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 (<20 M) 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.Since the late 1970s, it has been known that animal mitochondria can experience a sudden increase in the permeability of their inner membrane to low and medium molecular weight compounds via the opening of a pore (1-3). This mitochondrial permeability transition pore (PTP) 1 is viewed as a multiprotein complex composed at least of the voltage-dependent anion channel, the adenine nucleotide translocator (AdNT), and cyclophilin-D, at the contact sites between outer and inner membranes (4). When the pore opens, solutes up to about 1.5 kDa can pass through the inner membrane, a process known as the mitochondrial permeability transition (MPT). Subsequently, the membrane potential (⌬⌿) decays, oxidative phosphorylation is uncoupled from electron flow, intramitochondrial ions and metabolites are released, and a large amplitude swelling can occur, disrupting the outer membrane and releasing intermembrane compounds.Although pore opening primarily requires the accumulation of Ca 2ϩ in the mitochondrial matrix, it is also modulated by numerous factors. For instance, P i , low ⌬⌿, thiol-oxidizing reagents, low ATP level, fatty acids, anoxia, and reaeration stress all favor pore opening, whereas thiol-reducing agents, low pH, high ⌬⌿, and divalent cations other than Ca 2ϩ counteract it (5). Inhibition of MPT is readily achieved with submicromolar concentrations of cyclosporin A (CsA) (6, 7). This highly specific effect has decisively contributed to the acceptance of the pore theory (6, 7) and is used today as the primary diagnostic trait of the classical MPT (5). The implication of mitochondria and PTP in mammalian cell death gave a new impetus to the research. For instance, cytochrome c has been shown to be released from the mitochondrial intermembrane space into the cytosol (8, 9), where it can trigger apoptosis (10). How ...
SummaryRapid pollen tube growth requires a high rate of sugar metabolism to meet energetic and biosynthetic demands. Previous work on pollen sugar metabolism showed that tobacco pollen carry out ef®cient ethanolic fermentation concomitantly with a high rate of respiration (Bucher et al., 1995). Here we show that the products of fermentation, acetaldehyde and ethanol, are further metabolised in a pathway that bypasses mitochondrial PDH. The enzymes involved in this pathway are pyruvate decarboxylase, aldehyde dehydrogenase and acetyl-CoA synthetase. Radiolabelling experiments show that during tobacco pollen tube growth label of 14 C-ethanol is incorporated into CO 2 as well as into lipids and other higher molecular weight compounds. A role for the glyoxylate cycle appears unlikely since activity of malate synthase, a key enzyme of the glyoxylate cycle, could not be detected.
In this paper we report on our study of the changes in biomass, lipid composition, and fermentation end products, as well as in the ATP level and synthesis rate in cultivated potato (Solanum tuberosum) cells submitted to anoxia stress. During the first phase of about 12 h, cells coped with the reduced energy supply brought about by fermentation and their membrane lipids remained intact. The second phase (12-24 h), during which the energy supply dropped down to 1% to 2% of its maximal theoretical normoxic value, was characterized by an extensive hydrolysis of membrane lipids to free fatty acids. This autolytic process was ascribed to the activation of a lipolytic acyl hydrolase. Cells were also treated under normoxia with inhibitors known to interfere with energy metabolism. Carbonyl-cyanide-4-trifluoromethoxyphenylhydrazone did not induce lipid hydrolysis, which was also the case when sodium azide or salicylhydroxamic acid were fed separately. However, the simultaneous use of sodium azide plus salicylhydroxamic acid or 2-deoxy-D-glucose plus iodoacetate with normoxic cells promoted a lipid hydrolysis pattern similar to that seen in anoxic cells. Therefore, a threshold exists in the rate of ATP synthesis (approximately 10 mol g ؊1 fresh weight h ؊1 ), below which the integrity of the membranes in anoxic potato cells cannot be preserved.O 2 deprivation becomes a frequent stress for plants submitted to unpredictable heavy rainfalls and flooding. The diffusion of O 2 to their submerged underground organs is severely limited, so that plants must cope with hypoxic or even anoxic conditions. Whereas ATP is produced with a high efficiency by respiration in nongreen cells, its synthesis is much lower under anoxia, with fermentation as the sole energy provider. For most higher plants, the latter condition eventually becomes lethal. The multifarious effects of O 2 deprivation stress on plants sensitive and resistant to anoxia are fairly well understood and have been extensively reviewed in this decade (Armstrong et al., 1994; Sachs, 1994; Ratcliffe, 1995; Crawford and Braendle, 1996; Drew, 1997; Vartapetian and Jackson, 1997).Aside from the obvious interest given to the responses of energy metabolism, the role of macromolecules, and in particular, of gene expression and protein synthesis, has received considerable attention (Sachs, 1994; Drew, 1997). In contrast, the behavior of membrane lipids under anoxia has scarcely been investigated. This is surprising knowing how important it is for a living cell to maintain its membrane integrity. In potato (Solanum tuberosum) tubers, for instance, changes in membrane lipids have mostly been studied during aging, and have been related to overall lipid unsaturation, lipid degradation, and peroxidation processes (Knowles and Knowles, 1989; Spychalla and Desborough, 1990a, 1990b; Kumar and Knowles, 1993; Dipierro and De Leonardis, 1997). Although these effects are not directly relevant to anoxia (Kumar and Knowles, 1996), they are probably related to the effects likely to occur unde...
This article reviews the relationship between the energy status of plant cells under O(2) stress (e.g. waterlogging) and the maintenance of membrane intactness, using information largely derived from suspension cultures of anoxia-intolerant potato cells. Energy-related parameters measured were fermentation end-products (ethanol, lactate, alanine), respiratory rate, ATP, adenylate energy charge, nitrate reductase activity and biomass. ATP synthesis rates were calculated from the first four parameters. Reactive oxygen species were estimated from H(2)O(2) and superoxide levels, and the enzymatic detoxification potential from the activity levels of catalase and superoxide dismutase. Structure-related parameters were total fatty acids, free fatty acids (FFAs), lipid hydroperoxides, total phospholipids, N-acylphosphatidylethanolamine (NAPE) and cell viability. The following issues are addressed in this review: (1) what is the impact of anoxia on membrane lipids and how does this relate to energy status; (2) does O(2) per se play a role in these changes; (3) under which conditions and to what extent does lipid peroxidation occur upon re-aeration; and (4) can the effects of re-aeration be distinguished from those of anoxia? The emerging picture is a reappraisal of the relative contributions of anoxia and re-aeration. Two successive phases (pre-lytic and lytic) characterize potato cells under anoxia. They are connected by a threshold in ATP production rate, below which membrane lipids are hydrolysed to FFAs, and NAPE increases. Since lipid peroxidation occurs only when cells are reoxygenated during the lytic phase, its biological relevance in an already damaged system is questionable.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.