Search

A total of 47 rice accessions collected from Kenya were investigated the genetic variations and classified into two cluster groups, A and B, by polymorphism data of 65 simple sequence repeat (SSR) markers. Clusters A and B corresponded to Japonica and Indica Groups, respectively. The number of Japonica Group accessions was limited in comparison with those of the Indica Group. Based on their patterns of reaction to standard differential blast isolates (SDBIs), these accessions and 57 control cultivars including differential varieties and several accessions harboring partial resistance genes were classified again into three cluster groups: Ia (high resistance), Ib (intermediate resistance) and II (susceptible). The rice accessions from Kenya were classified only into groups Ia and Ib. The accessions from Kenya were finally classified into three categories, A-Ia, B-Ia and B-Ib, based on the two classifications of polymorphism of SSR markers and resistance. The Indica Group accessions had wider genetic variation for blast resistance than did the Japonica Group accessions. The three leading cultivars (Basmati 217, Basmati 370 and ITA 310) categorized into Cluster group Ia were susceptible to some SDBIs from Kenya. The genetic variation for blast resistance in Kenya was demonstrated as the first report using SDBIs.

The fungal pathogen Pyricularia oryzae causes blast, a severe disease of rice (Oryza sativa L.). Improving blast resistance is important in rice breeding programs. Inoculation tests have been used to select for resistance genotypes, with DNA marker-based selection becoming an efficient alternative. No comprehensive DNA marker system for race-specific resistance alleles in the Japanese rice breeding program has been developed because some loci contain multiple resistance alleles. Here, we used the Fluidigm SNP genotyping platform to determine a set of 96 single nucleotide polymorphism (SNP) markers for 10 loci with race-specific resistance. The markers were then used to evaluate the presence or absence of 24 resistance alleles in 369 cultivars; results were 93.5% consistent with reported inoculation test-based genotypes in japonica varieties. The evaluation system was successfully applied to high-yield varieties with indica genetic backgrounds. The system includes polymorphisms that distinguish the resistant alleles at the tightly linked Pita and Pita-2 loci, thereby confirming that all the tested cultivars with Pita-2 allele carry Pita allele. We also developed and validated insertion/deletion (InDel) markers for ten resistance loci. Combining SNP and InDel markers is an accurate and efficient strategy for selection for race-specific resistance to blast in breeding programs.

Characterization of resistance genes is essential for the development of a new variety resistant to blast (Magnaporthe grisea) in rice (Oryza sativa L.). In this study, we found that an indica variety, Kasalath, harbored a resistance gene that showed a broad spectrum of resistance reactions against standard differential blast isolates from the Philippines and Japan. Quantitative trait locus (QTL) analysis was conducted using backcross inbred lines derived from crosses between Kasalath and the japonica cultivar, Koshihikari, to detect the QTL for blast resistance. QTLs were detected on chromosomes 6 and 8. Further, QTL analysis of chromosome segment substitution lines of Kasalath in the Koshihikari genetic background revealed that the Kasalath allele at the QTL on chromosome 6 strongly contributed to blast resistance. These results suggested that the gene locus might be identical to Pi13(t) that had already been reported. By marker-assisted breeding, a near-isogenic line, Koshihikari Toyama BL7, with a Kasalath chromosome segment containing Pi13(t) introduced into Koshihikari genetic background, was developed. The reaction of Koshihikari Toyama BL7 to the differential blast isolates indicated that Pi13(t) was a broad-spectrum and moderate resistance gene. Since Pi13(t) showed single-gene segregation and was characterized, the gene was re-designated as Pi13.

We precisely mapped the Pb1 locus for the panicle blast resistence of a quantitative nature on rice chromosome 11 using RFLP markers. Based on the cosegregation of the resistant genotypes of Pb1 and Stvb-i, a rice stripe virus (RSV) resistance gene derived from an Indica cultivar Modan, we examined the linkage relationships between Pb1 and 13 Stvb-i-linked RFLP markers located on the long arm of chromosome 11. The Pb1 locus was mapped in the Modan-derived chromosomal region in the middle part of the long arm of chromosome 11. Pb1 was located on the telomere side in relation to the Stvb-i locus. The Pb1 gene was closely located at 1.2cM from three RFLP markers: S723, CDO226, C189. Then, we examined the graphical genotypes of 34 Modan-derived RSV-resistant cultivars with or without panicle blast resistance, and 12 susceptible Japonica cultivars, using 21 RFLP markers. Among them, cultivars with panicle blast resistance were classified into four types: A, B, C and D, and those without it into six types: E, F, G, H, I and J. In all of the panicle blast-resistant cultivars, the Modan-type bands were observed in S723, CDO226 and C189. On the other hand, in all the panicle blast-susceptible cultivars, the Japonica-type band was observed in S723, whereas the Modan-type bands were noticed in CDO226 and C189 in Type E cultivars. Consequently, only the genotypes of the S723 locus completely coincided with the genotypes of the Pb1 locus. Although, it was determined, based on linkage analysis, that S723, CDO226 and C189 were located at the same locus, graphical genotyping analysis, using many progeny cultivars derived from Modan, revealed that S723 was the closest marker to Pb1 among the three.