Ogura cytoplasmic male sterility (CMS) in radish ( Raphanus sativus ) is caused by an aberrant mitochondrial gene, Orf138 , that prevents the production of functional pollen without affecting female fertility. Rfo , a nuclear gene that restores male fertility, alters the expression of Orf138 at the post‐transcriptional level. The Ogura CMS/ Rfo two‐component system is a useful model for investigating nuclear–cytoplasmic interactions, as well as the physiological basis of fertility restoration. Using a combination of positional cloning and microsynteny analysis of Arabidopsis thaliana and radish, we genetically and physically delimited the Rfo locus to a 15‐kb DNA segment. Analysis of this segment shows that Rfo is a member of the pentatricopeptide repeat (PPR) family. In Arabidopsis , this family contains more than 450 members of unknown function, although most of them are predicted to be targeted to mitochondria and chloroplasts and are thought to have roles in organellar gene expression.

Cytoplasmic male sterility is a maternally inherited trait in higher plants that prevents the production of functional pollen. Ogura cytoplasmic male sterility in radish (Raphanus sativus) is regulated by the orf138 mitochondrial locus. Male fertility can be restored when orf138 accumulation is suppressed by the nuclear Rfo locus, which consists of three genes putatively encoding highly similar pentatricopeptide repeat proteins (PPR-A, -B, and -C). We produced transgenic rapeseed (Brassica napus) plants separately expressing PPR-A and PPR-B and demonstrated that both encoded proteins accumulated preferentially in the anthers of young flower buds. Immunodetection of ORF138 showed that, unlike PPR-B, PPR-A had no effect on the synthesis of the sterility protein. Moreover, immunolocalization experiments indicated that complete elimination of ORF138 from the tapetum of anthers correlated with the restoration of fertility. Thus, the primary role of PPR-B in restoring fertility is to inhibit ORF138 synthesis in the tapetum of young anthers. In situ hybridization experiments confirmed, at the cellular level, that PPR-B has no effect on the accumulation of orf138 mRNA. Lastly, immunoprecipitation experiments demonstrated that PPR-B, but not PPR-A, is associated with the orf138 RNA in vivo, linking restoration activity with the ability to directly or indirectly interact with the orf138 RNA. Together, our data support a role for PPR-B in the translational regulation of orf138 mRNA.