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

Alternative NADH dehydrogenase: A complex I backup, a drug target, and a tool for mitochondrial gene therapy

Alternative NADH dehydrogenase, also known as type II NADH dehydrogenase (NDH-2), catalyzes the same redox reaction as mitochondrial respiratory chain complex I. Specifically, it oxidizes reduced nicotinamide adenine dinucleotide (NADH) while simultaneously reducing ubiquinone to ubiquinol. However, unlike complex I, this enzyme is non-proton pumping, comprises of a single subunit, and is resistant to rotenone. Initially identified in bacteria, fungi and plants, NDH-2 was subsequently discovered in protists and certain animal taxa including sea squirts. The gene coding for NDH-2 is also present in the genomes of some annelids, tardigrades, and crustaceans. For over two decades, NDH-2 has been investigated as a potential substitute for defective complex I. In model organisms, NDH-2 has been shown to ameliorate a broad spectrum of conditions associated with complex I malfunction, including symptoms of Parkinson's disease. Recently, lifespan extension has been observed in animals expressing NDH-2 in a heterologous manner. A variety of mechanisms have been put forward by which NDH-2 may extend lifespan. Such mechanisms include the activation of pro-longevity pathways through modulation of the NAD+/NADH ratio, decreasing production of reactive oxygen species (ROS) in mitochondria, or then through moderate increases in ROS production followed by activation of defense pathways (mitohormesis). This review gives an overview of the latest research on NDH-2, including the structural peculiarities of NDH-2, its inhibitors, its role in the pathogenicity of mycobacteria and apicomplexan parasites, and its function in bacteria, fungi, and animals.

Mitochondrial complex I subunit NDUFS8.2 modulates responses to stresses associated with reduced water availability

Mitochondria are important sources of energy in plants and are implicated in coordination of a number of metabolic and physiological processes including stabilization of redox balance, synthesis and turnover of a number of metabolites, and control of programmed cell death. Mitochondrial electron transport chain (mETC) is the backbone of the energy producing process which can influence other processes as well. Accumulating evidence suggests that mETC can affect responses to environmental stimuli and modulate tolerance to extreme conditions such as drought or salinity. Screening for stress responses of 13 Arabidopsis mitochondria-related T-DNA insertion mutants, we identified ndufs8.2-1 which has an increased ability to withstand osmotic and oxidative stresses compared to wild type plants. Insertion in ndufs8.2-1 disrupted the gene that encodes the NADH dehydrogenase [ubiquinone] fragment S subunit 8 (NDUFS8) a component of Complex I of mETC. ndufs8.2-1 tolerated reduced water availability, retained photosynthetic activity and recovered from severe water stress with higher efficiency compared to wild type plants. Several mitochondrial functions were altered in the mutant including oxygen consumption, ROS production, ATP and ADP content as well as activities of genes encoding alternative oxidase 1A (AOX1A) and various alternative NAD(P)H dehydrogenases (ND). Our results suggest that in the absence of NDUFS8.2 stress-induced ROS generation is restrained leading to reduced oxidative damage and improved tolerance to water deficiency. mETC components can be implicated in redox and energy homeostasis and modulate responses to stresses associated with reduced water availability.

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.

Legume Alternative Oxidase Isoforms Show Differential Sensitivity to Pyruvate Activation

Alternative oxidase (AOX) is an important component of the plant respiratory pathway, enabling a route for electrons that bypasses the energy-conserving, ROS-producing complexes of the mitochondrial electron transport chain. Plants contain numerous isoforms of AOX, classified as either AOX1 or AOX2. AOX1 isoforms have received the most attention due to their importance in stress responses across a wide range of species. However, the propensity for at least one isoform of AOX2 to accumulate to very high levels in photosynthetic tissues of all legumes studied to date, suggests that this isoform has specialized roles, but we know little of its properties. Previous studies with sub-mitochondrial particles of soybean cotyledons and roots indicated that differential expression of GmAOX1, GmAOX2A, and GmAOX2D across tissues might confer different activation kinetics with pyruvate. We have investigated this using recombinantly expressed isoforms of soybean AOX in a previously described bacterial system (Selinski et al., 2016, Physiologia Plantarum 157, 264-279). Pyruvate activation kinetics were similar between the two GmAOX2 isoforms but differed substantially from those of GmAOX1, suggesting that selective expression of AOX1 and 2 could determine the level of AOX activity. However, this alone cannot completely explain the differences seen in sub-mitochondrial particles isolated from different legume tissues and possible reasons for this are discussed.

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

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

Effect of Salt Stress on the Expression and Promoter Methylation of the Genes Encoding the Mitochondrial and Cytosolic Forms of Aconitase and Fumarase in Maize

The influence of salt stress on gene expression, promoter methylation, and enzymatic activity of the mitochondrial and cytosolic forms of aconitase and fumarase has been investigated in maize (Zea mays L.) seedlings. The incubation of maize seedlings in 150-mM NaCl solution resulted in a several-fold increase of the mitochondrial activities of aconitase and fumarase that peaked at 6 h of NaCl treatment, while the cytosolic activity of aconitase and fumarase decreased. This corresponded to the decrease in promoter methylation of the genes Aco1 and Fum1 encoding the mitochondrial forms of these enzymes and the increase in promoter methylation of the genes Aco2 and Fum2 encoding the cytosolic forms. The pattern of expression of the genes encoding the mitochondrial forms of aconitase and fumarase corresponded to the profile of the increase of the stress marker gene ZmCOI6.1. It is concluded that the mitochondrial and cytosolic forms of aconitase and fumarase are regulated via the epigenetic mechanism of promoter methylation of their genes in the opposite ways in response to salt stress. The role of the mitochondrial isoforms of aconitase and fumarase in the elevation of respiration under salt stress is discussed.

Participation of calcium ions in induction of respiratory response caused by variation potential in pea seedlings

Electrical signals in plants caused by external stimuli are capable of inducing various physiological responses. The mechanisms of transformation of a long-distance electrical signal (ES) into a functional response remain largely unexplored and require additional research. In this work, we investigated the role of calcium ions in the development of ES-induced respiratory response. Gradual heating of the leaf causes the propagation of variation potential (VP) in the pea seedling. The propagation of VP leads to a transient activation of respiration in an unaffected leaf. During the VP generation, a transient increase in the intracellular calcium concentration takes place. A calcium channel blocker inhibits the respiratory response, and a calcium ionophore induces the activation of respiration. Inhibitory analysis has showed that the VP-induced increase in respiration activity is probably associated with calcium-mediated activation of rotenone-insensitive alternative NADPH dehydrogenases in mitochondria.