Mapping and characterization of Rf 5: a new gene conditioning pollen fertility restoration in A1 and A2 cytoplasm in sorghum (Sorghum bicolor (L.) Moench).

With an aim to further characterize the cytoplasmic male sterility-fertility restoration system in sorghum, a major fertility restoration gene was mapped along with a second locus capable of partial restoration of pollen fertility. The major fertility restoration gene, Rf(5), was located on sorghum chromosome SBI-05, and was capable of restoring pollen fertility in both A(1) and A(2) male sterile cytoplasms. Depending on the restorer parent, mapping populations exhibited fertility restoration phenotypes that ranged from nearly bimodal distribution due to the action of Rf(5), to a more normalized distribution reflecting the action of Rf(5) and additional modifier/partial restoration genes. A second fertility restoration locus capable of partially restoring pollen fertility in A(1) cytoplasm was localized to chromosome SBI-04. Unlike Rf(5), this modifier/partial restorer gene acting alone resulted in less than 10% seed set in both A(1) and A(2) cytoplasms, and modified the extent of restoration conditioned by the major restorer Rf(5) in A(1) cytoplasm. In examining the genomic regions spanning the Rf(5) locus, a cluster of pentatricopeptide gene family members with high homology to rice Rf (1) and sorghum Rf (2) were identified as potential candidates encoding Rf(5).