Stem Rust Resistance Genes Sr24 Research Articles

Characterization of Puccinia graminis f. sp. tritici isolates derived from an unusual wheat stem rust outbreak in Germany in 2013

An unusual stem rust infestation occurred in German wheat fields in summer 2013. This study analysed 48 isolates derived from 17 Puccinia graminis f. sp. tritici (Pgt) samples and six races were identified: TKTTF, TKKTF, TKPTF, TKKTP, PKPTF and MMMTF. Infection type and genotypic data confirmed that none of these races belonged to the TTKS (Ug99) race group. German isolates of race TKTTF are phenotypically different to the ones responsible for the stem rust epidemic in Ethiopia in 2013–2014. Forty isolates were genotyped using a custom SNP array. Phylogenetic analysis showed that these 40 isolates represented two distinct lineages (clade IV and clade V). Thirty‐eight isolates clustered into clade IV, which previously was defined by Ethiopian isolates of race TKTTF. Race TKKTP is of special concern due to its combined virulence to stem rust resistance genes Sr24, SrTmp and Sr1RSAmigo. The vulnerability to race TKKTP in US and international winter wheat was confirmed as 55% of North American and international cultivars and breeding lines resistant to race TTKSK (Ug99) became susceptible to TKKTP. Races identified in Germany in 2013 confirmed the presence of virulence to important resistance genes that are effective against race TTKSK. This information should be useful for breeders to select diverse and effective resistance genes in order to provide more durable stem rust resistance and reduce the use of fungicides.

The application of DNA marker and doubled-haploid technology for stacking multiple stem rust resistance genes in wheat

In wheat, the use of gene “pyramids” or “stacks” of major genes that confer resistance to all local strains of the fungal stem rust pathogen Puccinia graminis f. sp. tritici (Pgt) can increase durability of resistance where wheat cultivars with the single gene components are not widely deployed. Stacking two or more resistance genes becomes a breeding challenge, particularly when pathogen races that discriminate the genes are not available. The use of DNA markers and doubled-haploid technology provides a route for producing lines homozygous for multiple resistance genes. We have applied this approach to produce gene pyramids of two or more of the stem rust resistance genes Sr24 and new sources of SrR, Sr31 and Sr26 on reduced alien chromatin in the genetic backgrounds of Westonia and Pavon wheat. These genes, which are all derived from “alien” sources (SrR and Sr31 from rye, Sr24 and Sr26 from Agropyron elongatum) each provide resistance to all currently known pathotypes of Pgt in Australia, and SrR and Sr26 also provide resistance against all the variants of stem rust race Ug99 (TTKS group).

Genotyping of U.S. Wheat Germplasm for Presence of Stem Rust Resistance Genes Sr24, Sr36 and Sr1RSAmigo

The stem rust resistance genes Sr24, Sr26, Sr36, and Sr1RSAmigo confer resistance to race TTKSK (= Ug99) of Puccinia graminis f. sp. tritici Pers. (Pgt). A collection of 776 cultivars and breeding lines of wheat (Triticum aestivum L.) from all growing regions of the United States were screened with simple sequence repeat and sequence tagged site markers linked to Sr24, Sr26, Sr36, and Sr1RSAmigo to determine frequencies of these genes in U.S. wheat germplasm. Marker efficacy in predicting the presence of these genes was evaluated via comparison with assayed seedling infection type. Among the lines evaluated, the most predominant gene is Sr24, present in hard winter, hard spring, and soft winter wheat lines. Resistance in soft winter wheat is primarily due to Sr36 The 1RS·1AL rye translocation carrying Sr1RSAmigo is present at equal frequencies in hard winter and soft winter wheat. Utilization of marker‐assisted selection for stem rust resistance genes can hasten the development of wheat cultivars resistant to TTKSK and its variants and allow for the development of resistance gene pyramids for more durable stem rust resistance.

Development of PCR markers for the selection of wheat stem rust resistance genes Sr24 and Sr26 in diverse wheat germplasm

The use of major resistance genes is the most cost-effective strategy for preventing stem rust epidemics in Australian wheat crops. The long-term success of this strategy is dependent on combining resistance genes that are effective against all predominant races of the pathogen, a task greatly assisted by the use of molecular markers linked to individual resistance genes. The wheat stem rust resistance genes Sr24 and Sr26 (derived from Agropyron elongatum) and SrR and Sr31 (derived from rye) are available in wheat as segments of alien chromosome translocated to wheat chromosomes. Each of these genes provides resistance to all races of wheat stem rust currently found in Australia . We have developed robust PCR markers for Sr24 and Sr26 (this study) and SrR and Sr31 (previously reported) that are applicable across a wide selection of Australian wheat germplasm. Wheat lines have recently become available in which the size of the alien segments containing Sr26, SrR and Sr31 has been reduced. Newly developed PCR-markers can be used to identify the presence of the shorter alien segment in all cases. Assuming that these genes have different gene-for-gene specificities and that the wheat industry will discourage the use of varieties carrying single genes only, the newly developed PCR markers will facilitate the incorporation of two or more of the genes Sr24, Sr26, SrR and Sr31 into wheat lines and have the potential to provide durable control to stem rust in Australia and elsewhere.