Impact of oxygen stress and energy availability on membrane stability of plant cells.

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.

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.