Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2006
Publication Date: 7/1/2006
Citation: Kuhlman, L.C., Pring, D.R., Rooney, W.L., Tang, H.V. 2006. Allelic frequency at the Rf3 and Rf4 loci, and the genetics of A3 cytoplasmic fertility restoration in converted sorghum lines. Crop Science. 46:1576-1580. Interpretive Summary: Cytoplasmic-nuclear male sterility (CMS) of higher plants is widely used to efficiently produce hybrid seed of many U.S. crops. CMS is mandatory for hybrid grain sorghum production. One source of CMS in sorghum, from the line IS1112C, is difficult to use because two genes are required to restore fertility for field grain production. We constructed two independent sorghum lines, each carrying one of the two genes required to restore fertility. Using these two critical lines to screen U.S. Germplasm, 140 important lines were surveyed for presence of either of the two required genes. Nine lines were identified. Three lines were found to carry both genes, two lines carried one of the two required genes, and four lines carry at least one of the two genes. Among the nine lines identified, two lines restored fertility in a novel manner, indicating an alternative fertility restoration mechanism. The rare lines identified in this study will be of value to sorghum geneticists and breeders in evaluating the possibility that the CMS associated with IS1112C might be developed as a single-gene fertility restorer system for utilization, thus providing diversity to U.S. hybrid grain sorghum production.
Technical Abstract: Cytoplasmic-nuclear male sterility (CMS) in sorghum (Sorghum bicolor [L.] Moench) is integral to hybrid seed production. One such CMS, A3, is controlled by a two gene gametophytic fertility restoration system involving two restoration alleles, Rf3 and Rf4, which are required for individual male gamete viability. An allelic frequency survey was undertaken using 140 sorghum lines from the Sorghum Conversion Program. These lines were hybridized to two testers, A3FL3 (Rf3Rf3rf4rf4) and A3FL4 (rf3rf3Rf4Rf4). The resulting hybrids were evaluated for I2-KI pollen stainability and seed set to determine their fertility restoration reaction. Transcript analyses were performed on lines which contain restoration alleles to determine whether they fit the expected genetic model of fertility restoration. Three lines (SC306, SC512, and SC215) were found to restore fertility with both testers and thus contain both restoration alleles. Six other lines contained dominant alleles at only one of the restoration loci. The allelic frequency of Rf3 in the surveyed population was 5.1% and the frequency of Rf4 was 4.8%. Using transcript analyses, two exceptional lines were identified (SC306 and SC215) that carried Rf3 but did not confer enhanced transcript processing activity (TPA) of mitochondrial orf107. Enhanced TPA was thought to be tightly linked to or represent gene action of Rf3, but identification of these lines demonstrates that enhanced TPA is not required for fertility restoration.