Localization of the Rf3 restorer-of-fertility gene for maize S-type cytoplasmic male sterility

Maize S-type cytoplasmic male sterility (CMS-S) is a maternally inherited trait that prevents pollen grains from developing to maturity. CMS-S is associated with the high levels of a novel mitochondrial transcript, orf355/orf77. Cleavage of this RNA, mediated by the nuclear restorer Rf3, reverses the sterility. Rf3 was previously mapped on the long arm of chromosome 2. The goals of this research were to fine-map the locus and to identify Rf3 using a candidate gene approach. Genotyping of nearisogenic lines (NILs) mapped Rf3 to a 1.98 Mb region of 2L. Six candidate genes, all predicted to code for mitochondrially targeted pentatricopeptide repeat proteins (PPR), were PCR-amplified, sequenced, and compared from multiple Rf3-containing NILs and non-restoring rf3 inbreds. One PPR-Rf3 candidate gene had two consistent differences between multiple restoring and non-restoring lines. Gene expression in pre-emergent tassels from the fertility-restored and non-restored plants was compared. Within the 3 Mb region surrounding Rf3, 9 genes were differentially expressed between restoring and non-restoring lines, including genes that could code for an ATP-binding protein, an ATPase, and four PPR proteins. Although Rf3 has not yet been identified, this study has revealed five promising candidates.