Submitted to: Journal of Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 28, 2000
Publication Date: N/A
Interpretive Summary: Blast disease can reduce rice yields by 50%. There are several rice genes that provide complete resistance to one or several of the hundreds of blast races, but such major gene resistance in host plants can be overcome through genetic shifts in pathogen populations, sometimes in less than two years. Partial or field resistance to blast also exists in rice. Partial resistance is not generally race specific and is believed to be a more durable form of disease resistance than is major gene resistance. Ideally, varieties would contain genes for both complete resistance and partial resistance genes to protect the crop if /when the major gene(s) resistance is overcome. Seedling tests are available to detect complete resistance to various blast races. The results from such tests become complicated when test varieties contain multiple genes for resistance and/or genes for partial or quantitative resistance. Determination of linkage between molecular markers and the multiple and incomplete resistance genes would allow breeders to reliably select for and combine blast resistance genes even in early breeding generations based on reliable molecular marker information. 'Lemont' has been durably resistant to blast in the U.S. since the early 1980's. 'Teqing', a Chinese variety, exhibits complete resistance to all U.S. races of blast. We collected phenoytpic and molecular marker data from inbred lines from Lemont/Teqing and moecluarly mapped nine quantitative trait loci (QTLs) for field. One QTL was from Lemont while three correlated with major resistance genes previously mapped from Teqing. Our results support the theory that ideal disease resistance can be obtained from a combination of major genes and QTLs and identifies molecular markers to facilitate efforts to combine blast resistance genes.
Lines from a Lemont x Teqing recombinant inbred population were evaluated for three traits associated with dilatory resistance to rice blast disease: the Standard Evaluation System for rating leaf blast, percentage diseased leaf area, and the area under a disease progress curve. RFLP mapping using 175 well distributed loci revealed nine quantitative trait loci (QTLs) distributed one each on chromosomes 1, 2, 3, 4, 6, 7, and 9, with two loci chromosome 12. All nine putative QTLs were associated with the calculated areas disease progress curves, while six QTLs associated with with the data on both percentage diseased leaf area and the Standard Evaluation System. Teqing contributed the resistance allele for all but one of these loci. Individual QTLs accounted for 5 to 32 percent of the observed phenotypic variation and combined QTL models accounted for 43 to 53 percent. Three QTLs were located near three of the four major resistance genes previously identified in this population. The resistances of both Lemont and Teqing were attributable to both major genes capable of inducing hypersensitive reactions and minor genes causing less distinctive phenotypic differences. Interactions were noted between QTLs and major genes. Our findings are in support of the strategy of pyramiding major genes and QTLs in carefully selected combinations to develop improved varieties with resistance to the blast fungus that is both broad in spectrum and durable.