Subcellular heterogeneity of mitochondrial membrane potential in anther tapetum

Studies on animal material have revealed that changes in the mitochondrial permeability transition pore (PTP), which cause a reduction in the mitochondrial transmembrane potential (DeltaPsi(m)) followed by release of cytochrome c, belong to the earliest manifestations of some types of apoptosis. We have attempted to monitor the DeltaPsi(m) of mitochondria during programmed cell death (PCD) of the secretory tapetum using JC-1, a fluorochrome dye that detects mitochondrial membrane potential and to relate changes in this potential to mitochondrial ultrastructure. Analysis of tapetal cells isolated from Ornithogalum virens anthers revealed that the DeltaPsi(m) of mitochondria in the tapetal cells alters during development; the change, however, is not uniform in the mitochondrial population within a single tapetal cell. In young tapetal cells, at the tetrad stage, we detected only the red fluorescence of JC-1 aggregates in all tapetal mitochondria, which indicates highly negative DeltaPsi(m). In an advanced stage of PCD at the late microspore stage, in each tapetal cell we detected both mitochondria with red (as formerly) and mitochondria with green fluorescence. The green fluorescence of JC-1 monomers indicates mitochondria with depolarised membranes. These changes in DeltaPsi(m) are related to observed changes in mitochondria ultrastructure. This is the first documentation of intracellular heterogeneity of DeltaPsi(m) during anther tapetum development. Alteration in DeltaPsi(m) suggests a relationship between mitochondrial function and PCD processes in tapetal cells.