Reactive oxygen species inhibit the succinate oxidation-supported generation of membrane potential in wheat mitochondria

Wheat omics: Classical breeding to new breeding technologies

Wheat is an important cereal crop, and its significance is more due to compete for dietary products in the world. Many constraints facing by the wheat crop due to environmental hazardous, biotic, abiotic stress and heavy matters factors, as a result, decrease the yield. Understanding the molecular mechanism related to these factors is significant to figure out genes regulate under specific conditions. Classical breeding using hybridization has been used to increase the yield but not prospered at the desired level. With the development of newly emerging technologies in biological sciences i.e., marker assisted breeding (MAB), QTLs mapping, mutation breeding, proteomics, metabolomics, next-generation sequencing (NGS), RNA_sequencing, transcriptomics, differential expression genes (DEGs), computational resources and genome editing techniques i.e. (CRISPR cas9; Cas13) advances in the field of omics. Application of new breeding technologies develops huge data; considerable development is needed in bioinformatics science to interpret the data. However, combined omics application to address physiological questions linked with genetics is still a challenge. Moreover, viroid discovery opens the new direction for research, economics, and target specification. Comparative genomics important to figure gene of interest processes are further discussed about considering the identification of genes, genomic loci, and biochemical pathways linked with stress resilience in wheat. Furthermore, this review extensively discussed the omics approaches and their effective use. Integrated plant omics technologies have been used viroid genomes associated with CRISPR and CRISPR-associated Cas13a proteins system used for engineering of viroid interference along with high-performance multidimensional phenotyping as a significant limiting factor for increasing stress resistance in wheat.

UVC light modulates vitamin C and phenolic biosynthesis in acerola fruit: role of increased mitochondria activity and ROS production

The effects of ultraviolet-C light (UVC) on vitamin C and phenolic compounds in acerola during postharvest storage were investigated in order to elucidate the mechanism inducing the antioxidant systems. The fruits, stored at 10 °C for 7 days after a hormetic UVC irradiation (two pulses of 0.3 J/cm²), showed significantly less degradation of vitamin C and phenolic compounds than the control without the UVC challenge. UVC activated the L-galactono-1,4-lactone dehydrogenase (GalDH), a key enzyme for vitamin C biosynthesis, and altered the composition of phenolic compounds, through phenolic biosynthesis, in acerola during postharvest storage. UVC also induced reactive oxygen species (ROS) productions at immediate (day 0) and late (day 7) times during postharvest storage through the mitochondrial electron transport chain and NADPH oxidase, respectively. Results suggest that UVC helps in the retention of vitamin C and phenolic content in acerola by altering ascorbic acid and phenolic metabolism through an increase in mitochondrial activity and a ROS-mediated mechanism. Data showed the beneficial effects of UVC on maintenance of nutraceutical quality in acerola during postharvest storage and supplied new insights into understanding the mechanism by which UVC irradiation enhance the antioxidant system in fruits.

First Evidence of a Protective Effect of Plant Bioactive Compounds against H2O2-Induced Aconitase Damage in Durum Wheat Mitochondria

In order to contribute to the understanding of the antioxidant behavior of plant bioactive compounds with respect to specific subcellular targets, in this study, their capability to protect aconitase activity from oxidative-mediated dysfunction was evaluated for the first time in plant mitochondria. Interest was focused on the Krebs cycle enzyme catalyzing the citrate/isocitrate interconversion via cis-aconitate, as it possesses a [4Fe-4S]²⁺ cluster at the active site, making it an early and highly sensitive target of reactive oxygen species (ROS)-induced oxidative damage. In particular, the effect on the aconitase reaction of five natural phenols, including ferulic acid, apigenin, quercetin, resveratrol, and curcumin, as well as of the isothiocyanate sulforaphane, was investigated in highly purified mitochondria obtained from durum wheat (DWM). Interestingly, a short-term (10 min) DWM pre-treatment with all investigated compounds, applied at 150 µM (75 µM in the case of resveratrol), completely prevented aconitase damage induced by a 15 min exposure of mitochondria to 500 µM H₂O₂. Curcumin and quercetin were also found to completely recover DWM-aconitase activity when phytochemical treatment was performed after H₂O₂ damage. In addition, all tested phytochemicals (except ferulic) induced a significant increase of aconitase activity in undamaged mitochondria. On the contrary, a relevant protective and recovery effect of only quercetin treatment was observed in terms of the aconitase activity of a commercial purified mammalian isoform, which was used for comparison. Overall, the results obtained in this study may suggest a possible role of phytochemicals in preserving plant mitochondrial aconitase activity, as well as energy metabolism, against oxidative damage that may occur under environmental stress conditions. Further investigations are needed to elucidate the physiological role and the mechanism responsible for this short-term protective effect.

Modulation of Potassium Channel Activity in the Balance of ROS and ATP Production by Durum Wheat Mitochondria-An Amazing Defense Tool Against Hyperosmotic Stress

In plants, the existence of a mitochondrial potassium channel was firstly demonstrated about 15 years ago in durum wheat as an ATP-dependent potassium channel (PmitoKATP). Since then, both properties of the original PmitoKATP and occurrence of different mitochondrial potassium channels in a number of plant species (monocotyledonous and dicotyledonous) and tissues/organs (etiolated and green) have been shown. Here, an overview of the current knowledge is reported; in particular, the issue of PmitoKATP physiological modulation is addressed. Similarities and differences with other potassium channels, as well as possible cross-regulation with other mitochondrial proteins (Plant Uncoupling Protein, Alternative Oxidase, Plant Inner Membrane Anion Channel) are also described. PmitoKATP is inhibited by ATP and activated by superoxide anion, as well as by free fatty acids (FFAs) and acyl-CoAs. Interestingly, channel activation increases electrophoretic potassium uptake across the inner membrane toward the matrix, so collapsing membrane potential (ΔΨ), the main component of the protonmotive force (Δp) in plant mitochondria; moreover, cooperation between PmitoKATP and the K(+)/H(+) antiporter allows a potassium cycle able to dissipate also ΔpH. Interestingly, ΔΨ collapse matches with an active control of mitochondrial reactive oxygen species (ROS) production. Fully open channel is able to lower superoxide anion up to 35-fold compared to a condition of ATP-inhibited channel. On the other hand, ΔΨ collapse by PmitoKATP was unexpectedly found to not affect ATP synthesis via oxidative phosphorylation. This may probably occur by means of a controlled collapse due to ATP inhibition of PmitoKATP; this brake to the channel activity may allow a loss of the bulk phase Δp, but may preserve a non-classically detectable localized driving force for ATP synthesis. This ability may become crucial under environmental/oxidative stress. In particular, under moderate hyperosmotic stress (mannitol or NaCl), PmitoKATP was found to be activated by ROS, so inhibiting further large-scale ROS production according to a feedback mechanism; moreover, a stress-activated phospholipase A2 may generate FFAs, further activating the channel. In conclusion, a main property of PmitoKATP is the ability to keep in balance the control of harmful ROS with the mitochondrial/cellular bioenergetics, thus preserving ATP for energetic needs of cell defense under stress.

Transport pathways--proton motive force interrelationship in durum wheat mitochondria

In durum wheat mitochondria (DWM) the ATP-inhibited plant mitochondrial potassium channel (PmitoK(ATP)) and the plant uncoupling protein (PUCP) are able to strongly reduce the proton motive force (pmf) to control mitochondrial production of reactive oxygen species; under these conditions, mitochondrial carriers lack the driving force for transport and should be inactive. However, unexpectedly, DWM uncoupling by PmitoK(ATP) neither impairs the exchange of ADP for ATP nor blocks the inward transport of Pi and succinate. This uptake may occur via the plant inner membrane anion channel (PIMAC), which is physiologically inhibited by membrane potential, but unlocks its activity in de-energized mitochondria. Probably, cooperation between PIMAC and carriers may accomplish metabolite movement across the inner membrane under both energized and de-energized conditions. PIMAC may also cooperate with PmitoK(ATP) to transport ammonium salts in DWM. Interestingly, this finding may trouble classical interpretation of in vitro mitochondrial swelling; instead of free passage of ammonia through the inner membrane and proton symport with Pi, that trigger metabolite movements via carriers, transport of ammonium via PmitoK(ATP) and that of the counteranion via PIMAC may occur. Here, we review properties, modulation and function of the above reported DWM channels and carriers to shed new light on the control that they exert on pmf and vice-versa.

The existence of phospholipase A(2) activity in plant mitochondria and its activation by hyperosmotic stress in durum wheat (Triticum durum Desf.)

The activity of mitochondrial phospholipase A(2) (PLA(2)) was shown for the first time in plants. It was observed in etiolated seedlings from durum wheat, barley, tomato, spelt and green seedlings of maize, but not in potato and topinambur tubers and lentil etiolated seedlings. This result was achieved by a novel spectrophotometric assay based on the coupled PLA(2)/lipoxygenase reactions using 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine as substrate; the mitochondrial localisation was assessed by checking recovery of marker enzymes. Durum wheat mitochondrial PLA(2) (DWM-PLA(2)) showed maximal activity at pH 9.0 and 1mM Ca(2+), hyperbolic kinetics (K(m)=90±6μM, V(max)=29±1nmolmin(-1)mg(-1) of protein) and inhibition by methyl arachidonyl fluorophosphonate, 5-(4-benzyloxyphenyl)-4S-(7-phenylheptanoylamino)pentanoic acid and palmityl trifluoromethyl ketone. Reactive oxygen species had no effect on DWM-PLA(2), that instead was activated by about 50% and 95%, respectively, under salt (0.21M NaCl) and osmotic (0.42M mannitol) stress imposed during germination. Contrarily, a secondary Ca(2+)-independent activity, having optimum at pH 7.0, was stress-insensitive. We propose that the activation of DWM-PLA(2) is responsible for the strong increase of free fatty acids recently measured in mitochondria under the same stress conditions [Laus, et al., J. Exp. Bot. 62 (2011) 141-154] that, in turn, activate potassium channel and uncoupling protein, able to counteract hyperosmotic stress.

Potassium channel-oxidative phosphorylation relationship in durum wheat mitochondria from control and hyperosmotic-stressed seedlings

Durum wheat mitochondria (DWM) possess an ATP-inhibited K(+) channel, the plant mitoK(ATP) (PmitoK(ATP) ), which is activated under environmental stress to control mitochondrial ROS production. To do this, PmitoK(ATP) collapses membrane potential (ΔΨ), thus suggesting mitochondrial uncoupling. We tested this point by studying oxidative phosphorylation (OXPHOS) in DWM purified from control seedlings and from seedlings subjected both to severe mannitol and NaCl stress. In severely-stressed DWM, the ATP synthesis via OXPHOS, continuously monitored by a spectrophotometric assay, was about 90% inhibited when the PmitoK(ATP) was activated by KCl. Contrarily, in control DWM, although PmitoK(ATP) collapsed ΔΨ, ATP synthesis, as well as coupling [respiratory control (RC) ratio and ratio between phosphorylated ADP and reduced oxygen (ADP/O)] checked by oxygen uptake experiments, were unaffected. We suggest that PmitoK(ATP) may play an important defensive role at the onset of the environmental/oxidative stress by preserving energy in a crucial moment for cell and mitochondrial bioenergetics. Consistently, under moderate mannitol stress, miming an early stress condition, the channel may efficiently control reactive oxygen species (ROS) generation (about 35-fold from fully open to closed state) without impairing ATP synthesis. Anyway, if the stress significantly proceeds, the PmitoK(ATP) becomes fully activated by decrease of ATP concentration (25-40%) and increase of activators [free fatty acids (FFAs) and superoxide anion], thus impairing ATP synthesis.

An Arabidopsis mitochondrial uncoupling protein confers tolerance to drought and salt stress in transgenic tobacco plants

BACKGROUND

Plants are challenged by a large number of environmental stresses that reduce productivity and even cause death. Both chloroplasts and mitochondria produce reactive oxygen species under normal conditions; however, stress causes an imbalance in these species that leads to deviations from normal cellular conditions and a variety of toxic effects. Mitochondria have uncoupling proteins (UCPs) that uncouple electron transport from ATP synthesis. There is evidence that UCPs play a role in alleviating stress caused by reactive oxygen species overproduction. However, direct evidence that UCPs protect plants from abiotic stress is lacking.

METHODOLOGY/PRINCIPAL FINDINGS

Tolerances to salt and water deficit were analyzed in transgenic tobacco plants that overexpress a UCP (AtUCP1) from Arabidopsis thaliana. Seeds of AtUCP1 transgenic lines germinated faster, and adult plants showed better responses to drought and salt stress than wild-type (WT) plants. These phenotypes correlated with increased water retention and higher gas exchange parameters in transgenic plants that overexpress AtUCP1. WT plants exhibited increased respiration under stress, while transgenic plants were only slightly affected. Furthermore, the transgenic plants showed reduced accumulation of hydrogen peroxide in stressed leaves compared with WT plants.

CONCLUSIONS/SIGNIFICANCE

Higher levels of AtUCP1 improved tolerance to multiple abiotic stresses, and this protection was correlated with lower oxidative stress. Our data support previous assumptions that UCPs reduce the imbalance of reactive oxygen species. Our data also suggest that UCPs may play a role in stomatal closure, which agrees with other evidence of a direct relationship between these proteins and photosynthesis. Manipulation of the UCP protein expression in mitochondria is a new avenue for crop improvement and may lead to crops with greater tolerance for challenging environmental conditions.

Reactive oxygen species and antioxidant machinery in abiotic stress tolerance in crop plants

Various abiotic stresses lead to the overproduction of reactive oxygen species (ROS) in plants which are highly reactive and toxic and cause damage to proteins, lipids, carbohydrates and DNA which ultimately results in oxidative stress. The ROS comprises both free radical (O(2)(-), superoxide radicals; OH, hydroxyl radical; HO(2), perhydroxy radical and RO, alkoxy radicals) and non-radical (molecular) forms (H(2)O(2), hydrogen peroxide and (1)O(2), singlet oxygen). In chloroplasts, photosystem I and II (PSI and PSII) are the major sites for the production of (1)O(2) and O(2)(-). In mitochondria, complex I, ubiquinone and complex III of electron transport chain (ETC) are the major sites for the generation of O(2)(-). The antioxidant defense machinery protects plants against oxidative stress damages. Plants possess very efficient enzymatic (superoxide dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX; glutathione reductase, GR; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; glutathione peroxidase, GPX; guaicol peroxidase, GOPX and glutathione-S- transferase, GST) and non-enzymatic (ascorbic acid, ASH; glutathione, GSH; phenolic compounds, alkaloids, non-protein amino acids and α-tocopherols) antioxidant defense systems which work in concert to control the cascades of uncontrolled oxidation and protect plant cells from oxidative damage by scavenging of ROS. ROS also influence the expression of a number of genes and therefore control the many processes like growth, cell cycle, programmed cell death (PCD), abiotic stress responses, pathogen defense, systemic signaling and development. In this review, we describe the biochemistry of ROS and their production sites, and ROS scavenging antioxidant defense machinery.

Antioxidant response of wheat roots to drought acclimation

Wheat (Triticum aestivum L.) seedlings of a drought-resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods. The antioxidant defense in terms of redox metabolites and enzymes in root cells and mitochondria was examined in relation to membrane damage. Acclimated seedlings exhibited higher relative water content and were able to limit the accumulation of H(2)O(2) and membrane damage during subsequent severe water stress conditions. This was due to systematic up-regulation of superoxide dismutase, ascorbate peroxidase (APX), catalase, peroxidases, and ascorbate-glutathione cycle components at both the whole cell level as well as in mitochondria. In contrast, direct exposure of severe water stress to non-acclimated seedlings caused greater water loss, excessive accumulation of H(2)O(2) followed by elevated lipid peroxidation due to the poor antioxidant enzyme response particularly of APX, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and ascorbate-glutathione redox balance. Mitochondrial antioxidant defense was found to be better than the cellular defense in non-acclimated roots. Termination of stress followed by rewatering leads to a rapid enhancement in all the antioxidant defense components in non-acclimated roots, which suggested that the excess levels of H(2)O(2) during severe water stress conditions might have inhibited or down-regulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced tolerance toward oxidative stress in the root tissue of wheat seedlings due to both reactive oxygen species restriction and well-coordinated induction of antioxidant defense.

Reactive oxygen species and nitric oxide in plant mitochondria: origin and redundant regulatory systems

Plant mitochondria differ from their mammalian counterparts in many respects, which are due to the unique and variable surroundings of plant mitochondria. In green leaves, plant mitochondria are surrounded by ample respiratory substrates and abundant molecular oxygen, both resulting from active photosynthesis, while in roots and bulky rhizomes and fruit carbohydrates may be plenty, whereas oxygen levels are falling. Several enzymatic complexes in mitochondrial electron transport chain (ETC) are capable of reactive oxygen species (ROS) formation under physiological and pathological conditions. Inherently connected parameters such as the redox state of electron carriers in the ETC, ATP synthase activity and inner mitochondrial membrane potential, when affected by external stimuli, can give rise to ROS formation via complexes I and III, and by reverse electron transport (RET) from complex II. Superoxide radicals produced are quickly scavenged by superoxide dismutase (MnSOD), and the resulting H(2)O(2) is detoxified by peroxiredoxin-thioredoxin system or by the enzymes of ascorbate-glutathione cycle, found in the mitochondrial matrix. Arginine-dependent nitric oxide (NO)-releasing activity of enzymatic origin has been detected in plant mitochondria. The molecular identity of the enzyme is not clear but the involvement of mitochondria-localized enzymes responsible for arginine catabolism, arginase and ornithine aminotransferase has been shown in the regulation of NO efflux. Besides direct control by antioxidants, mitochondrial ROS production is tightly controlled by multiple redundant systems affecting inner membrane potential: NAD(P)H-dependent dehydrogenases, alternative oxidase (AOX), uncoupling proteins, ATP-sensitive K(+) channel and a number of matrix and intermembrane enzymes capable of direct electron donation to ETC. NO removal, on the other hand, takes place either by reactions with molecular oxygen or superoxide resulting in peroxynitrite, nitrite or nitrate ions or through interaction with non-symbiotic hemoglobins or glutathione. Mitochondrial ROS and NO production is tightly controlled by multiple redundant systems providing the regulatory mechanism for redox homeostasis and specific ROS/NO signaling.

Wheat defense genes in fungal (Puccinia striiformis) infection

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most important diseases of wheat worldwide. To isolate defense-related genes against the pathogen, a suppression subtractive hybridization library was constructed for an incompatible interaction. From the library, 652 sequences were determined to be unigenes, of which 31 were determined as genes involved in signal transduction and 77 were predicted to encode defense-related proteins. Expression patterns of 12 selected signal transduction and defense-related genes were determined using quantitative real-time polymerase chain reaction. Signal transduction genes started increasing their expression at 12 h post inoculation (hpi), and expressions of the most of the transport and resistance-related genes were induced at 18 hpi. The gene expression results indicate specific molecular and cellular activities during the incompatible interaction between wheat and the stripe rust pathogen. In general, the expression increase of wheat signal transduction genes soon after inoculation with the pathogen inducing various defense-related genes, including reactive oxygen species, ATP-binding cassette (ABC) transporters, pathogenesis-related proteins, and genes involved in the phenylpropanoid pathway. The activities of these defense genes work in a sequential and concerted manner to result in a hypersensitive response.

Photosynthesis and cell respiration modulated by water deficit in grapevine (Vitis vinifera L.) cv. Cabernet Sauvignon

Winegrape productivity and quality has been related to the regulated deficit irrigation, with important implications for the plant bioenergetics. When water deficit is imposed to grapevine plants, alterations in overall photosynthesis and cell respiration are observed. The aim of this study was to assess the modulations promoted by water stress on photosynthesis and respiration in leaves of the cv. Cabernet Sauvignon (cv. CS) for better understanding the physiological responses related to its drought tolerance and quality improvement under water deficit. For this purpose, measurements of photosynthetic efficiency, leaf water potential, gas exchange and O2 consumption were carried out. Leaf water potential, photosynthesis, stomatal conductance, transpiration and internal carbon concentration were significantly reduced upon stress, suggesting that plants of cv. CS present higher water use efficiency (A N/E) and lower carboxylative capacity (A N/Ci) under this condition. On the other hand, cell respiration increased more than 70 % as estimated by the increase of O2 consumption measured 12 days after suspension of irrigation. Most of this effect was related to a four-fold increase of the mitochondrial alternative oxidase (AOX) activity. These data indicate a key role for the AOX pathway in the physiological responses of grapevines to water deficit, and it implies that analyses of the AOX activation patterns should be useful for programs aiming to improve the consistency of fruit production and quality of winegrape cultivars by regulated deficit irrigation.

Plant inner membrane anion channel (PIMAC) function in plant mitochondria

To date, the existence of the plant inner membrane anion channel (PIMAC) has been shown only in potato mitochondria, but its physiological role remains unclear. In this study, by means of swelling experiments in K(+) and ammonium salts, we characterize a PIMAC-like anion-conducting pathway in mitochondria from durum wheat (DWM), a monocotyledonous species phylogenetically far from potato. DWM were investigated since they possess a very active potassium channel (PmitoK(ATP)), so implying a very active matching anion uniport pathway and, possibly, a coordinated function. As in potato mitochondria, the electrophoretic uptake of chloride and succinate was inhibited by matrix [H(+)], propranolol, and tributyltin, and was insensitive to Mg(2+), N,N'-dicyclohexylcarbodiimide (DCCD) and mercurials, thus showing PIMAC's existence in DWM. PIMAC actively transports dicarboxylates, oxodicarboxylates, tricarboxylates and Pi. Interestingly, a novel mechanism of swelling in ammonium salts of isolated plant mitochondria is reported, based on electrophoretic anion uptake via PIMAC and ammonium uniport via PmitoK(ATP). PIMAC is inhibited by physiological compounds, such as ATP and free fatty acids, by high electrical membrane potential (Delta Psi), but not by acyl-CoAs or reactive oxygen species. PIMAC was found to cooperate with dicarboxylate carrier by allowing succinate uptake that triggers succinate/malate exchange in isolated DWM. Similar results were obtained using mitochondria from the dicotyledonous species topinambur, so suggesting generalization of results. We propose that PIMAC is normally inactive in vivo due to ATP and Delta Psi inhibition, but activation may occur in mitochondria de-energized by PmitoK(ATP) (or other dissipative systems) to replace or integrate the operation of classical anion carriers.

Succinate dehydrogenase flavoprotein subunit expression in Saccharomyces cerevisiae--involvement of the mitochondrial FAD transporter, Flx1p

The mitochondrial FAD transporter, Flx1p, is a member of the mitochondrial carrier family responsible for FAD transport in Saccharomyces cerevisiae. It has also been suggested that it has a role in maintaining the normal activity of mitochondrial FAD-binding enzymes, including lipoamide dehydrogenase and succinate dehydrogenase flavoprotein subunit Sdh1p. A decrease in the amount of Sdh1p in the flx1Delta mutant strain has been determined here to be due to a post-transcriptional control that involves regulatory sequences located upstream of the SDH1 coding sequence. The SDH1 coding sequence and the regulatory sequences located downstream of the SDH1 coding region, as well as protein import and cofactor attachment, seem to be not involved in the decrease in the amount of protein.

Ectopic expression of GmPAP3 alleviates oxidative damage caused by salinity and osmotic stresses

The primary biochemical reaction of purple acid phosphatases (PAP) is to catalyze the hydrolysis of phosphate esters and anhydrides. However, the soybean GmPAP3 gene expression is induced by NaCl, osmotic, and oxidative treatments, indicating a possible role of PAP in abiotic stress responses. Confocal and electron microscopic studies demonstrated that GmPAP3 protein is mainly localized in mitochondria, a primary site for reactive oxygen species (ROS) production. When subjected to NaCl and polyethylene glycol (PEG) treatments, ectopic expression of GmPAP3 in transgenic tobacco BY-2 cells mimicked the protective effects exhibited by the antioxidant ascorbic acid: increase in the percentage of cells with active mitochondria; reduction in the percentage of dead cells; and reduced accumulation of ROS. In addition, when GmPAP3 transgenic Arabidopsis thaliana seedlings were subjected to NaCl, PEG, and paraquat (PQ) treatments, the percentage of root elongation was significantly higher than the wild type. Furthermore, PQ-induced lipid peroxidation in these transgenic seedlings was also reduced. In summary, the mitochondrial localized GmPAP3 may play a role in stress tolerance by enhancing ROS scavenging.

Seawater stress applied at germination affects mitochondrial function in durum wheat (Triticum durum) early seedlings

Seawater stress effects on mitochondrial ATP synthesis and membrane potential (ΔΨ) were investigated in germinating durum wheat seedlings under moderate (22% seawater osmolarity, -0.62 MPa) and severe (37% seawater osmolarity, -1.04 MPa) stress. To estimate the osmotic component of salt stress, mannitol solutions (0.25 and 0.42 m) iso-osmotic with the saline ones were used. Moderate stress intensity only delayed mean germination time (MGT), whereas higher seawater osmolarity reduced germination percentage as well. In contrast, Na⁺ and Cl^- accumulation showed a sharp increase under moderate stress and only a small further increase under severe stress, which was more pronounced for Cl^-. Only severe stress significantly damaged succinate-dependent oxidative phosphorylation, which may be related to the stress-induced alteration in inner mitochondrial membrane permeability, as indicated by changes in ΔΨ profiles. Proline-dependent oxidative phosphorylation, however, was inhibited under moderate stress. This suggests the occurrence of an adaptation mechanism leading to proline accumulation as an osmoprotectant. Moreover, both the osmotic and the toxic components of seawater stress were detrimental to oxidative phosphorylation. Damage to germination and MGT, in contrast, were mainly caused by osmotic stress. Therefore, mitochondrial function appears to be a more sensitive target of toxic stress than growth. In conclusion, the effects of seawater stress on mitochondrial ATP synthesis vary in relation to the substrate oxidised and stress level, inducing both adaptive responses and damage.

Plant uncoupling protein in mitochondria from aged-dehydrated slices of Jerusalem artichoke tubers becomes sensitive to superoxide and to hydrogen peroxide without increase in protein level

We investigated the occurrence of the plant Uncoupling Protein (UCP) in mitochondria isolated from both fresh (f-JAM) and aged-dehydrated (a-d-JAM) slices of Jerusalem artichoke tubers (Helianthus tuberosus L.). The presence of UCP was shown by immunological analysis and its function was investigated by measuring the decrease of the mitochondrial membrane potential due to linoleic acid (LA) and its inhibition by purine nucleotides under conditions in which the adenine nucleotide translocator (ANT) was inhibited by atractyloside (Atr). f-JAM and a-d-JAM had the same protein content, but differed from one another with respect to purine nucleotide inhibition, substrate specificity, and sensitivity to ROS. Hydrogen peroxide and superoxide anion, generated in situ by xanthine plus xanthine oxidase, caused a significant increase in the UCP function in a-d-JAM, but not in f-JAM. This occurred in a manner sensitive to ATP, but not to Atr, thus showing that ANT has no role in the process. The dependence of the rate of membrane potential decrease on increasing LA concentrations, either in the absence or the presence of ROS, showed a sigmoidal saturation both in f-JAM and a-d-JAM. However, addition of ROS in a-d-JAM resulted in about 40% increase of the Vmax value, with no change in the K0.5 (about 20 microM), whereas in f-JAM no effect on either the Vmax or K0.5 (about 28 microM) was found. Furthermore, a decreased ROS production as a result of LA addition was found in both f-JAM and a-d-JAM, the effect being more marked in a-d-JAM.

Mitochondria/nuclear signaling of alternative oxidase gene expression occurs through distinct pathways involving organic acids and reactive oxygen species

Cultured cells of tobacco (Nicotiana tabacum L. cv Petit Havana) were used to investigate signals regulating the expression of the "model" nuclear gene encoding the alternative oxidase (AOX) (AOX1), the terminal oxidase of the mitochondrial alternative respiratory pathway. Several conditions shown to induce AOX1 mRNA accumulation also result in an increase in cellular citrate concentrations, suggesting that citrate and/or other tricarboxylic acid (TCA) cycle intermediates may be important signal metabolites. In addition, mitochondrial reactive oxygen species (ROS) production has recently been shown to be a factor mediating mitochondria-to-nucleus signaling for the expression of AOX1. We found that the exogenously supplied TCA cycle organic acids citrate, malate and 2-oxoglutarate caused rapid and dramatic increases in the steady-state level of AOX1 mRNA at low, near physiological concentrations (0.1 mM). Furthermore, an increase in AOX1 induced by the addition of organic acids occurs independently of mitochondrial ROS formation. Our results demonstrate that two separate pathways for mitochondria-to-nucleus signaling of AOX1 may exist, one involving ROS and the other organic acids.

Isolated durum wheat and potato cell mitochondria oxidize externally added NADH mostly via the malate/oxaloacetate shuttle with a rate that depends on the carrier-mediated transport

We investigated whether and how mitochondria from durum wheat (Triticum durum Desf.) and potato (Solanum tuberosum), isolated from etiolated shoots and a cell suspension culture, respectively, oxidize externally added NADH via the mitochondrial shuttles; in particular, we compared the shuttles and the external NADH dehydrogenase (NADH DHExt) with respect to their capacity to oxidize external NADH. We found that external NADH and NADPH can be oxidized via two separate DHExt, whereas under conditions in which the activities of NAD(P)H DHExt are largely prevented, NADH (but not NADPH) is oxidized in the presence of external malate (MAL) and MAL dehydrogenase, in a manner sensitive to several non-penetrant compounds according to the occurrence of the MAL/oxaloacetate (OAA) shuttle. In durum wheat mitochondria and potato cell mitochondria, the rate of NADH oxidation was limited by the rate of a novel carrier, the MAL/OAA antiporter, which is different from other carriers thought to transport OAA across the mitochondrial membrane. No NAD(P)H oxidation occurred arising from the MAL/Aspartate and the alpha-glycerophosphate/dihydroxyacetonphosphate shuttles. We determined the kinetic parameters of the enzymes and the antiporter involved in NADH oxidation, and, on the basis of a kinetic analysis, we showed that, at low physiological NADH concentrations, oxidation via the MAL/OAA shuttle occurred with a higher efficiency than that due to the NADH DHExt (about 100- and 10-fold at 1 microm NADH in durum wheat mitochondria and in potato cell mitochondria, respectively). The NADH DHExt contribution to NADH oxidation increased with increasing NADH concentration.

Riboflavin uptake and FAD synthesis in Saccharomyces cerevisiae mitochondria: involvement of the Flx1p carrier in FAD export

We have studied the functional steps by which Saccharomyces cerevisiae mitochondria can synthesize FAD from cytosolic riboflavin (Rf). Riboflavin uptake into mitochondria took place via a mechanism that is consistent with the existence of (at least two) carrier systems. FAD was synthesized inside mitochondria by a mitochondrial FAD synthetase (EC 2.7.7.2), and it was exported into the cytosol via an export system that was inhibited by lumiflavin, and which was different from the riboflavin uptake system. To understand the role of the putative mitochondrial FAD carrier, Flx1p, in this pathway, an flx1Delta mutant strain was constructed. Coupled mitochondria isolated from flx1Delta mutant cells were compared with wild-type mitochondria with respect to the capability to take up Rf, to synthesize FAD from it, and to export FAD into the extramitochondrial phase. Mitochondria isolated from flx1Delta mutant cells specifically lost the ability to export FAD, but did not lose the ability to take up Rf, FAD, or FMN and to synthesize FAD from Rf. Hence, Flx1p is proposed to be the mitochondrial FAD export carrier. Moreover, deletion of the FLX1 gene resulted in a specific reduction of the activities of mitochondrial lipoamide dehydrogenase and succinate dehydrogenase, which are FAD-binding enzymes. For the flavoprotein subunit of succinate dehydrogenase we could demonstrate that this was not due to a changed level of mitochondrial FAD or to a change in the degree of flavinylation of the protein. Instead, the amount of the flavoprotein subunit of succinate dehydrogenase was strongly reduced, indicating an additional regulatory role for Flx1p in protein synthesis or degradation.