Fine Mapping Of Major Genes And Analysis Of QTLs Conferring Resistance To Leaf Blast In Two Rice Varieties (Oryza Sativa L.)

Rice blast disease, caused by Pyricularia grisea (Cooke) Sacc., teleomorph Magaporthe grisea (Hebert) Barr, is one of the most destructive diseases worldwide. Utilization of resistance genes in rice breeding programs is considered an effective and economical strategy to control the disease. However, cultivars carrying a single R gene can only last for a short period of time until the advent of new dominant pathogenic isolates due to the high variability and pathogenicity of M. grisea population. Hence, exploiting wide-spectrum R genes and quantitative resistant loci, and pyramiding them into rice cultivars for developing broad-spectrum and durable resistance to rice blast by marker-assisted selection is a priority in rice breeding programs. In this study, we analyzed the genotypes of blast resistance in cv. IR24 and cv. DV85, and mapped three major blast R genes and some quantitative resistant loci. The main results were as follows:1. The resistance of cv. IR24 to Chinese blast isolates are mainly conditioned by R gene Pi20(t) and Pib, among which the Pi20(t) can be used as an superior R gene in rice breeding programs as it confer resistance to a broad spectrum of japonica-isolates (94.2% of 160 isolates) and a moderate spectrum of indica-isolates (52.6% of 160 isolates) in China. The isolates 98095 which could specifically differentiate the Pi20(t) was selected from a total of 160 blast isolates tested, and used to inoculate the recombinant inbred lines(RILs) and F2 population from the cross between Asominori and IR24 to finely map the Pi20(t). As a result, two flanking and three co-segregating SSR markers for Pi20(t), nearby the centromere region of chromosome 12, were identified. These five markers can tag Pi20(t) over 98% accuracy and show high polymorphisms between Pi20(t) donor IR24 and a series of elite cultivars in China. We suggest that these SSR markers can be useful in marker assisted selection for rapidly introducing Pi20(t) into japonica varieties or pyramiding Pi20(t) with other resistance genes into indica varieties.2. The indica cv. DV85 has a high level of resistance to Chinese blast isolates as it confer resistance to 92.9% of 98 japonica-derived isolates and 54.0% of 50 indica-derived isolates, this indicate it can also be used in rice breeding programs for blast resistance. Based on the results of genetic analysis and preliminary gene mapping from RILs, BC1F1(Kinmaze/DV85//Kinmaze) and F2 population derived from the cross of Kinmaze(susceptible) and DV85(resistant), we confirm that DV85 harbors a new dominant R gene conferring resistance to the Chinese japonica-derived isolate 97-27-2. The novel R gene was located on the terminal region of long arm of chromosome 1, tentatively designed as Pidv(t). By using an enlarged mapping population from F2 individuals, and by developing InDel markers combined with the released SSR markers, we further finely map the Pidv(t) gene to an 66.9kb interval defined by two markers, C4 and RM12182, with a genetic distance of 0.5 and 0.1 cM, respectively. The fine-mapping of Pidv(t) is favorable to marker-assisted selection and map-based cloning of the gene.3 Seventy-one RILs of Asominori×IR24 (populationⅠ) and 81 RILs of Kinmaze×DV85 (populationⅡ) were inoculated with six isolates 91-17-2,97-27-2,59-3, L64-1, CH26 and TH16, respectively. Fifty-eight QTLs were detected in the populations I at the LOD 2.5, of which 34 QTLs derived from the alleles of resistant parent IR24,24 QTLs derived from the alleles of susceptible parent Asominori, and 16,13,12 and 17 QTLs were related to lesion number (LN), lesion length (LL), lesion area (LA) and lesion degree (LD), respectively. In the populationⅡ, fifty-nine QTLs were detected at the LOD 2.5, including 29 QTLs derived from the alleles of resistant parent DV85,30 QTLs derived from the alleles of susceptible parent Kinmaze, and 16,12,14 and 17 QTLs related to LN, LL, LA and LD. The QTLs conferring major effects were mainly located in Pib and Pidv(t) region, thus, QTLs together with the major genes are responsible for the broad-spectrum resistance in IR24 and in DV85, respectively. QTLs are often clustered in the region of major genes, meanwhile, most QTLs detected for different traits for the same isolate are co-located in the same loci. Only few QTLs conferred resistance to 2-3 isolates, and no QTLs conferred resistance to more than 3 isolates. By comparative analysis for QTLs from two RILs populations,17 loci from populationⅠand 15 loci from populationⅡwere found to locate within or near to the same region as other QTLs or major genes identified by other researchers using different mapping populations. Eleven QTLs identified from the two RIL populations were located in the same loci in this study, including 5 loci corresponding to the same location of QTLs identified in other mapping populations. This indicates that some consensus QTLs for blast resistance were harbored in rice genome,which are helpful for fine mapping and map-based cloning of quantitative resistance loci.