On the role of plant mitochondrial metabolism and its impact on photosynthesis in both optimal and sub-optimal growth conditions

Given that the pathways of photosynthesis and respiration catalyze partially opposing processes, it follows that their relative activities must be carefully regulated within plant cells. Recent evidence has shown that the components of the mitochondrial electron transport chain are essential for the proper maintenance of intracellular redox gradients, to allow considerable rates of photorespiration and in turn efficient photosynthesis. Thus considerable advances have been made in understanding the interaction between respiration and photosynthesis during the last decades and the potential mechanisms linking mitochondrial function and photosynthetic efficiency will be reviewed. Despite the fact that manipulation of various steps of mitochondrial metabolism has been demonstrated to alter photosynthesis under optimal growth conditions, it is likely that these changes will, by and large, not be maintained under sub-optimal situations. Therefore producing plants to meet this aim remains a critical challenge. It is clear, however, that although there have been a range of studies analysing changes in respiratory and photosynthetic rates in response to light, temperature and CO₂, our knowledge of the environmental impact on these processes and its linkage still remains fragmented. We will also discuss the metabolic changes associated to plant respiration and photosynthesis as important components of the survival strategy as they considerably extend the period that a plant can withstand to a stress situation.     

植物线粒体呼吸状态的研究方法及其在植物生物学中的应用

线粒体呼吸状态是指当线粒体中呼吸底物及ADP以不同浓度存在时线粒体的呼吸速率,它是用来研究线粒体功能的重要指标。本文结合我们的研究经验,详细阐述了线粒体呼吸状态的研究方法,介绍了该方法在分析线粒体电子传递链功能和氧化磷酸化活性中的应用,此外概述了线粒体呼吸状态的测定和分析方法在植物生理生态研究中的具体应用。     

Mitochondrial respiration in blood platelets of depressive patients

Recent evidences include mitochondrial dysfunctions in pathophysiology of mood disorders. We examined association between depressive disorders and mitochondrial respiration using both intact and permeabilized blood platelets. In intact platelets, physiological respiration, maximal capacity of electron transport system and respiratory rate after complex I inhibition were decreased in depressive patients, who reached partial remission, compared to healthy controls. Respiratory rates were unchanged in several respiratory states in permeabilized platelets. Results indicate that changes in respiratory rate in intact platelets can be used as biological marker of depressive disorder. The hypothesis that decreased mitochondrial respiratory rate participate in pathophysiology of depression was supported.     

Mitochondrial fatty acid synthesis is required for normal mitochondrial morphology and function in Trypanosoma brucei

Trypanosoma brucei use microsomal elongases for de novo synthesis of most of its fatty acids. In addition, this parasite utilizes an essential mitochondrial type II synthase for production of octanoate (a lipoic acid precursor) as well as longer fatty acids such as palmitate. Evidence from other organisms suggests that mitochondrially synthesized fatty acids are required for efficient respiration but the exact relationship remains unclear. In procyclic form trypanosomes, we also found that RNAi depletion of the mitochondrial acyl carrier protein, an important component of the fatty acid synthesis machinery, significantly reduces cytochrome-mediated respiration. This reduction was explained by RNAi-mediated inhibition of respiratory complexes II, III and IV, but not complex I. Other effects of RNAi, such as changes in mitochondrial morphology and alterations in membrane potential, raised the possibility of a change in mitochondrial membrane composition. Using mass spectrometry, we observed a decrease in total and mitochondrial phosphatidylinositol and mitochondrial phosphatidylethanolamine. Thus, we conclude that the mitochondrial synthase produces fatty acids needed for maintaining local phospholipid levels that are required for activity of respiratory complexes and preservation of mitochondrial morphology and function.     

Regulation of thermogenesis in flowering Araceae: the role of the alternative oxidase

The inflorescences of several members of the Arum lily family warm up during flowering and are able to maintain their temperature at a constant level, relatively independent of the ambient temperature. The heat is generated via a mitochondrial respiratory pathway that is distinct from the cytochrome chain and involves a cyanide-resistant alternative oxidase (AOX). In this paper we have used flux control analysis to investigate the influence of temperature on the rate of respiration through both cytochrome and alternative oxidases in mitochondria isolated from the appendices of intact thermogenic Arum maculatum inflorescences. Results are presented which indicate that at low temperatures, the dehydrogenases are almost in full control of respiration but as the temperature increases flux control shifts to the AOX. On the basis of these results a simple model of thermoregulation is presented that is applicable to all species of thermogenic plants. The model takes into account the temperature characteristics of the separate components of the plant mitochondrial respiratory chain and the control of each process. We propose that 1) in all aroid flowers AOX assumes almost complete control over respiration, 2) the temperature profile of AOX explains the reversed relationship between ambient temperature and respiration in thermoregulating Arum flowers, 3) the thermoregulation process is the same in all species and 4) variations in inflorescence temperatures can easily be explained by variations in AOX protein concentrations.     

The Influence of Mitochondrial Concentration and Storage on the Respiratory Control of Isolated Plant Mitochondria

The respiration of mitochondria isolated from various plant tissues was studied over a range of mitochondrial concentrations and at various times after isolation. Respiration at 25 C expressed as nanomoles of O₂ per minute per milligram of protein was constant for mitochondrial concentrations higher than some critical amount, usually 0.25 to 1.0 milligram of protein per reaction. Below this concentration the state 3 respiration rate declined and the mitochondria appeared to lose respiratory control. The respiration of isolated mitochondria stored in ice but measured at 25 C generally declined over long time periods although mitochondria from some tissues showed an initial increase. The results indicate that valid comparisons of the respiratory activity of mitochondria isolated from different tissues or from different parts of the same tissue cannot be made at least until the influence of the above factors has been evaluated.     

Respiration in postharvest sugarbeet roots is not limited by respiratory capacity or adenylates

Control of respiration has largely been studied with growing and/or photosynthetic tissues or organs, but has rarely been examined in harvested and stored plant products. As nongrowing, heterotrophic organs that are reliant on respiration to provide all of their metabolic needs, harvested plant products differ dramatically in their metabolism and respiratory needs from growing and photosynthetically active plant organs, and it cannot be assumed that the same mechanism controls respiration in both actively growing and harvested plant organs. To elucidate mechanisms of respiratory control for a harvested and stored plant product, sugarbeet (Beta vulgaris L.) root respiration was characterized with respect to respiratory capacity, adenylate levels and cellular energy status in roots whose respiration was altered by wounding or cold treatment (1 degrees C) and in response to potential effectors of respiration. Respiration rate was induced by wounding in roots stored at 10 degrees C and by cold temperature in roots stored at 1 degrees C for 11-13d. Alterations in respiration rate due to wounding or storage temperature were unrelated to changes in total respiratory capacity, the capacities of the cytochrome c oxidase (COX) or alternative oxidase (AOX) pathways, adenylate concentrations or cellular energy status, measured by the ATP:ADP ratio. In root tissue, respiration was induced by exogenous NADH indicating that respiratory capacity was capable of oxidizing additional electrons fed into the electron transport chain via an external NADH dehydrogenase. Respiration was not induced by addition of ADP or a respiratory uncoupler. These results suggest that respiration rate in stored sugarbeet roots is not limited by respiratory capacity, ADP availability or cellular energy status. Since respiration in plants can be regulated by substrate availability, respiratory capacity or energy status, it is likely that a substrate, other than ADP, limits respiration in stored sugarbeet roots.     

Simultaneous evaluation of substrate-dependent oxygen consumption rates and mitochondrial membrane potential by TMRM and safranin in cortical mitochondria

Mitochondrial membrane potential (mtMP) is critical for maintaining the physiological function of the respiratory chain to generate ATP. The present study characterized the inter-relationship between mtMP, using safranin and tetramethyl rhodamine methyl ester (TMRM), and mitochondrial respiratory activity and established a protocol for functional analysis of mitochondrial bioenergetics in a multi-sensor system. Coupled respiration was decreased by 27 and 30–35% in the presence of TMRM and safranin respectively. Maximal respiration was higher than coupled with Complex I- and II-linked substrates in the presence of both dyes. Safranin showed decreased maximal respiration at a higher concentration of carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone (FCCP) compared with TMRM. FCCP titration revealed that maximal respiration in the presence of glutamate and malate was not sustainable at higher FCCP concentrations as compared with pyruvate and malate. Oxygen consumption rate (OCR) and mtMP in response to mitochondrial substrates were higher in isolated mitochondria compared with tissue homogenates. Safranin exhibited higher sensitivity to changes in mtMP than TMRM. This multi-sensor system measured mitochondrial parameters in the brain of transgenic mice that model Alzheimer''s disease (AD), because mitochondrial dysfunction is believed to be a primary event in the pathogenesis of AD. The coupled and maximal respiration of electron transport chain were decreased in the cortex of AD mice along with the mtMP compared with age-matched controls. Overall, these data demonstrate that safranin and TMRM are suitable for the simultaneous evaluation of mtMP and respiratory chain activity using isolated mitochondria and tissue homogenate. However, certain care should be taken concerning the selection of appropriate substrates and dyes for specific experimental circumstances.     

Effect of (+)-pulegone and other oil components of Mentha x Piperita on cucumber respiration

Peppermint (Mentha x piperita L.) essential oil and main components were assessed for their ability to interfere with plant respiratory functions. Tests were conducted on both root segments and mitochondria isolated by etiolated seedlings of cucumber (Cucumis sativus L.). Total essential oil inhibited 50% of root and mitochondrial respiration (IC50) when used at 324 and 593 ppm, respectively. (+)-Pulegone was the most toxic compound, with a 0.08 and 0.12 mM IC50 for root and mitochondrial respiration, respectively. (-)-Menthone. followed (+)-pulegone in its inhibitory action (IC50 values of 1.11 and 2.30 mM for root and mitochondrial respiration respectively), whereas (-)-menthol was the less inhibitory compound (IC50 values of 1.85 and 3.80 mM respectively). A positive correlation was found for (+)-pulegone, (-)-menthone and (-)-menthol between water solubility and respiratory inhibition. The uncoupling agent. carbonyl-cyanide-m-chlorophenyl-hydrazone (CCCP), lowered (-)-menthol and (-)menthone inhibition and annulled (+)-pulegone inhibition of mitochondrial respiration, whereas salicyl-hydroxamic acid (SHAM) 2-hydroxybenzohydroxamic acid, the alternative oxidase (AO) inhibitor, increased (-)-menthone inhibition and annulled both (+)-pulegone and (-)-menthol inhibitory activity. The possible interaction of (-)-pulegone and (-)-menthol with AO and the mechanism of action of(+)-pulegone, (-)-menthone and (-)-menthol on mitochondrial respiration are discussed.     

Analysis of the sodium chloride‐dependent respiratory kinetics of wheat mitochondria reveals differential effects on phosphorylating and non‐phosphorylating electron transport pathways

A number of previous studies have documented the gross response of mitochondrial respiration to salinity treatment, but it is unclear how NaCl directly affects the kinetics of plant phosphorylating and non‐phosphorylating electron transport pathways. This study investigates the direct effects of NaCl upon different respiratory pathways in wheat, by measuring rates of isolated mitochondrial oxygen consumption across different substrate oxidation pathways in saline media. We also profile the abundance of respiratory proteins by using targeted selected reaction monitoring (SRM) mass spectrometry of mitochondria isolated from control and salt‐treated wheat plants. We show that all pathways of electron transport were inhibited by NaCl concentrations above 400 mM; however electron transfer chains showed divergent responses to NaCl concentrations between 0 and 200 mM. Stimulation of oxygen consumption was measured in response to NaCl in scenarios where exogenous NADH was provided as substrate and electron flow was coupled to the generation of a proton gradient across the inner membrane. Protein abundance measurements show that several enzymes with activities less affected by NaCl are induced by salinity, whereas enzymes with activities inhibited by NaCl are depleted. These data deepen our understanding of how plant respiration responds to NaCl, offering new mechanistic explanations for the divergent salinity responses of whole‐plant respiratory rate in the literature.     

Changes in respiratory mitochondrial machinery and cytochrome and alternative pathway activities in response to energy demand underlie the acclimation of respiration to elevated CO2 in the invasive Opuntia ficus-indica

Studies on long-term effects of plants grown at elevated CO(2) are scarce and mechanisms of such responses are largely unknown. To gain mechanistic understanding on respiratory acclimation to elevated CO(2), the Crassulacean acid metabolism Mediterranean invasive Opuntia ficus-indica Miller was grown at various CO(2) concentrations. Respiration rates, maximum activity of cytochrome c oxidase, and active mitochondrial number consistently decreased in plants grown at elevated CO(2) during the 9 months of the study when compared to ambient plants. Plant growth at elevated CO(2) also reduced cytochrome pathway activity, but increased the activity of the alternative pathway. Despite all these effects seen in plants grown at high CO(2), the specific oxygen uptake rate per unit of active mitochondria was the same for plants grown at ambient and elevated CO(2). Although decreases in photorespiration activity have been pointed out as a factor contributing to the long-term acclimation of plant respiration to growth at elevated CO(2), the homeostatic maintenance of specific respiratory rate per unit of mitochondria in response to high CO(2) suggests that photorespiratory activity may play a small role on the long-term acclimation of respiration to elevated CO(2). However, despite growth enhancement and as a result of the inhibition in cytochrome pathway activity by elevated CO(2), total mitochondrial ATP production was decreased by plant growth at elevated CO(2) when compared to ambient-grown plants. Because plant growth at elevated CO(2) increased biomass but reduced respiratory machinery, activity, and ATP yields while maintaining O(2) consumption rates per unit of mitochondria, we suggest that acclimation to elevated CO(2) results from physiological adjustment of respiration to tissue ATP demand, which may not be entirely driven by nitrogen metabolism as previously suggested.     

Non-ohmic proton conductance of the mitochondrial inner membrane in hepatocytes

The mitochondrial membrane potential in isolated hepatocytes was measured using the distribution of the lipophilic cation triphenylmethylphosphonium (TPMP+) with appropriate corrections for plasma membrane potential, cytoplasmic and mitochondrial binding of TPMP+, and other factors. The relationship between mitochondrial membrane potential and respiration rate in hepatocytes was examined as the respiratory chain was titrated with myxothiazol in the presence of oligomycin. This relationship was nonproportional and similar to results with isolated mitochondria respiring on succinate. This shows that there is an increased proton conductance of the mitochondrial inner membrane in situ at high values of membrane potential. From the respiration rate and mitochondrial membrane potential of hepatocytes in the absence of oligomycin, we estimate that the passive proton permeability of the mitochondrial inner membrane accounts for 20-40% of the basal respiration rate of hepatocytes. The relationship between log[TPMP+]tot/[TPMP+]e and respiration rate in thymocytes was also nonproportional suggesting that the phenomenon is not peculiar to hepatocytes. There is less mitochondrial proton leak in hepatocytes from hypothyroid rats. A large proportion of the difference in basal respiration rate between hepatocytes from normal and hypothyroid rats can be accounted for by differences in the proton permeability characteristics of the mitochondrial inner membrane.     

Effect of ethanol intake on rat liver mitochondrial respiration and oxidative phosphorylation

The effect of ethanol intake on liver mitochondrial functions was investigated by feeding rats with a liquid isocaloric diet containing various concentrations of ethanol. We found that after feeding the liquid diet for 2 to 3 months, the body weight of rats did not show a significant difference between treated and control groups. However, the mitochondrial respiration rate decreased significantly with the increase of ethanol concentration in the diet. We found that when the rats were fed on 10.8% ethanol, the average succinate-supported State 3 respiration rate decreased from 54.5 to 44.8 nmol O2/min/mg and the glutamate-malate-supported State 3 respiration rate decreased from 38.8 to 23.6 nmol O2/min/mg as compared with the control. Interestingly, we noted that ethanol intake caused a more drastic effect on State 3 respiration than on State 4 respiration, irrespective of the substrate utilized by the mitochondria. In addition, the respiratory control and ADP/O ratios were found to decrease concomitantly with the increase of ethanol level in the diet. Moreover, we found that the effect of ethanol on both respiratory control and ADP/O ratios of liver mitochondria was more pronounced in glutamate-malate-supported respiration than succinate-supported respiration. These results clearly demonstrate that ethanol intake by the rat can cause impairment of liver mitochondrial respiration and oxidative phosphorylation, and that these effects are exerted through damage to mitochondrial membranes.