| Powdery mildew, caused by the obligate fungus Blumeria graminis f. sp tritici (Bgt), is causing increasing damage to wheat production because of semi-dwarf, nitrogen fertilizer and the climate, especially in the past decade. To combat the disease, a continuing need exists to discover new genes and for powdery mildew resistance and deploy those genes for breeding programs. To date, about 79 formally designated Pm genes have been cataloged at 50 loci, but only Pm3b, Pm21 and Pm38 have been cloned. The cloning of Pm genes will be helpful both for accelerating the research progress of Pm genes in wheat and improving the utilization of resistance genes in wheat breeding. The Pm40 gene, conferring resistance to many currently important races of Blumeria graminis f.sp tritici (Bgt) in China, was still effective in controlling the wheat powdery mildew, such as, Shandong shouguang-3, Shandong pingyi-3 and Henan xinxiang-3, etc. Therefore, it’s necessary to paying more attention on the Pm40 gene.The Pm40 gene was previously mapped on wheat chromosome7BS with low-density molecular markers. In the present research, genomes of Brachypodium distachyon and Oryza sativa japonica were used to develop conserved EST-STS molecular markers to increase the marker density of the chromosomal region containing the Pm40 locus, which will provide a sound foundation for further fine mapping and map-based cloning. In this study,601 F2 plants and F2:3 lines of population derived from cross of L693/L1034 were used for fine mapping of Pm40. Wheat expressed sequence tags (ESTs) located in wheat 7BS chromosome were used to develop sequence tagged site (STS) markers. The EST-STS markers flanking Pm40 were used to characterize collinear regions of model crop.The main results obtained from this study are as follow:1. To determine inheritance of the Pm40 gene in wheat line L693, a total of 601 F2 plants and F2:3 progenies were inoculated with Bgt pathotypes E28 at the seeding stage in the greenhouse and adult stage. We found the all F1 plants resistant to powdery mildew; the F2 population segregated in 450 resistant and 151 susceptible among 601 plants, fitting a 3:1 radio (x23:1= 0.005, P=0.943), the segregation of the F2:3 fit a 1:2:1 radio (x21:2:1=0.005, P=0.997), indicating that the powdery mildew resistance in the L693/L1034 population were controlled by a single dominant gene. The reactions were same to Bgt28 both seeding stage and adult stage.2. Previous reported SSR markers linked with Pm40 were firstly used to test for polymorphism in present population. Four markers (Xwmc335, Xgwm297, Xwmc364, Xwmc476) showed clear polymorphisms and were linked with Pm40. Two additional markers (Xwmc662, Xgwm43) out of 87 SSR markers mapped on chromosome 7BS showed polymorphisms and were linked with Pm40. The order of the 6 SSR markers on wheat 7BS is Xwmc335, Xgwm297, Xwmc662, Xwmc476, Xwmc364, Xgwm43. The genetic distance between Pm40 and the markers were 0.58 cM,0.74 cM,0.24 cM,0.37 cM,0.33 cM and 0.86 cM, respectively. The Pm40 is flanked by Xwmc335 and Xgwm297.3. To saturate the chromosomal region containing the Pm40 locus with more markers,351 EST-STS markers were developed according to the wheat ESTs mapped on chromosome 7BS. Results showed that 22 markers were polymorphic and seven were linked with Pm40 in the F2 population. Each marker locus segregated in 1:2:1 or 3:1 ratios. A linkage map flanking wheat chromosome 7BS was constructed with six SSR markers and seven EST-STSs. Pm40 is flanked by BF291338 and Xwmc335 at genetic distances of 0.26 cM and 0.58 cM, respectively. The genetic map spans 6.181cM and the average distance is 0.44cM between markers.4. To accurately characterize the collinearity between the Pm40 region and the genomic regions of Brachypodium distachyon, Oryza sativa japonica and Sorghum bicolor, three sequences (BE423064, BE446359, BF291338) corresponding to the mapped wheat ESTs were used as queries to perform a BLAST search against the Brachypodium distachyon, Oryza sativa japonica and Sorghum bicolor genome sequences. Comparative genomic analysis established the collinearity of the Pm40 genomic region with a 1.09 Mbp genomic region on chromosome 3 in Brachypodium distachyon, a 1.16 Mbp genomic region on chromosome 8 in Oryza sativa japonica and a 1.62 Mbp genomic region on chromosome 7 in Sorghum bicolor.5. The high density genetic map of Pm40 is capable of providing excellent genetic markers for marker assistant slection breeding, it can speed up the breeding progress; The Pm40 region orthologous coliner sequence on Brachypodium distachyon, Oryza sativa japonica and Sorghum bicolor discoveryed in this study could paly an important role in seeking for candidate gene and homologous cloning. |
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