Location Of Wheat Powdery Mildew Resistance Genes With Molecular Marker And Marker-Assisted Selection

Powdery mildew of commom wheat (Triticum aestivum L.), caused by Blumeria graminis (DC.) E.O. Speer f. sp. tritici Em. Marchal (syn. Erysiphe graminis f. sp. tritici), is one of the major disease of wheat worldwide, especially in highly productive areas with a maritime or semi-continental climate. In china, wheat powdery mildew used to occur only occasionally in the southwestern plateau and coastal area of Shandong Province before the 1970s. However, its severity increased markedly, mainly due to high application rates of nitrogenous fertilizers, production of semidwarf wheat cultivars and expansion of irrigated area. Utilization of resistant cultivars is the most economical, efficient and environmentally friendly approach to control this disease, as long as sources of resistance are available. Molecualr markers tightly linked to resistance genes not only facilitates the identification and mapping of resistance genes, but also can be used for marker-assisted selection (MAS). MAS greatly enhances the breeding efficiency and expedites the process. In the present study, we had constructed the genetic maps of Pm3b, Pm4b, Pm12, Pm1c and Pm23, discussed the application of the tightly linked markers in gene pyramiding and backcrossing breeding.1. The powdery mildew resistance gene Pm23, identified in the common wheat Line 81-7241 and originally assigned to wheat chromosome 5A, was relocated on chromosome 2AL with the aid of molecular markers. Mapping of microsatellite markers in two wheat crosses segregating for Pm23 and Pm4b, respectively, in combination with the reported mapping of Pm4a, indicated that the three genes were all linked to the marker Xgwm356 with a distance of 3-5 cM. Allelism between Pm4b and Pm23 was then confirmed, when the progenies of a cross between VPM1 (Pm4b) and Line 81-7241, were shown to be all resistant to a B. graminis isolate avirulent to the both parents. Pm23 is therefore a new allele of the Pm4 locus, and was redesignated as Pm4c.2. Pm1c was located on the segment of the tightly linked resistance genes familie of 7AL chromosome. Preferred small group (PSG) was used to search SSR markers linked to the resistance gene. 7 markers (Xgwm282, Xgwm332, Xgwm344, Xgwm63, Xcfa2040, Xwmc525 and Xpsp3094) located on chromosome 7AL, were identified with a little larger genetic distances, whereas, the markers Xbarc70 and Xbarc78, physically assigned to chromosome 4AL, were tightly linked to Pm1c with genetic distances of 0.9 cM and 0.8 cM, respectively. This indicated the possible translocation or chromosomal rearrangement had occured between chromosome 4AL and 7AL. And the study has provided molecular evidence for the special phenomenon, also will benefit the next research for the origin of Pm1c.3. Separately using the SSR markers located on chromosome 1AS (57 SSRs) and 6B (66 SSRs), together with the PSG method, 2 and 4 microsatellite markers were identificated linked to Pm3b and Pm12, respectively. The markers sequence and genetic distances are Xgdm33-3.6cM-Pm3b-2.7cM-Xwmc104 for Pm3b, and Xbarc76-2.7cM-Xwmc487-2.4cM- Xswes180-4.9cM-Pm12-4.1cM-Xswes222 for Pm12. The tightly linked markers can be effectively used in MAS for gene pyramiding.4. Using the markers tightly linked to powdery mildew resistance genes, we genotyped the cultivars or lines possessing the different Pm genes to discuss the possible application in MAS. For the markers Xgdm33 and Xwmc104 linked to Pm3b, it was effective to identify Pm3b in pyramiding populations of Pm3b,Pm4b,Pm12,Pm1c and Pm19, except for the Pm23. When using the linked markers Xswes180 and Xswes222, we can determine the existance of Pm12 in the pyramiding of 6 genes. And the marker Xbarc78 linked to Pm1c exhibited the similar genotype to Pm19, however, it was a good choice to differentiate Pm1c from other 4 genes. For the markers Xbarc122 and Xgwm356, they were both linked to Pm4b and Pm23, as the fact that Pm4b and Pm23 were allele, thus the both markers were effective to identify Pm4 loci (Pm4b and Pm4c) from Pm3b, Pm12, Pm1c and Pm19.5. Line Shannong 2618 is a promising wheat line with high yield and good agronomical characeteristics, together with HMW-GS 5 + 10 and 7 + 8, but shows susceptible to wheat powdery mildew. The molecular markers Xbarc122375 tightly linked to Pm23 and AS-PCR marker Dx5-470bp linked to Dx5 were used to select the ideal plants in backcrossing population from Shannong2618/Pm23. And the improved Line Shannong 2618 with better resistance to wheat powdery mildew were easily obtained.