Mapping a Resistance Gene to Puccinia graminis f. sp. tritici in the Bread Wheat Cultivar 'Matlabas'.

Puccinia graminis f. sp. tritici race TTKSF+ was collected from the South African wheat cultivar 'Matlabas' in 2010. F2 and F3 populations derived from a Matlabas × Line 37-07 cross segregated for a single resistance gene to race TTKSF that is avirulent to Matlabas. In screening genomic DNA bulks of susceptible or resistant F2 plants with simple sequence repeat (SSR) markers, three chromosome arm 2BS markers and one multilocus marker amplified alleles present only in the resistant bulks and Matlabas. Additional 2B-specific SSR markers, incorporating markers spanning regions containing Sr9h, SrWLR, Sr28, and Sr47, were screened in the parental lines and mapped in the F2 population. Linkage and QTL mapping showed that the gene is located between Xbarc160 in the centromeric region and Xgwm47 on the long arm of chromosome 2B. When 2B-specific SNP markers were mapped, the area of interest was delimited to a 15.3 cM region on chromosome arm 2BL, with XIWA543-HRM and Xgwm47 as flanking loci. Matlabas, Webster, and related Sr9h lines all produced a similar, low infection type to race TTKSF, but were susceptible to race TTKSF+. Phenotypic data and allelic studies suggested that stem rust resistance in Matlabas was derived from an Sr9h source.

Stem Rust Resistance in a Geographically Diverse Collection of Spring Wheat Lines Collected from Across Africa.

Following the emergence of the Ug99 lineage of Puccinia graminis f. sp. tritici (Pgt) a collective international effort has been undertaken to identify new sources of wheat stem rust resistance effective against these races. Analyses were undertaken in a collection of wheat genotypes gathered from across Africa to identify stem rust resistance effective against the Pgt races found in Eastern and Southern Africa. The African wheat collection consisted of historic genotypes collected in Kenya, South Africa, Ethiopia, Sudan, Zambia, Morocco, and Tunisia, and current South African breeding lines. Both Bayesian cluster and principal coordinate analyses placed the wheat lines from Sudan in a distinct group, but indicated a degree of genetic relatedness among the other wheat lines despite originating from countries across Africa. Seedling screens with Pgt race PTKST, pedigree information and marker haplotype analysis confirmed the presence of Sr2, Sr36, Sr24, Sr31, and Lr34/Yr18/Sr57 in a number of the lines. A genome-wide association study (GWAS) undertaken with Diversiry Arrays Technology (DArT) and stem rust (Sr) gene associated markers and Stem Area Infected (SAI) and Reaction Type (RT) field phenotypes, collected from trials carried out across two seasons in Kenya in 2009 and in South Africa in 2011, identified 29 marker-trait associations (MTA). Three MTA were in common between SAI and RT, with the biggest effect MTA being found on chromosome 6AS. Two wheat lines, W1406 and W6979 that exhibited high levels of adult plant stem rust resistance were selected to generate bi-parental mapping populations. Only the MTA on chromosomes 6AS and 3BS, and the locus Lr34/Yr18/Sr57 were confirmed following QTL mapping. Additional stem rust resistance QTL, not detected by the GWAS, were found on chromosomes 2BS, 2DL, 3DL, and 4D.