Fine Mapping of Two Homoeologous Wheat Genes Conferring Effector-Triggered Susceptibility to the Necrotrophic Pathogen Stagonospora nodorum

The development of the wheat disease Stagonospora nodorum blotch (SNB) is dependent on compatible interactions between wheat sensitivity genes and corresponding necrotrophic effectors (NEs), also known as host-selective toxins, produced by the fungal pathogen Stagonospora nodorum.  We previously showed that the orthologous wheat genes Snn3-B1 and Snn3-D1 recognize the same NE, SnTox3, to confer effector-triggered susceptibility. Saturation and comparative mapping with rice and Brachypodium led to the delineation of both Snn3-B1 and Snn3-D1 to small intervals, and markers flanking Snn3-D1 were anchored to two separate Ae. tauschii BAC contigs. Here, we used BAC end sequences, the available wheat, barley and Ae. tauschii sequencing resources, and sequencing of the BAC contigs flanking the Snn3-D1 region using Ion-torrent sequencing technology for further marker development and fine-mapping. The Snn3-B1 and Snn3-D1 loci have been narrowed to 1.3 and 0.25 cM intervals, respectively, and the Snn3-D1 candidate region has been delineated to three overlapping BACs spanning 260 kb. Four genes were identified from one of the overlapping BACs and are considered good candidates. In addition, a number of EMS-induced mutants were generated for both genes. To isolate these two genes further high-resolution mapping, marker development, and sequence analysis of the EMS-mutants will be pursued. Cloning of these two wheat genes will provide necessary foundation to study their evolutionary history and investigate their functional roles in mediating recognition of SnTox3.