The PET1-CMS mitochondrial mutation in sunflower is associated with premature programmed cell death and cytochrome c release

In mammals, mitochondria have been shown to play a key intermediary role in apoptosis, a morphologically distinct form of programmed cell death (PCD), for example, through the release of cytochrome c, which activates a proteo lytic enzyme cascade, resulting in specific nuclear DNA degradation and cel l death. In plants, PCD is a feature of normal development, including the p enultimate stage of anther development, leading to dehiscence and pollen re lease. However, there is little evidence that plant mitochondria are involv ed in PCD. In a wide range of plant species, anther and/or pollen developme nt is disrupted in a class of mutants termed CMS (for cytoplasmic male ster ility), which is associated with mutations in the mitochondrial genome. On the basis of the manifestation of a number of morphological and biochemical markers of apoptosis, we have shown that the PET1-CMS cytoplasm in sunflow er causes premature PCD of the tapetal cells, which then extends to other a nther tissues. These features included cell condensation, oligonucleosomal cleavage of nuclear DNA, separation of chromatin into delineated masses, an d initial persistence of mitochondria. In addition, immunocytochemical anal ysis revealed that cytochrome c was released partially from the mitochondri a into the cytosol of tapetal cells before the gross morphological changes associated with PCD. The decrease in cytochrome c content in mitochondria i solated from male sterile florets preceded a decrease in the integrity of t he outer mitochondrial membrane and respiratory control ratio. Our data sug gest that plant mitochondria, like mammalian mitochondria, play a key role in the induction of PCD. The tissue-specific nature of the CMS phenotype is discussed with regard to cellular respiratory demand and PCD during normal anther development.