MITOCHONDRIAL INVOLVEMENT IN PETAL AND STAMEN DEVELOPMENT STUDIED BY SEXUAL AND SOMATIC HYBRIDIZATION OF NICOTIANA SPECIES

The interspecific origin, frequency of occurrence, and variation in phenotype of cytoplasmic male-sterile cultivars of Nicotiana tabacum are reviewed in order to provide a framework for understanding cell biological investigations of the interaction between mitochondria and the nucleus in regulating development of floral organs in these cultivars. Analyses of mitochondrial protein synthesis, mitochondrial DNA, and RNA transcripts have shown that nuclear effects on mitochondrial activities are very common. Cybridization of cytoplasmic male-sterile cultivars has produced heritable alterations in phenotype which can be correlated with changes in mitochondrial DNA and RNA transcripts. A hypothesis is presented that the different morphologies of cytoplasmic male-sterile cultivars are indicative of variation in activity of several different mitochondrial genes, not yet identified. According to this hypothesis, transcripts from these genes differ in each cultivar, in response to nuclear genes of N. tabacum. The particular pattern of differential expression accounts for the variations in phenotype that characterize each different interspecific combination of N. tabacum nucleus and Nicotiana sp. mitochondria. Evidence in support of this hypothesis has been assembled by comparisons of male-sterile cultivars with cultivars restored to fertility either by nuclear restorers or by somatic cybridization.