Two nuclear-directed means of suppressing a dominant mitochondrial mutation in common bean

Abstract

Plant mitochondria have demonstrated remarkable differences in genome size, organization, recombinational activity, and even gene expression compared to their fungal and animal counterparts. The plant mitochondrial genome is characterized by a high degree of recombinational activity whereby DNA exchange across direct repeats throughout the genome results in the generation of subgenomic DNA molecules. These molecules encode only a portion of the total genetic information and, taken together, create a highly redundant genome structure. The subdivision of the genome into redundant subgenomic forms may offer the opportunity to accumulate quite significant genetic variation within the cell. Mitochondrial DNA molecules maintained substoichiometrically have been reported in a number of plant species (Bonhomme et al. 1992; Gutierres et al. 1997; Janska et al. 1998 Kanazawa et al. 1994; Small et al. 1987; Suzuki et al. 1996; Yesodi et al. 1995). It has been suggested that the retention of these molecules at unusually low copy numbers represents a means of retaining genetic variability (Small et al. 1989). Evidence consistent with this hypothesis has demonstrated that molecules maintained substoichiometrically can be reamplified to detectable copy number; moreover, molecules maintained substoichiometrically are apparently transcriptionally and translationally silent (Janska et al. 1998). The cytoplasmic male sterility (CMS) phenotype in common bean is characterized by pollen abortion arising from incomplete cytokinesis and associated with a mitochondrial mutation designated pvs (for Phaseolus vulgaris sterility sequence) (Abad et al. 1995; Johns et al. 1992). The CMS common bean mitochondrial genome has been mapped by overlapping cosmid clone analysis (Janska and Mackenzie 1993). The mapping data indicate that the genome is a tripartite redundant genome with interrecombining molecules of 394, 257, and 210 kb. The 210 kb molecule contains the 2.4 kb pvs region, in which two unique open reading frames are present. These are designated pvs-orf98 and pvsorf239 and encode 98and 239-amino acid peptides, respectively (Chase and Ortega 1992; Johns et al. 1992). Only one translation product of 27 kDa, corresponding to the product of pvs-orf239, has been detected and directly associated with sterility (Abad et al. 1995). Of interest, this ORF239 polypeptide accumulates in reproductive tissues only, with no detectable translation product present in vegetative tissues (Abad et al. 1995). This tissue-specific expression of ORF239 likely represents one means of nuclear-directed suppression of mitochondrial gene expression, as will be discussed here. Reversion to fertility in the male-sterile common bean line occurs spontaneously (Mackenzie et al. 1988) or in association with the introduction of a nuclear gene designated Fr (Mackenzie and Chase 1990). In either case, the fertile phenotype is associated with the disappearance of the pvs-containing 210 kb DNA molecule (Janska and Mackenzie 1993). This presumable disappearance of the pvs-containing molecule, a consequence of genomic shifting of the molecule to substoichiometric levels, likely represents a second means by which nuclear suppression of mitochondrial expression can occur. The details of the reversion process will also be described.