Male-sterile tobacco displays abnormal mitochondrial atp1 transcript accumulation and reduced floral ATP/ADP ratio

Alloplasmic male sterility is commonly obtained in Nicotiana by combining t he nucleus from N. tabacum with the cytoplasm from other Nicotiana species. Besides being male-sterile, most of these cultivars also display changes i n floral organ structure. Flowers from male-sterile plants containing the n ucleus from N. tabacum combined with N. repanda cytoplasm develop stamen wi th shortened filaments and shrivelled anthers capped with stigmatoids. Male fertility and normal floral development can be restored by introduction of a restorer chromosome fragment from the cytoplasmic donor N. repanda into the N. tabacum nucleus. A novel reading frame, orf274, located upstream of atp1 in the mitochondrial genomes of both N. tabacum and N. repanda, as wel l as in the male-sterile and fertility-restored plants was identified. Co-t ranscripts of orf274 and atp1 were detected by RT-PCR in all four cultivars , but these transcripts accumulate to levels detectable by northern hybridi zation only in male-sterile plants. These co-transcripts neither generate d etectable levels of an ORF274 polypeptide nor lead to altered expression of the ATP synthase subunit alpha. Measurement of ATP and ADP steady-state le vels, however, revealed that the ATP/ADP ratio is significantly lower in yo ung floral buds of male-sterile plants than in fertile plants.