Mitochondrial contribution to the anoxic Ca2+ signal in maize suspension-cultured cells

Anoxia induces a rapid elevation of the cytosolic Ca2+ concentration ([Ca2+](cyt)) in maize (Zea mays L.) cells, which is caused by the release of the ion from intracellular stores. This anoxic Ca2+ release is important for gene activation and survival in O-2-deprived maize seedlings and cells. In this study we examined the contribution of mitochondrial Ca2+ to the anoxic [Ca2+](cyt) elevation in maize cells. Imaging of intramitochondrial Ca2+ levels showed that a majority of mitochondria released their Ca2+ in response to anoxia and took up Ca2+ upon reoxygenation. We also investigated whether the mitochondrial Ca2+ release contributed to the increase in [Ca2+](cyt) under anoxia. Analysis of the spatial association between anoxic [Ca2+](cyt) changes and the distribution of mitochondrial and other intracellular Ca2+ stores revealed that the largest [Ca2+](cyt) increases occurred close to mitochondria and away from the tonoplast. In addition, carbonylcyanide p-trifluoromethoxyphenyl hydrazone treatment depolarized mitochondria and caused a mild elevation of [Ca2+](cyt) under aerobic conditions but prevented a [Ca2+](cyt) increase in response to a subsequent anoxic poise. These results suggest that mitochondria play an important role in the anoxic elevation of [Ca2+](cyt) and participate in the signaling of O-2 deprivation.