Alternative oxidase: a defence against metabolic fluctuations?

The Role of TCA Cycle Enzymes in Plants

As one of the iconic pathways in plant metabolism, the tricarboxylic acid (TCA) cycle is commonly thought to not only be responsible for the oxidization of respiratory substrate to drive ATP synthesis but also provide carbon skeletons to anabolic processes and contribute to carbon-nitrogen interaction and biotic stress responses. The functions of the TCA cycle enzymes are characterized by a saturation transgenesis approach, whereby the constituent expression of proteins is knocked out or reduced in order to investigate their function in vivo. The alteration of TCA cycle enzyme expression results in changed plant growth and photosynthesis under controlled conditions. Moreover, improvements in plant performance and postharvest properties are reported by overexpression of either endogenous forms or heterologous genes of a number of the enzymes. Given the importance of the TCA cycle in plant metabolism regulation, here, the function of each enzyme and its roles in different tissues are discussed. This article additionally highlights the recent finding that the plant TCA cycle, like that of mammals and microbes, dynamically assembles functional substrate channels or metabolons and discusses the implications of this finding to the current understanding of the metabolic regulation of the plant TCA cycle.

Effects of OsAOX1a Deficiency on Mitochondrial Metabolism at Critical Node of Seed Viability in Rice

Mitochondrial alternative oxidase 1a (AOX1a) plays an extremely important role in the critical node of seed viability during storage. However, the regulatory mechanism is still poorly understood. The aim of this study was to identify the regulatory mechanisms by comparing OsAOX1a-RNAi and wild-type (WT) rice seed during artificial aging treatment. Weight gain and time for the seed germination percentage decreased to 50% (P50) in OsAOX1a-RNAi rice seed, indicating possible impairment in seed development and storability. Compared to WT seeds at 100%, 90%, 80%, and 70% germination, the NADH- and succinate-dependent O₂ consumption, the activity of mitochondrial malate dehydrogenase, and ATP contents all decreased in the OsAOX1a-RNAi seeds, indicating that mitochondrial status in the OsAOX1a-RNAi seeds after imbibition was weaker than in the WT seeds. In addition, the reduction in the abundance of Complex I subunits showed that the capacity of the mitochondrial electron transfer chain was significantly inhibited in the OsAOX1a-RNAi seeds at the critical node of seed viability. The results indicate that ATP production was impaired in the OsAOX1a-RNAi seeds during aging. Therefore, we conclude that mitochondrial metabolism and alternative pathways were severely inhibited in the OsAOX1a-RNAi seeds at critical node of viability, which could accelerate the collapse of seed viability. The precise regulatory mechanism of the alternative pathway at the critical node of viability needs to be further analyzed. This finding might provide the basis for developing monitoring and warning indicators when seed viability declines to the critical node during storage.

Ca2+-dependent oxidation of exogenous NADH and NADPH by the mitochondria of spring wheat and its relation with AOX capacity and ROS content at high temperatures

Mitochondria are sources of reactive oxygen species (ROS) in a plant cell under high temperature. Mitochondrial alternative NAD(P)H dehydrogenases (type II NAD(P)H DHs) and cyanide-resistant oxidase (AOX) can regulate ROS production, but their role at high temperatures is unknown. This study investigates the influence heat acclimation (37 °C) and heat shock (50 °C) temperatures on ROS content, activity and protein abundance of external Ca²⁺-dependent NAD(P)H DHs (NDB) and AOX in mitochondria of 4- and 8-day-old seedlings of spring wheat (Triticum aestivum L., var. Novosibirskya 29). The shoots of 4-day-old seedlings contained more carbohydrates, had a higher rate of total respiration and a high rate of oxidation of exogenous NADH, a greater AOX capacity and a lower of ROS content, as compared to leaves of 8-day-old seedlings, and were more resistant to heat shock. The activity of external NADH DH was higher than the one of NADPH DH in mitochondria of both shoots and leaves. At 37 °C, high NADH oxidation was associated with increased AOX capacity in mitochondria of both shoots and leaves, whereas NADPH oxidation with COX capacity. At 50 °C, the NADPH oxidation by shoots' mitochondria increased and the NADH oxidation stayed high. The content of NDB and AOX proteins depends on heat treatments and differs between mitochondria of shoots and leaves. Our data indicate that Ca²⁺-dependent type II NAD(P)H DHs can regulate the ROS content and together with AOX are involved in heat tolerance, depending on the development phase of spring wheat and is, probably, tissue-specific.

Suppression of mitochondrial alternative oxidase can result in upregulation of the ROS scavenging network: some possible mechanisms underlying the compensation effect

Mitochondrial alternative oxidase is an important protein involved in maintaining cellular metabolic and energy balance, especially under stress conditions. AOX genes knockout is aimed at revealing the functions of AOX genes. Under unfavourable conditions, AOX-suppressed plants (mainly based on Arabidopsis AOX1a-knockout lines) usually experience strong oxidative stress. However, a compensation effect, which consists of the absence of AOX1a leading to an increase in defence response mechanisms, concomitant with a decrease in ROS content, has also been demonstrated. This review briefly describes the possible mechanisms underlying the compensation effect upon the suppression of AOX1a. Information about mitochondrial retrograde regulation of AOX is given. The importance of ROS and mitochondrial membrane potential in triggering the signal transmission from mitochondria in the absence of AOX or disturbance of mitochondrial electron transport chain functions is indicated. The few available data on the response of the cell to the absence of AOX at the level of changes in the hormonal balance and the reactions of chloroplasts are presented. The decrease in the relative amount of reduced ascorbate at stable ROS levels as a result of compensation in AOX1a-suppressed plants is proposed as a sign of stress development. Obtaining direct evidence on the mechanisms and signalling pathways involved in AOX modulation in the genome should facilitate a deeper understanding of the role of AOX in the integration of cellular signalling pathways.

Transcriptome Analyses in a Selected Gene Set Indicate Alternative Oxidase (AOX) and Early Enhanced Fermentation as Critical for Salinity Tolerance in Rice

Plants subjected to stress need to respond rapidly and efficiently to acclimatize and survive. In this paper, we investigated a selected gene set potentially involved in early cell reprogramming in two rice genotypes with contrasting salinity tolerance (Pokkali tolerant and IR29 susceptible) in order to advance knowledge of early molecular mechanisms of rice in dealing with salt stress. Selected genes were evaluated in available transcriptomic data over a short period of 24 h and involved enzymes that avoid ROS formation (AOX, UCP and PTOX), impact ATP production (PFK, ADH and COX) or relate to the antioxidant system. Higher transcript accumulation of AOX (ROS balancing), PFK and ADH (alcohol fermentation) was detected in the tolerant genotype, while the sensitive genotype revealed higher UCP and PTOX transcript levels, indicating a predominant role for early transcription of AOX and fermentation in conferring salt stress tolerance to rice. Antioxidant gene analyses supported higher oxidative stress in IR29, with transcript increases of cytosolic CAT and SOD from all cell compartments (cytoplasm, peroxisome, chloroplast and mitochondria). In contrast, Pokkali increased mRNA levels from the AsA-GSH cycle as cytosolic/mitochondrial DHAR was involved in ascorbate recovery. In addition, these responses occurred from 2 h in IR29 and 10 h in Pokkali, indicating early but ineffective antioxidant activity in the susceptible genotype. Overall, our data suggest that AOX and ADH can play a critical role during early cell reprogramming for improving salt stress tolerance by efficiently controlling ROS formation in mitochondria. We discuss our results in relation to gene engineering and editing approaches to develop salinity-tolerant crops.

Cyanide resistant respiration and the alternative oxidase pathway: A journey from plants to mammals

In a large number of organisms covering all phyla, the mitochondrial respiratory chain harbors, in addition to the conventional elements, auxiliary proteins that confer adaptive metabolic plasticity. The alternative oxidase (AOX) represents one of the most studied auxiliary proteins, initially identified in plants. In contrast to the standard respiratory chain, the AOX mediates a thermogenic cyanide-resistant respiration; a phenomenon that has been of great interest for over 2 centuries in that energy is not conserved when electrons flow through it. Here we summarize centuries of studies starting from the early observations of thermogenicity in plants and the identification of cyanide resistant respiration, to the fascinating discovery of the AOX and its current applications in animals under normal and pathological conditions.

The Lack of Alternative Oxidase 1a Restricts in vivo Respiratory Activity and Stress-Related Metabolism for Leaf Osmoprotection and Redox Balancing Under Sudden Acute Water and Salt Stress in Arabidopsis thaliana

In plants salt and water stress result in an induction of respiration and accumulation of stress-related metabolites (SRMs) with osmoregulation and osmoprotection functions that benefit photosynthesis. The synthesis of SRMs may depend on an active respiratory metabolism, which can be restricted under stress by the inhibition of the cytochrome oxidase pathway (COP), thus causing an increase in the reduction level of the ubiquinone pool. However, the activity of the alternative oxidase pathway (AOP) is thought to prevent this from occurring while at the same time, dissipates excess of reducing power from the chloroplast and thereby improves photosynthetic performance. The present research is based on the hypothesis that the accumulation of SRMs under osmotic stress will be affected by changes in folial AOP activity. To test this, the oxygen isotope-fractionation technique was used to study the in vivo respiratory activities of COP and AOP in leaves of wild-type Arabidopsis thaliana plants and of aox1a mutants under sudden acute stress conditions induced by mannitol and salt treatments. Levels of leaf primary metabolites and transcripts of respiratory-related proteins were also determined in parallel to photosynthetic analyses. The lack of in vivo AOP response in the aox1a mutants coincided with a lower leaf relative water content and a decreased accumulation of crucial osmoregulators. Additionally, levels of oxidative stress-related metabolites and transcripts encoding alternative respiratory components were increased. Coordinated changes in metabolite levels, respiratory activities and photosynthetic performance highlight the contribution of the AOP in providing flexibility to carbon metabolism for the accumulation of SRMs.

Genes from oxidative phosphorylation complexes II-V and two dual-function subunits of complex I are transcribed in Viscum album despite absence of the entire mitochondrial holo-complex I

Mistletoes (Viscum) and close relatives are unique among flowering plants in having a drastically altered electron transport chain. Lack of complex I genes has previously been reported for the mitochondrial genome, and here we report an almost complete absence of nuclear-encoded complex I genes in the transcriptome of Viscum album. Compared to Arabidopsis with approximately 40 nuclear complex I genes, we recover only transcripts of two dual-function genes: gamma carbonic anhydrase and L-galactono-1,4-lactone dehydrogenase. The complement of genes belonging to complexes II-V of the oxidative phosphorylation pathway appears to be in accordance with other vascular plants. Additionally, transcripts encoding alternative NAD(P)H dehydrogenases and alternative oxidase were found. Despite sequence divergence, structural modeling suggests that the encoded proteins are structurally conserved. Complex I loss is a special feature in Viscum species and relatives, as all other parasitic flowering plants investigated to date seem to have a complete OXPHOS system. Hence, Viscum offers a unique system for specifically investigating molecular consequences of complex I absence, such as the role of complex I subunits involved in secondary functions.

Plant mitochondria - past, present and future

The study of plant mitochondria started in earnest around 1950 with the first isolations of mitochondria from animal and plant tissues. The first 35 years were spent establishing the basic properties of plant mitochondria and plant respiration using biochemical and physiological approaches. A number of unique properties (compared to mammalian mitochondria) were observed: (i) the ability to oxidize malate, glycine and cytosolic NAD(P)H at high rates; (ii) the partial insensitivity to rotenone, which turned out to be due to the presence of a second NADH dehydrogenase on the inner surface of the inner mitochondrial membrane in addition to the classical Complex I NADH dehydrogenase; and (iii) the partial insensitivity to cyanide, which turned out to be due to an alternative oxidase, which is also located on the inner surface of the inner mitochondrial membrane, in addition to the classical Complex IV, cytochrome oxidase. With the appearance of molecular biology methods around 1985, followed by genomics, further unique properties were discovered: (iv) plant mitochondrial DNA (mtDNA) is 10-600 times larger than the mammalian mtDNA, yet it only contains approximately 50% more genes; (v) plant mtDNA has kept the standard genetic code, and it has a low divergence rate with respect to point mutations, but a high recombinatorial activity; (vi) mitochondrial mRNA maturation includes a uniquely complex set of activities for processing, splicing and editing (at hundreds of sites); (vii) recombination in mtDNA creates novel reading frames that can produce male sterility; and (viii) plant mitochondria have a large proteome with 2000-3000 different proteins containing many unique proteins such as 200-300 pentatricopeptide repeat proteins. We describe the present and fairly detailed picture of the structure and function of plant mitochondria and how the unique properties make their metabolism more flexible allowing them to be involved in many diverse processes in the plant cell, such as photosynthesis, photorespiration, CAM and C4 metabolism, heat production, temperature control, stress resistance mechanisms, programmed cell death and genomic evolution. However, it is still a challenge to understand how the regulation of metabolism and mtDNA expression works at the cellular level and how retrograde signaling from the mitochondria coordinates all those processes.

From Plant Survival Under Severe Stress to Anti-Viral Human Defense - A Perspective That Calls for Common Efforts

Reprogramming of primary virus-infected cells is the critical step that turns viral attacks harmful to humans by initiating super-spreading at cell, organism and population levels. To develop early anti-viral therapies and proactive administration, it is important to understand the very first steps of this process. Plant somatic embryogenesis (SE) is the earliest and most studied model for de novo programming upon severe stress that, in contrast to virus attacks, promotes individual cell and organism survival. We argued that transcript level profiles of target genes established from in vitro SE induction as reference compared to virus-induced profiles can identify differential virus traits that link to harmful reprogramming. To validate this hypothesis, we selected a standard set of genes named 'ReprogVirus'. This approach was recently applied and published. It resulted in identifying 'CoV-MAC-TED', a complex trait that is promising to support combating SARS-CoV-2-induced cell reprogramming in primary infected nose and mouth cells. In this perspective, we aim to explain the rationale of our scientific approach. We are highlighting relevant background knowledge on SE, emphasize the role of alternative oxidase in plant reprogramming and resilience as a learning tool for designing human virus-defense strategies and, present the list of selected genes. As an outlook, we announce wider data collection in a 'ReprogVirus Platform' to support anti-viral strategy design through common efforts.

Mitochondrial bioenergetics and enzymatic antioxidant defense differ in Paraná pine cell lines with contrasting embryogenic potential

Araucaria angustifolia is classified as a critically endangered species by the International Union for Conservation of Nature. This threat is worsened by the inefficiency of methods for ex-situ conservation and propagation. In conifers, somatic embryogenesis (SE) associated with cryopreservation is an efficient method to achieve germplasm conservation and mass clonal propagation. However, the efficiency of SE is highly dependent on genotype responsivity to the artificial stimulus used in vitro during cell line proliferation and later during somatic embryo development. In this study, we evaluated the activity of antioxidant enzymes and characterized mitochondrial functions during the proliferation of embryogenic cells of A. angustifolia responsive (SE1) and non-responsive (SE6) to the development of somatic embryos. The activities of the antioxidant enzymes GR (EC 1.6.4.2), MDHAR (EC 1.6.5.4), and POX (EC 1.11.1.7) were increased in SE1 culture, while in SE6 culture, only the activity of DHAR (EC 1.8.5.1) was significantly higher. Additionally, SE6 culture presented a higher number of mitochondria, which agreed with the increased rate of oxygen consumption compared to responsive SE1 culture; however, the mitochondrial volume was lower. Although the ATP levels did not differ, the NAD(P)H levels were higher in SE1 cells. NDs, AOX, and UCP were less active in responsive SE1 than in non-responsive cells. Our results show significant differences between SE1 and SE6 embryogenic cells regarding mitochondrial functions and antioxidant enzyme activities, which may be intrinsic to the in vitro proliferation phase of both cell lines, possessing a crucial role for the induction of in vitro maturation process.

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

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

Role of mitochondrial alternative oxidase in the regulation of cellular homeostasis during development of photosynthetic function in greening leaves

Here, recent publications on the role of mitochondrial non-phosphorylating pathways (NPhPs) in the electron transport chain during the de-etiolation of wheat leaves are reviewed. Among NPhPs, the alternative oxidase (AOX) pathway is the most effective pathway in maintaining cellular redox and energy balance, especially under stress conditions, including light stress. AOX is considered to dissipate excess reductants produced in the chloroplasts, and thereby prevent photooxidation. However, when etiolated wheat plants were exposed to a physiologically relevant light level, AOX was rapidly induced and increased, although the etioplasts did not produce excess reductants and have their own strong photoprotective mechanisms. The present study provides further insights into the role of AOX in greening cells and highlights the importance of AOX in the integration of cellular signalling pathways.

On the use of n-octyl gallate and salicylhydroxamic acid to study the alternative oxidase role

The alternative oxidase (AOX) catalyzes the transfer of electrons from ubiquinol to oxygen without the translocation of protons across the inner mitochondrial membrane. This enzyme has been proposed to participate in the regulation of cell growth, sporulation, yeast-mycelium transition, resistance to reactive oxygen species, infection, and production of secondary metabolites. Two approaches have been used to evaluate AOX function: incubation of cells for long periods of time with AOX inhibitors or deletion of AOX gene. However, AOX inhibitors might have different targets. To test non-specific effects of n-octyl gallate (nOg) and salicylhydroxamic acid (SHAM) on fungal physiology we measured the growth and respiratory capacity of two fungal strains lacking (Ustilago maydis-Δaox and Saccharomyces cerevisiae) and three species containing the AOX gene (U. maydis WT, Debaryomyces hansenii, and Aspergillus nidulans). For U. maydis, a strong inhibition of growth and respiratory capacity by SHAM was observed, regardless of the presence of AOX. Similarly, A. nidulans mycelial growth was inhibited by low concentrations of nOg independently of AOX expression. In contrast, these inhibitors had no effect or had a minor effect on S. cerevisiae and D. hansenii growth. These results show that nOg and SHAM have AOX independent effects which vary in different microorganisms, indicating that studies based on long-term incubation of cells with these inhibitors should be considered as inconclusive.

In Vivo Metabolic Regulation of Alternative Oxidase under Nutrient Deficiency-Interaction with Arbuscular Mycorrhizal Fungi and Rhizobium Bacteria

The interaction of the alternative oxidase (AOX) pathway with nutrient metabolism is important for understanding how respiration modulates ATP synthesis and carbon economy in plants under nutrient deficiency. Although AOX activity reduces the energy yield of respiration, this enzymatic activity is upregulated under stress conditions to maintain the functioning of primary metabolism. The in vivo metabolic regulation of AOX activity by phosphorus (P) and nitrogen (N) and during plant symbioses with Arbuscular mycorrhizal fungi (AMF) and Rhizobium bacteria is still not fully understood. We highlight several findings and open questions concerning the in vivo regulation of AOX activity and its impact on plant metabolism during P deficiency and symbiosis with AMF. We also highlight the need for the identification of which metabolic regulatory factors of AOX activity are related to N availability and nitrogen-fixing legume-rhizobia symbiosis in order to improve our understanding of N assimilation and biological nitrogen fixation.

Mitochondrial NAD(P)H oxidation pathways and nitrate/ammonium redox balancing in plants

Plant mitochondrial oxidative phosphorylation is characterised by alternative electron transport pathways with different energetic efficiencies, allowing turnover of cellular redox compounds like NAD(P)H. These electron transport chain pathways are profoundly affected by soil nitrogen availability, most commonly as oxidized nitrate (NO₃^-) and/or reduced ammonium (NH₄⁺). The bioenergetic strategies involved in assimilating different N sources can alter redox homeostasis and antioxidant systems in different cellular compartments, including the mitochondria and the cell wall. Conversely, changes in mitochondrial redox systems can affect plant responses to N. This review explores the integration between N assimilation, mitochondrial redox metabolism, and apoplast metabolism.

Identification of Alternative Mitochondrial Electron Transport Pathway Components in Chickpea Indicates a Differential Response to Salinity Stress between Cultivars

All plants contain an alternative electron transport pathway (AP) in their mitochondria, consisting of the alternative oxidase (AOX) and type 2 NAD(P)H dehydrogenase (ND) families, that are thought to play a role in controlling oxidative stress responses at the cellular level. These alternative electron transport components have been extensively studied in plants like Arabidopsis and stress inducible isoforms identified, but we know very little about them in the important crop plant chickpea. Here we identify AP components in chickpea (Cicer arietinum) and explore their response to stress at the transcript level. Based on sequence similarity with the functionally characterized proteins of Arabidopsis thaliana, five putative internal (matrix)-facing NAD(P)H dehydrogenases (CaNDA1-4 and CaNDC1) and four putative external (inter-membrane space)-facing NAD(P)H dehydrogenases (CaNDB1-4) were identified in chickpea. The corresponding activities were demonstrated for the first time in purified mitochondria of chickpea leaves and roots. Oxidation of matrix NADH generated from malate or glycine in the presence of the Complex I inhibitor rotenone was high compared to other plant species, as was oxidation of exogenous NAD(P)H. In leaf mitochondria, external NADH oxidation was stimulated by exogenous calcium and external NADPH oxidation was essentially calcium dependent. However, in roots these activities were low and largely calcium independent. A salinity experiment with six chickpea cultivars was used to identify salt-responsive alternative oxidase and NAD(P)H dehydrogenase gene transcripts in leaves from a three-point time series. An analysis of the Na:K ratio and Na content separated these cultivars into high and low Na accumulators. In the high Na accumulators, there was a significant up-regulation of CaAOX1, CaNDB2, CaNDB4, CaNDA3 and CaNDC1 in leaf tissue under long term stress, suggesting the formation of a stress-modified form of the mitochondrial electron transport chain (mETC) in leaves of these cultivars. In particular, stress-induced expression of the CaNDB2 gene showed a striking positive correlation with that of CaAOX1 across all genotypes and time points. The coordinated salinity-induced up-regulation of CaAOX1 and CaNDB2 suggests that the mitochondrial alternative pathway of respiration is an important facet of the stress response in chickpea, in high Na accumulators in particular, despite high capacities for both of these activities in leaf mitochondria of non-stressed chickpeas.

Dissecting the interaction of photosynthetic electron transfer with mitochondrial signalling and hypoxic response in the Arabidopsis rcd1 mutant

The Arabidopsis mutant rcd1 is tolerant to methyl viologen (MV). MV enhances the Mehler reaction, i.e. electron transfer from Photosystem I (PSI) to O₂, generating reactive oxygen species (ROS) in the chloroplast. To study the MV tolerance of rcd1, we first addressed chloroplast thiol redox enzymes potentially implicated in ROS scavenging. NADPH-thioredoxin oxidoreductase type C (NTRC) was more reduced in rcd1. NTRC contributed to the photosynthetic and metabolic phenotypes of rcd1, but did not determine its MV tolerance. We next tested rcd1 for alterations in the Mehler reaction. In rcd1, but not in the wild type, the PSI-to-MV electron transfer was abolished by hypoxic atmosphere. A characteristic feature of rcd1 is constitutive expression of mitochondrial dysfunction stimulon (MDS) genes that affect mitochondrial respiration. Similarly to rcd1, in other MDS-overexpressing plants hypoxia also inhibited the PSI-to-MV electron transfer. One possible explanation is that the MDS gene products may affect the Mehler reaction by altering the availability of O₂. In green tissues, this putative effect is masked by photosynthetic O₂ evolution. However, O₂ evolution was rapidly suppressed in MV-treated plants. Transcriptomic meta-analysis indicated that MDS gene expression is linked to hypoxic response not only under MV, but also in standard growth conditions. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

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.

Mitochondrial Mechanisms Underlying Tolerance to Fluctuating Oxygen Conditions: Lessons from Hypoxia-Tolerant Organisms

Oxygen (O2) is essential for most metazoan life due to its central role in mitochondrial oxidative phosphorylation (OXPHOS), which generates >90% of the cellular adenosine triphosphate. O2 fluctuations are an ultimate mitochondrial stressor resulting in mitochondrial damage, energy deficiency, and cell death. This work provides an overview of the known and putative mechanisms involved in mitochondrial tolerance to fluctuating O2 conditions in hypoxia-tolerant organisms including aquatic and terrestrial vertebrates and invertebrates. Mechanisms of regulation of the mitochondrial OXPHOS and electron transport system (ETS) (including alternative oxidases), sulphide tolerance, regulation of redox status and mitochondrial quality control, and the potential role of hypoxia-inducible factor (HIF) in mitochondrial tolerance to hypoxia are discussed. Mitochondrial phenotypes of distantly related animal species reveal common features including conservation and/or anticipatory upregulation of ETS capacity, suppression of reactive oxygen species (ROS)-producing electron flux through ubiquinone, reversible suppression of OXPHOS activity, and investment into the mitochondrial quality control mechanisms. Despite the putative importance of oxidative stress in adaptations to hypoxia, establishing the link between hypoxia tolerance and mitochondrial redox mechanisms is complicated by the difficulties of establishing the species-specific concentration thresholds above which the damaging effects of ROS outweigh their potentially adaptive signaling function. The key gaps in our knowledge about the potential mechanisms of mitochondrial tolerance to hypoxia include regulation of mitochondrial biogenesis and fusion/fission dynamics, and HIF-dependent metabolic regulation that require further investigation in hypoxia-tolerant species. Future physiological, molecular and genetic studies of mitochondrial responses to hypoxia, and reoxygenation in phylogenetically diverse hypoxia-tolerant species could reveal novel solutions to the ubiquitous and metabolically severe problem of O2 deficiency and would have important implications for understanding the evolution of hypoxia tolerance and the potential mitigation of pathological states caused by O2 fluctuations.

Altered levels of AOX1a expression result in changes in metabolic pathways in Arabidopsis thaliana plants acclimated to low dose rates of ultraviolet B radiation

UV-B is a damaging component of solar radiation that inevitably reaches the Earth's surface. Plants have developed response mechanisms to adapt to UVB exposure. The alternative oxidase (AOX) catalyzes the ATP-uncoupling cyanide-resistant alternative pathway (AP) in plant mitochondria and is thought to be an important part of the cellular defense network under stress conditions. This study aimed to unravel the poorly understood functional significance of AOX1a induction in Arabidopsis thaliana leaves exposed to ecologically relevant doses of UVB radiation, by comparing wild-type (WT) plants with plants with modified expression of the AOX1a gene, either downregulated by antisense (AS-12) or overexpressed (XX-2). UVB exposure resulted in a phenotypic difference between lines. AOX1a overexpression resulted in the highest induction of AOX1A synthesis and MnSOD activity, and the lowest ROS level without pronounced changes in the phenotype relative to other genotypes. In AS-12 plants, expression of the majority of the genes encoding AOX was detected, other non-phosphorylating pathway components and antioxidant enzymes increased along with anthocyanin accumulation in leaves, and the ROS content was lower than in the WT. In addition to the expected AOX1 protein size (34 kDa), an AOX1 30 kDa band appeared under UVB exposure in all genotypes. However, in AS-12, the alterations in the transcript level and in the abundance of AOX1 protein isoforms induced by UVB could not fully functionally compensate for the lack of AOX1A. This was confirmed by the observed low AP capacity and increased levels of the oxidized form of ascorbate. These results highlight the importance of AOX in plant response to UVB for the control of a balanced metabolism, and indicate that AOX1a plays a key role in the regulation of the stress response.

The Lack of Mitochondrial Thioredoxin TRXo1 Affects In Vivo Alternative Oxidase Activity and Carbon Metabolism under Different Light Conditions

The alternative oxidase (AOX) constitutes a nonphosphorylating pathway of electron transport in the mitochondrial respiratory chain that provides flexibility to energy and carbon primary metabolism. Its activity is regulated in vitro by the mitochondrial thioredoxin (TRX) system which reduces conserved cysteines residues of AOX. However, in vivo evidence for redox regulation of the AOX activity is still scarce. In the present study, the redox state, protein levels and in vivo activity of the AOX in parallel to photosynthetic parameters were determined in Arabidopsis knockout mutants lacking mitochondrial trxo1 under moderate (ML) and high light (HL) conditions, known to induce in vivo AOX activity. In addition, 13C- and 14C-labeling experiments together with metabolite profiling were performed to better understand the metabolic coordination between energy and carbon metabolism in the trxo1 mutants. Our results show that the in vivo AOX activity is higher in the trxo1 mutants at ML while the AOX redox state is apparently unaltered. These results suggest that mitochondrial thiol redox systems are responsible for maintaining AOX in its reduced form rather than regulating its activity in vivo. Moreover, the negative regulation of the tricarboxylic acid cycle by the TRX system is coordinated with the increased input of electrons into the AOX pathway. Under HL conditions, while AOX and photosynthesis displayed similar patterns in the mutants, photorespiration is restricted at the level of glycine decarboxylation most likely as a consequence of redox imbalance.

Mitochondrial Adaptations to Variable Environments and Their Role in Animals' Stress Tolerance

Mitochondria are the key organelles involved in energy and redox homeostasis, cellular signaling, and survival. Animal mitochondria are exquisitely sensitive to environmental stress, and stress-induced changes in the mitochondrial integrity and function have major consequences for the organismal performance and fitness. Studies in the model organisms such as terrestrial mammals and insects showed that mitochondrial dysfunction is a major cause of injury during pathological conditions and environmental insults such as hypoxia, ischemia-reperfusion, and exposure to toxins. However, animals from highly stressful environments (such as the intertidal zone of the ocean) can maintain mitochondrial integrity and function despite intense and rapid fluctuations in abiotic conditions and associated changes in the intracellular milieu. Recent studies demonstrate that mitochondria of intertidal organisms (including mollusks, crustaceans, and fish) are capable of maintaining activity of mitochondrial electron transport system (ETS), ATP synthesis, and mitochondrial coupling in a broad range of temperature, osmolarity, and ion content. Mitochondria of intertidal organisms such as mollusks are also resistant to hypoxia-reoxygenation injury and show stability or even upregulation of the mitochondrial ETS activity and ATP synthesis capacity during intermittent hypoxia. In contrast, pH optima for mitochondrial ATP synthesis and respiration are relatively narrow in intertidal mollusks and may reflect adaptation to suppress metabolic rate during pH shifts caused by extreme stress. Sensitivity to anthropogenic pollutants (such as trace metals) in intertidal mollusks appears similar to that of other organisms (including mammals) and may reflect the lack of adaptation to these evolutionarily novel stressors. The mechanisms of the exceptional mitochondrial resilience to temperature, salinity, and hypoxic stress are not yet fully understood in intertidal organisms, yet recent studies demonstrate that they may involve rapid modulation of the ETS capacity (possibly due to post-translation modification of mitochondrial proteins), upregulation of antioxidant defenses in anticipation of oxidative stress, and high activity of mitochondrial proteases involved in degradation of damaged mitochondrial proteins. With rapidly developing molecular tools for non-model organisms, future studies of mitochondrial adaptations should pinpoint the molecular sites associated with the passive tolerance and/or active regulation of mitochondrial activity during stress exposures in intertidal organisms, investigate the roles of mitochondria in transduction of stress signals, and explore the interplay between bioenergetics and mitochondrial signaling in facilitating survival in these highly stressful environments.

Photorespiration is complemented by cyclic electron flow and the alternative oxidase pathway to optimize photosynthesis and protect against abiotic stress

Optimization of photosynthetic performance and protection against abiotic stress are essential to sustain plant growth. Photorespiratory metabolism can help plants to adapt to abiotic stress. The beneficial role of photorespiration under abiotic stress is further strengthened by cyclic electron flow (CEF) and alternative oxidase (AOX) pathways. We have attempted to critically assess the literature on the responses of these three phenomena-photorespiration, CEF and AOX, to different stress situations. We emphasize that photorespiration is the key player to protect photosynthesis and upregulates CEF as well as AOX. Then these three processes work in coordination to protect the plants against photoinhibition and maintain an optimal redox state in the cell, while providing ATP for metabolism and protein repair. H₂O₂ generated during photorespiratory metabolism seems to be an important signal to upregulate CEF or AOX. Further experiments are necessary to identify the signals originating from CEF or AOX to modulate photorespiration. The mutants deficient in CEF or AOX or both could be useful in this regard. The mutual interactions between CEF and AOX, so as to keep their complementarity, are also to be examined further.

Downregulation of mitochondrial alternative oxidase affects chloroplast function, redox status and stress response in a marine diatom

Diatom dominance in contemporary aquatic environments indicates that they have developed unique and effective mechanisms to cope with the rapid and considerable fluctuations that characterize these environments. In view of their evolutionary history from a secondary endosymbiosis, inter-organellar regulation of biochemical activities may be of particular relevance. Diatom mitochondrial alternative oxidase (AOX) is believed to play a significant role in supplying chloroplasts with ATP produced in the mitochondria. Using the model diatom Phaeodactylum tricornutum we generated AOX knockdown lines, and followed sensitivity to stressors, photosynthesis and transcriptome and metabolome profiles of wild-type and knockdown lines. We show here that expression of the AOX gene is upregulated by various stresses including H₂ O₂ , heat, high light illumination, and iron or nitrogen limitation. AOX knockdown results in hypersensitivity to stress. Knockdown lines also show significantly reduced photosynthetic rates and their chloroplasts are more oxidized. Comparisons of transcriptome and metabolome profiles suggest a strong impact of AOX activity on gene expression, which is carried through to the level of the metabolome. Our data provide evidence for the involvement of mitochondrial AOX in processes central to the cell biology of diatoms, revealing that cross-talk between mitochondria and chloroplasts is crucial for maintaining sensitivity to changing environments.

Get the Balance Right: ROS Homeostasis and Redox Signalling in Fruit

Plant central metabolism generates reactive oxygen species (ROS), which are key regulators that mediate signalling pathways involved in developmental processes and plant responses to environmental fluctuations. These highly reactive metabolites can lead to cellular damage when the reduction-oxidation (redox) homeostasis becomes unbalanced. Whilst decades of research have studied redox homeostasis in leaves, fundamental knowledge in fruit biology is still fragmentary. This is even more surprising when considering the natural profusion of fruit antioxidants that can process ROS and benefit human health. In this review, we explore redox biology in fruit and provide an overview of fruit antioxidants with recent examples. We further examine the central role of the redox hub in signalling during development and stress, with particular emphasis on ascorbate, also referred to as vitamin C. Progress in understanding the molecular mechanisms involved in the redox regulations that are linked to central metabolism and stress pathways will help to define novel strategies for optimising fruit nutritional quality, fruit production and storage.

The role of HEXOKINASE1 in Arabidopsis leaf growth

Here, we used a hxk1 mutant in the Col-0 background. We demonstrated that HXK1 regulates cell proliferation and expansion early during leaf development, and that HXK1 is involved in sucrose-induced leaf growth stimulation independent of GPT2. Furthermore, we identified KINγ as a novel HXK1-interacting protein. In the last decade, extensive efforts have been made to unravel the underlying mechanisms of plant growth control through sugar availability. Signaling by the conserved glucose sensor HEXOKINASE1 (HXK1) has been shown to exert both growth-promoting and growth-inhibitory effects depending on the sugar levels, the environmental conditions and the plant species. Here, we used a hxk1 mutant in the Col-0 background to investigate the role of HXK1 during leaf growth in more detail and show that it is affected in both cell proliferation and cell expansion early during leaf development. Furthermore, the hxk1 mutant is less sensitive to sucrose-induced cell proliferation with no significant increase in final leaf growth after transfer to sucrose. Early during leaf development, transfer to sucrose stimulates expression of GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSPORTER2 (GPT2) and represses chloroplast differentiation. However, in the hxk1 mutant GPT2 expression was still upregulated by transfer to sucrose although chloroplast differentiation was not affected, suggesting that GPT2 is not involved in HXK1-dependent regulation of leaf growth. Finally, using tandem affinity purification of protein complexes from cell cultures, we identified KINγ, a protein containing four cystathionine β-synthase domains, as an interacting protein of HXK1.

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.

On the role of the tricarboxylic acid cycle in plant productivity

The tricarboxylic acid (TCA) cycle is one of the canonical energy pathways of living systems, as well as being an example of a pathway in which dynamic enzyme assemblies, or metabolons, are well characterized. The role of the enzymes have been the subject of saturated transgenesis approaches, whereby the expression of the constituent enzymes were reduced or knocked out in order to ascertain their in vivo function. Some of the resultant plants exhibited improved photosynthesis and plant growth, under controlled greenhouse conditions. In addition, overexpression of the endogenous genes, or heterologous forms of a number of the enzymes, has been carried out in tomato fruit and the roots of a range of species, and in some instances improvement in fruit yield and postharvest properties and plant performance, under nutrient limitation, have been reported, respectively. Given a number of variants, in nature, we discuss possible synthetic approaches involving introducing these variants, or at least a subset of them, into plants. We additionally discuss the likely consequences of introducing synthetic metabolons, wherein certain pairs of reactions are artificially permanently assembled into plants, and speculate as to future strategies to further improve plant productivity by manipulation of the core metabolic pathway.

An L,L-diaminopimelate aminotransferase mutation leads to metabolic shifts and growth inhibition in Arabidopsis

Lysine (Lys) connects the mitochondrial electron transport chain to amino acid catabolism and the tricarboxylic acid cycle. However, our understanding of how a deficiency in Lys biosynthesis impacts plant metabolism and growth remains limited. Here, we used a previously characterized Arabidopsis mutant (dapat) with reduced activity of the Lys biosynthesis enzyme L,L-diaminopimelate aminotransferase to investigate the physiological and metabolic impacts of impaired Lys biosynthesis. Despite displaying similar stomatal conductance and internal CO2 concentration, we observed reduced photosynthesis and growth in the dapat mutant. Surprisingly, whilst we did not find differences in dark respiration between genotypes, a lower storage and consumption of starch and sugars was observed in dapat plants. We found higher protein turnover but no differences in total amino acids during a diurnal cycle in dapat plants. Transcriptional and two-dimensional (isoelectric focalization/SDS-PAGE) proteome analyses revealed alterations in the abundance of several transcripts and proteins associated with photosynthesis and photorespiration coupled with a high glycine/serine ratio and increased levels of stress-responsive amino acids. Taken together, our findings demonstrate that biochemical alterations rather than stomatal limitations are responsible for the decreased photosynthesis and growth of the dapat mutant, which we hypothesize mimics stress conditions associated with impairments in the Lys biosynthesis pathway.

Alternative Type II NAD(P)H Dehydrogenases in the Mitochondria of Protists and Fungi

Plants, fungi, and some protists possess a more branched electron transport chain in their mitochondria compared to canonical one. In these organisms, the electron transport chain contains several rotenone-insensitive NAD(P)H dehydrogenases. Some are located on the outer surface, and others are located on the inner surface of the inner mitochondrial membrane. The putative role of these enzymes still remains elusive, but they may prevent the overreduction of the electron transport chain components and decrease the production of reaction oxygen species as a consequence. The last two decades resulted in the discovery of alternative rotenone-insensitive NAD(P)H dehydrogenases present in representatives of fungi and protozoa. The aim of this review is to gather and focus on current information concerning molecular and functional properties, regulation, and the physiological role of fungal and protozoan alternative NAD(P)H dehydrogenases.

More to NAD+ than meets the eye: A regulator of metabolic pools and gene expression in Arabidopsis

Since its discovery more than a century ago, nicotinamide adenine dinucleotide (NAD⁺) is recognised as a fascinating cornerstone of cellular metabolism. This ubiquitous energy cofactor plays vital roles in metabolic pathways and regulatory processes, a fact emphasised by the essentiality of a balanced NAD⁺ metabolism for normal plant growth and development. Research on the role of NAD in plants has been predominantly carried out in the model plant Arabidopsis thaliana (Arabidopsis) with emphasis on the redox properties and cellular signalling functions of the metabolite. This review examines the current state of knowledge concerning how NAD can regulate both metabolic pools and gene expression in Arabidopsis. Particular focus is placed on recent studies highlighting the complexity of metabolic regulations involving NAD, more particularly in the mitochondrial compartment, and of signalling roles with respect to interactions with environmental fluctuations most specifically those involving plant immunity.

An In Vivo Perspective of the Role(s) of the Alternative Oxidase Pathway

Despite intense research on the in vitro characterization of regulatory factors modulating the alternative oxidase (AOX) pathway, the regulation of its activity in vivo is still not fully understood. Advances concerning in vivo regulation of AOX based on the oxygen-isotope fractionation technique are reviewed, and regulatory factors that merit future research are highlighted. In addition, we review and discuss the main biological functions assigned to the plant AOX, and suggest future experiments involving in vivo activity measurements to test different hypothesized physiological roles.

Short-term ammonium supply induces cellular defence to prevent oxidative stress in Arabidopsis leaves

Plants can assimilate nitrogen from soil pools of both ammonium and nitrate, and the relative levels of these two nitrogen sources are highly variable in soil. Long-term ammonium nutrition is known to cause damage to Arabidopsis that has been linked to mitochondrial oxidative stress. Using hydroponic cultures, we analysed the consequences of rapid shifts between nitrate and ammonium nutrition. This did not induce growth retardation, showing that Arabidopsis can compensate for the changes in redox metabolism associated with the variations in nitrogen redox status. During the first 3 h of ammonium treatment, we observed distinct transient shifts in reactive oxygen species (ROS), low-mass antioxidants, ROS-scavenging enzymes, and mitochondrial alternative electron transport pathways, indicating rapid but temporally separated changes in chloroplastic, mitochondrial and cytosolic ROS metabolism. The fast induction of antioxidant defences significantly lowered intracellular H₂ O₂ levels, and thus protected Arabidopsis leaves from oxidative stress. On the other hand elevated extracellular ROS production in response to ammonium supply may be involved in signalling. The response pattern displays an intricate plasticity of Arabidopsis redox metabolism to minimise stress in responses to nutrient changes.

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.

Cotton cytosolic pyruvate kinase GhPK6 participates in fast fiber elongation regulation in a ROS-mediated manner

Cotton cytosolic pyruvate kinase GhPK6 is preferentially expressed in the late stage of fiber elongation process, transgenic experiments indicated that its expression level was negatively correlated to cell expansion rate. Pyruvate kinase (PK) plays vital regulatory roles in rapid cell growth in mammals. However, the function of PK in plant cell growth remains unclear. In allotetraploid upland cotton (Gossypium hirsutum L.), a total of 33 PK genes are encoded by the genome. Analysis of the transcriptome data indicated that only two cytosolic PK genes, GhPK6 and its duplicated gene GhPK26, are preferentially expressed in elongating cotton fiber cells. RT-qPCR and western blot analyses revealed that the expression of GhPK6 was negatively correlated with fiber elongation rate, which well explains the observed sharp increase of cytosolic PK activity at the end of fast fiber elongation process. Furthermore, virus-induced gene silencing of GhPK6 in cotton plants resulted in increased fiber cell elongation and reduced reactive oxygen species (ROS) accumulation. On the contrary, Arabidopsis plants ectopically expressing GhPK6 exhibited ROS-mediated growth inhibition, whereas the addition of ROS scavenging reagents could partly rescue this inhibition. These data collectively suggested that GhPK6 might play an important role in regulating cotton fiber elongation in a ROS-dependent inhibition manner.

The origin of cytosolic ATP in photosynthetic cells

In photosynthetically active cells, both chloroplasts and mitochondria have the capacity to produce ATP via photophosphorylation and oxidative phosphorylation, respectively. Thus, theoretically, both organelles could provide ATP for the cytosol, but the extent, to which they actually do this, and how the process is regulated, both remain unclear. Most of the evidence discussed comes from experiments with rapid fractionation of isolated protoplasts subjected to different treatments in combination with application of specific inhibitors. The results obtained indicate that, under conditions where ATP demand for photosynthetic CO2 fixation is sufficiently high, the mitochondria supply the bulk of ATP for the cytosol. In contrast, under stress conditions where CO2 fixation is severely limited, ATP will build up in chloroplasts and it can then be exported to the cytosol, by metabolite shuttle mechanisms. Thus, depending on the conditions, either mitochondria or chloroplasts can supply the bulk of ATP for the cytosol. This supply of ATP is discussed in relation to the idea that mitochondrial functions may be tuned to provide an optimal environment for the chloroplast. By balancing cellular redox states, mitochondria can contribute to an optimal photosynthetic capacity.

The evolution of substrate specificity-associated residues and Ca(2+) -binding motifs in EF-hand-containing type II NAD(P)H dehydrogenases

Most eukaryotic organisms, except some animal clades, have mitochondrial alternative electron transport enzymes that allow respiration to bypass the energy coupling in oxidative phosphorylation. The energy bypass enzymes in plants include the external type II NAD(P)H dehydrogenases (DHs) of the NDB family, which are characterized by an EF-hand domain for Ca(2+) binding. Here we investigate these plant enzymes by combining molecular modeling with evolutionary analysis. Molecular modeling of the Arabidopsis thaliana AtNDB1 with the yeast ScNDI1 as template revealed distinct similarities in the core catalytic parts, and highlighted the interaction between the pyridine nucleotide and residues correlating with NAD(P)H substrate specificity. The EF-hand domain of AtNDB1 has no counterpart in ScNDI1, and was instead modeled with Ca(2+) -binding signal transducer proteins. Combined models displayed a proximity of the AtNDB1 EF-hand domain to the substrate entrance side of the catalytic part. Evolutionary analysis of the eukaryotic NDB-type proteins revealed ancient and recent reversions between the motif observed in proteins specific for NADH (acidic type) and NADPH (non-acidic type), and that the clade of enzymes with acidic motifs in angiosperms derives from non-acidic-motif NDB-type proteins present in basal plants, fungi and protists. The results suggest that Ca(2+) -dependent external NADPH oxidation is an ancient process, indicating that it has a fundamental importance for eukaryotic cellular redox metabolism. In contrast, the external NADH DHs in plants are products of a recent expansion, mirroring the expansion of the alternative oxidase family.

Stress-Induced Accumulation of DcAOX1 and DcAOX2a Transcripts Coincides with Critical Time Point for Structural Biomass Prediction in Carrot Primary Cultures (Daucus carota L.)

Stress-adaptive cell plasticity in target tissues and cells for plant biomass growth is important for yield stability. In vitro systems with reproducible cell plasticity can help to identify relevant metabolic and molecular events during early cell reprogramming. In carrot, regulation of the central root meristem is a critical target for yield-determining secondary growth. Calorespirometry, a tool previously identified as promising for predictive growth phenotyping has been applied to measure the respiration rate in carrot meristem. In a carrot primary culture system (PCS), this tool allowed identifying an early peak related with structural biomass formation during lag phase of growth, around the 4th day of culture. In the present study, we report a dynamic and correlated expression of carrot AOX genes (DcAOX1 and DcAOX2a) during PCS lag phase and during exponential growth. Both genes showed an increase in transcript levels until 36 h after explant inoculation, and a subsequent down-regulation, before the initiation of exponential growth. In PCS growing at two different temperatures (21°C and 28°C), DcAOX1 was also found to be more expressed in the highest temperature. DcAOX genes' were further explored in a plant pot experiment in response to chilling, which confirmed the early AOX transcript increase prior to the induction of a specific anti-freezing gene. Our findings point to DcAOX1 and DcAOX2a as being reasonable candidates for functional marker development related to early cell reprogramming. While the genomic sequence of DcAOX2a was previously described, we characterize here the complete genomic sequence of DcAOX1.

Alternative Oxidase Gene Family in Hypericum perforatum L.: Characterization and Expression at the Post-germinative Phase

Alternative oxidase (AOX) protein is located in the inner mitochondrial membrane and is encoded in the nuclear genome being involved in plant response upon a diversity of environmental stresses and also in normal plant growth and development. Here we report the characterization of the AOX gene family of Hypericum perforatum L. Two AOX genes were identified, both with a structure of four exons (HpAOX1, acc. KU674355 and HpAOX2, acc. KU674356). High variability was found at the N-terminal region of the protein coincident with the high variability identified at the mitochondrial transit peptide. In silico analysis of regulatory elements located at intronic regions identified putative sequences coding for miRNA precursors and trace elements of a transposon. Simple sequence repeats were also identified. Additionally, the mRNA levels for the HpAOX1 and HpAOX2, along with the ones for the HpGAPA (glyceraldehyde-3-phosphate dehydrogenase A subunit) and the HpCAT1 (catalase 1), were evaluated during the post-germinative development. Gene expression analysis was performed by RT-qPCR with accurate data normalization, pointing out HpHYP1 (chamba phenolic oxidative coupling protein 1) and HpH2A (histone 2A) as the most suitable reference genes (RGs) according to GeNorm algorithm. The HpAOX2 transcript demonstrated larger stability during the process with a slight down-regulation in its expression. Contrarily, HpAOX1 and HpGAPA (the corresponding protein is homolog to the chloroplast isoform involved in the photosynthetic carbon assimilation in other plant species) transcripts showed a marked increase, with a similar expression pattern between them, during the post-germinative development. On the other hand, the HpCAT1 (the corresponding protein is homolog to the major H2O2-scavenging enzyme in other plant species) transcripts showed an opposite behavior with a down-regulation during the process. In summary, our findings, although preliminary, highlight the importance to investigate in more detail the participation of AOX genes during the post-germinative development in H. perforatum, in order to explore their functional role in optimizing photosynthesis and in the control of reactive oxygen species (ROS) levels during the process.

Physiological role of alternative oxidase (from yeasts to plants)

Mitochondria of all so far studied organisms, with the exception of Archaea, mammals, some yeasts, and protists, contain, along with the classical phosphorylating cytochrome pathway, a so-called cyanide-insensitive alternative oxidase (AOX) localized on the matrix side of the mitochondrial inner membrane, and electron transport through which is not coupled with ATP synthesis and energy accumulation. Mechanisms underlying plentiful functions of AOX in organisms at various levels of organization ranging from yeasts to plants are considered. First and foremost, AOX provides a chance of cell survival after inhibiting the terminal components of the main respiratory chain or losing the ability to synthesize these components. The vitally important role of AOX is obvious in thermogenesis of thermogenic plant organs where it becomes the only terminal oxidase with a very high activity, and the energy of substrate oxidation by this respiratory pathway is converted into heat, thus promoting evaporation of volatile substances attracting pollinating insects. AOX plays a fundamentally significant role in alleviating or preventing oxidative stress, thus ensuring the defense against a wide range of stresses and adverse environmental conditions, such as changes in temperature and light intensities, osmotic stress, drought, and attack by incompatible strains of bacterial pathogens, phytopathogens, or their elicitors. Participation of AOX in pathogen survival during its existence inside the host, in antivirus defense, as well as in metabolic rearrangements in plants during embryogenesis and cell differentiation is described. Examples are given to demonstrate that AOX might be an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals.

The alternative respiratory pathway is involved in brassinosteroid-induced environmental stress tolerance in Nicotiana benthamiana

Brassinosteroids (BRs), plant steroid hormones, play essential roles in modulating cell elongation, vascular differentiation, senescence, and stress responses. However, the mechanisms by which BRs regulate plant mitochondria and resistance to abiotic stress remain largely unclear. Mitochondrial alternative oxidase (AOX) is involved in the plant response to a variety of environmental stresses. In this report, the role of AOX in BR-induced tolerance against cold, polyethylene glycol (PEG), and high-light stresses was investigated. Exogenous applied brassinolide (BL, the most active BR) induced, while brassinazole (BRZ, a BR biosynthesis inhibitor) reduced alternative respiration and AOX1 expression in Nicotiana benthamiana. Chemical scavenging of H2O2 and virus-induced gene silencing (VIGS) of NbRBOHB compromised the BR-induced alternative respiratory pathway, and this result was further confirmed by NbAOX1 promoter analysis. Furthermore, inhibition of AOX activity by chemical treatment or a VIGS-based approach decreased plant resistance to environmental stresses and compromised BR-induced stress tolerance. Taken together, our results indicate that BR-induced AOX capability might contribute to the avoidance of superfluous reactive oxygen species accumulation and the protection of photosystems under stress conditions in N. benthamiana.

Alternative oxidase: distribution, induction, properties, structure, regulation, and functions

The respiratory chain in the majority of organisms with aerobic type metabolism features the concomitant existence of the phosphorylating cytochrome pathway and the cyanide- and antimycin A-insensitive oxidative route comprising a so-called alternative oxidase (AOX) as a terminal oxidase. In this review, the history of AOX discovery is described. Considerable evidence is presented that AOX occurs widely in organisms at various levels of organization and is not confined to the plant kingdom. This enzyme has not been found only in Archaea, mammals, some yeasts and protists. Bioinformatics research revealed the sequences characteristic of AOX in representatives of various taxonomic groups. Based on multiple alignments of these sequences, a phylogenetic tree was constructed to infer their possible evolution. The ways of AOX activation, as well as regulatory interactions between AOX and the main respiratory chain are described. Data are summarized concerning the properties of AOX and the AOX-encoding genes whose expression is either constitutive or induced by various factors. Information is presented on the structure of AOX, its active center, and the ubiquinone-binding site. The principal functions of AOX are analyzed, including the cases of cell survival, optimization of respiratory metabolism, protection against excess of reactive oxygen species, and adaptation to variable nutrition sources and to biotic and abiotic stress factors. It is emphasized that different AOX functions complement each other in many instances and are not mutually exclusive. Examples are given to demonstrate that AOX is an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals. This is the first comprehensive review on alternative oxidases of various organisms ranging from yeasts and protists to vascular plants.

Early events induced by the toxin deoxynivalenol lead to programmed cell death in Nicotiana tabacum cells

Deoxynivalenol (DON) is a mycotoxin affecting animals and plants. This toxin synthesized by Fusarium culmorum and Fusarium graminearum is currently believed to play a decisive role in the fungal phytopathogenesis as a virulence factor. Using cultured cells of Nicotiana tabacum BY2, we showed that DON-induced programmed cell death (PCD) could require transcription and translation processes, in contrast to what was observed in animal cells. DON could induce different cross-linked pathways involving (i) reactive oxygen species (ROS) generation linked, at least partly, to a mitochondrial dysfunction and a transcriptional down-regulation of the alternative oxidase (Aox1) gene and (ii) regulation of ion channel activities participating in cell shrinkage, to achieve PCD.

Antioxidative and proteolytic systems protect mitochondria from oxidative damage in S-deficient Arabidopsis thaliana

We examined the functioning of the antioxidative defense system in Arabidopsis thaliana under sulphur (S) deficiency with an emphasis on the role of mitochondria. In tissue extracts and in isolated mitochondria from S-deficient plants, the concentration of non-protein thiols declined but protein thiols did not change. Superoxide anion and hydrogen peroxide were accumulated in leaf blades and the generation of superoxide anion by isolated mitochondria was higher. Lower abundance of reduced (GSH) plus oxidized (GSSG) glutathione in the leaf and root tissues, and leaf mitochondria from S-deficient plants was accompanied by a decrease in the level of GSH and the changes in the GSH/GSSG ratios. In the chloroplasts, the total level of glutathione decreased. Lower levels of reduced (AsA) and oxidized (DHA) ascorbate were reflected in much higher ratios of AsA/DHA. Sulphur deficiency led to an increase in the activity of cytosolic, mitochondrial and chloroplastic antioxidative enzymes, peroxidases, catalases and superoxide dismutases. The protein carbonyl level was higher in the leaves of S-deficient plants and in the chloroplasts, while in the roots, leaf and root mitochondria it remained unchanged. Protease activity in leaf extracts of S-deficient plants was higher, but in root extracts it did not differ. The proteolytic system reflected subcellular specificity. In leaf and root mitochondria the protease activity was higher, whereas in the chloroplasts it did not change. We propose that the preferential incorporation of S to protein thiols and activation of antioxidative and proteolytic systems are likely important for the survival of S-deficient plants and that the mitochondria maintain redox homeostasis.

Modulation of alternative oxidase to enhance tolerance against cold stress of chickpea by chemical treatments

The alternative oxidase (AOX) is the enzyme responsible for the alternative respiratory pathway. This experiment was conducted to examine the influence on cold tolerance ability of chickpea (Cicer aurentium cv. Müfitbey) seedlings of AOX activator (pyruvate), AOX inhibitor (salicylhydroxamic acid (SHAM)) and an inhibitor of the cytochrome pathway of respiration (antimycin A) treatments. 5mM pyruvate, 2μM antimycin A and 4mM SHAM solutions were exogenously applied to thirteen-day-old chickpea leaves and then the seedlings were transferred to a different plant growth chamber arranged to 10/5°C (day/night) for 48h. Cold stress markedly increased the activities of antioxidant enzymes compared to controls. Pyruvate and antimycin A significantly increased the cold-induced increase in antioxidant activity but SHAM decreased it. Cold-induced increases in superoxide anion, hydrogen peroxide, and lipid peroxidation levels were significantly reduced by pyruvate and antimycin A, but increased by SHAM treatment. Pyruvate and antimycin A application increased both the activity and protein expression of AOX in comparison to cold stress alone. However, SHAM significantly decreased activity of AOX but did not affect its expression. Total cellular respiration values (TCRV) supported the changes in activity and expression of AOX. While TCRV were increased by cold and pyruvate, they were significantly reduced by SHAM and especially antimycin A. These results indicate that pyruvate and antimycin A applications were effective in reducing oxidative stress by activating the alternative respiratory pathway as well as antioxidant activity. Furthermore, direct activation of AOX, rather than inhibition of the cytochrome pathway, was the most effective way to mitigate cold stress.

Protein phosphatase 2A (PP2A) regulatory subunit B'γ interacts with cytoplasmic ACONITASE 3 and modulates the abundance of AOX1A and AOX1D in Arabidopsis thaliana

Organellar reactive oxygen species (ROS) signalling is a key mechanism that promotes the onset of defensive measures in stress-exposed plants. The underlying molecular mechanisms and feedback regulation loops, however, still remain poorly understood. Our previous work has shown that a specific regulatory B'γ subunit of protein phosphatase 2A (PP2A) is required to control organellar ROS signalling and associated metabolic adjustments in Arabidopsis thaliana. Here, we addressed the mechanisms through which PP2A-B'γ impacts on organellar metabolic crosstalk and ROS homeostasis in leaves. Genetic, biochemical and pharmacological approaches, together with a combination of data-dependent acquisition (DDA) and selected reaction monitoring (SRM) MS techniques, were utilized to assess PP2A-B'γ-dependent adjustments in Arabidopsis thaliana. We show that PP2A-B'γ physically interacts with the cytoplasmic form of aconitase, a central metabolic enzyme functionally connected with mitochondrial respiration, oxidative stress responses and regulation of cell death in plants. Furthermore, PP2A-B'γ impacts ROS homeostasis by controlling the abundance of specific alternative oxidase isoforms, AOX1A and AOX1D, in leaf mitochondria. We conclude that PP2A-B'γ-dependent regulatory actions modulate the functional status of metabolic enzymes that essentially contribute to intracellular ROS signalling and metabolic homeostasis in plants.

Biogenesis of mitochondria in cauliflower (Brassica oleracea var. botrytis) curds subjected to temperature stress and recovery involves regulation of the complexome, respiratory chain activity, organellar translation and ultrastructure

The biogenesis of the cauliflower curd mitochondrial proteome was investigated under cold, heat and the recovery. For the first time, two dimensional fluorescence difference gel electrophoresis was used to study the plant mitochondrial complexome in heat and heat recovery. Particularly, changes in the complex I and complex III subunits and import proteins, and the partial disintegration of matrix complexes were observed. The presence of unassembled subunits of ATP synthase was accompanied by impairment in mitochondrial translation of its subunit. In cold and heat, the transcription profiles of mitochondrial genes were uncorrelated. The in-gel activities of respiratory complexes were particularly affected after stress recovery. Despite a general stability of respiratory chain complexes in heat, functional studies showed that their activity and the ATP synthesis yield were affected. Contrary to cold stress, heat stress resulted in a reduced efficiency of oxidative phosphorylation likely due to changes in alternative oxidase (AOX) activity. Stress and stress recovery differently modulated the protein level and activity of AOX. Heat stress induced an increase in AOX activity and protein level, and AOX1a and AOX1d transcript level, while heat recovery reversed the AOX protein and activity changes. Conversely, cold stress led to a decrease in AOX activity (and protein level), which was reversed after cold recovery. Thus, cauliflower AOX is only induced by heat stress. In heat, contrary to the AOX activity, the activity of rotenone-insensitive internal NADH dehydrogenase was diminished. The relevance of various steps of plant mitochondrial biogenesis to temperature stress response and recovery is discussed.