Ca2+-induced high amplitude swelling and cytochrome c release from wheat (Triticum aestivum L.) mitochondria under anoxic stress

Under stress conditions, mitochondria sense metabolic changes, e.g. in pH. cytoplasmic Ca2+, 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 shogun. 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 Ca2+. Ca2+ uptake by mitochondria was dependent on the presence of inorganic phosphate. Treatment of mitochondria with high micromolar and millimolar Ca2+ (but not Mg2+) 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 Ca2+ ions together caused cytochrome c release in a CsA-insensitive manner. This process correlated positively with Ca2+ concentration and required Ca2+ localization in the mitochondrial matrix. Functional characteristics of wheat root mitochondria, such as membrane potential, Ca2+ transport, swelling, and cytochrome c release under lack of oxygen are discussed in relation to PCD. (C) 2002 Annals of Botany Company.