|San Miguel, Phillip|
Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2006
Publication Date: 3/14/2007
Citation: Nagy, E.D., Lee, T., Ramakrishna, W., Xu, Z., Klein, P.E., San Miguel, P., Cheng, C., Li, J., Devos, K.M., Schertz, K., Dunkle, L.D., Bennetzen, J.L. 2007. Fine mapping of a gene controlling the reaction to a fungal pathogen and its host-selective toxin, the Pc locus of sorghum bicolor. Theoretical and Applied Genetics. 114(6) 961-970. Interpretive Summary: Milo disease is a classical disease of sorghum caused by a soil-borne fungus that threatened cultivation of the crop in North America during the 1930s. Susceptibility to milo disease is controlled by a single gene, termed Pc, which spontaneously converts to a resistant form (pc) at an unusually high frequency. The nature of the Pc gene and the mechanism by which it changes to confer resistance are not known. In this work, DNA markers were used to map the gene to a chromosomal region containing several genes with sequences that are very similar to previously characterized disease resistance genes. The results were consistent with the conclusion that the Pc gene is present in multiple copies and that mutations to resistance arise through the loss of one or more copies of the gene. This information provides significant clues as to the origin of disease resistance and about mechanisms of mutations at the level of chromosomes. In addition to providing fundamental genetic information useful to plant breeders, geneticists, and molecular biologists, this discovery will facilitate efforts to clone and definitively identify the Pc gene and determine how it functions to protect sorghum against fungal pathogens.
Technical Abstract: Milo disease in sorghum is caused by isolates of the soil-borne fungus Periconia circinata that produce PC-toxin. Susceptibility to milo disease is conditioned by a single, semi-dominant gene, termed Pc. The susceptible allele (Pc) converts to a resistant form (pc) spontaneously at a gametic frequency of 10-3 to 10-4. A high-density genetic map was constructed around the Pc locus using DNA markers, allowing the Pc gene to be delimited to a 0.9 cM region on the short arm of sorghum chromosome 9. Physically, the Pc-region was covered by a single BAC clone. Sequence analysis of this BAC revealed twelve gene candidates. Several of the predicted genes in the region are homologous to disease resistance loci, including one NBS-LRR resistance gene analogue that is present in multiple tandem copies. Analysis of pc isolines derived from Pc/Pc sorghum suggests that one or more members of this NBS-LRR gene family is the Pc gene that conditions susceptibility.