on securing global food demand, which urges an immediate need to enhance the current yield of major food crops by two-fold to feed the increasing population. As a fourth major food crop, enhancing potato productivity is important for food security of an increasing population. However, potato plant is highly prone to high temperature, drought, soil salinity, as well as insect and diseases. In order to maintain a sustainable potato production, we must adapt our cultivation practices and develop stress tolerant potato cultivars that are appropriately engineered for changing environment. Yet the lack of data on the underlying mechanisms of potato plant resistance to abiotic and biotic stress and the ability to predict future outcomes constitutes a major knowledge gap. It is a challenge for plant scientists to pinpoint means of improving tuber yield under increasing CO2, high temperature and drought stress including the changing patterns of pest and pathogen infestations. Understanding stress-related physiological, biochemical and molecular processes is crucial to develop screening procedures for selecting crop cultivars that can better adapt to changing growth conditions. Elucidation of such mechanism may offer new insights into the identification of specific characteristics that may be useful in breeding new cultivars aimed at maintaining or even enhancing potato yield under changing climate. This paper discusses the recent progress on the mechanism by which potato plants initially sense the changes in their surrounding CO2, temperature, water status, soil salinity and consequently respond to these changes at the molecular, biochemical and physiological levels. We suggest that future research needs to be concentrated on the identification and characterization of signaling molecules and target genes regulating stress tolerance and crop yield potential.