Isolation of mitochondria from embryogenic cultures of Picea abies (L.) Karst. and Abies cephalonica Loud.: characterization of a K+(ATP) channel

SSR1 is involved in maintaining the function of mitochondria electron transport chain and iron homeostasis upon proline treatment in Arabidopsis

Although increasing intracellular proline under stressed condition could help the plants survive, treating plant with high level of proline under normal condition could be inhibitory to plant growth. Among other possible mechanisms, proline-induced mitochondrial reactive oxygen species (ROS) production due to electron overflow in mitochondria electron transport chain (mETC) caused by elevated proline degradation may contribute to the proline toxicity. However, direct evidences are still elusive. Here, we reported a functional characterization of SSR1, encoding a protein localized in mitochondria matrix, in maintaining the function of mETC through analyzing the proline hypersensitive phenotype of an Arabidopsis mutant ssr1-1 with a truncated SSR1 protein. Our analysis demonstrated that upon proline treatment, there were higher mitochondrial ROS, lower ATP content, reduced activity of mETC complex I and II, and reduced iron content in ssr1-1, in comparison to the wild type. Therefore, SSR1 is involved in maintaining normal capacity of mETC in transporting electrons in a way that related to iron homeostasis. Our results also supported that normal mETC activity is required for alleviating the proline toxicity.

Mitochondrial calcium transport and permeability transition as rational targets for plant protection

The mitochondrial permeability transition (MPT) is a death-inducing mechanism that collapses electrochemical gradients across inner mitochondrial membranes. Several studies in model plants have detailed potential MPT-dependent cell death upon abiotic stress in response to heat shock, ultraviolet radiation, heavy metal toxicity and waterlogging. However, the molecular specifics of the MPT and its possible role on plant cell death remain controversial. This review addresses previous and recent developments on the role(s) of the MPT in plants. Considering these advances, MPT targeting can constitute a plausible strategy to ameliorate cell death in plants upon abiotic stress.

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.

Analysis of subcellular metabolite distributions within Arabidopsis thaliana leaf tissue: a primer for subcellular metabolomics

Every biological organism relies for its proper function on interactions between a multitude of molecular entities like RNA, proteins, and metabolites. The comprehensive measurement and the analysis of all these entities would therefore provide the basis for our functional and mechanistic understanding of most biological processes. Next to their amount and identity, it is most crucial to also gain information about the subcellular distribution and the flux of the measured compounds between the cellular compartments. That is, we want to understand not only the individual functions of cellular components but also their functional implications within the whole organism. While the analysis of macromolecules like DNA, RNA, and proteins is quite established and robust, analytical techniques for small metabolites, which are prone to diffusion and degradation processes, provide a host of unsolved challenges. The major limitations here are the metabolite conversion and relocation processes. In this protocol we describe a methodological workflow which includes a nonaqueous fractionation method, a fractionated two-phase liquid/liquid extraction protocol, and a software package, which together allow extracting and analyzing starch, proteins, and especially polar and lipophilic metabolites from a single sample towards the estimation of their subcellular distributions.

Changes in ATP, glucose-6-phosphate and NAD(P)H cellular levels during the proliferation and maturation phases of Abies alba Mill. embryogenic cultures

The aim of the present study was to evaluate the adenosine triphospate (ATP), glucose-6-phosphate (glu-6P) and reduced form of nicotinamide adenine dinucleotide phosphate (NAD(P)H) cellular levels during the proliferation and maturation phases of Abies alba Mill. somatic embryos. For a better understanding of the dynamics of these parameters during the proliferation cycle, four embryonic cell lines were tested. During the maturation period, three independent experiments were conducted, focused on the effects of PEG-4000 (5 or 10% (w/v)) and abscisic acid (16, 32 or 64 μM) applied together (Experiments A and B) or with addition of gibberellic acid (Experiment C) on the dynamics of bio-energetic molecules and on the mean number of cotyledonary somatic embryos. Our results demonstrated that the cellular levels of bio-energetic molecules strongly depended on the composition of maturation media. Generally, the higher the number of cotyledonary embryos produced, the higher the level of ATP observed after a 2-week maturation period. The cellular level of ATP, glu-6P and NAD(P)H increased, particularly after the transition from the proliferation to the maturation phase when the differentiation and growth of somatic embryos occurred.

The uniqueness of the plant mitochondrial potassium channel

The ATP-inhibited Plant Mitochondrial K(+) Channel (PmitoKATP) was discovered about fifteen years ago in Durum Wheat Mitochondria (DWM). PmitoKATP catalyses the electrophoretic K(+) uniport through the inner mitochondrial membrane; moreover, the co-operation between PmitoKATP and K(+)/H(+) antiporter allows such a great operation of a K(+) cycle to collapse mitochondrial membrane potential (ΔΨ) and ΔpH, thus impairing protonmotive force (Δp). A possible physiological role of such ΔΨ control is the restriction of harmful reactive oxygen species (ROS) production under environmental/oxidative stress conditions. Interestingly, DWM lacking Δp were found to be nevertheless fully coupled and able to regularly accomplish ATP synthesis; this unexpected behaviour makes necessary to recast in some way the classical chemiosmotic model. In the whole, PmitoKATP may oppose to large scale ROS production by lowering ΔΨ under environmental/oxidative stress, but, when stress is moderate, this occurs without impairing ATP synthesis in a crucial moment for cell and mitochondrial bioenergetics.

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.

Activation of the plant mitochondrial potassium channel by free fatty acids and acyl-CoA esters: a possible defence mechanism in the response to hyperosmotic stress

The effect of free fatty acids (FFAs) and acyl-CoA esters on K(+) uptake was studied in mitochondria isolated from durum wheat (Triticum durum Desf.), a species that has adapted well to the semi-arid Mediterranean area and possessing a highly active mitochondrial ATP-sensitive K(+) channel (PmitoK(ATP)), that may confer resistance to environmental stresses. This was made by swelling experiments in KCl solution under experimental conditions in which PmitoK(ATP) activity was monitored. Linoleate and other FFAs (laurate, palmitate, stearate, palmitoleate, oleate, arachidonate, and the non-physiological 1-undecanesulphonate and 5-phenylvalerate), used at a concentration (10 μM) unable to damage membranes of isolated mitochondria, stimulated K(+) uptake by about 2-4-fold. Acyl-CoAs also promoted K(+) transport to a much larger extent with respect to FFAs (about 5-12-fold). In a different experimental system based on safranin O fluorescence measurements, the dissipation of electrical membrane potential induced by K(+) uptake via PmitoK(ATP) was found to increase in the presence of 5-phenylvalerate and palmitoyl-CoA, both unable to elicit the activity of the Plant Uncoupling Protein. This result suggests a direct activation of PmitoK(ATP). Stimulation of K(+) transport by FFAs/acyl-CoAs resulted in a widespread phenomenon in plant mitochondria from different mono/dicotyledonous species (bread wheat, barley, triticale, maize, lentil, pea, and topinambur) and from different organs (root, tuber, leaf, and shoot). Finally, an increase in mitochondrial FFAs up to a content of 50 nmol mg(-1) protein, which was able to activate PmitoK(ATP) strongly, was observed under hyperosmotic stress conditions. Since PmitoK(ATP) may act against environmental/oxidative stress, its activation by FFAs/acyl-CoAs is proposed to represent a physiological defence mechanism.

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.

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.