Mapping Stem Rust Resistance on Chromosome 3BL in Thatcher That Interacts with Lr34

Stem rust, caused by Puccinia graminis f.sp. tritici (Pgt), is a potentially destructive disease of wheat that has been well controlled by deploying cultivars carrying stem rust resistance (Sr) genes.  Following the emergence of virulent Pgt races in Africa (Ug99) there has been increased investment in research aimed at understanding the genetics of resistance.  Lr34 confers resistance to all three wheat rusts and has also been shown to interact with other rust resistance genes.  Previously, Lr34 has been shown to act as a nonsuppressor of seedling stem rust resistance (Sr) genes found in the cultivar Thatcher.  Furthermore, NILs of Thatcher carrying Lr34 were more resistant to Ug99 in field tests in Kenya compared to Thatcher.  The goal of our study was to determine which Sr gene(s) were interacting with Lr34 in Thatcher.  Two populations segregated for Pgt resistance inherited from Thatcher but were fixed for Lr34.  Mapping with SNP markers located an Sr gene responsible for field resistance on chromosome 3BL.  A physical map of Chinese Spring harbours one NB-LRR gene-like sequence in the interval between flanking SNP markers.  A SNP marker based on the NB-LRR sequence could explain much of stem rust severity observed in field testing.  The marker was also associated with seedling resistance to Pgt in the presence of Lr34.  Work is ongoing to clone the Sr gene on 3BL, determine if the gene differs from previously identified Sr genes, and understand its interaction with Lr34.