The role of mild uncoupling and non-coupled respiration in the regulation of hydrogen peroxide generation by plant mitochondria

The uncoupling of respiration in plant mitochondria: keeping reactive oxygen and nitrogen species under control

Plant mitochondrial respiration involves the operation of various alternative pathways. These pathways participate, both directly and indirectly, in the maintenance of mitochondrial functions though they do not contribute to energy production, being uncoupled from the generation of an electrochemical gradient across the mitochondrial membrane and thus from ATP production. Recent findings suggest that uncoupled respiration is involved in reactive oxygen species (ROS) and nitric oxide (NO) scavenging, regulation, and homeostasis. Here we discuss specific roles and possible functions of uncoupled mitochondrial respiration in ROS and NO metabolism. The mechanisms of expression and regulation of the NDA-, NDB- and NDC-type non-coupled NADH and NADPH dehydrogenases, the alternative oxidase (AOX), and the uncoupling protein (UCP) are examined in relation to their involvement in the establishment of the stable far-from-equilibrium state of plant metabolism. The role of uncoupled respiration in controlling the levels of ROS and NO as well as inducing signaling events is considered. Secondary functions of uncoupled respiration include its role in protection from stress factors and roles in biosynthesis and catabolism. It is concluded that uncoupled mitochondrial respiration plays an important role in providing rapid adaptation of plants to changing environmental factors via regulation of ROS and NO.

Integrative role of plant mitochondria facing oxidative stress: The case of ozone

Ozone is a secondary air pollutant, which causes oxidative stress in plants by producing reactive oxygen species (ROS) starting by an external attack of leaf apoplast. ROS have a dual role, acting as signaling molecules, regulating different physiological processes and response to stress, but also inducing oxidative damage. The production of ROS in plant cells is compartmented and regulated by scavengers and specific enzyme pathways. Chronic doses of ozone are known to trigger an important increase of the respiratory process while decreasing photosynthesis. Mitochondria, which normally operate with usual levels of intracellular ROS, would have to play a prominent role to cope with an enhanced ozone-derived ROS production. It is thus needed to compile the available literature on the effects of ozone on mitochondria to precise their strategy facing oxidative stress. An overview of the mitochondrial fate in three steps is proposed, i) starting with the initial responses of the mitochondria for alleviating the overproduction of ROS by the enhancement of existing antioxidant metabolism and adjustments of the electron transport chain, ii) followed by the setting up of detoxifying processes through exchanges between mitochondria and the cell, and iii) ending by an accelerated senescence initiated by mitochondrial membrane permeability and leading to programmed cell death.

Effects of Salicylic Acid on the Metabolism of Mitochondrial Reactive Oxygen Species in Plants

Different abiotic and biotic stresses lead to the production and accumulation of reactive oxygen species (ROS) in various cell organelles such as in mitochondria, resulting in oxidative stress, inducing defense responses or programmed cell death (PCD) in plants. In response to oxidative stress, cells activate various cytoprotective responses, enhancing the antioxidant system, increasing the activity of alternative oxidase and degrading the oxidized proteins. Oxidative stress responses are orchestrated by several phytohormones such as salicylic acid (SA). The biomolecule SA is a key regulator in mitochondria-mediated defense signaling and PCD, but the mode of its action is not known in full detail. In this review, the current knowledge on the multifaceted role of SA in mitochondrial ROS metabolism is summarized to gain a better understanding of SA-regulated processes at the subcellular level in plant defense responses.

Properties of the Permeability Transition of Pea Stem Mitochondria

In striking analogy with Saccharomyces cerevisiae, etiolated pea stem mitochondria did not show appreciable Ca²⁺ uptake. Only treatment with the ionophore ETH129 (which allows electrophoretic Ca²⁺ equilibration) caused Ca²⁺ uptake followed by increased inner membrane permeability, membrane depolarization and Ca²⁺ release. Like the permeability transition (PT) of mammals, yeast and Drosophila, the PT of pea stem mitochondria was stimulated by diamide and phenylarsine oxide and inhibited by Mg-ADP and Mg-ATP, suggesting a common underlying mechanism; yet, the plant PT also displayed distinctive features: (i) as in mammals it was desensitized by cyclosporin A, which does not affect the PT of yeast and Drosophila; (ii) similarly to S. cerevisiae and Drosophila it was inhibited by Pi, which stimulates the PT of mammals; (iii) like in mammals and Drosophila it was sensitized by benzodiazepine 423, which is ineffective in S. cerevisiae; (iv) like what observed in Drosophila it did not mediate swelling and cytochrome c release, which is instead seen in mammals and S. cerevisiae. We find that cyclophilin D, the mitochondrial receptor for cyclosporin A, is present in pea stem mitochondria. These results indicate that the plant PT has unique features and suggest that, as in Drosophila, it may provide pea stem mitochondria with a Ca²⁺ release channel.

The occurrence and control of nitric oxide generation by the plant mitochondrial electron transport chain

The plant mitochondrial electron transport chain (ETC) is bifurcated such that electrons from ubiquinol are passed to oxygen via the usual cytochrome path or through alternative oxidase (AOX). We previously showed that knockdown of AOX in transgenic tobacco increased leaf concentrations of nitric oxide (NO), implying that an activity capable of generating NO had been effected. Here, we identify the potential source of this NO. Treatment of leaves with antimycin A (AA, Qi -site inhibitor of Complex III) increased NO amount more than treatment with myxothiazol (Myxo, Qo -site inhibitor) despite both being equally effective at inhibiting respiration. Comparison of nitrate-grown wild-type with AOX knockdown and overexpression plants showed a negative correlation between AOX amount and NO amount following AA. Further, Myxo fully negated the ability of AA to increase NO amount. With ammonium-grown plants, neither AA nor Myxo strongly increased NO amount in any plant line. When these leaves were supplied with nitrite alongside the AA or Myxo, then the inhibitor effects across lines mirrored that of nitrate-grown plants. Hence the ETC, likely the Q-cycle of Complex III generates NO from nitrite, and AOX reduces this activity by acting as a non-energy-conserving electron sink upstream of Complex III.

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.

Mitochondrial bioenergetics linked to the manifestation of programmed cell death during somatic embryogenesis of Abies alba

The present work reports changes in bioenergetic parameters and mitochondrial activities during the manifestation of two events of programmed cell death (PCD), linked to Abies alba somatic embryogenesis. PCD, evidenced by in situ nuclear DNA fragmentation (TUNEL assay), DNA laddering and cytochrome c release, was decreased in maturing embryogenic tissue with respect to the proliferation stage. In addition, the major cellular energetic metabolites (ATP, NAD(P)H and glucose-6-phosphate) were highered during maturation. The main mitochondrial activities changed during two developmental stages. Mitochondria, isolated from maturing, with respect to proliferating cell masses, showed an increased activity of the alternative oxidase, external NADH dehydrogenase and fatty-acid mediated uncoupling. Conversely, a significant decrease of the mitochondrial K (ATP)(+) channel activity was observed. These results suggest a correlation between mitochondrial activities and the manifestation of PCD during the development of somatic embryos. In particular, it is suggested that the K (ATP)(+) channel activity could induce an entry of K(+) into the matrix, followed by swelling and a release of cytochrome c during proliferation, whereas the alternative pathways, acting as anti-apoptotic factors, may partially counteract PCD events occurring during maturation of somatic embryos.

The fate and the role of mitochondria in Fe-deficient roots of strategy I plants

In well aerated soils, iron exists, mainly as scarcely soluble oxides and oxi-hydroxides and, therefore, not freely available to plants uptake, notwithstanding its abundance. Multifaceted strategies involving reductase activities, proton processes, specialized storage proteins, and other, act in concert to mobilize iron from the environment, to take it up and to distribute it inside the plant. Because of its fundamental role in plant productivity several questions concerning homeostasis of iron in plants are currently a matter of intense debate. We discuss some recent studies on Strategy I responses in dicotyledonous plants focusing on metabolic change induced by iron deficiency, mainly concerning the involvement of mitochondria.

Activity of a KATP+ channel in Arum spadix mitochondria during thermogenesis

This report demonstrates that mitochondria isolated from thermogenic Arum spadices possess an ATP-sensitive potassium channel--responsible for electrical potential (DeltaPsi) collapse and mitochondrial swelling--whose characteristics are similar to those previously described in pea and wheat mitochondria. In order to study the relationship between this K(ATP)(+) channel and the uncoupled respiration, linked to thermogenesis, K(+) transport activities were compared with those of mitochondria that were isolated from pea stems, soybean suspension cell cultures and Arum tubers. The channel from Arum spadices is highly active and its major features are (i) potassium flux is performed primarily in an inward-rectifying manner; (ii) the influx of K(+) is associated with a matrix volume increase in both energized and non-energized mitochondria; and (iii) its activity depends on the redox state of electron transport chain (ETC) and oxygen availability. In particular, this paper shows that the K(ATP)(+) channel is inwardly activated in parallel with the alternative oxidase (AO). The activation is linked to an ETC-oxidized state and to high oxygen consumption. The putative role of this K(ATP)(+) channel is discussed in relation to flowering of thermogenic Arum spadices.

The transcript levels of two plant mitochondrial uncoupling protein (pUCP)-related genes are not affected by hyperosmotic stress in durum wheat seedlings showing an increased level of pUCP activity

Etiolated early seedlings of durum wheat submitted to moderate and severe salt (NaCl) and osmotic (mannitol) stress showed no relevant increase of both transcript levels of two plant uncoupling protein (pUCP)-related genes and maximal pUCP activity in purified mitochondria (which estimates protein level); contrarily, pUCP functioning due to endogenous free fatty acids strongly increased. These results show that pUCP activation under hyperosmotic stress may be due to modulation of pUCP reaction rather than to an increased protein synthesis. Finally, a properly developed method, based on a single membrane potential measurement, to evaluate both pUCP maximal activity and functioning, is reported.

Fatty acid efficiency profile in uncoupling of Acanthamoeba castellanii mitochondria

A profile of free fatty acid (FFA) specificity in Acanthamoeba castellanii mitochondrial uncoupling is described. The FFA uncoupling specificity was observed as different abilities to stimulate resting respiration, to decrease resting membrane potential, and to decrease oxidative phosphorylation efficiency. Tested unsaturated FFA (C18-20) were more effective as uncouplers and protonophores when compared to tested saturated FFA (C8-18), with palmitic acid (C16:0) as the most active. As FFA efficiency in mitochondrial uncoupling is related to physiological changes of fatty acid composition (and thereby FFA availability) during growth of amoeba cells, it could be a way to regulate the activity of an uncoupling protein and thereby the efficiency of oxidative phosphorylation during a cell life of this unicellular organism.

Changes in plant mitochondrial electron transport alter cellular levels of reactive oxygen species and susceptibility to cell death signaling molecules

Transgenic tobacco (Nicotiana tabacum) lacking mitochondrial alternative oxidase (AOX) have been compared with wild-type (Wt) tobacco using two different systems, either suspension cell cultures or leaves. In both systems, a lack of AOX was accompanied by an increase in some anti-oxidant defenses, consistent with the hypothesis that a lack of AOX increases the mitochondrial generation of reactive oxygen species (ROS). In most cases, this increase in anti-oxidant defenses could more than offset the presumed increased rate of ROS generation, resulting paradoxically in a lower steady-state level of ROS than was found in Wt leaves or suspension cells. We also found that the amount of cell death induced by salicylic acid or nitric oxide correlated strongly with the level of ROS (irrespective of the level of AOX), while death induced by azide was dependent upon the presence or absence of AOX. These results suggest that susceptibility to cell death by signaling molecules (salicylic acid and nitric oxide) is dependent upon the steady-state cellular level of ROS and that AOX levels clearly contribute to this steady state, perhaps by influencing the rate of mitochondrial-generated ROS and hence the cellular level of anti-oxidant defenses.

Certain aspects of uncoupling due to mitochondrial uncoupling proteins in vitro and in vivo

Thermogenic uncoupling has been proven only for UCP1 in brown adipose tissue. All other isoforms of UCPs are potentially acting in suppression of mitochondrial reactive oxygen species (ROS) production. In this contribution we show that BAT mitochondria can be uncoupled by lauric acid in the range of approximately 100 nM when endogenous fatty acids are combusted by carnitine cycle and beta-oxidation is properly separated from the uncoupling effect. Respiration increased up to 3 times when related to the lowest fatty acid content (BSA present plus carnitine cycle). We also illustrated that any effect leading to more coupled states leads to enhanced H2O2 generation and any effect resulting in uncoupling gives reduced H2O2 generation in BAT mitochondria. Finally, we report doubling of plant UCP transcript in cells as well as amount of protein detected by 3H-GTP-binding sites in mitochondria of shoots and roots of maize seedlings subjected to the salt stress.

Uncoupling the relationship between fatty acids and longevity

Research into different species has verified the negative correlation between longevity and the level of reactive oxygen species (ROS) production. ROS creates oxidative damage and, consequently, fuels the aging process. As such, the astonishing longevity of avian species correlates well with their lower levels of ROS production, in comparison to mammals of similar size. Apart from this inter-species difference, caloric restriction (CR) is a widely-documented means of increasing intra-species longevity, and it works by decreasing ROS production. However, little is known about the mechanisms responsible, either for the retardation of aging in CR or for the longevity of long-living species. Recent findings have shown an increase in uncoupling protein (UCP) activity with lower ROS levels, after CR stress. These UCPs are stimulated by fatty acids. Moreover, in numerous studies, fatty acids have been demonstrated to generate a reduction in ROS generation. Thus, the decreased ROS production seen in both CR and longer lifespan may occur via up-regulation of free fatty acid stimulation of UCP activity. Consequently, free fatty acids may play an important regulatory role in longevity, by reducing ROS, via actions on UCPs.

Light-induced decrease in DCF fluorescence of wheat leaves in the presence of salicyl hydroxamate

5-(and-6)-Carboxy-2',7'-dichlorodihydrofluorescein diacetate (DCF-DA), a permeative indicator of oxidative stress, was loaded into dissected leaves of wheat in order to monitor the temporal development of reactive oxygen species. DCF fluorescence was found to be constant under dark conditions. Upon loading the leaves with salicyl hydroxamate, a blocker of the alternative oxidase, DCF fluorescence linearly increased in the dark. This indicates a function of alternative oxidase in preventing reactive oxygen radicals in the mitochondria. Upon illumination, the DCF signal decreased within 5 min. As illuminated chloroplasts would increase the load of reactive oxygen species, the observed decrease cannot be assigned to the production of reactive oxygen species in the chloroplasts. Three different putative mechanisms are considered which all assign an important role to light-induced delivery of NAD(P)H: (1) direct quenching of DCF fluorescence by light-generated NAD(P)H, (2) light-stimulated activation of scavenging enzymes, or (3) redirection of mitochondrial electron fluxes as caused by the delivery of excess redox equivalents (NADH) from the chloroplasts.

Activation of alternative oxidase and uncoupling protein lowers hydrogen peroxide formation in amoebaAcanthamoeba castellaniimitochondria

Mitochondria of amoeba Acanthamoeba castellanii were used to determine the role of two energy-dissipating systems, i.e., a free fatty acid (FFA)-activated, purine nucleotide-inhibited uncoupling protein (AcUCP) and a FFA-insensitive, purine nucleotide-activated ubiquinol alternative oxidase (AcAOX), in decreasing reactive oxygen species production in unicellular organisms. It is shown that the activation of AcUCP by externally added FFA resulted in a strong decrease in H2O2 production, whilst the inhibition of the FFA acid-induced AcUCP activity by GDP or addition of bovine serum albumin (BSA) enhanced production of H2O2. Similarly, the activation of antimycin-resistant AcAOX-mediated respiration by GMP significantly lowered H2O2 production, while inhibition of the oxidase by benzohydroxamate cancelled the GMP-induced effect on H2O2 production. When active together, both energy-dissipating systems revealed a cumulative effect on decreasing H2O2 formation. The results suggest that protection against mitochondrial oxidative stress may be a physiological role of AOX and UCP in unicellulars, such as A. castellanii.

Evidence for the presence of ferritin in plant mitochondria

In this work, evidence for the presence of ferritins in plant mitochondria is supplied. Mitochondria were isolated from etiolated pea stems and Arabidopsis thaliana cell cultures. The proteins were separated by SDS/PAGE. A protein, with an apparent molecular mass of approximately 25-26 kDa (corresponding to that of ferritin), was cross-reacted with an antibody raised against pea seed ferritin. The mitochondrial ferritin from pea stems was also purified by immunoprecipitation. The purified protein was analyzed by MALDI-TOF mass spectrometry and the results of both mass finger print and peptide fragmentation by post source decay assign the polypeptide sequence to the pea ferritin (P < 0.05). The mitochondrial localization of ferritin was also confirmed by immunocytochemistry experiments on isolated mitochondria and cross-sections of pea stem cells. The possible role of ferritin in oxidative stress of plant mitochondria is discussed.

Protein oxidation in plant mitochondria as a stress indicator

Plant mitochondria produce reactive oxygen species (ROS) as an unavoidable side product of aerobic metabolism, but they have mechanisms for regulating this production such as the alternative oxidase. Once produced, ROS can be removed by several different enzyme systems. Finally, should the first two strategies fail, the ROS produced can act as a signal to the rest of the cell and/or cause damage to DNA, lipids and proteins. Proteins are modified in a variety of ways by ROS, some direct, others indirect e.g. by conjugation with breakdown products of fatty acid peroxidation. Reversible oxidation of cysteine and methionine side chains is an important mechanism for regulating enzyme activity. Mitochondria from both mammalian and plant tissues contain a number of oxidised proteins, but the relative abundance of these post-translationally modified forms is as yet unknown, as are the consequences of the modification for the properties and turnover time of the proteins. Specific proteins appear to be particularly vulnerable to oxidative carbonylation in the matrix of plant mitochondria; these include several enzymes of the Krebs cycle, glycine decarboxylase, superoxide dismutase and heat shock proteins. Plant mitochondria contain a number of different proteases, but their role in removing oxidatively damaged proteins is, as yet, unclear.

GmPAP3, a novel purple acid phosphatase-like gene in soybean induced by NaCl stress but not phosphorus deficiency

Purple acid phosphatases (PAPs) are commonly found in plants, but the physiological functions of different classes of PAPs are not thoroughly understood. In the present study, we identified a novel gene, GmPAP3, from salt-stressed soybean using suppression subtractive hybridization (SSH) techniques. Protein sequence alignment studies and phylogenetic analysis strongly suggested that GmPAP3 belongs to the group of plant PAPs and PAP-like proteins that are distinct from those of fungi and animals. In addition, the invariable consensus metal binding residues of PAPs were all conserved in GmPAP3. Surprisingly, analysis of protein sorting signals showed that a putative mitochondrion targeting transit peptide is present on GmPAP3. Northern blot analysis revealed that NaCl stress causes a general induction of GmPAP3 expression in both roots and leaves of various cultivated (Glycine max) and wild (Glycine soja) soybean varieties. Further test using two genetically unrelated cultivated soybean varieties showed that the expression pattern of GmPAP3 is distinct from other PAP genes in soybeans. NaCl stress and oxidative stress but not phosphorus (P) starvation induces the expression of GmPAP3. These results suggest that the physiological role of GmPAP3 might be related to the adaptation of soybean to NaCl stress, possibly through its involvement in reactive oxygen species (ROS) forming and/or scavenging or stress-responding signal transduction pathways.

Mitochondrial functional state in clonal pancreatic beta-cells exposed to free fatty acids

Excessive free fatty acid (FFA) exposure represents a potentially important diabetogenic condition that can impair insulin secretion from pancreatic beta-cells. Because mitochondrial oxidative phosphorylation is a main link between glucose metabolism and insulin secretion, in the present work we investigated the effects of the FFA oleate (OE) on mitochondrial function in the clonal pancreatic beta-cell line, MIN6. Both the long term (72 h) and short term (immediately after application) impact of OE exposure on beta-cells was investigated. After 72 h of exposure to OE (0.4 mm, 0.5% bovine serum albumin) cells were washed and permeabilized, and mitochondrial function (respiration, phosphorylation, membrane potential formation, production of reactive oxygen species) was measured in the absence or presence of OE. MIN6 cells exposed to OE for 72 h showed impaired glucose-stimulated insulin secretion and decreased cellular ATP. Mitochondria in OE-exposed cells retained normal functional characteristics in FFA-free medium; however, they were significantly more sensitive to the acute uncoupling effect of OE treatment. The mitochondria of OE-exposed cells displayed increased depolarization caused by acute OE treatment, which is attributable to the elevation in the FFA-transporting function of uncoupling protein 2 and the dicarboxylate carrier. These cells also had an increased production of reactive oxygen species in complex I of the mitochondrial respiratory chain that could be activated by FFA. A high level of reduction of respiratory complex I augmented acute FFA-induced uncoupling in a way compatible with activation of mitochondrial uncoupling protein by intramitochondrial superoxide. A stronger augmentation was observed in OE-exposed cells. Together, these events may underlie FFA-induced depression of the ATP/ADP ratio in beta-cells, which accounts for the defective glucose-stimulated insulin secretion associated with lipotoxicity.

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

In order to gain a first insight into the effects of reactive oxygen species (ROS) on plant mitochondria, we studied the effect of the ROS producing system consisting of xanthine plus xanthine oxidase on the rate of membrane potential (DeltaPsi) generation due to either succinate or NADH addition to durum wheat mitochondria as monitored by safranin fluorescence. We show that the early ROS production inhibits the succinate-dependent, but not the NADH-dependent, DeltaPsi generation and oxygen uptake. This inhibition appears to depend on the impairment of mitochondrial permeability to succinate. It does not involve mitochondrial thiol groups sensitive to either mersalyl or N-ethylmaleimide and might involve both protein residues and/or membrane lipids, as suggested by the mixed nature. We propose that, during oxidative stress, early generation of ROS can affect plant mitochondria by impairing metabolite transport, thus preventing further substrate oxidation, DeltaPsi generation and consequent large-scale ROS production.

Effects of cold exposure in vivo and uncouplers and recouplers in vitro on potato tuber mitochondria

Effects of cold exposure in vivo and treatment with laurate, carboxyatractylate, atractylate, nucleotides, and BSA in vitro on potato tuber mitochondria have been studied. Cold exposure of tubers for 48-96 h resulted in some uncoupling that could be reversed completely by BSA and partially by ADP, ATP, UDP, carboxyatractylate, and atractylate. UDP was less effective than ADP and ATP, and atractylate was less effective than carboxyatractylate. The recoupling effects of nucleotides were absent when the nucleotides were added after carboxyatractylate. GDP, UDP, and CDP did not recouple mitochondria from either the control or the cold-exposed tubers. This indicates that the cold-induced fatty acid-mediated uncoupling in potato tuber mitochondria is partially due to the operation of the ATP/ADP antiporter. As to the plant uncoupling protein, its contribution to the uncoupling in tuber is negligible or, under the conditions used, somehow desensitized to nucleotides.

Occurrence and characteristics of the mitochondrial permeability transition in plants

The behavior of purified potato mitochondria toward the main effectors of the animal mitochondrial permeability transition has been studied by light scattering, fluorescence, SDS-polyacrylamide gel electrophoresis, and immunoblotting techniques. The addition of Ca(2+) induces a phosphate-dependent swelling that is fully inhibited by cyclosporin A if dithioerythritol is present. Mg(2+) cannot be substituted for Ca(2+) but competes with it. Disruption of the outer membrane and release of several proteins, including cytochrome c, occur upon completion of swelling. Ca(2+)-induced swelling is delayed and its rate is decreased when pH is shifted from 7.4 to 6.6. It is accelerated by diamide, phenylarsine oxide, and linolenic acid. In the absence of Ca(2+), however, linolenic acid (< or =20 microm) rapidly dissipates the succinate-driven membrane potential while having no effect on mitochondrial volume. Anoxic conditions favor in vitro swelling and the concomitant release of cytochrome c and of other proteins in a pH-dependent way. These data indicate that the classical mitochondrial permeability transition occurs also in plants. This may have important implications for our understanding of cell stress and death processes.

Alternative oxidase in durum wheat mitochondria. Activation by pyruvate, hydroxypyruvate and glyoxylate and physiological role

In order to gain a first insight into the alternative oxidase (AO) function in durum wheat mitochondria (DWM), we investigated some activation pathways of this enzyme in DWM purified from both etiolated shoots and green leaves. AO was activated when DWM were added with either pyruvate, known as an AO activator in other plant mitochondria, or alanine plus 2-oxoglutarate, which can generate intramitochondrial pyruvate and glutamate via transamination. In contrast, no AO activity was observed during oxidation of malate plus glutamate or succinate (which can generate malate). In this regard DWM differ from other plant mitochondria. Moreover, DWM were found: (i) to have a very low malic enzyme (ME) activity, (ii) to release oxaloacetate rather than pyruvate during malate oxidation and (iii) to poorly oxidise malate in the absence of glutamate, which removes oxaloacetate via transamination. Therefore, we show that, unlike other plant mitochondria, no pyruvate is generated inside DWM from malate via ME, allowing no AO activity. Other AO activators, alternative to pyruvate, were checked by evaluating the capability of several compounds to induce oxygen uptake and/or electrical membrane potential (Delta Psi) in cyanide-treated DWM. Hydroxypyruvate and glyoxylate, photorespiratory cycle intermediates, were found to be powerful AO activators, capable of inducing a maximal rate of cyanide-insensitive oxygen uptake 1.7 times and 2.3 times higher than pyruvate, respectively. These results suggest that in durum wheat a link may exist between AO activity and photorespiratory metabolism rather than malate metabolism. Moreover, we observed that AO activation resulted in both a partially coupled respiration and a reduction by half of the rate of superoxide anion generation; therefore, AO is expected to work as an antioxidative defence system when the photorespiratory cycle is highly active, as under environmental stress.

The expression of alternative oxidase and uncoupling protein during fruit ripening in mango

The expression of alternative oxidase (Aox) and uncoupling proteins (Ucp) was investigated during ripening in mango (Mangifera indica) and compared with the expression of peroxisomal thiolase, a previously described ripening marker in mango. The multigene family for the Aox in mango was expressed differentially during ripening. Abundance of Aox message and protein both peaked at the ripe stage. Expression of the single gene for the Ucp peaked at the turning stage and the protein abundance peaked at the ripe stage. Proteins of the cytochrome chain peaked at the mature stage of ripening. The pattern of protein accumulation suggested that increases in cytochrome chain components played an important role in facilitating the climacteric burst of respiration and that the Aox and Ucp may play a role in post-climacteric senescent processes. Because both message and protein for the Aox and Ucp increased in a similar pattern, it suggests that their expression is not controlled in a reciprocal manner but may be active simultaneously.

PLANT MITOCHONDRIA AND OXIDATIVE STRESS: Electron Transport, NADPH Turnover, and Metabolism of Reactive Oxygen Species

The production of reactive oxygen species (ROS), such as O2- and H2O2, is an unavoidable consequence of aerobic metabolism. In plant cells the mitochondrial electron transport chain (ETC) is a major site of ROS production. In addition to complexes I-IV, the plant mitochondrial ETC contains a non-proton-pumping alternative oxidase as well as two rotenone-insensitive, non-proton-pumping NAD(P)H dehydrogenases on each side of the inner membrane: NDex on the outer surface and NDin on the inner surface. Because of their dependence on Ca2+, the two NDex may be active only when the plant cell is stressed. Complex I is the main enzyme oxidizing NADH under normal conditions and is also a major site of ROS production, together with complex III. The alternative oxidase and possibly NDin(NADH) function to limit mitochondrial ROS production by keeping the ETC relatively oxidized. Several enzymes are found in the matrix that, together with small antioxidants such as glutathione, help remove ROS. The antioxidants are kept in a reduced state by matrix NADPH produced by NADP-isocitrate dehydrogenase and non-proton-pumping transhydrogenase activities. When these defenses are overwhelmed, as occurs during both biotic and abiotic stress, the mitochondria are damaged by oxidative stress.