The wheat (Triticum aestivum L. and T. turgidum L.) stem rust resistance gene Sr6 confers a high level of resistance against a wide range of races of Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. and E. Henn. in North America. In this study, we report the identification of flanking markers for Sr6. We used a population of 139 recombinant inbred lines from the cross MN99394 * MN98550. A partial linkage map of chromosome 2D comprised Xgwm484, Xcfd77, Xcfd43, Xwmc453, Sr6, XwPt_4381, XwPt_0330, Xgpw94049, and Xgwm102. Four markers mapped distal to Sr6, while the other four were proximal within distances of 4.8 and 6.5 cM, respectively. Marker-assisted selection and pyramiding of Sr6 with other stem rust resistance genes could benefit from the availability of Sr6-flanking markers.
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Puccinia graminis f. sp. tritici is the causal agent of stem rust of wheat. A new race designated TTKSK (also known as Ug99) and its variants (TTKST and TTTSK) are virulent to most of the stem rust resistance genes currently deployed in wheat cultivars worldwide. Therefore, identification, mapping, and deployment of effective resistance genes are critical components of global efforts to mitigate this threat. Multipathotype seedling tests demonstrated that resistance gene Sr35 is effective against the three TTKS variants and another broadly virulent race from Yemen, TRTTF. Two genetic maps of Sr35 are presented in diploid (Triticum monococcum) and two in hexaploid wheat (T aestivum). The Sr35 resistance to TRTTF and RKQQC races was mapped in diploid wheat within a 2.2 to 3.1 cM interval on the long arm of chromosome 3A(m) between markers XBF483299 and XCJ656351. This interval corresponds to a 174-kb region in Brachypodium that includes 16 annotated genes. The Sr35 map location was confirmed in two backcross-derived hexaploid populations segregating for Sr35. Recombination between diploid and hexaploid chromosomes was 10-fold lower than between homologous chromosomes, but was sufficient to reduce the introgressed diploid segment. These maps provide markers closely linked to Sr35 that will be useful to accelerate its deployment and pyramiding with other stem rust resistance genes.
Race Ug99 of the fungus Puccinia graminis tritici that causes stem or black rust disease on wheat was first detected in Uganda in 1998. Seven races belonging to the Ug99 lineage are now known and have spread to various wheat-growing countries in the eastern African highlands, as well as Zimbabwe, South Africa, Sudan, Yemen, and Iran. Because of the susceptibility of 90% of the wheat varieties grown worldwide, the Ug99 group of races was recognized as a major threat to wheat production and food security. Its spread, either wind-mediated or human-aided, to other countries in Africa, Asia, and beyond is evident. Screening in Kenya and Ethiopia has identified a low frequency of resistant wheat varieties and breeding materials. Identification and transfer of new sources of race-specific resistance from various wheat relatives is underway to enhance the diversity of resistance. Although new Ug99-resistant varieties that yield more than current popular varieties are being released and promoted, major efforts are required to displace current Ug99 susceptible varieties with varieties that have diverse race-specific or durable resistance and mitigate the Ug99 threat.
Chromosome engineering is a useful strategy for transfer of alien genes from wild relatives into modern crops. However, this strategy has not been extensively used for alien gene introgression in most crops due to low efficiency of conventional cytogenetic techniques. Here, we report an improved scheme of chromosome engineering for efficient elimination of a large amount of goatgrass (Aegilops speltoides) chromatin surrounding Sr39, a gene that provides resistance to multiple stem rust races, including Ug99 (TTKSK) in wheat. The wheat ph1b mutation, which promotes meiotic pairing between homoeologous chromosomes, was employed to induce recombination between wheat chromosome 2B and goatgrass 2S chromatin using a backcross scheme favorable for inducing and detecting the homoeologous recombinants with small goatgrass chromosome segments. Forty recombinants with Sr39 with reduced surrounding goatgrass chromatin were quickly identified from 1,048 backcross progenies through disease screening and molecular marker analysis. Four of the recombinants carrying Sr39 with a minimal amount of goatgrass chromatin (2.87-9.15% of the translocated chromosomes) were verified using genomic in situ hybridization. Approximately 97% of the goatgrass chromatin was eliminated in one of the recombinants, in which a tiny goatgrass chromosome segment containing Sr39 was retained in the wheat genome. Localization of the goatgrass chromatin in the recombinants led to rapid development of three molecular markers tightly linked to Sr39. The new wheat lines and markers provide useful resources for the ongoing global effort to combat Ug99. This study has demonstrated great potential of chromosome engineering in genome manipulation for plant improvement.
Using simple sequence repeat (SSR) and amplified fragment length polymorphism (AFLP) marker analyses, the genetic structure of selected South African wheat stem rust races was compared with Ug99. SSR analysis divided the population into two distinct groups with 24.5% similarity between them. A local race, UVPgt55 (North American race notation TTKSF), grouped with Ug99 (TTKSK) with a 100% similarity. When AFLP data were included, the same groups were found, but with an increased similarity of 66.7%. Although the SSR data were unable to distinguish between all individual isolates, the AFLP data alone and in combination with the SSR data discriminated between the isolates. The grouping of individual isolates resembled the pathogenicity profile of the different races. On the basis of its similarity with Ug99, it was concluded that UVPgt55 was most probably an exotic introduction into South Africa, whereas the other races specialized locally through mutational adaptation.
Wheat stem rust, caused by Puccinia graminis f. sp. tritici, has been effectively controlled through the use of genetic resistance. P graminis f. sp. tritici race TTKSK (Ug99) possesses virulence to many resistance genes that have been used in wheat breeding worldwide. One strategy to aid breeders in developing resistant cultivars is to utilize resistance genes transferred from wild relatives to wheat. Stem rust resistance genes have previously been introgressed from Triticum monococcum to wheat. In order to identify additional resistance genes, we screened 1,061 accessions of T monococcum and 205 accessions of T urartu against race TTKSK and four additional P. graminis f. sp. triad races: TTTTF, TRTTF, QFCSC, and MCCFC. A high frequency of the accessions (78.7% of T monococcum and 93.0% of T urartu) were resistant to P graminis f. sp. tritici race TTKSK, with infection types ranging from 0 to 2+. Among these resistant accessions, 55 T monococcum accessions (6.4% of the total) were also resistant to the other four races. Associations of resistance in T monococcum germ-plasm to different races indicated the presence of genes conferring resistance to multiple races. Comparing the observed infection type patterns to the expected patterns of known genes indicated that previously uncharacterized genes
Identification of wheat stem rust (Puccinia graminis f. sp. tritici) isolate Ug99 has raised international concern. Since initial detection, seven races have been identified in the Ug99 lineage and occurrence is confirmed in nine countries. During rust surveys of Eritrea in 2010, stem rust was widespread in the highland wheat growing areas. Stem rust samples were analyzed for race identity and identified as TTKST (four isolates) and PTKST (five isolates). Both races belong to the Ug99 lineage and both exhibit combined Sr31 and Sr24 virulence. This first confirmation of TTKST and PTKST in Eritrea is important as it represents further geographical spread of Ug99-related races. Based on postulated migration routes of the original Ug99, the confirmed presence of TTKST and PTKST in Eritrea increases the possibility for range expansion out of Africa by crossing the Red Sea and into the Arabian Peninsula. Future spread of TTKST and PTKST to western Asia is considered highly likely.; Identification of wheat stem rust (Puccinia graminis f. sp. tritici) isolate Ug99 has raised international concern. Since initial detection, seven races have been identified in the Ug99 lineage and occurrence is confirmed in nine countries. During rust surveys of Eritrea in 2010, stem rust was widespread in the highland wheat growing areas. Stem rust samples were analyzed for race identity and identified as TTKST (four isolates) and PTKST (five isolates). Both races belong to the Ug99 lineage and both exhibit combined Sr31 and Sr24 virulence. This first confirmation of TTKST and PTKST in Eritrea is important as it represents further geographical spread of Ug99-related races. Based on postulated migration routes of the original Ug99, the confirmed presence of TTKST and PTKST in Eritrea increases the possibility for range expansion out of Africa by crossing the Red Sea and into the Arabian Peninsula. Future spread of TTKST and PTKST to western Asia is considered highly likely.
Wheat stem rust (Puccinia graminis f. sp. tritici) race TTKSK (Ug99), with virulence to the majority of the world's wheat (Triticum aestivum) cultivars, has spread from Uganda throughout eastern Africa, Yemen, and Iran. The identification and spread of variants of race TTKSK with virulence to additional stem rust resistance genes has reminded breeders and pathologists of the danger of deploying major resistance genes alone. In order to protect wheat from this rapidly spreading and adapting pathogen, multiple resistance genes are needed, preferably from improved germplasm. Preliminary screening of over 700 spring wheat breeding lines and cultivars developed at least 20 years ago identified 88 accessions with field resistance to Ug99. We included these resistant accessions in the stem rust screening nursery in Njoro, Kenya for two additional seasons. The accessions were also screened with a bulk of North American isolates of P. graminis f. sp. tritici in the field in St. Paul, MN. In order to further characterize the resistance in these accessions, we obtained seedling phenotypes for 10 races of P. graminis f. sp. tritici, including two races from the race TTKSK complex. This phenotyping led to the identification of accessions with either adult-plant or all-stage resistance to race TTKSK, and often North American races of P. graminis f. sp. tritici as well. These Ug99 resistant accessions can be obtained by breeders and introgressed into current breeding germplasm.
The race Ug99 of Puccinia graminis f. sp. tritici causing stem rust disease of wheat was initially identified in Uganda in 1998. It was designated as TTKSK based on the North American nomenclature and has caused periodic losses to wheat crops in East Africa. Ug99 has recently moved out of Africa to Yemen and West Asia.
Isolates of Puccinia graminis f. sp. tritici belonging to the Ug99 race group are virulent to a broad spectrum of resistance genes, rendering most of the world's wheat germplasm susceptible to stem rust (3). Following the initial detection of Ug99 (TTKSK, North American [NA] race notation) in Uganda, virulence to the widely used Sr31 resistance gene has been reported from Kenya, Ethiopia, Sudan, and Iran (2,3). In November 2009, a wheat genotype suspected to carry Sr31 showed a susceptible response to stem rust in a disease nursery (29°08′05.02′′S, 30°38′29.18′′E), inoculated with race TTKSP, near Greytown in KwaZulu-Natal, South Africa.