Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/1997
Publication Date: N/A
Citation: Interpretive Summary: Cytoplasmic male-sterile (CMS) maize contains a unique gene, T-urf13, that functions to prevent fertile pollen formation, an important component in producing hybrid seed. To identify a common mechanism by which pollen fertility is restored, other genes which suppress the effects of T-urf13 were investigated. Two new genes which restore pollen fertility were discovered and characterized. These were designated Rf8 and Rf*. The Rf genes appear to suppress the sterility effect by recognizing a specific sequence in the T-urf13 RNA, the genetic material that contains the instructions to make URF13 protein. The Rf genes cut the RNA into smaller pieces, which prevents the synthesis of URF13 protein, and thereby restore pollen fertility. A survey of T-cytoplasm maize lines revealed that Rf8 and Rf* are rare in maize germplasm. However, these results are an important clue revealing a mechanism by which other similar CMS systems may yfunction in other economically important crops as well. These results wil be of interest to plant scientists, especially those investigating male-sterile systems in other crop plants.
Technical Abstract: Rf8 is a newly-described restorer gene that can substitute for Rf1 to partially restore pollen fertility to male-sterile, T-cytoplasm maize. Families segregating for Rf8 were used to investigate the mechanism of this fertility restoration and to compare it to the restoration conditioned by Rf1. Rf8 is unlinked to the rf1 locus, however, it has similar, but unique, effects on T-urf13 transcript accumulation. Like the 1.6- and 0.6-kb T-urf13 transcripts that accumulate in T-cytoplasm plants carrying Rf1, 1.42- and 0.42-kb transcripts (that are absent in Rf1 containing plants) cosegregate with partial fertility restoration in families segregating for Rf8. Additionally, plants that carry Rf8 have a slight reduction in accumulation of the URF13 protein. A survey of T-cytoplasm maize lines, inbreds, and F1 hybrids by mitochondrial RNA gel blot analyses revealed that Rf8 is rare in maize germplasm. These surveys revealed the presence of another rare, weak restorer factor, Rf*, which is associated with the accumulation of 1.4- and 0.4-kb T-urf13 transcripts. Primer extension analyses indicate that the 5' ends of the 1.42- and 1.4-kb transcripts are +140 and +160 bases, respectively, 3' of the AUG initiation codon of the T-urf13 open reading frame. Comparison of sequences containing each identified 5' end of Rf1-, Rf8, and Rf*-associated mitochondrial transcripts revealed a consensus sequences, 5'-CNACNNU-3', that is positioned adjacent to the 5' nucleotide of each transcript. These results demonstrate that multiple nuclear genes can have similar and specific effects on the expression of the unique T-urf13 mitochondrial coding sequence to restore pollen fertility to T-cytoplasm maize.