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United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: Identification and Utilization of Ug99 Resistance Genes from Wild Relatives of Wheat

Location: Cereal Crops Research

2012 Annual Report


1a.Objectives (from AD-416):
The objectives of this cooperative research project are to:.
1)identify Ug99 resistance genes from wild relatives of wheat,.
2)introgress alien species-derived Ug99 resistance genes into wheat, and.
3)deploy Ug99 resistance genes into wheat for the Northern Plains region.


1b.Approach (from AD-416):
Approximately 500 accessions of wild species closely related to wheat and 46 wheat-wild species derivatives will be evaluated for reactions to Ug99 and additional virulent races. The wild species include five diploid Aegilops species with C, D, M, S, and U genomes, four polyploid Aegilops species with the D genome (Ae. cylindrica, Ae. ventricosa, Ae. crassa, and Ae. juvenalis), and five Thinopyrum species (Th. junceum, Th. intermedium, Th. bessarabicum, Th. elongatum, and Th. ponticum). The wheat-wild species derivatives include 23 wheat-Th. intermedium disomic addition lines and 23 introgression lines with small Th. ponticum chromosome segments derived from somatic cell hybridization. They will first be tested for reactions to two African races, TTKSK (Ug99) and TRTTF, and the North American race TTTTF, all with broad virulence spectra at the seedling stage. The genotypes showing resistance to TTKSK will be further characterized with eight races, including three races in the Ug99 lineage (TTKSK, TTKST, and TTTSK), and 3 - 5 North American races (QFCS, QTHJ, RCRS, RKQQ and TPMK). The accessions of the wild relatives identified as resistant to the Ug99 lineage, TRTTF, and TTTTF will be crossed to the durum ‘Rusty’ and common wheat ‘Chinese Spring’. Forty new amphiploids will be synthesized from these crosses using embryo rescue and chromosome doubling techniques. Amphiploids with high levels of resistance will be used to develop alien chromosome addition, substitution, and translocation lines. Stem rust testing and molecular marker analysis will be used to facilitate this introgression process. In addition, about 100 new synthetic hexaploid wheat (SHW) lines will be developed from crosses of Rusty with the Ae. tauschii accessions possessing resistance to the Ug99 races, as well as from crosses of susceptible Ae. tauschii accessions to Triticum carthlicum, T. dicoccum, T. polonicum, T. turanicum, and T. turgidum accessions possessing high levels of resistance to the three races. The SHW lines with high levels of Ug99 resistance will be directly used as donors for developing adapted or elite wheat germplasm through conventional backcross procedures. The currently available Sr genes that are effective against Ug99, including Sr22, Sr25, Sr33, Sr35, Sr36, Sr37, Sr39, Sr42, and Sr45, will be deployed in current durum and bread wheat cultivars adapted to the Northern Plains. Some of these genes have been tagged by molecular markers and/or can be identified using a local pathotype of stem rust; these genes will be directly transferred into the recipients through 3-5 consecutive backcrosses using marker-assisted selection and/or stem rust testing. For the other Sr genes, we will first develop BC1F1 seeds (~30 - 50 seeds for each donor), which will be quickly advanced to BC1F5 through single seed descent. The resistant BC1F5-derived lines will be selected by Ug99 testing. In addition, the resistance genes in one accession from each of the five tetraploid wheat species with high levels of resistance to the Ug99 races, TRTTF, and TTTTF, will be characterized using molecular markers and will be used to develop adapted durum and bread wheat germplasm.


3.Progress Report:

Identification of stem rust resistance genes from wild relatives of wheat. We evaluated 239 accessions belonging to five Thinopyrum species (Th. bessarabicum, Th. elongatum, Th. intermedium, Th. junceum, and Th. ponticum) for their seedling resistance to six major stem rust races (TMLKC, RTQQC, TPPKC, QFCSC, TCMJC, and TPMKC) in the U.S. Results showed that the accessions were resistant to all or most of the races, with the exception of a Th. elongatum accession that was susceptible to all six races. Among the five species, Th. elongatum and Th. ponticum exhibited the highest level of resistance and most of the accessions of the two species showed near-immunity to all of the races. The accessions belonging to the other three species (Th. bessarabicum, Th. intermedium, and Th. junceum) had varied levels of resistance ranging from near-immunity to moderate resistance. The results from this study substantiate that the Thinopyrum species are a rich source of stem rust resistance. Association mapping of Ug99 resistance in cultivated emmer using high throughput SNP markers. We previously evaluated 359 T. dicoccum accessions for resistance to TTKSK, TRTT, and TTTT. A subset of 181 accessions was genotyped with wheat 9K SNP markers. We identified 2,356 polymorphic SNP markers distributed across the A and B genomes. After filtering markers/accessions on missing data points and minor allele frequency, 1,289 mapped SNPs and 178 emmer accessions were used for linkage disequilibrium and association analysis. The results showed that a number of SNP markers located on chromosomes 5A and 5B were significantly associated with TTKSK resistance. No known Sr genes were previously mapped to these regions, suggesting that some emmer accessions may carry novel Sr genes. The accessions with high levels of TTKSK resistance represent useful resources for genetic studies and breeding for stem rust resistance. Identification and introgression of alien species-derived Ug99 resistance genes into wheat. We previously identified five Ae. tauschii accessions (CIae 12, CIae 25, RL5271, TA1659, and TA2377) and identified 81 cultivated emmer (T. dicoccum) accessions with resistance to Ug99. To transfer the resistance genes into hexaploid wheat, about 100 new SHW lines have been developed by crossing Ug99 resistant cultivated emmer accessions to Ae. tauschii. To identify the resistance gene in CIae 25, we previously evaluated 800 F2 plants from the Ae. tauschii cross CIae 25 × AL8/78 (AL8/78 is an Ae. tauschii accession susceptible to stem rust) for resistance to TPMK and identified 192 susceptible plants. By bulked segregant analysis, we identified a new Sr gene in the distal region of Ae. tauschii chromosome 2DS. Deployment of Ug99 resistance genes into wheat for the Northern Plains region. We initiated the introgression of the Sr genes that were effective against Ug99 to durum and bread wheat cultivars in the previous year. In 2012, a total of 31 cross combinations were made in three classes of wheat (durum, hard red spring, and hard red winter wheat) for eight genes (Sr2, Sr22, Sr25, Sr26, Sr35, Sr36, Sr39, Sr40, Sr47, and Sr-Cad). Seven, 12, and 12 cross combinations were further made for durum, spring common wheat, and winter common wheat, respectively. Twelve BC2F1’s and ten BC1F1’s have been produced. Seven F1’s have been reproduced due to hybrid sterility or poor germination of the hybrid seeds.


Last Modified: 8/22/2014
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