| Blast, caused by Magnaporthe grisea, is one of the most serious diseases of rice. Resistance is often lost in a few years after cultivars are released. Developing durable resistance cultivars with several resistance genes is an effective approach for blast control. It is difficult to pyramid several resistance genes into one cultivar with traditional breeding method because of environment and variability of the rice blast fungus. Marker assisted selection (MAS) could overcome these problems and breed cultivars with durable resistance in a short period of time. This study focused on the selection of resistiance cultivars and mapping of the resistance genes for breeding cultivars with durable resistance .66 rice cultivars and 31 monogenic lines with blast resistance were challenged with 45 isolates of Magnaporthe grisea. All the cultivars belonged to wide spectrum of resistance. Comparision of the resistance spectrum of the cultivars with that of monogenic lines indicated that most of the cultivars could contain more than one resistance gene and most of them should harbor new resistance genes.Simple sequence repeat (SSR) analysis was performed using bulked-segregant analysis (BSA) in combination with recessive-class analysis (RCA). Three resistance genes in Xinguangmei(XGM) and two in Yanxian No.1(YX) were characterized and mapped through linkage analysis. Resistance genes in XGM were named tentatively as Pi-x(t), Pi-x1(t) and Pi-x2(t), respectively. Pi-x(t) was located on rice chromosome 2, linked to RM3535 and RM3248 at 3.4 cM and 1.5 cM, respectively. This resistance gene was located on the same region with Pi-g(t) and Pi-b. But it was not allelic to Pi-g(t) through allelisim test, and also different from Pi-b through Pi-b gene specific PCR detection. Both Pi-x1(t) and Pi-x2(t) were located on chromosome 12 flanked by RM101 and RM277. Pi-xl(t) was flanked by the two markers at 4.3 cM and 12.3 cM, respectively. Pi-x2(t) was at a distance of 4.3 and 12.0 cM to the two markers, respectively. It needs to be further proved whether Pi-x1(t) and Pi-x2(t) are different or not because they are very close. XGM might carry Pi-12,Pi-19, and Pi-20 on chromosome 12 through comparision between the resistance spectrum of XGM and that of monogenic lines. So, it is very necessary to further determine the relations between Pi-x1(t) and Pi-x2(t) with these known resistance genes.Resistance genes in YX were named tentatively as Pi-y(t) and Pi-y1(t), respectively. Both resistance genes were mapped on chromsome 2 flanked by RM3248 and RM208. Pi-y(t) was at the same distance of 0.8 cM to the both markers. And Pi-y1(t) was at a distance of 1.3cM and 1.7cM to the two markers, respectively. It needs to be further proved whether Pi-y(t) and Pi-y1(t) are different or not because their genetic distances are very closed. Pi-y(t) and Pi-y1(t) were close to the known resistance genes Pi-x(t), Pi-g(t), and Pi-b on chromosome 2. Allelism test showed that Pi-y(t) and Pi-y1(t) were not allelic to Pi-x(t) and Pi-g(t). In Pi-b gene specific PCR detection, YX could be applified a specific product to Pi-b. This indicated YX might possess Pi-b gene. So, Pi-y(t) and Pi-y1(t) might be or allelic to Pi-b. It is very necessary to further dertimine the relations between Pi-b and Pi-y(t) or...
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