QTL Mapping For Adult-plant Resistance To Stripe Rust And Powdery Mildew In Common Wheat

Stripe rust and powdery mildew, caused by Puccinia striiformis f. sp. tritici and Blumeria graminis f. sp. tritici, respectively, are the devastating diseases in common wheat (Triticum aestivum L.) worldwide. Deployment of resistant cultivars is the most environmentally and economical way for controlling these diseases, particularly for cultivars with adult-plant resistance (APR). Therefore, the aims of this study were to identify the quantitative trait loci (QTLs) for APR to stripe rust and powdery mildew in common wheat, and to assess the stability of the QTL effects detected across 3 to 5 environments. This knowledge could lead to a more efficient strategy in wheat breeding for durable resistance to stripe rust and powdery mildew.In the present study, 1724 simple sequence repeat (SSR) markers were used to map QTLs for APR to stripe rust in two populations derived from the crosses Strampelli/Huixianhong and Pingyuan 50/Mingxian 169, respectively, and to powdery mildew in the DH population from Bainong 64/Jingshuang 16. The Strampelli/Huixianhong and Pingyuan 50/Mingxian 169 populations were planted in Beijing, Gansu and Sichuan during the 2004-05, 2005-06, 2006-07 and 2007-08 cropping seasons. Artificial inoculations were carried out in the three locations with the prevalent Chinese race CYR32. Disease severities were scored for 1-3 times in different locations, and the maximum disease severities (MDS) were used for QTL analysis. The Bainong 64/Jingshuang 16 DH lines were planted in Beijing and Anyang during the 2005-06 and 2007-08 cropping seasons. Artificial inoculations were carried out in Beijing using the highly virulent Blumeria graminis f. sp. tritici isolate E20. Disease severities on penultimate leaves were scored twice in Beijing, whereas at Anyang, MDS were scored under natural infection. Field trials were planted in randomized complete blocks with three replicates. Plots consisted of single 1.5-2.0 m rows with 25 cm between rows. QTLs were detected by composite interval mapping (CIM) and/or inclusive composite interval mapping (ICIM) using the software Cartographer 2.5 and IciMapping 2.2, respectively.1. In the F3 population of 252 lines derived from the cross Strampelli×Huixianhong, correlations for MDS ranged from 0.58 to 0.74 (P<0.0001) among five environments, and the broad-sense heritabilities of MDS was 0.97, based on the mean values averaged across environments. Joint and single–environment analyses by composite interval mapping identified three QTLs in Strampelli for reduceing stripe rust severity, designated QYr.caas-4BL, QYr.caas-5BL and QYr.caas-7DS, explaining 4.5%, 2.9-5.5% and 17.1-39.1% of phenotypic variance, respectively, across five environments. Three epistatic interactions between different pairs of QTLs accounted for 6.1-35.0% of the phenotypic variance, which is for the first time to have detected the epistasis of APR genes to stripe rust. The QTL QYr.caas-7DS flanked by markers csLV34 and Xgwm295 showed the largest effect for resistance to stripe rust. Sequence analyses confirmed that QYr.caas-7DS was Lr34/Yr18/Pm38 gene. The SSR markers Xgwm165 and Xgwm149, Xwmc415 and Xwmc537, and csLV34, closely linked to QYr.caas-4BL, QYr.caas-5BL and QYr.caas-7DS, with genetic distances of 2.8-3.6 cM, 2.5-4.3 cM and 6.8-17.0 cM, respectively. Therefore, SSR markers Xgwm165, Xwmc415, Xwmc537 and csLV34 could be used for marker-assisted selection in wheat breeding.2. The 540 SSR markers were screened to map QTLs for APR to stripe rust in a doubled haploid (DH) population of 137 lines derived from the cross Pingyuan 50×Mingxian 169. Broad-sense heritability of MDS was 0.91, based on the mean value averaged across four environments. Three QTLs for APR to stripe rust have been detected by inclusive composite interval mapping method on chromosomes 2BS, 5AL and 6BS, designated QYr.caas-2BS, QYr.caas-5AL and QYr.caas-6BS, respectively, explaining from 4.5 to 19.9% of the phenotypic variance. All QTLs were from Pingyuan 50. QYr.caas-5AL, different from all QTLs reported previously, was flanked by SSR markers Xwmc410 and Xbarc261, with a genetic distance of 0.6-6.8 cM, and it accounted for 5.0-19.9% of phenotypic variance. Moreover, Xgwm361 and Xbarc136 were closely linked to QYr.caas-6BS with a genetic disatnce of 0.2 cM. Thereby, SSR markers Xwmc410, Xbarc261, Xgwm361 and Xbarc136 closely linked to the two QTLs could be used in marker-assisted selection for APR to striper sust in wheat breeding programs.3. In the DH population of 181 lines derived from the cross Bainong 64×Jingshuang 16, the MDS scores were significantly correlated (P < 0.0001) across three environments, with correlation coefficients ranging from 0.61 to 0.76. The MDS and area under the disease progress curve (AUDPC) were significantly correlated for the test in Beijing over 2 years (r = 0.89, P < 0.0001). Broad-sense heritabilities of MDS and AUDPC were 0.89 and 0.77, respectively, based on the mean values averaged across environments. Composite interval mapping detected four QTLs for APR to powdery mildew on chromosomes 1A, 4DL, 6BS and 7A, and these were designated QPm.caas-1A, QPm.caas-4DL, QPm.caas-6BS and QPm.caas-7A, respectively, and explained from 6.3-22.7% of the phenotypic variance. All QTLs identified in this population were from the resistant parent Bainong 64. QTLs QPm.caas-4DL and QPm.caas-6BS were stable across environments with high genetic effects on powdery mildew response, accounting for 15.2-22.7% and 9.0-13.2% of the phenotypic variance, and Xwmc331 and Xbarc79 were closely linked to QPm.caas-4DL and QPm.caas-6BS with genetic distances of 0.8-11.0 cM and 0-11.9 cM, respectively. Therefore, Xwmc331 and Xbarc79 should be useful for the improvement of powdery mildew resistance in wheat.4. The 21 resistant F6 lines developed from a cross between Chinese winter wheat cultivars Bainong 64 and Lumai 21 with APR genes were tested for powdery mildew resistance in the field and 20 SSR markers. Five QTLs for APR to powdery mildew were found in BFB5, BFB9 and BFB16, and 11 lines possessed 4 QTLs. This validated the possibity and effectiveness for pyramiding the APR genes to powdery mildew in comme wheat using closely linked molecuar markers, and indicated that combination of 4-5 APR QTLs to powdery mildew can provide a near-immune level of adult-plant resistance. According to the above results, we can make crosses between Strampelli, Pingyuan 50, Bainong 64 and Chinese commercial susceptable cultivars with good agronomical traits and backcross 1-2 times with the susceptable cultivars. Molecular markers identified in this study will facilitate the selection of APR genes among the segregating progenies for breeding durable resistant wheat cultivars.