| Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici, is a serious disease worldwide,especially in humid areas, where it causes partial or total loss of grain yield on susceptible varieties when climatic conditions are favorable. The most effective strategy for controlling powdery mildew would be to make full use of resistance cultivars. Identifying powdery mildew resistance genes and developing reliable molecular markers are essential for breeding powdery mildew resistance cultivars using marker assisted selection (MAS) approach in wheat breeding program. Wild emmer (Triticum turgidum var. dicoccoides, 2n =4x = 28, AABB), which has the same genome formula as durum wheat and has contributed two genomes to bread wheat, is the progenitor of cultivated wheat. It possesses many important beneficial traits, including powdery mildew resistance. In this study, powdery mildew resistance line 3D232, a hexaploid progeny of wild emmer and common wheat,was used to cross with susceptible common wheat line XueZao to generate segregating population for fine mapping the powdery mildew resistance gene Ml3D232 in line 3D232. High-resolution genetic linkage map of Ml3D232 was constructed via SSR, EST and comparative genetics analysis. By screening wild emmer BAC library, the physical map of powdery mildew resistance gene Ml3D232 region was constructed.Through BAC sequencing, gene annotation, and association mapping analyses, the candidate gene of M13D232 was determined. The main results obtained are as the following:1. Comparative genomics analysis using ESTs flanking the powdery mildew resistant gene M13D232 against the Brachypodium genome sequences revealed that the M13D232 genomic region is highly co-linearity with the distal region of Brachypodium chromosome 4 from genes Bradi4g37380 to Bradi4g37980. Comparative genetic linkage maps showed that powdery mildew resistance genes MlWE4, Pm36 and Ml3D232 were mapped in the same genetic interval on chromosome arm 5BL and co-segregated with markers XBD37670 and XBD37680, indicating they are likely the same gene or alleles in the same locus.2. The flanking markers XBD37380 and XBD37980 were used to screen a 4300 F2 plants separating populations of XueZao/3D232. A total of 170 recombination plants were identified between the two markers and a fine genetic linkage map of powdery mildew resistance gene Ml3D232 was constructed. Ml3D232 was located in a 0.035cM interval between markers XBD37670 and XBD37680.3. By screening wild emmer BAC library and performing chromosome walking, a physical map including 7 overlapping BAC clones 62B24, 389B11,814, 15117, 805D23, 848122 and 225L10 was constructed for the powdery mildew resistance gene M13D232 genomic region. After BAC sequencing and gene annotation, a BTB-BACK domain gene cluster including 7 members was identified in the 551 kb genomic regions, and was thereafter named BBK1, BBK2, BBK3, BBK4, BBK5, BBK6 and BBK7.New markers WGGC3637, WGGC3712, WGGC5183 and WGGC5188 developed from the BAC contig sequences were co-segregated with the powdery mildew resistance gene M13D232.4. Association mapping was performed on the Al3D232 candidate genomic region using a panel of 192 wild emmer accessions collected from Israeli. Based on high-resolution mapping, haplotype analysis and the B. graminis f. sp. tritici inoculation results of different Bgt isolates on the wild emmer,the BBKI was proposed as the only candidate gene for the powdery mildew resistance gene M13D232.5. Two tightly-linked and 4 co-segregated molecular markers were tested on a set of new wheat breeding lines and diversified wheat lines containing different powdery mildew resistance genes to assess their potential of marker-assisted selection for Ml3D232 in wheat breeding program. The results indicated that 5 markers (XBD37670, WGGC5183, WGGC3712, WGGC5188 and WGGC5192) could be used as diagnostic markers in selection of Ml3D232 in breeding programs. WGGC3712 was the most reliable and user-friendly marker for M13D232. |
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