Molecular Mapping and Validation of SrND643: A New Wheat Gene for Resistance to the Stem Rust Pathogen Ug99 Race Group.

This study reports the identification of a new gene conferring resistance to the Ug99 lineage of races of Puccinia graminis f. sp. tritici in wheat (Triticum aestivum L.). Because the virulent races of stem rust pathogen continue to pose a serious threat in global wheat production, identification and molecular characterization of new resistance genes remains of utmost important to enhance resistance diversity and durability in wheat germplasm. Advanced wheat breeding line 'ND643/2*Weebill1' carries a stem rust resistance gene, temporarily designated as SrND643, effective against the Ug99 group of P. graminis f. sp. tritici races at both seedling and adult growth stages. This study was conducted to map the chromosomal location of SrND643 and identify closely linked molecular markers to allow its selection in breeding populations. In total, 123 recombinant inbred lines, developed by crossing ND643/2*Weebill1 with susceptible line 'Cacuke', were evaluated for stem rust response in field nurseries at Njoro, Kenya, during two growing seasons in 2010, and were genotyped with DNA markers, including Diversity Arrays Technology, simple sequence repeats (SSR), and single-nucleotide polymorphisms. Linkage mapping tagged SrND643 at the distal end of chromosome 4AL, showing close association with SSR markers Xgwm350 (0.5 centimorgans [cM]), Xwmc219 (4.1 cM), and Xwmc776 (2.9 cM). The race specificity of SrND643 is different from that of Sr7a and Sr7b, indicating that the resistance is conferred by a gene at a new locus or by a new allele of Sr7. The flanking markers Xgwm350 and Xwmc219 were predictive of the presence of SrND643 in advanced germplasm, thus validating the map location and their use in marker-assisted selection.