Mapping resistance to the Ug99 race group of the stem rust pathogen in a spring wheat landrace

Authors: Babiker, Ebrahiem; Bonman, John; Gordon, Tyler; Chao, Shiaoman; Newcomb, Maria; Rouse, Matthew - Matt; Jin, Yue; WANYERA, RUTH - Kenya Agricultural And Livestock Research Organization; ACEVEDO, MARICELIS - North Dakota State University; Brown-Guedira, Gina; WILLIAMSON, SHARON - North Carolina State University

Submitted to: Journal of Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2015
Publication Date: 1/20/2015
Citation: Babiker, E.M., Bonman, J.M., Gordon, T.C., Chao, S., Newcomb, M.S., Rouse, M.N., Jin, Y., Wanyera, R., Acevedo, M., Brown Guedira, G.L., Williamson, S. 2015. Mapping resistance to the Ug99 race group of the stem rust pathogen in a spring wheat landrace. Journal of Theoretical and Applied Genetics. 128:605-612. doi:10.1007/s00122-015-2456-6.

Interpretive Summary: Stem rust is an important disease of wheat worldwide. Breeding resistant cultivars are the most economical means of managing the disease and resistance that is genetically complex is more long lasting. However, new races of the stem rust pathogen, commonly called ‘Ug99’, have arisen in Africa. These new races are a threat to wheat production in the US and elsewhere because many popular wheat varieties are susceptible to the new races. To reduce the threat posed by Ug99, we seek to discover and genetically map new resistance genes that provide protection from the new races. The objective of this study was to elucidate the inheritance of Ug99 resistance in a wheat landrace accession called PI 374670, which was resistant in field experiments in East Africa where Ug99 is endemic. Our results indicated a single gene controlled seedling resistance to Ug99 and several other races of the stem rust pathogen. Newly developed wheat molecular markers, called single nucleotide polymorphisms (SNPs), were used to ‘genotype’ a population of plants developed by crossing the resistant accession PI 374670 with a susceptible line. With this method we found a major locus for both Ug99 seedling resistance and East African field resistance on wheat chromosome 7AL. The SNP markers identified in this study could possibly be used in marker assisted selection to combine the resistance from PI 374670 with other effective resistance genes. Thus, our results could help US wheat breeders produce new varieties with genetically complex resistance against Ug99.

Technical Abstract: Wheat landrace PI 374670 has seedling and field resistance to stem rust caused by Puccinia graminis f. sp tritici Eriks. & E. Henn (Pgt) race TTKSK. To elucidate the inheritance of resistance, 216 BC1F2 families, 192 double haploid (DH) lines, and 185 recombinant inbred lines (RILs) were developed by crossing PI 374670 and the susceptible line LMPG-6. The parents and progeny were evaluated for seedling resistance to Pgt races TTKSK, MCCFC, and TPMKC. The DH lines were tested in field stem rust nurseries in Kenya and Ethiopia. The DH lines were genotyped with the 90K wheat iSelect SNP genotyping platform. Goodness-of-fit tests indicated that a single dominant gene in PI 374670 conditioned seedling resistance to the three Pgt races. The seedling resistance locus mapped to the long arm of chromosome 7A and this result was verified in the RIL population screened with the flanking SNP markers using KASP assays. In the same region, a major QTL for field resistance was detected in a 7.7 cM interval and explained 34-54% and 29-36% of the variation in Kenya and Ethiopia, respectively. Results from tests with specific Pgt races and the csIH81 marker showed that the resistance was not due to Sr22. Results from the KASP assay of PI 374670 and ‘Norka’, an accession with Sr15, showed that the four markers linked to the resistance locus were all monomorphic between the two accessions. Thus, a new source of Ug99 stem rust resistance, either closely linked or allelic to Sr15, is responsible for the seedling and field resistance of PI 374670.