An RGA-like marker detects all known Lr21 leaf rust resistance gene familymembers in Aegilops tauschii and wheat

Leaf rust is one of the most important diseases of wheat worldwide, particu larly in the Great Plains region of the USA. One long-term strategy for the control of this disease may be through durable genetic resistance by gene pyramiding. An important step in this strategy is identifying molecular mar kers linked to different leaf rust-resistance genes. Here we report the mol ecular tagging of a leaf rust-resistance gene that may have the potential f or durable resistance through further genetic manipulation and gene pyramid ing. Lr39 was previously designated for a leaf rust-resistance gene introgr essed from Aegilops tauschii accession TA1675 into the common wheat germpla sm WGRC2. Lr40 was designated for a gene derived from Ae. tauschii accessio n TA1649 and is present in germplasm WGRC7. These genes are now believed to be allelic to Lr21, which was transferred to wheat from a different access ion of Ae. tauschii. Molecular mapping of Lr39 and Lr40 indicates that both genes come from TA1649. WGRC2 and WRGC7 also have a similar infection type against rust culture PRTUS6. We suggest the designation of the gene in WGR C2 should be changed to Lr40. RFLP marker KSUD14 (locus Xksud14) was found 0.2-cM proximal to Lr40 in a WGRC2/Wichita F-2 population (218 individuals) , and co-segregated with the gene in a WGRC7/Wichita F-2 population (165 in dividuals). A PCR-based molecular marker developed from the sequence-tagged -site (STS) of XksudI4 was mapped to the same locus as the RFLP marker KSUD 14 in both populations. KSUD14 has the structure of a resistance gene analo g (RGA) including kinase2a and kinase3 domains similar to the Cre3 gene of wheat and the rust resistance gene Rpl-D of maize. When the PCR products am plified from KSU14 STS were cleaved with restriction enzyme MspI, an 885-bp fragment was found in WGRC2, WGRC7, the Lr21 near-isogenic line, and eight accessions of Ae. tauschii shown to have resistance gene alleles at the Lr 21 locus. The KSUD14 PCR-based assay provides an excellent marker for Lr40 and Lr21 in diverse wheat breeding and wild Ae. tauschii populations.