Inheritance And Mapping Of A Thinopyrum-derived Powdery Mildew Resistance Gene Introgressed Into Wheat

Powdery mildew,caused by Blumeria graminis f.sp.tritici.is an important disease of wheat(Triticum aestivum)worldwide.Since the late 1970s.powdery mildew has gadually spread in China as a consequence of changing crop practices such as irrigation, increased nitrogen fertilization,and the use of semi-dwarf cultivars.Especially during 1980s-1990s,mainly due to the wide employment of 1B/1R translocations contained the single gene Pm8 and their derivatives in wheat breeding programs,the high frequency of virulence to Pm8 in Chinese B.graminis f.sp.tritici populations has remained stable at about 94%,resulting in the wheat area affected by powdery mildew increased to 12.0 million ha in 1996 from 2.9 million ha in 1981.Resistant wheat cultivars remain the most cost efficient and effective means for powdery mildew control,while the introgression of alien resistance gone from relatives of wheat is one of the efficient ways to deal with a 'short live" problem of monogenic resistance.Among the Thinopyrum species,the hexaploid Th.intermedium(2n = 6x = 42, JJ~SS) and decaploid Th.ponticum(2n = 10x = 70,JJJJ~sJ~s) are two of the most important perennial Triticeae species in wheat improvement.The wheat-Thinopyrum partial amphiploids and their derivatives have been widely used in wheat breeding programs throughout the world,as a valuable source of multiple disease and mite resistance for increasing genetic diversity available to wheat breeders.In 1992,the inter-genus hybridization between wheat and Thinopyrum programs at Institute of Crop Genetics of Shanxi Academy of Agricultural Sciences initiated a program of the introgression of Thinopyrum-derived alien resistance genes for powdery mildew and yellow rust,crossing susceptible wheat cultivars with resistant wheat-Thinopyrum partial amphiploids as donor parents.To date a lot of derived lines have been developed with multi-resistance genes incorporated from Th.intermedium and Th.ponticum.The objectives of this study are:1) To determine the inheritance of resistance to powdery mildew and identify the number of resistance genes in the germplasm lines CH5026 and CH7034:and 2) To identify chromosomal location of the resistance genes introgressed from Th.intermedium and Th. ponticum using SSR markers,which would be helpful in employing new resistance genes, especially for pyramidal complexes containing several resistance genes. 1:CH5026,a wheat breeding line developed by backcrossing a partial wheat-Th. intermedium amphiploid TAI7045 with susceptible wheat cultivar,exhibits the resistance to both powdery mildew(Pro) and yellow rust.Greenhouse evaluation of resistance reactions at seedling and adult stage showed that CH5026 had Pm-resistance similar to its donor TAI7045 and Th.intermedium,whereas the wheat parents of both CH5026 and TAI7045 were susceptible,indicating that the Pm-resistance gene present in CH5026 derived from Th.intermedium,the grass parent of TAI7045.To investigate the inheritance mode of its Pro-resistance,CH5026 was crossed with the susceptible(S) cv.Jintai 170 and line CH5065.Parents and the resulted F1 plants,F2 and BC1 populations,and F2-derived F3(F2:3) lines were tested with mixed powdery mildew races,including Egt isolate E09, in the greenhouse.The F2 populations from two crosses segregated in a ratio of 3R:1S. and the BC1 population from a backcross of CH 5026(R) / Jintai170(S) // Jintai170(S) in a ratio of 1R:1S.The F2:3 lines from CH5026(R)×CH5065(S) exhibited a segregation pattern of 1(all R):2 segregating:1(all S).According to disease ratings of parents,and the F1,F2,and BC1 generations,and the F3 lines,the Pm-resistance in CH5026 was conferred by a single dominant gene.Using bulked segregant analysis(BAS),a CH5026(R)×CH5065(S) F2 population consisting of 161 individuals was screened with 349 wheat microsatellite(SSR) primer pairs to detect molecular markers linked to gene responsible for powdery mildew resistance.The results indicated that the microsatellite primer Xwmc41 and Xgwm539 on chromosomal arm 2DL was linked with the resistance gene,with the genetic distance 14.8 cM and 7.9cM2:Wheat germplasm line CH7034,developed by backcrossing a wheat-Th,ponticum partial amphiploid Xiaoyan7430 with susceptible wheat cultivars,was found to be resistant to both powdery mildew(Pm) and yellow rust.To determine the inheritance and chromosomal location of powdery mildew resistance gene,CH7034 was crossed with a susceptive wheat cultivar SY95-71.The seedling and adult tests of F1,F2 progeny plus parents were inoculated with Egt isolate E09 in greenhouse.F1 plants were consistent with their resistant parent CH7034 and resistant to Pro.In the F2 populations,the observed phenotypic classes for powdery mildew reaction satisfactorily fitted a 3R:1S segregation ratio,suggesting that the powdery mildew resistance in CH7034 was genetically controlled by a single dominant gene.Using bulked segregation analysis(BSA),a total of 148 F2 plants from the cross CH7034(R)/SY95-71(S) was screened with 307 wheat microsatellite(SSR) primer pairs to detect molecular markers linked to Pm resistance gene. The microsatellite marker linked to Pm resistance gene position were conformed using Chinese Spring nullitetrasomic and ditelosomic lines.The results indicated that the Pm resistance gene present in CH7034 was linked to microsatellite marker Xgwm311-2A with the genetic distance of 12.4cM and mapped to chromosome 2AL.