Under stress conditions, mitochondria sense metabolic changes, e.g. in pH, cytoplasmic Ca(2+), energy status, and reactive oxygen species (ROS), and respond by induction of the permeability transition pore (PTP) and by releasing cytochrome c, thus initiating the programmed cell death (PCD) cascade in animal cells. In plant cells, the presence of all the components of the cascade has not yet been shown. In wheat (Triticum aestivum L.) root mitochondria, the onset of anoxia caused rapid dissipation of the inner membrane potential, initial shrinkage of the mitochondrial matrix and the release of previously accumulated Ca(2+). Ca(2+) uptake by mitochondria was dependent on the presence of inorganic phosphate. Treatment of mitochondria with high micromolar and millimolar Ca(2+) (but not Mg(2+)) concentrations induced high amplitude swelling, indicative of PTP opening. Alterations in mitochondrial volume were confirmed by transmission electron microscopy. Mitochondrial swelling was not sensitive to cyclosporin A (CsA)-an inhibitor of mammalian PTP. The release of cytochrome c was monitored under lack of oxygen. Anoxia alone failed to induce cytochrome c release from mitochondria. Oxygen deprivation and Ca(2+) ions together caused cytochrome c release in a CsA-insensitive manner. This process correlated positively with Ca(2+) concentration and required Ca(2+) localization in the mitochondrial matrix. Functional characteristics of wheat root mitochondria, such as membrane potential, Ca(2+) transport, swelling, and cytochrome c release under lack of oxygen are discussed in relation to PCD.
