Cytoplasmic male sterility (CMS) is a widespread, maternally inherited trait that results in an inability of plants to produce functional pollen. The Ogura CMS system originated in radish, but has since been transferred to, and confers male sterility on, plants in the related genus Brassica . A gene which restores male fertility is needed for the Ogura CMS system to be exploited commercially for hybrid seed production in oilseed species such as Brassica napus. The restorer gene Rfo is a dominant radish nuclear gene that restores the male fertility to plants with Ogura cytoplasm. This gene has been transferred into Brassica napus through intergeneric crosses; however the introgressed segment of radish DNA contains an unknown number of genes, some of which confer undesirable traits, such as an elevated content of seed glucosinolates, antinutritive compounds that render the seed meal unusable as animal feed. A fine scale linkage map of the region in radish containing Rfo was constructed, and a map-based cloning approach relying on synteny between radish and Arabidopsis was used to clone Rfo. A radish gene encoding a 687 amino acid protein with a predicted mitochondrial targeting presequence was found to confer male fertility upon transformation into Ogura CMS B. napus . This gene, codes for a pentatricopeptide repeat (PPR)-containing protein with multiple, in this case 16, PPR domains. Two similar genes that do not appear to function as Rfo flank this gene. A transcript representing a non-functional allele (rfo) was detected in sterile radish plants. Comparison of the Rfo region with the syntenic Arabidopsis region indicates that a PPR gene is not present at the Rfo-equivalent site in Arabidopsis , although a smaller and related PPR gene is found about 40 kb from this site.

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