Mitochondrial alternative pathway is associated with development of freezing tolerance in common wheat

24-Epibrassinolide Reduces Drought-Induced Oxidative Stress by Modulating the Antioxidant System and Respiration in Wheat Seedlings

Brassinosteroids (BRs) represent a group of plant signaling molecules with a steroidal skeleton that play an essential role in plant adaptation to different environmental stresses, including drought. In this work, the effect of pretreatment with 0.4 µM 24-epibrassinolide (EBR) on the oxidant/antioxidant system in 4-day-old wheat seedlings (Triticum aestivum L.) was studied under moderate drought stress simulated by 12% polyethylene glycol 6000 (PEG). It was revealed that EBR-pretreatment had a protective effect on wheat plants as evidenced by the maintenance of their growth rate, as well as the reduction in lipid peroxidation and electrolyte leakage from plant tissues under drought conditions. This effect was likely due to the ability of EBR to reduce the stress-induced accumulation of reactive oxygen species (ROS) and modulate the activity of antioxidant enzymes. Meanwhile, EBR pretreatment enhanced proline accumulation and increased the barrier properties of the cell walls in seedlings by accelerating the lignin deposition. Moreover, the ability of EBR to prevent a drought-caused increase in the intensity of the total dark respiration and the capacity of alternative respiration contributes significantly to the antistress action of this hormone.

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.

Identification and characterization of a natural SNP variant in ALTERNATIVE OXIDASE gene associated with cold stress tolerance in watermelon

Alternative oxidase (AOX) is a mitochondrial enzyme encoded by a small nuclear gene family, which contains the two subfamilies, AOX1 and AOX2. In the present study on watermelon (Citrullus lanatus), only one ClAOX gene, belonging to AOX2 subfamily but having a similar gene structure to AtAOX1a, was found in the watermelon genome. The expression analysis suggested that ClAOX had the constitutive expression feature of AOX2 subfamily, but was cold inducible, which is normally considered an AOX1 subfamily feature. Moreover, one single nucleotide polymorphism (SNP) in ClAOX sequence, which led to the change from Lys (N) to Asn (K) in the 96^th amino acids, was found among watermelon subspecies. Ectopic expression of two ClAOX alleles in the Arabidopsis aox1a knock-out mutant indicated that ClAOX^K-expressing plants had stronger cold tolerance than aox1a mutant and ClAOX^N-expressing plants. Our findings suggested watermelon genome contained a single ClAOX that possessed the expression features of both AOX1 and AOX2 subfamilies. A naturally existing SNP in ClAOX differentiated the cold tolerance of transgenic Arabidopsis plants, impling a possibility this gene might be a functional marker for stress-tolerance breeding.

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.

Hydrogen Peroxide and Nitric Oxide Crosstalk Mediates Brassinosteroids Induced Cold Stress Tolerance in Medicago truncatula

Brassinosteroids (BRs) play pivotal roles in modulating plant growth, development, and stress responses. In this study, a Medicago truncatula plant pretreated with brassinolide (BL, the most active BR), enhanced cold stress tolerance by regulating the expression of several cold-related genes and antioxidant enzymes activities. Previous studies reported that hydrogen peroxide (H₂O₂) and nitric oxide (NO) are involved during environmental stress conditions. However, how these two signaling molecules interact with each other in BRs-induced abiotic stress tolerance remain largely unclear. BL-pretreatment induced, while brassinazole (BRZ, a specific inhibitor of BRs biosynthesis) reduced H₂O₂ and NO production. Further, application of dimethylthiourea (DMTU, a H₂O₂ and OH- scavenger) blocked BRs-induced NO production, but BRs-induced H₂O₂ generation was not sensitive to 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO, a scavenger of NO). Moreover, pretreatment with DMTU and PTIO decreased BL-induced mitochondrial alternative oxidase (AOX) and the photosystem capacity. However, pretreatment with PTIO was found to be more effective than DMTU in reducing BRs-induced increases in Valt, Vt, and MtAOX1 gene expression. Similarly, BRs-induced photosystem II efficiency was found in NO dependent manner than H₂O₂. Finally, we conclude that H₂O₂ was involved in NO generation, whereas NO was found to be crucial in BRs-induced AOX capacity, which further contributed to the protection of the photosystem under cold stress conditions in Medicago truncatula.

Alternative oxidase (AOX) over-expression improves cell expansion and elongation in cotton seedling exposed to cool temperatures

KEY MESSAGE

Evidence that supports a relation between AOX expression and improvement in plant height, internode length, and total leaf area under cool temperature is shown. Cell expansion and elongation appear to be enhanced when AOX expression was increased. Cotton growth is sensitive to cool temperature during germination and early seedling development. Delayed emergence, seedling damage, and increased risk to disease are common. Late seasonal cool weather is a major factor limiting the consistent production of high-quality cotton lint in West Texas. Alternative oxidase functions in the inner membrane of the mitochondria via an alternative respiration pathway and serves as a multifunctional system for amelioration of abiotic and biotic stresses. Cotton seedling emergence and growth exposed to cool temperatures was examined in plants with enhanced AOX expression. Thirteen T1 seed lines showed 3 to 1 segregation for the T-DNA containing the tobacco AOX1 gene. Two over-expressing, single-copy, homozygous AOX lines (94-20T and 66-6T) and Null line (94-3N) were selected for examination. The transcript levels were ≈ 2 to 6 fold higher in the AOX lines compared to those of the Null line and wild-type in stem, leaf, root and boll tissues. The research examined the hypothesis that transgenic cotton with enhanced AOX expression will have enhanced growth traits under suboptimal cool temperatures. Improved plant height, internode length, plant height and internode length from second node, and total leaf area under cool temperatures were observed in AOX over-expression lines. This may be attributed to improved cell expansion and elongation characteristics in the AOX line.

Genome-wide identification and analysis of the ALTERNATIVE OXIDASE gene family in diploid and hexaploid wheat

A comprehensive understanding of wheat responses to environmental stress will contribute to the long-term goal of feeding the planet. ALERNATIVE OXIDASE (AOX) genes encode proteins involved in a bypass of the electron transport chain and are also known to be involved in stress tolerance in multiple species. Here, we report the identification and characterization of the AOX gene family in diploid and hexaploid wheat. Four genes each were found in the diploid ancestors Triticum urartu, and Aegilops tauschii, and three in Aegilops speltoides. In hexaploid wheat (Triticum aestivum), 20 genes were identified, some with multiple splice variants, corresponding to a total of 24 proteins for those with observed transcription and translation. These proteins were classified as AOX1a, AOX1c, AOX1e or AOX1d via phylogenetic analysis. Proteins lacking most or all signature AOX motifs were assigned to putative regulatory roles. Analysis of protein-targeting sequences suggests mixed localization to the mitochondria and other organelles. In comparison to the most studied AOX from Trypanosoma brucei, there were amino acid substitutions at critical functional domains indicating possible role divergence in wheat or grasses in general. In hexaploid wheat, AOX genes were expressed at specific developmental stages as well as in response to both biotic and abiotic stresses such as fungal pathogens, heat and drought. These AOX expression patterns suggest a highly regulated and diverse transcription and expression system. The insights gained provide a framework for the continued and expanded study of AOX genes in wheat for stress tolerance through breeding new varieties, as well as resistance to AOX-targeted herbicides, all of which can ultimately be used synergistically to improve crop yield.

Nitric oxide is involved in brassinosteroid-induced alternative respiratory pathway in Nicotiana benthamiana seedlings' response to salt stress

Recent studies reported that brassinosteroids (BRs) can induce plant tolerance to different environmental stresses via the nitric oxide (NO) signaling pathway. Previous reports have indicated that alternative oxidase (AOX) plays an important role in plants under various stresses. The mechanisms governing how NO is involved as a signal molecule which connects BR with AOX in regulating stress tolerance are still unknown. Recently, we found that Nicotiana benthamiana seedlings which were pretreated with BR have more tolerance to salt stress, accompanied with an increase of CN-resistant respiration. Our results suggested that pretreatment with 0.1 μM brassinolide (BL, the most active brassinosteroid) alleviated salt-induced oxidative damage and increased the NbAOX1 transcript level. Application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-1-oxyl-3-oxide (cPTIO, an NO scavenger) or virus-induced gene silencing of nitrate reductase (NR) and nitric oxide synthase (NOS)-like enzyme compromised the BRs-induced alternative respiratory pathway. Furthermore, pretreatment with specific chemical inhibitors of NR and NOS or gene silencing experiments decreased plant resistance to salt stress which also compromised BRs-induced salt stress tolerance. In conclusion, NO is involved in BRs-induced AOX capability which plays essential roles in salt tolerance in N. benthamiana seedlings.

Contributions of photosynthetic and non-photosynthetic cell types to leaf respiration in Vicia faba L. and their responses to growth temperature

In intact leaves, mitochondrial populations are highly heterogeneous among contrasting cell types; how such contrasting populations respond to sustained changes in the environment remains, however, unclear. Here, we examined respiratory rates, mitochondrial protein composition and response to growth temperature in photosynthetic (mesophyll) and non-photosynthetic (epidermal) cells from fully expanded leaves of warm-developed (WD) and cold-developed (CD) broad bean (Vicia faba L.). Rates of respiration were significantly higher in mesophyll cell protoplasts (MCPs) than epidermal cell protoplasts (ECPs), with both protoplast types exhibiting capacity for cytochrome and alternative oxidase activity. Compared with ECPs, MCPs contained greater relative quantities of porin, suggesting higher mitochondrial surface area in mesophyll cells. Nevertheless, the relative quantities of respiratory proteins (normalized to porin) were similar in MCPs and ECPs, suggesting that ECPs have lower numbers of mitochondria yet similar protein complement to MCP mitochondria (albeit with lower abundance serine hydroxymethyltransferase). Several mitochondrial proteins (both non-photorespiratory and photorespiratory) exhibited an increased abundance in response to cold in both protoplast types. Based on estimates of individual protoplast respiration rates, combined with leaf cell abundance data, epidermal cells make a small but significant (2%) contribution to overall leaf respiration which increases twofold in the cold. Taken together, our data highlight the heterogeneous nature of mitochondrial populations in leaves, both among contrasting cell types and in how those populations respond to growth temperature.

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.

Mitochondrial energy-dissipating systems (alternative oxidase, uncoupling proteins, and external NADH dehydrogenase) are involved in development of frost-resistance of winter wheat seedlings

Gene expression, protein synthesis, and activities of alternative oxidase (AOX), uncoupling proteins (UCP), adenine nucleotide translocator (ANT), and non-coupled NAD(P)H dehydrogenases (NDex, NDPex, and NDin) were studied in shoots of etiolated winter wheat (Triticum aestivum L.) seedlings after exposure to hardening low positive (2°C for 7 days) and freezing (-2°C for 2 days) temperatures. The cold hardening efficiently increased frost-resistance of the seedlings and decreased the generation of reactive oxygen species (ROS) during further cold shock. Functioning of mitochondrial energy-dissipating systems can represent a mechanism responsible for the decrease in ROS under these conditions. These systems are different in their response to the action of the hardening low positive and freezing temperatures. The functioning of the first system causes induction of AOX and UCP synthesis associated with an increase in electron transfer via AOX in the mitochondrial respiratory chain and also with an increase in the sensitivity of mitochondrial non-phosphorylating respiration to linoleic and palmitic acids. The increase in electron transfer via AOX upon exposure of seedlings to hardening freezing temperature is associated with retention of a high activity of NDex. It seems that NDex but not the NDPex and NDin can play an important role in maintaining the functional state of mitochondria in heterotrophic tissues of plants under the influence of freezing temperatures. The involvement of the mitochondrial energy-dissipating systems and their possible physiological role in the adaptation of winter crops to cold and frost are discussed.

Light regulation of mitochondrial alternative oxidase pathway during greening of etiolated wheat seedlings

This study deals with effects of de-etiolation (48h) of spring wheat (Triticum aestivum L., var. Irgina) seedlings on differential expression of AOX1 genes, levels of AOX protein and the alternative respiratory pathway (AP) capacity. As a result of exposure to continuous irradiation of dark-grown wheat seedlings, the respiratory activity and AP capacity in leaves significantly increased during the first 6h of studies. Expression of AOX1a was up-regulated by light and proved consistent with changes in the AP capacity. Effects on expression of AOX1c were less pronounced. Immunoblot analysis showed three distinct bands of AOX with molecular weights of 34, 36 and 38kDa, with no significant changes in the relative levels during de-etiolation. The lack of a clear correlation between AOX protein amount, AOX1a expression, and AP capacity suggests post-translational control of the enzyme activation. The AOX1a suppression and a decrease in the AP capacity correlated with the sugar pool depletion after 24h of the de-etiolation, which may mean a possible substrate dependence of the AOX activity in the green cells. More efficient malate oxidation by mitochondria as well as the higher AOX capacity during the first 6h of de-etiolation was detected, whereas respiration and AOX capacity with exogenous NADH and glycine increased after 6 and 24h, respectively. We conclude that AOX plays an important role during development of an actively photosynthesizing cell, and can rapidly adapt to changes in metabolism and photosynthesis.

Expression and signal regulation of the alternative oxidase genes under abiotic stresses

Plants in their natural environment frequently face various abiotic stresses, such as drought, high salinity, and chilling. Plant mitochondria contain an alternative oxidase (AOX), which is encoded by a small family of nuclear genes. AOX genes have been shown to be highly responsive to abiotic stresses. Using transgenic plants with varying levels of AOX expression, it has been confirmed that AOX genes are important for abiotic stress tolerance. Although the roles of AOX under abiotic stresses have been extensively studied and there are several excellent reviews on this topic, the differential expression patterns of the AOX gene family members and the signal regulation of AOX gene(s) under abiotic stresses have not been extensively summarized. Here, we review and discuss the current progress of these two important issues.

Alternative oxidase: a mitochondrial respiratory pathway to maintain metabolic and signaling homeostasis during abiotic and biotic stress in plants

Alternative oxidase (AOX) is a non-energy conserving terminal oxidase in the plant mitochondrial electron transport chain. While respiratory carbon oxidation pathways, electron transport, and ATP turnover are tightly coupled processes, AOX provides a means to relax this coupling, thus providing a degree of metabolic homeostasis to carbon and energy metabolism. Beside their role in primary metabolism, plant mitochondria also act as "signaling organelles", able to influence processes such as nuclear gene expression. AOX activity can control the level of potential mitochondrial signaling molecules such as superoxide, nitric oxide and important redox couples. In this way, AOX also provides a degree of signaling homeostasis to the organelle. Evidence suggests that AOX function in metabolic and signaling homeostasis is particularly important during stress. These include abiotic stresses such as low temperature, drought, and nutrient deficiency, as well as biotic stresses such as bacterial infection. This review provides an introduction to the genetic and biochemical control of AOX respiration, as well as providing generalized examples of how AOX activity can provide metabolic and signaling homeostasis. This review also examines abiotic and biotic stresses in which AOX respiration has been critically evaluated, and considers the overall role of AOX in growth and stress tolerance.

A novel role for cyanide in the control of cucumber (Cucumis sativus L.) seedlings response to environmental stress

The effects of potassium cyanide (KCN) pretreatment on the response of cucumber (Cucumis sativus L.) plants to salt, polyethylene glycol (PEG) and cold stress were investigated in the present study. Here, we found that KCN pretreatment improved cucumber seedlings tolerance to stress conditions with maximum efficiency at a concentration of 20 µM. The results showed that pretreatment with 20 µM KCN alleviated stress-induced oxidative damage in plant cells and clearly induced the activity of alternative oxidase (AOX) and the ethylene production. Furthermore, the structures of thylakoids and mitochondria in the KCN-pretreated seedlings were less damaged by the stress conditions, which maintained higher total chlorophyll content, photosynthetic rate and photosystem II (PSII) proteins levels than the control. Importantly, the addition of the AOX inhibitor salicylhydroxamic acid (1 mm; SHAM) decreased plant resistance to environmental stress and even compromised the cyanide (CN)-enhanced stress tolerance. Therefore, our findings provide a novel role of CN in plant against environmental stress and indicate that the CN-enhanced AOX might contribute to the reactive oxygen species (ROS) scavenging and the protection of photosystem by maintaining energy charge homoeostasis from chloroplast to mitochondria.

Autoimmune response and repression of mitotic cell division occur in inter-specific crosses between tetraploid wheat and Aegilops tauschii Coss. that show low temperature-induced hybrid necrosis

Common wheat is an allohexaploid species originating from a naturally occurring inter-specific cross between tetraploid wheat and the diploid wild wheat Aegilops tauschii Coss. Artificial allopolyploidization can produce synthetic hexaploid wheat. However, synthetic triploid hybrids show four types of hybrid growth abnormalities: type II and III hybrid necrosis, hybrid chlorosis, and severe growth abortion. Of these hybrid abnormalities, type II necrosis is induced by low temperature. Under low temperature, elongation of stems and expansion of new leaves is repressed in type II necrosis lines, which later exhibit necrotic symptoms. Here, we characterize type II necrosis in detail. Comparative transcriptome analysis showed that a number of defense-related genes were highly up-regulated in seedling leaves that showed type II necrosis. Transmission electron microscopy revealed extensive cell death in the leaves under low-temperature conditions, accompanied by abundant generation of reactive oxygen species. In addition, down-regulation of cell cycle-related genes was observed in shoot apices of type II necrosis lines under low-temperature conditions. Quantitative RT-PCR and in situ hybridization showed repression of accumulation of histone H4 transcripts in the shoot apical meristem of type II necrosis lines. These results strongly suggest that an autoimmune response-like reaction and repression of cell division in the shoot apical meristem are associated with the abnormal growth phenotype in type II necrosis lines.

Impact of mitochondrial alternative oxidase expression on the response of Nicotiana tabacum to cold temperature

The plant mitochondrial electron transport chain (ETC) includes a non-energy conserving alternative oxidase (AOX) thought to dampen reactive oxygen species (ROS) generation by the ETC and/or facilitate carbon metabolism by uncoupling it from ATP turnover. When wild-type (WT) Nicotiana tabacum grown at 28°C/22°C (light/dark) were transferred to 12°C/5°C, they showed a large induction of leaf Aox1a mRNA and AOX protein within 24 h. Transfer to cold also resulted in a large accumulation of monosaccharides, an increase in transcript level of genes encoding important ROS-scavenging enzymes and a moderate increase in lipid peroxidation. Transgenic plants with suppressed AOX level showed less cold-induced sugar accumulation than WT while transgenic plants with enhanced AOX levels showed enhanced sugar accumulation. This is inconsistent with the hypothesis that AOX acts to burn excess carbohydrate, but rather suggests a role for AOX to aid sugar accumulation, at least during cold stress. At 28°C/22°C, plants with suppressed AOX had elevated levels of lipid peroxidation compared with WT, while plants with enhanced AOX had reduced lipid peroxidation. This is consistent with the hypothesis that AOX dampens ROS generation and oxidative damage. However, this inverse relationship between AOX level and lipid peroxidation did not hold upon shift to cold. Under this stress condition, plants with strong suppression of AOX show enhanced induction of ROS-scavenging enzymes compared with WT and decline in lipid peroxidation. These data suggest that, under stress conditions, the lack of AOX enhances a mitochondrial stress-signaling pathway able to increase the ROS-scavenging capacity of the cell.

Leaf respiration and alternative oxidase in field-grown alpine grasses respond to natural changes in temperature and light

• We report the first investigation of changes in electron partitioning via the alternative respiratory pathway (AP) and alternative oxidase (AOX) protein abundance in field-grown plants and their role in seasonal acclimation of respiration. • We sampled two alpine grasses native to New Zealand, Chionochloa rubra and Chionochloa pallens, from field sites of different altitudes, over 1 yr and also intensively over a 2-wk period. • In both species, respiration acclimated to seasonal changes in temperature through changes in basal capacity (R₁₀) but not temperature sensitivity (E₀). In C. pallens, acclimation of respiration may be associated with a higher AOX : cytochrome c oxidase (COX) protein abundance ratio. Oxygen isotope discrimination (D), which reflects relative changes in AP electron partitioning, correlated positively with daily integrated photosynthetically active radiation (PAR) in both species over seasonal timescales. Respiratory parameters, the AOX : COX protein ratio and D were stable over a 2-wk period, during which significant temperature changes were experienced in the field. • We conclude that respiration in Chionochloa spp. acclimates strongly to seasonal, but not to short-term, temperature variation. Alternative oxidase appears to be involved in the plant response to both seasonal changes in temperature and daily changes in light, highlighting the complexity of the function of AOX in the field.

An analytical model of non-photorespiratory CO₂release in the light and dark in leaves of C₃species based on stoichiometric flux balance

Leaf respiration continues in the light but at a reduced rate. This inhibition is highly variable, and the mechanisms are poorly known, partly due to the lack of a formal model that can generate testable hypotheses. We derived an analytical model for non-photorespiratory CO₂ release by solving steady-state supply/demand equations for ATP, NADH and NADPH, coupled to a widely used photosynthesis model. We used this model to evaluate causes for suppression of respiration by light. The model agrees with many observations, including highly variable suppression at saturating light, greater suppression in mature leaves, reduced assimilatory quotient (ratio of net CO₂ and O₂ exchange) concurrent with nitrate reduction and a Kok effect (discrete change in quantum yield at low light). The model predicts engagement of non-phosphorylating pathways at moderate to high light, or concurrent with processes that yield ATP and NADH, such as fatty acid or terpenoid synthesis. Suppression of respiration is governed largely by photosynthetic adenylate balance, although photorespiratory NADH may contribute at sub-saturating light. Key questions include the precise diel variation of anabolism and the ATP : 2e⁻ ratio for photophosphorylation. Our model can focus experimental research and is a step towards a fully process-based model of CO₂ exchange.

Cell physiology of plants growing in cold environments

The life of plants growing in cold extreme environments has been well investigated in terms of morphological, anatomical, and ecophysiological adaptations. In contrast, long-term cellular or metabolic studies have been performed by only a few groups. Moreover, a number of single reports exist, which often represent just a glimpse of plant behavior. The review draws together the literature which has focused on tissue and cellular adaptations mainly to low temperatures and high light. Most studies have been done with European alpine plants; comparably well studied are only two phanerogams found in the coastal Antarctic. Plant adaptation in northern polar regions has always been of interest in terms of ecophysiology and plant propagation, but nowadays, this interest extends to the effects of global warming. More recently, metabolic and cellular investigations have included cold and UV resistance mechanisms. Low-temperature stress resistance in plants from cold environments reflects the climate conditions at the growth sites. It is now a matter of molecular analyses to find the induced genes and their products such as chaperones or dehydrins responsible for this resistance. Development of plants under snow or pollen tube growth at 0 degrees C shows that cell biology is needed to explain the stability and function of the cytoskeleton. Many results in this field are based on laboratory studies, but several publications show that it is not difficult to study cellular mechanisms with the plants adapted to a natural stress. Studies on high light and UV loads may be split in two parts. Many reports describe natural UV as harmful for the plants, but these studies were mainly conducted by shielding off natural UV (as controls). Other experiments apply additional UV in the field and have had practically no negative impact on metabolism. The latter group is supported by the observations that green overwintering plants increase their flavonoids under snow even in the absence of UV. Thus, their defense and antioxidant role dominates. Ultrastructural comparisons were unable to find special light adaptations in plants taken from polar regions vs. high alpine species. The only adaptation found at the subcellular level for most alpine and polar plants are protrusions of the chloroplast envelopes. They are seen as a demand for fast membrane transport requiring additional membrane surface area, whereby the increase in stroma volume may help to support carbohydrate formation. Plants forming such protrusions have to cope with a short vegetation time. These observations are connected to the question as to how photosynthesis works quite well even at or under zero temperatures. The interplay between plastids, mitochondria, and peroxisomes, known as photorespiration, seems to be more intense than in lowland plants. This organelle cooperation serves as a valve for a surplus in solar energy input under cold conditions. Additional metabolic acclimations are under investigation, such as the role of an alternative plastid terminal oxidase. Plants from cold environments may also be seen as ideal objects for studying the combined effects of high light plus cold resistance-from the molecular level to the whole plant adaptation. Modern instrumentation makes it possible to perform vital metabolic measurements under outdoor conditions, and research stations in remote polar and alpine areas provide support for scientists in the preparation of samples for later cellular studies in the home laboratory.

The expression, function and regulation of mitochondrial alternative oxidase under biotic stresses

To survive, plants possess elaborate defence mechanisms to protect themselves against virus or pathogen invasion. Recent studies have suggested that plant mitochondria may play an important role in host defence responses to biotic stresses. In contrast with animal mitochondria, plant mitochondria possess a unique respiratory pathway, the cyanide-insensitive alternative pathway, which is catalysed by the alternative oxidase (AOX). Much work has revealed that the genes encoding AOX, AOX protein and the alternative respiratory pathway are frequently induced during plant-pathogen (or virus) interaction. This raises the possibility that AOX is involved in host defence responses to biotic stresses. Thus, a key to the understanding of the role of mitochondrial respiration under biotic stresses is to learn the function and regulation of AOX. In this article, we focus on the theoretical and experimental progress made in the current understanding of the function and regulation of AOX under biotic stresses. We also address some speculative aspects to aid further research in this area.

Aox gene structure, transcript variation and expression in plants

Alternative oxidase (Aox) has been proposed as a functional marker for breeding stress tolerant plant varieties. This requires presence of polymorphic Aox allele sequences in plants that affect plant phenotype in a recognizable way. In this review, we examine the hypothesis that organization of genomic Aox sequences and gene expression patterns are highly variable in relation to the possibility that such a variation may allow development of Aox functional markers in plants. Aox is encoded by a small multigene family, typically with four to five members in higher plants. The predominant structure of genomic Aox sequences is that of four exons interrupted by three introns at well conserved positions. Evolutionary intron loss and gain has resulted in the variation of intron numbers in some Aox members that may harbor two to four introns and three to five exons in their sequence. Accumulating evidence suggests that Aox gene structure is polymorphic enough to allow development of Aox markers in many plant species. However, the functional significance of Aox structural variation has not been examined exhaustively. Aox expression patterns display variability and typically Aox genes fall into two discrete subfamilies, Aox1 and Aox2, the former being present in all plants and the latter restricted in eudicot species. Typically, although not exclusively, the Aox1-type genes are induced by many different kinds of stress, whereas Aox2-type genes are expressed in a constitutive or developmentally regulated way. Specific Aox alleles are among the first and most intensively stress-induced genes in several experimental systems involving oxidative stress. Differential response of Aox genes to stress may provide a flexible plan of plant defense where an energy-dissipating system in mitochondria is involved. Evidence to link structural variation and differential allele expression patterns is scarce. Much research is still required to understand the significance of polymorphisms within AOX gene sequences for gene regulation and its potential for breeding on important agronomic traits. Association studies and mapping approaches will be helpful to advance future perspectives for application more efficiently.

Alternative oxidase: what information can protein sequence comparisons give us?

The finding that alternative oxidase (AOX) is present in most kingdoms of life has resulted in a large number of AOX sequences that are available for analyses. Multiple sequence alignments of AOX proteins from evolutionarily divergent organisms represent a valuable tool and can be used to identify amino acids and domains that may play a role in catalysis, membrane association and post-translational regulation, especially when these data are coupled with the structural model for the enzyme. I validate the use of this approach by demonstrating that it detects the conserved glutamate and histidine residues in AOX that initially led to its identification as a di-iron carboxylate protein and the generation of a structural model for the protein. A comparative analysis using a larger dataset identified 35 additional amino acids that are conserved in all AOXs examined, 30 of which have not been investigated to date. I hypothesize that these residues will be involved in the quinol terminal oxidase activity or membrane association of AOX. Major differences in AOX protein sequences between kingdoms are revealed, and it is hypothesized that two angiosperm-specific domains may be responsible for the non-covalent dimerization of AOX, whereas two indels in the aplastidic AOXs may play a role in their post-translational regulation. A scheme for predicting whether a particular AOX protein will be recognized by the alternative oxidase monoclonal antibody generated against the AOX of Sauromatum guttatum (Voodoo lily) is presented. The number of functional sites in AOX is greater than expected, and determining the structure of AOX will prove extremely valuable to future research.

Temperature downshift induces antioxidant response in fungi isolated from Antarctica

Although investigators have been studying the cold-shock response in a variety of organisms for the last two decades or more, comparatively little is known about the difference between antioxidant cell response to cold stress in Antarctic and temperate microorganisms. The change of environmental temperature, which is one of the most common stresses, could be crucial for their use in the biotechnological industry and in ecological research. We compared the effect of short-term temperature downshift on antioxidant cell response in Antarctic and temperate fungi belonging to the genus Penicillium. Our study showed that downshift from an optimal temperature to 15 degrees or 6 degrees C led to a cell response typical of oxidative stress: significant reduction of biomass production; increase in the levels of oxidative damaged proteins and accumulation of storage carbohydrates (glycogen and trehalose) in comparison to growth at optimal temperature. Cell response against cold stress includes also increase in the activities of SOD and CAT, which are key enzymes for directly scavenging reactive oxygen species. This response is more species-dependent than dependent on the degree of cold-shock. Antarctic psychrotolerant strain Penicillium olsonii p14 that is adapted to life in extremely cold conditions demonstrated enhanced tolerance to temperature downshift in comparison with both mesophilic strains (Antarctic Penicillium waksmanii m12 and temperate Penicillium sp. t35).