The Respiratory Chain of Plant Mitochondria: VI. Flavoprotein Components of the Respiratory Chain of Mung Bean Mitochondria

Redox changes of the flavoproteins of mung bean (Phaseolus aureus) mitochondria were measured by differential absorbance at 468 to 493 nanometers and by fluorescence emission above 500 nanometers excited at 436 nanometers. Four flavoproteins are distinguishable by the ratio of their fluorescence to absorbance changes, and by their requirement, or lack of it, for energy-linked reverse electron transport for reduction by succinate. Two flavoproteins are reduced by succinate in fully depleted mitochondria which lack the capacity for reverse electron transport. These are designated Fp_ha and Fp_hf and have fluorescence to absorbance ratios of 0 and 1.4, respectively. The two flavoproteins have the same half-time for oxidation, but Fp_hf is reduced more slowly than Fp_ha by substrate in the presence of cyanide. One flavoprotein with a fluorescence to absorbance ratio of 0 is not reduced by succinate in anaerobic, fully depleted mitochondria, but is rapidly reduced on subsequent addition of malate; it is designated Fp_m. The fourth distinguishable flavoprotein component is reducible by succinate in an energy-linked reaction, even in partially depleted mitochondria. This component has a fluorescence to absorbance ratio of 3.8 and is designated Fp_1f. In addition to these four flavoproteins reducible by substrates, there is a highly fluorescent flavin-containing component in or associated with these mitochondria, which is rapidly reduced by dithionite.     

The Respiratory Chain of Plant Mitochondria: XVII. Flavoprotein-Cytochrome b(562) Interaction in Antimycin-treated Skunk Cabbage Mitochondria

During the transition from the aerobic steady state with succinate as substrate to anaerobiosis, in suspensions of skunk cabbage (Symplocarpus foetidus) mitochondria treated with antimycin A, cytochrome b(562) becomes reoxidized to the extent of about 20%, synchronously with the reduction of cytochrome c(549). This reoxidation occurs in both the absence and presence of m-chlorobenzhydroxamic acid, a specific inhibitor for the alternate terminal oxidase of plant mitochondria. A flavoprotein component, amounting to 13% to 15% of the total nonfluorescent mitochondrial flavoprotein, undergoes reduction synchronously with the oxidation of cytochrome b(562) during the aerobic to anaerobic transition with succinate as substrate in the presence of both antimycin A and m-chlorobenzhydroxamic acid. This flavoprotein component remains reduced in the presence of cyanide. The half-time for reduction of the flavoprotein component and cytochrome c(549) and for oxidation of cytochrome b(562) during the aerobic to anaerobic transition with succinate as substrate in the presence of both antimycin A and m-chlorobenzhydroxamic acid is 2 seconds. The half-times for oxidation of cytochrome c(549) and the flavoprotein component are 2.1 and 170 milliseconds, respectively, during the anaerobic to aerobic transition induced by addition of 14 mum O(2) to the mitochondrial suspensions. The half-time for reduction of cytochrome b(562) under these conditions is 150 milliseconds, synchronous with the flavoprotein component. The synchrony of the flavoprotein oxidation and of the cytochrome b(562) reduction at a rate much slower than that of cytochrome c(549) oxidation implies that, in antimycin-treated plant mitochondria, the state of the cytochrome b(562)/antimycin complex is regulated by the redox state of this flavoprotein component, rather than by cytochrome c(549). It is tentatively suggested that these two components are not part of the main sequence of the respiratory chain, but may be part of a multienzyme complex active in the hydroxylation reactions required for ubiquinone biosynthesis in the inner mitochondrial membrane.     

The Respiratory Chain of Plant Mitochondria: VIII. Reduction Kinetics of the Respiratory Chain Carriers of Mung Bean Mitochondria with Reduced Nicotinamide Adenine Dinucleotide

Addition of 90 micromolar reduced nicotinamide adenine dinucleotide (NADH) in the presence of cyanide to a suspension of aerobic mung bean (Phaseolus aureus) mitochondria depleted with ADP and uncoupler gives a cycle of reduction of electron transport carriers followed by reoxidation, as NADH is oxidized to NAD⁺ through the cyanide-insensitive, alternate oxidase by excess oxygen in the reaction medium. Under these conditions, cytochrome b₅₅₃ and the nonfluorescent, high potential flavoprotein Fp_ha of the plant respiratory chain become completely reduced with half-times of 2.5 to 2.8 seconds for both components. Reoxidation of flavoprotein Fp_ha on exhaustion of NADH is more rapid than that of cytochrome b₅₅₃. There is a lag of 1.5 seconds after NADH addition before any reduction of ubiquinone can be observed, whereas there is no lag perceptible in the reduction of flavoprotein Fp_ha and cytochrome b₅₅₃. The half-time for ubiquinone reduction is 4.5 seconds, and the extent of reduction is 90% or greater. About 30% of cytochrome b₅₅₇ is reduced under these conditions with a half-time of 10 seconds; both cytochrome b₅₆₂ and the fluorescent, high potential flavoprotein Fp_hf show little, if any, reduction. The two cytochromes c in these mitochondria, c₅₄₇ and c₅₄₉, are reduced in synchrony with a half-time of 0.8 second. These two components are already 60% reduced in the presence of cyanide but absence of substrate, and they become completely reduced on addition of NADH. These results indicated that reducing equivalents enter the respiratory chain from exogenous NADH at flavoprotein Fp_ha and are rapidly transported through cytochrome b₅₅₃ to the cytochromes c; once the latter are completely reduced, reduction of ubiquinone begins. Ubiquinone appears to act as a storage pool for reducing equivalents entering the respiratory chain on the substrate side of coupling site 2. It is suggested that flavoprotein Fp_ha and cytochrome b₅₅₃ together may act as the branching point in the plant respiratory chain from which forward electron transport can take place to oxygen through the cytochrome chain via cytochrome oxidase, or to oxygen through the alternate, cyanide-insensitive oxidase via the fluorescent, high potential flavoprotein Fp_hf.     

The Respiratory Chain of Plant Mitochondria: XVI. Interaction of Cytochrome b(562) with the Respiratory Chain of Coupled and Uncoupled Mung Bean Mitochondria: Evidence for Its Exclusion from the Main Sequence of the Chain

Cytochromes b₅₅₃, b₅₅₇, and b₅₆₂ of mung bean (Phaseolus aureus) mitochondria become partially reduced with endogenous substrate on addition of antimycin A to the aerobic mitochondrial suspension. Addition of ATP causes partial reoxidation of the three cytochromes. This partial oxidation by ATP is inhibited by oligomycin and reversed by uncoupler. Ubiquinone does not appear to act as electron acceptor for the oxidation reaction, but a nonfluorescent flavoprotein, or possibly ironsulfur protein, component does appear to act as acceptor. This is consistent with reverse electron transport driven by ATP across the first site of energy conservation of the respiratory chain. Endogenous pyridine nucleotide and the fluorescent flavoprotein with E_m7.2 = −155mv (midpoint potential at pH 7.2, referred to normal hydrogen electrode) in uncoupled mitochondria become reduced in anaerobiosis attained by oxidation of succinate in the absence of respiratory inhibitors of the cytochrome chain, provided that Pi and ATP are present. Under these same conditions, cytochrome b₅₅₇ is completely reduced but cytochrome b₅₆₂ remains nearly completely oxidized. There is no equilibration across the first site of energy conservation between the carriers on the low potential side and cytochrome b₅₆₂ with E_m7.2 = −77mv on the high potential side. It is concluded that cytochrome b₅₆₂ is not a part of the main sequence of electron transport carriers of the mitochondrial respiratory chain of plants; it can participate in redox reactions with the respiratory chain in coupled mitochondria but not in uncoupled mitochondria unless antimycin A is present.     

The Respiratory Chain of Plant Mitochondria. II. Oxidative Phosphorylation in Skunk Cabbage Mitochondria

Mitochondria were prepared from the spadices of skunk cabbage (Symplocarpus foetidus) whose respiratory rate with succinate and malate showed 15% to 30% sensitivity to cyanide inhibition, and which showed respiratory control by added ADP. The observed respiratory control ratios ranged from 1.1 to 1.4. The change in pH of the mitochondrial suspension was recorded simultaneously with oxygen uptake: alkalinization of the medium, expected for phosphorylation of ADP, coincided with the period of acceleration in oxygen uptake caused by addition of an ADP aliquot. The ADP/O ratios obtained were 1.3 for succinate and 1.9 for malate. In the presence of 0.3 mm cyanide, the ADP/O ratio for succinate was zero, while that for malate was 0.7. These results are consistent with the existence of an alternate oxidase which interacts with the flavoprotein and pyridine nucleotide components of the respiratory chain and which, in the presence of cyanide, allows the first phosphorylation site to function with an efficiency of about 70%. In the absence of respiratory inhibitors, the efficiency of each phosphorylation site is also about 70%. This result implies that diversion of reducing equivalents through the alternate oxidase, thereby bypassing the 2 phosphorylation sites associated with the cytochrome components of these mitochondria, occurs to a negligible extent during the oxidative phosphorylation of ADP or State 3.Addition of ADP or uncoupler to skunk cabbage mitochondria respiring in the controlled state or State 4, results in reduction of cytochrome c and the oxidation of the cytochromes b, ubiquinone and pyridine nucleotide. A site of interaction of ADP with the respiratory chain between cytochromes b and cytochrome c is thereby identified by means of the crossover theorem. Flavoprotein measured by fluorescence is also oxidized upon addition of ADP or uncoupler, but flavoprotein measured by optical absorbance changes becomes more reduced under these conditions. Depletion of the mitochondria by pretreatment with ADP and uncoupler prevents reduction of most of the fluorescent flavoprotein by succinate. These results indicate that skunk cabbage mitochondria contain both high and low potential flavo-proteins characterized by different fluorescence/absorbance ratios similar to those demonstrated to be part of the respiratory chain in mitochondria from animal tissues.     

The Respiratory Chain of Plant Mitochondria: VII. Kinetics of Flavoprotein Oxidation in Skunk Cabbage Mitochondria

The oxidation kinetics of the two high potential flavo-proteins, one (Fp_hf) fluorescent and the other (Fp_ha) nonfluorescent, in mitochondria from skunk cabbage (Symplocarpus foetidus) spadices have been measured by combined spectrophotometry and fluorimetry. In the absence of respiratory inhibitors, both flavoproteins are oxidized at nearly the same rate with half-times between 120 and 160 milliseconds at 24 C. When slight differences in rate are observed, it is Fp_ha which consistently has the shorter half-time. The presence of 0.3 millimolar KCN has no perceptible effect on the oxidation rate of either component. Antimycin A (2 nanomoles per milligram of protein) increases the oxidation half-time of Fp_ha about 3-fold, but it has no effect on the oxidation half-time of Fp_hf. In contrast to these two inhibitors, m-chlorobenzhydroxamic acid—an inhibitor specific to the cyanide insensitive, alternate oxidase pathway in these mitochondria—increases the oxidation half-time of Fp_hf 10-fold to about 2 seconds, while increasing that of Fp_ha only some 20%. This result implies that the flavoprotein Fp_hf mediates electron transport to the alternate oxidase from the region of the mitochondrial respiratory chain encompassing Fp_ha, ubiquinone, and the cytochromes b. The oxidation rate of cytochrome b₅₅₇ is unaffected by either m-chlorobenzhydroxamic acid or cyanide but is strongly inhibited by antimycin A. This result implies that cytochrome b₅₅₇ plays no direct role in the respiratory pathway to the alternate oxidase and is different from cytochrome b₇ found in mitochondria from the spadices of Arum maculatum.     

The Respiratory Chain of Plant Mitochondria: XIV. Ordering of Ubiquinone, Flavoproteins, and Cytochromes in the Respiratory Chain

The effect of initial oxygen concentration on the rate and extent of oxidation of the respiratory chain carriers of anaerobic mitochondria from mung bean (Phaseolus aureus) seedlings was examined. The substrate was succinate, with malonate added to give malonate to succinate ratios of 6 to 12, thereby minimizing the flow of reducing equivalents from substrate and insuring maximal extent of oxidation of the carriers. The ratio of oxidizing equivalents available from oxygen to reducing equivalents available from reduced ubiquinone, designated the equivalents ratio, varied from 30 to 1. Cytochromes aa₃ and c₅₄₇ have unaltered oxidation half-times, designated t_{½ on}, as the equivalents ratio is reduced from 30 to 3, and the extent of oxidation is decreased by about 25%. The time of the oxidation-reduction cycle induced by the oxygen pulse, calculated from the point of half oxidation to that of half reduction and designated t_{½ off}, decreases 200 fold with this reduction in equivalents ratio. The oxidation half-time, t_{½ on}, for ubiquinone is unaltered by decreasing the equivalents ratio from 6 to 1; the value of t_{½ off} decreases only 30% while the extent of oxidation decreases 50%. The values of t_{½ on} and t_{½ off} and the extent of oxidation of cytochrome b₅₅₃ and flavoprotein Fp_ha were all much reduced at low equivalents ratios. The results, plus results from previous studies, indicate that there is the following linear sequence of components in the plant respiratory chain:     

The Respiratory Chain of Plant Mitochondria: IV. Oxidation Rates of the Respiratory Carriers of Mung Bean Mitochondria in the Presence of Cyanide

The half-time for oxidation of cytochrome b(557) in mitochondria from etiolated mung bean (Phaseolus aureus) hypocotyls is 5.8 milliseconds at 24 Celsius in the absence or presence of 0.3 mm KCN, when the oxidation is carried out by injecting a small amount of oxygenated medium into a suspension of mitochondria made anaerobic in the presence of succinate plus malonate. Since oxygen is consumed by the alternate, cyanide-insensitive respiratory pathway of these mitochondria, cycles of oxidation and reduction can be obtained with the oxygen pulses when cyanide is present. Reduced cytochromes (a + a(3)) also become oxidized at nearly the uninhibited rate under these conditions, a(3) completely and a partially. The half-time for oxidation of c(547) is also unaffected by 0.3 mm KCN, but c(549) has a half-time equal to that of c(547) in the presence of KCN, compared to the shorter one observed in the absence of inhibitor. The maximum extent of oxidation of the cytochromes c is about 70% in the presence of 0.3 mm KCN; this oxidation is rapidly followed by an extensive reduction which is synchronous with the reduction of cytochrome a observed under the same conditions. In the presence of cyanide, it appears likely that the cytochromes c and b(557) are oxidized by cytochrome oxidase in oxygen pulse experiments, rather than by the alternate oxidase. The oxidation of cytochrome b(553) is partially inhibited by KCN, but complete oxidation is attained in the aerobic steady state with excess oxygen. If the oxygen pulse experiment is carried out in the presence of sufficient malonate so that entry of reducing equivalents into the respiratory chain occurs at a rate negligible compared to inter-carrier electron transport, the half-time for flavoprotein oxidation is unaffected by 0.3 mm KCN while that for ubiquinone oxidation is but 2-fold larger. The observed net oxidation rate of these two carriers in mung bean mitochondria is more sensitive to the entry rate of reducing equivalents, as set by succinate concentration and malonate to succinate ratio, then it is in skunk cabbage (Symplocarpus foetidus) mitochondria. These observations are interpreted in terms of a respiratory carrier Y, placed between flavoprotein plus ubiquinone and the cytochromes, which is the fork in the split respiratory pathway to the two terminal oxidases and which has lower electron transport capacity in mung bean mitochondria than in skunk cabbage mitochondria.     

The Respiratory Chain of Plant Mitochondria. I. Electron Transport Between Succinate and Oxygen in Skunk Cabbage Mitochondria

The kinetics of oxidation of ubiquinone, flavoprotein, cytochrome c, and the cytochrome b complex in skunk cabbage (Symplocarpus foetidus) mitochondria made anaerobic with succinate have been measured spectrophotometrically and fluorimetrically in the absence of respiratory inhibitor and in the presence of cyanide or antimycin A. No component identifiable by these means was oxidized rapidly enough in the presence of one or the other inhibitor to qualify for the role of alternate oxidase. Cycles of oxidation and rereduction of flavoprotein and ubiquinone obtained by injecting 12 mum oxygen into the anaerobic mitochondrial suspension were kinetically indistinguishable in the presence of cyanide or antimycin A, implying that these 2 components are part of a respiratory pathway between succinate and oxygen which does not involve the cytochromes and does involve a cyanide-insensitive alternate oxidase. The cytochrome b complex shows biphasic oxidation kinetics with half times of 0.018 sec and 0.4 sec in the absence of inhibitor, which increase to 0.2 sec and 1 sec in the presence of cyanide. In the presence of antimycin A, the oxidation of the cytochrome b complex shows an induction period of 1 sec and a half-time of 3.5 sec. A split respiratory chain with 2 terminal oxidases and a branch point between the cytochromes and flavoprotein and ubiquinone is proposed for these mitochondria.     

The Respiratory Chain of Plant Mitochondria: XI. Electron Transport from Succinate to Endogenous Pyridine Nucleotide in Mung Bean Mitochondria

Energy-linked reverse electron transport from succinate to endogenous NAD in tightly coupled mung bean (Phaseolus aureus) mitochondria may be driven by ATP if the two terminal oxidases of these mitochondria are inhibited, or may be driven by the free energy of succinate oxidation. This reaction is specific to the first site of energy conservation of the respiratory chain; it does not occur in the presence of uncoupler. If mung bean mitochondria become anaerobic during oxidation of succinate, their endogenous NAD becomes reduced in the presence of uncoupler, provided that both inorganic phosphate (P_i) and ATP are present. No reduction occurs in the absence of P_i, even in the presence of ATP added to provide a high phosphate potential. If fluorooxaloacetate is present in the uncoupled, aerobic steady state, no reduction of endogenous NAD occurs on anaerobiosis; this compound is an inhibitor of malate dehydrogenase. This result implies that endogenous NAD is reduced by malate formed from the fumarate generated during succinate oxidation. The source of free energy is most probably the endogenous energy stores in the form of acetyl CoA, or intermediates convertible to acetyl CoA, which removes the oxaloacetate formed from malate, thus driving the reaction towards reduction of NAD.     

Compartmentation of Alternative Oxidase in Plant Mitochondria

Capacity of alternative pathway mediated, CN-resistant respirationwas measured in mitochondria isolated from three plant species:Iris bulbs, potato tuber callus and Petunia cells, grown insuspension culture. Succinate, NADH or a combination of succinateand NADH were used as respiratory substrates. In all three plantspecies electrons from exogenous NADH appeared to have no accessto the alternative pathway connected with succinate mediatedrespiration and vice versa. Therefore, besides a (functional)compartmentation of Q-pools a compartmentation of alternativepathways in plant mitochondria is proposed as well. (Received October 19, 1985; Accepted January 29, 1986)     

The Respiratory Chain of Plant Mitochondria: V. Reaction of Reduced Cytochromes a and a3 in Mung Bean Mitochondria with Oxygen in the Presence of Cyanide

The half-times of oxidation by oxygen pulses of reduced cytochromes a and a(3) in mung bean mitochondria made anaerobic with succinate have been measured by means of a rapid mixing flow apparatus coupled to a dual wave length spectrophotometer in the presence and absence of cyanide. The absorbance changes at 438 to 455 millimicrons and 603 to 620 millimicrons are suitable for recording the time course of cytochrome a oxidation; the half-time is 2.0 milliseconds at 24 Celsius. This half-time does not change over the range 0 to 300 mum KCN, but the fraction of cytochrome a oxidized falls to a limiting value of 0.3 at the higher cyanide concentrations. The absorbance changes at 445 to 455 millimicrons record the time course of both cytochrome a and cytochrome a(3) oxidation; the former contributes 60% of the absorbance change and the latter 40%. The half-time for a(3) oxidation is calculated as 0.9 milliseconds at 24 Celsius. This half-time increases slightly to 1.3 milliseconds at 300 mum KCN. Reduced cytochrome a(3), whether uncomplexed or complexed with cyanide, becomes fully oxidized. The dissociation constant for the reduced cytochrome a(3)-cyanide complex is estimated to be 30 mum, whereas that for the oxidized a(3)-cyanide complex which inhibits electron transport is estimated to be 2 mum. This suggests two different binding sites for cyanide on the reduced and oxidized forms of cytochrome a(3). The fact that a limiting fraction of reduced cytochrome a can be oxidized at high cyanide concentrations implies that there is no interference by cyanide with electron transport from a to a(3), if cyanide remains bound to the site it occupies on reduced a(3) after this carrier becomes oxidized on reaction with molecular oxygen. Rearrangement of cyanide from this noninhibitory site to the inhibitory site occurs rapidly enough to compete with cytochrome a oxidation. The half-time for the rearrangement is calculated to be 0.9 milliseconds.     

The Respiratory Chain of Chlorella protothecoides: II. Isolation and Characterization of Mitochondria

Mitochondria were isolated from glucose-bleached Chlorella protothecoides Krüger, Indiana strain 25. These mitochondria oxidized succinate, NADH, and l-malate at high rates. Oxygen uptake with these substrates was partially inhibited by 1 mmm-chlorobenzhydroxamic acid (mCLAM). Respiratory control was seen with succinate as substrate in the presence of mCLAM. The apparent Km for succinate was determined to be 0.83 mm. Chlorella mitochondria catalyzed the oxidation of reduced horse heart cytochrome c.     

The Respiratory Chain of Plant Mitochondria: XII. Some Aspects of the Energy-linked Reverse Electron Transport from the Cytochromes c to the Cytochromes b in Mung Bean Mitochondria

The cytochromes c of mung bean (Phaseolus aureus) mitochondria become reduced when sulfide, a cytochrome oxidase inhibitor free from uncoupling side effects, is added to the aerobic mitochondrial suspension in the absence of added substrate. The cytochromes b remain largely oxidized. Subsequent addition of ATP results in partial oxidation of the cytochromes c and partial reduction of the cytochromes b due to ATP-driven reverse electron transport through the second site of energy conservation, or coupling site, of the respiratory chain. Cytochrome a is also oxidized under these conditions, but there is no concomitant reduction of the flavoprotein components, of ubiquinone, or of endogenous pyridine nucleotide. The reaction is abolished by oligomycin. The reducing equivalents transported from the cytochromes c and a in ATP-driven reverse electron transport are about 2-fold greater than those which appear in the cytochromes b. It is suggested that the equivalents not accounted for are present in a coupling site enzyme at the second site of energy conservation which interacts with the respiratory chain carriers by means of the dithiol-disulfide couple; this couple would not show absorbance changes with redox state over the wavelength range examined. With succinate present, reverse electron transport can be demonstrated at both coupling sites in both the aerobic steady state and in anaerobiosis. ATP-driven reverse electron transport in anaerobiosis maintains cytochrome a 30% oxidized while endogenous pyridine nucleotide is 50% reduced.     

Properties of Higher Plant Mitochondria. III. Effects of Respiratory Inhibitors

The effects of representative respiratory inhibitors were investigated on the coupled respiration of mung bean mitochondria using succinate and l-malate as substrates. The inhibitors studied were: (I) malonate, (II) amytal and rotenone, (III) antimycin A and 2-n-nonyl-4-hydroxyquinoline N-oxide (NOQNO), and (IV) cyanide and azide.     

Specificity of the Organic Acid Activation of Alternative Oxidase in Plant Mitochondria

The claim that succinate and malate can directly stimulate the activity of the alternative oxidase in plant mitochondria (A.M. Wagner, C.W.M. van den Bergen, H. Wincencjusz [1995] Plant Physiol 108: 1035-1042) was reinvestigated using sweet potato (Ipomoea batatas L.) mitochondria. In whole mitochondria, succinate (in the presence of malonate) and both L- and D-malate stimulated respiration via alternative oxidase in a pH- (and NAD+)-dependent manner. Solubilized malic enzyme catalyzed the oxidation of both L- and D-malate, although the latter at only a low rate and only at acid pH. In submitochondrial particle preparations with negligible malic enzyme activity, neither L- nor D-malate stimulated alternative oxidase activity. However, even in the presence of high malonate concentrations, some succinate oxidation was observed via the alternative oxidase, giving the impression of stimulation of the oxidase. Neither L-malate nor succinate (in the presence of malonate) changed the dependence of alternative oxidase activity on ubiquinone reduction state in submitochondrial particles. In contrast, a large change in this dependence was observed upon addition of pyruvate. Half-maximal stimulation of alternative oxidase by pyruvate occurred at less than 5 [mu]M in submitochondrial particles, one-twentieth of that reported for whole mitochondria, suggesting that pyruvate acts on the inside of the mitochondrion. We suggest that malate and succinate do not directly stimulate alternative oxidase, and that reports to the contrary reflect intra-mitochondrial generation of pyruvate via malic enzyme.     

Signals Regulating the Expression of the Nuclear Gene Encoding Alternative Oxidase of Plant Mitochondria

Suspension cells of tobacco (Nicotiana tabacum L. cv Bright Yellow) were used to investigate signals regulating the expression of the nuclear gene Aox1 encoding the mitochondrial alternative oxidase (AOX) protein responsible for cyanide-resistant respiration in plants. We found that an increase in the tricarboxylic acid cycle intermediate citrate (either after its exogenous supply to cells or after inhibition of aconitase by monofluoroacetate) caused a rapid and dramatic increase in the steady-state level of Aox1 mRNA and AOX protein. This led to a large increase in the capacity for AOX respiration, defined as the amount of salicylhydroxamic acid-sensitive O2 uptake by cells in the presence of potassium cyanide. The results indicate that citrate may be an important signal metabolite regulating Aox1 gene expression. A number of other treatments were also identified that rapidly induced the level of Aox1 mRNA and AOX capacity. These included short-term incubation of cells with 10 mM acetate, 2 [mu]M antimycin A, 5 mM H2O2, or 1 mM cysteine. For some of these treatments, induction of AOX occurred without an increase in cellular citrate level, indicating that other signals (possibly related to oxidative stress conditions) are also important in regulating Aox1 gene expression. The signals influencing Aox1 gene expression are discussed with regard to the potential function(s) of AOX to modulate tricarboxylic acid cycle metabolism and/or to prevent the generation of active oxygen species by the mitochondrial electron transport chain.