Durable Rust Resistance in Wheat

Objective 7: Reducing Linkage Drag Associated with Rust Resistance Genes

Cultivated wheat has a large number of related species and genera that represent an invaluable gene pool for wheat improvement. In the past 50 years, a number of effective stem rust (Sr) resistance genes identified from wild relatives of wheat have been incorporated into the wheat genome through genetic manipulation in the form of chromosome translocations and additions. Some of these genes, including Sr25, Sr26, and Sr43 from Thinopyrum elongatum, Sr37 and Sr40 from Triticum timopheevii, Sr32 and Sr39 from Aegilops speltoides, and Sr44 from Th. intermedium, have been found to be effective against Ug99. In addition, three novel Sr genes have been identified recently (temporally designated as Sr2S, Sr5S, and Sr6V) from Ae. speltoides and Haynaldia villosa. Deployment of combinations of these Sr genes in wheat can provide protection against the various races of the stem rust fungus, including Ug99. However, except for Sr25 and Sr26, all the other genes on the alien segments are associated with undesirable agronomic characteristics due to deleterious linkage drag. In addition, some of the available wheat lines containing these resistance genes are not genetically stable because large amounts of alien genetic materials have been integrated into these lines. These Sr resistance genes are not useful in their current forms.

Thus, the purpose of this Objective is to separate deleterious linkage drag from the Sr genes including Sr32, Sr37, Sr39, Sr40, Sr43, Sr44, Sr2S, Sr5S, and Sr6V to eliminate unwanted alien genetic materials. To remove unwanted alien DNA segments, homoeologous recombination will be induced by crossing and backcrossing the original translocation or addition lines carrying the Sr genes with the Ph1 (pairing homoeologous) gene mutant (ph1b) and Ph1-deficienct aneuploids susceptible to stem rust. Resultant progeny plants with reduced alien DNA segments will be selected via stem rust resistance testing, molecular markers, and fluorescent genomic in situ hybridization (FGISH). It is expected that useful genetic stocks carrying each of the nine Sr geneswill be developed and available for further germplasm enhancement efforts or immediately useful for breeding during the first 3-year funding period. At least four wheat linesin which the alien DNA segment carrying each of the Sr genes Sr32, Sr37, Sr39, and Sr40 is reduced to a minimal level will be developed. The Sr genes on the shortened translocations will be delivered to the wheat breeders to develop elite wheat germplasm with durable resistance to stem rust.

Objective 7: Reducing Linkage Drag Associated with Rust Resistance Genes
Activities	Outputs	Outcomes (Short- and Long-Term)
Activity 7.1 Separate Sr37, Sr39, Sr43, and Sr2BS from linkage drag and combine two resistance genes into a single linkage block	Two wheat germplasm lines each carrying the genes Sr37 and Sr39 on shortened alien chromosome segments. Improved genetic stocks with Sr43 and Sr2BS. Molecular markers closely linked to the Sr37 and Sr39. New and improved protocols and techniques for alien gene introgression.	Short term: Wheat germplasm lines each carrying the genes Sr37, Sr39, Sr43, and Sr2BS on shortened alien chromosome segments are available for breeding cultivars with durable stem rust resistance. Long term: Elite wheat lines carrying the genes Sr37, Sr39, Sr43, and Sr2BS are available for testing yield, quality, and stem rust resistance in the target regions.
Activity 7.2 Separate Sr32, Sr40, Sr44, Sr6V, and Sr5S from linkage drag	Genetic stocks carrying Sr32 and Sr40 on shortened alien chromosome segments. Compensating translocation lines with Sr44, Sr6V, and Sr5S. New and improved protocols and techniques for alien gene introgression.	Short term: Improved germplasm lines carrying the genes Sr32, Sr40, Sr44, Sr6V, and Sr5S are available for breeding cultivars with durable stem rust resistance. Long term: Enhanced diversity of agronomically acceptable sources of stem rust resistance.
Activity 7.3 Evaluate parental lines, segregation populations, and new translocation lines for reactions to stem rust	Evaluation data of parental lines, segregation populations, and new translocation lines for reactions to multiple pathotypes of stem rust.	The outcomes of this Activity are the same as those of Activities 7.1 and 7.2 because it provides the rust testing needed in the Activities 7.1 and 7.2.