Submitted to: Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 1998
Publication Date: N/A
Interpretive Summary: U.S. field crop production is substantially dependent on the use of hybrids. Hybrid sorghum production is totally dependent on the use of cytoplasmic male sterility (cms), but only one source is currently used. This results in cytoplasmic uniformity, which could predispose sorghum to vulnerabilities such as disease susceptibility. Agricultural Research Scientists in Gainesville, FL, are characterizing an alternative source of cms, the "A3" group, which may be useful if methods can be developed to allow its use. We show that two nuclear genes are required to restore fertility in hybrids, and have identified the probable role of one of the genes, Rf3. The effect of this gene is to cut a messenger RNA for a mitochondrial gene that appears to cause cms. The result is that the toxic gene product is probably no longer made, and the plants shed pollen. The identification of a gene that apparently causes cms and the nuclear gene involved in correcting this defect allows the rapid development of genetic markers. These markers could be used by geneticists to efficiently develop sorghum lines with this alternative source of cms.
Technical Abstract: The restoration of male fertility in sorghum carrying the A3 (IS1112C) source of cytoplasmic-nuclear male sterility (cms) is exacted through a novel two-gene gametophytic system. Fertility restoration involves complementary action by genes designated Rf3 and Rf4. Rf3 is tightly linked to, or represents, a single gene that tentatively regulates a transcript processing activity that cleaves transcripts of orf107, a chimeric mitochondrial open reading frame specific to IS1112C. No specific function of Rf4 can be inferred. The mitochondrial gene urf209 (orf25) is subject to nucleus-specific transcript processing 5' to the gene, conferred by single dominant gene designated Mmt2. Surprisingly, examinations of transcript patterns in F2's and two backcross progeny indicated co-segregation of the orf107 and urf209 enhanced processing activities. Several lines that do not restore fertility nor confer orf107 transcript processing do exhibit urf209 transcript processing, indicating that the activities are separable. We conclude that the gene(s) encoding orf107 and urf209 processing activities, or their regulation, are tightly linked in IS1112C. Alternatively, the similarity in apparent regulatory action of the genes may indicate allelic differences wherein the IS1112C Rf3 allele may differ from alleles of maintainer lines by the capability to regulate both orf107 and urf209 processing activities.