title: Association mapping of North American spring wheat breeding germplasm reveals loci conferring resistance to Ug99 and other African stem rust races.
abstract: BACKGROUND
  The recently identified Puccinia graminis f. sp. tritici (Pgt) race TTKSK (Ug99) poses a severe threat to global wheat production because of its broad virulence on several widely deployed resistance genes. Additional virulences have been detected in the Ug99 group of races, and the spread of this race group has been documented across wheat growing regions in Africa, the Middle East (Yemen), and West Asia (Iran). Other broadly virulent Pgt races, such as TRTTF and TKTTF, present further difficulties in maintaining abundant genetic resistance for their effective use in wheat breeding against this destructive fungal disease of wheat. In an effort to identify loci conferring resistance to these races, a genome-wide association study was carried out on a panel of 250 spring wheat breeding lines from the International Maize and Wheat Improvement Center (CIMMYT), six wheat breeding programs in the United States and three wheat breeding programs in Canada.
  
  
  RESULTS
  The lines included in this study were grouped into two major clusters, based on the results of principal component analysis using 23,976 SNP markers. Upon screening for adult plant resistance (APR) to Ug99 during 2013 and 2014 in artificial stem rust screening nurseries at Njoro, Kenya and at Debre Zeit, Ethiopia, several wheat lines were found to exhibit APR. The lines were also screened for resistance at the seedling stage against races TTKSK, TRTTF, and TKTTF at USDA-ARS Cereal Disease Laboratory in St. Paul, Minnesota; and only 9 of the 250 lines displayed seedling resistance to all the races. Using a mixed linear model, 27 SNP markers associated with APR against Ug99 were detected, including markers linked with the known APR gene Sr2. Using the same model, 23, 86, and 111 SNP markers associated with seedling resistance against races TTKSK, TRTTF, and TKTTF were identified, respectively. These included markers linked to the genes Sr8a and Sr11 providing seedling resistance to races TRTTF and TKTTF, respectively. We also identified putatively novel Sr resistance genes on chromosomes 3B, 4D, 5A, 5B, 6A, 7A, and 7B.
  
  
  CONCLUSIONS
  Our results demonstrate that the North American wheat breeding lines have several resistance loci that provide APR and seedling resistance to highly virulent Pgt races. Using the resistant lines and the SNP markers identified in this study, marker-assisted resistance breeding can assist in development of varieties with elevated levels of resistance to virulent stem rust races including TTKSK.
authors.affiliation: Department of Agronomy, Purdue University, 915 West State Street, West Lafayette, IN, 47907, USA. bajga002@umn.edu.|Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN, 55108, USA. bajga002@umn.edu.
authors.forename: P
authors.initials: P
authors.lastname: Bajgain
authors.affiliation: United States Department of Agriculture-Agricultural Research Service (USDA-ARS), Cereal Disease Laboratory, St. Paul, MN, 55108, USA. matthew.rouse@ars.usda.gov.|Department of Plant Pathology, University of Minnesota, St. Paul, MN, 55108, USA. matthew.rouse@ars.usda.gov.
authors.forename: M N
authors.initials: MN
authors.lastname: Rouse
authors.affiliation: Department of Crop and Soil Sciences, Washington State University, Pullman, WA, 99164, USA. peter.bulli@wsu.edu.
authors.forename: P
authors.initials: P
authors.lastname: Bulli
authors.affiliation: International Maize and Wheat Improvement Center (CIMMYT), ICRAF House, United Nations Avenue, Gigiri, Nairobi, Kenya. s.bhavani@cgiar.org.
authors.forename: S
authors.initials: S
authors.lastname: Bhavani
authors.affiliation: United States Department of Agriculture-Agricultural Research Service (USDA-ARS), Aberdeen, ID, 83210, USA. tyler.gordon@ars.usda.gov.
authors.forename: T
authors.initials: T
authors.lastname: Gordon
authors.affiliation: Kenya Agricultural and Livestock Research Organization (KALRO), Njoro, Kenya. wanyera@plantprotection.co.ke.
authors.forename: R
authors.initials: R
authors.lastname: Wanyera
authors.affiliation: Kenya Agricultural and Livestock Research Organization (KALRO), Njoro, Kenya. njaupnn@yahoo.com.
authors.forename: P N
authors.initials: PN
authors.lastname: Njau
authors.affiliation: Ethiopian Institute of Agricultural Research (EIAR), Pawe, Ethiopia. wasihunl@yahoo.com.
authors.forename: W
authors.initials: W
authors.lastname: Legesse
authors.affiliation: Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN, 55108, USA. ander319@umn.edu.
authors.forename: J A
authors.initials: JA
authors.lastname: Anderson
authors.affiliation: Department of Crop and Soil Sciences, Washington State University, Pullman, WA, 99164, USA. m.pumphrey@wsu.edu.
authors.forename: M O
authors.initials: MO
authors.lastname: Pumphrey
mesh_terms: D001487:Basidiomycota / Q000502:physiology*; D001947:Breeding; D002874:Chromosome Mapping / Q000379:methods*; D060467:Disease Resistance / Q000235:genetics*; D005787:Gene Frequency / Q000235:genetics; D056426:Genetic Loci; D015810:Linkage Disequilibrium / Q000235:genetics; D010641:Phenotype; D010935:Plant Diseases / Q000235:genetics / Q000382:microbiology*; D018547:Plant Stems / Q000382:microbiology; D020641:Polymorphism, Single Nucleotide / Q000235:genetics; D025341:Principal Component Analysis; D012621:Seasons; D036226:Seedlings / Q000235:genetics; D012639:Seeds / Q000235:genetics*; D014908:Triticum / Q000235:genetics* / Q000382:microbiology