The stem rust fungus, particularly race TTKSK (Ug99), poses a serious threat to world wheat production. Gene Sr42 or SrCad (which could be the same gene or an allele of Sr42) is effective against race TTKSK. However, known genetic markers for Sr42 are mostly SSR markers which are generally labor intensive to use. In this study, we mapped a race TTKSK resistance gene derived from PI 595667 at the same locus as Sr42 on chromosome 6DS. Based on position, pedigree and infection-type information, we propose that this gene is SrCad (Sr42). We enriched the genetic map for the Sr42 region using genotyping by sequencing (GBS) and array-derived SNP markers. In total, 21 SNP markers were discovered, spanning a genetic distance of 27.2 cM. Nine of them are derived from GBS and twelve from the Illumina iSelect 90K SNP assay. Ten of the twenty-one SNP markers are closely linked (<2.2 cM, or co-segregating) with Sr42. We converted five of the closely linked SNP markers into uniplex KASP assays which will better facilitate marker-assisted selection. We validated the KASP assay in a doubled haploid wheat population derived from a three-way cross between accessions PI 410954, RB07, and Faller that shared an uncharacterized resistance gene mapped at approximately the same locus as PI 595667. The development of closely linked (co-segregating), codominant, sequence-based SNP assays will aid marker-assisted selection and map-based cloning of Sr42.
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Most South African winter wheat varieties display all stage resistance (ASR) to stem rust caused by Puccinia graminis f. sp. tritici (Pgt). To study inheritance, four resistant varieties were crossed to a susceptible parent (Line 37) and F2 populations were phenotyped at the seedling stage with stem rust race PTKST (Ug99 lineage). Populations derived from varieties Koonap, Komati, Limpopo and SST 387 segregated in a 3:1 ratio, indicating that a single, dominant gene confers resistance in each population. Assessment of F2 seedlings of four intercrosses between these varieties failed to deliver susceptible segregants therefore suggesting that they carry the same resistance gene. Genotyping of F2 plants with microsatellite markers produced consistent linkage of resistance with markers on chromosome 6DS. Experiments are underway to determine the relationship between resistance in the four winter wheat varieties and resistance genes Sr42, SrCad and SrTmp, all located on 6DS. Current evidence shows that ASR in the South African winter wheat varieties Koonap, Komati, Limpopo and SST 387 is based on a single gene and thus vulnerable to pathogenic adaptation in Pgt.
Stem rust, caused by Puccinia graminis f. sp. tritici, is a highly destructive fungal disease of wheat. This pathogen has been effectively controlled in western Canada through resistance since the 1950s. In 1999, a new highly virulent race of stem rust was identified in Uganda. The new strain, named “Ug99”, was given the North American race designation TTKSK. In situ screening has demonstrated that approximately 75% of Canadian wheat cultivars are susceptible to this new race of stem rust. Fortunately, two cultivars, Peace and AC Cadillac, were highly resistant to Ug99. A doubled haploid population was generated from the cross: RL6071/Peace, where RL6071 was the stem rust susceptible parent. In 2008, 189 DH lines from this population were evaluated for response to Ug99 in Kenya. RL6071 and Peace were rated: 80 S and 5 R, respectively. Disease ratings of the DH lines, ranged from 80 S to 1 R. Mendelian evaluation of the stem rust scores indicated a two-gene model (X2=5.51; 0.25<P<0.10; d.f.=3) of inheritance. Peace has the positive allele for the diagnostic Lr34 DNA marker (csLVMS1) published by Spielmeyer et al. (2008). It is believed that Peace carries Lr34 and that this gene may be one of the genes responsible for Ug99 resistance in this cultivar. Molecular mapping of the Ug99 resistance in cultivar Peace is underway.
More than 80 % of the worldwide wheat (Triticum aestivum L.) area is currently sown with varieties susceptible to the Ug99 race group of stem rust fungus. However, wheat lines Niini, Tinkio, Coni, Pfunye, Blouk, and Ripper have demonstrated Ug99 resistance at the seedling and adult plant stages. We mapped stem rust resistance in populations derived from crosses of a susceptible parent with each of the resistant lines. The segregation of resistance in each population indicated the presence of a single gene. The resistance gene in Niini mapped to short arm of chromosome 6D and was flanked by SSR markers Xcfd49 at distances of 3.9 cM proximal and Xbarc183 8.4 cM distal, respectively. The chromosome location of this resistance was validated in three other populations: PBW343/Coni, PBW343/Tinkio, and Cacuke/Pfunye. Resistance initially postulated to be conferred by the SrTmp gene in Blouk and Ripper was also linked to Xcfd49 and Xbarc183 on 6DS, but it was mapped proximal to Xbarc183 at a similar position to previously mapped genes Sr42 and SrCad. Based on the variation in diagnostic marker alleles, it is possible that Niini and Pfunye may carry different resistance genes/alleles. Further studies are needed to determine the allelic relationships between various genes located on chromosome arm 6DS. Our results provide valuable molecular marker and genetic information for developing Ug99 resistant wheat varieties in diverse germplasm and using these markers to tag the resistance genes in wheat breeding.
Stem rust, caused by Puccinia graminis f. sp. tritici, is a devastating disease of wheat. The emergence of race TTKSK (Ug99) and new variants in Africa threatens wheat production worldwide. The best method of controlling stem rust is to deploy effective resistance genes in wheat cultivars. Few stem rust resistance (Sr) genes derived from the primary gene pool of wheat confer resistance to TTKSK. Norin 40, which carries Sr42, is resistant to TTKSK and variants TTKST and TTTSK. The goal of this study was to elucidate the inheritance of resistance to Ug99 in Norin 40 and map the Sr gene(s). A doubled haploid (DH) population of LMPG-6/Norin 40 was evaluated for resistance to the race TTKST. Segregation of 248 DH lines fitted a 1:1 ratio (χ 2 1:1= 0.58, p = 0.45), indicating a single gene in Norin 40 conditioned resistance to Ug99. This was confirmed by an independent F2:3 population also derived from the cross LMPG-6/Norin 40 where a 1:2:1 ratio (χ 21:2:1 = 0.69, p = 0.71) was observed following the inoculation with race TTKSK. Mapping with DNA markers located this gene to chromosome 6DS, the known location of Sr42. PCR marker FSD_RSA co-segregated with Sr42, and simple sequence repeat (SSR) marker BARC183 was closely linked (0.5 cM) to Sr42. A previous study found close linkage between FSD_RSA and SrCad, a temporarily designated gene that also confers resistance to Ug99, thus Sr42 may be the same gene or allelic. Marker FSD_RSA is suitable for marker-assisted selection (MAS) in wheat breeding programs to improve stem rust resistance, including Ug99.
Stem rust (caused by Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn.) has re-emerged as a threat to wheat production with the evolution of new pathogen races, namely TTKSK (Ug99) and its variants, in Africa. Deployment of resistant wheat cultivars has provided long-term control of stem rust. Identification of new resistance genes will contribute to future cultivars with broad resistance to stem rust. The related Canadian cultivars Peace and AC Cadillac show resistance to Ug99 at the seedling stage and in the field. The purpose of this study was to elucidate the inheritance and genetically map resistance to Ug99 in these two cultivars. Two populations were produced, an F2:3 population from LMPG/AC Cadillac and a doubled haploid (DH) population from RL6071/Peace. Both populations showed segregation at the seedling stage for a single stem rust resistance (Sr) gene, temporarily named SrCad. SrCad was mapped to chromosome 6DS in both populations with microsatellite markers and a marker (FSD_RSA) that is tightly linked to the common bunt resistance gene Bt10. FSD_RSA was the closest marker to SrCad (~1.6 cM). Evaluation of the RL6071/Peace DH population and a second DH population, AC Karma/87E03-S2B1, in Kenya showed that the combination of SrCad and leaf rust resistance gene Lr34 provided a high level of resistance to Ug99-type races in the field, whereas in the absence of Lr34 SrCad conferred moderate resistance. A survey confirmed that SrCad is the basis for all of the seedling resistance to Ug99 in Canadian wheat cultivars. While further study is needed to determine the relationship between SrCad and other Sr genes on chromosome 6DS, SrCad represents a valuable genetic resource for producing stem rust resistant wheat cultivars.