Map-Based Cloning of the Wheat Snn1 Gene Reveals a Wall-Associated Kinase Hijacked By the Necrotrophic Pathogen Stagonospora nodorum to Cause Disease

Stagonospora nodorum is an economically significant necrotrophic fungal pathogen of wheat worldwide. The pathogen produces numerous necrotrophic effectors (NEs), or host-selective toxins, that lead to effector-triggered susceptibility when recognized by specific host genes. Cloning of the wheat Tsn1 gene demonstrated that S. nodorum subverts classic NB-LRR-regulated resistance pathways to cause disease.  Here, we describe the cloning of another wheat gene, Snn1, which confers sensitivity to the NE SnTox1. Mapping in 17,000 gametes led to the development of markers tightly linked to Snn1, which were used to screen the Chinese Spring chromosome arm 1BS minimum tiling path BAC pools and identify a 2.5 Mb contig spanning the Snn1 locus. The gene was delineated to a segment of 0.16 cM spanned by four BACs, which contained six putative genes. Mapping eliminated five of the candidates by recombination, leaving only one gene, a putative wall-associated kinase (WAK), to cosegregate with Snn1.  Comparative sequence analysis of 15 EMS-induced SnTox1-insensitive mutants in the Chinese Spring background indicated that all harbored functional mutations within the WAK gene, thus providing final validation. Snn1 is 3,045 bp from start to stop codon with two introns.  The 714 deduced amino acid sequence contains an intracellular serine/threonine protein kinase domain and extracellular calcium binding epidermal growth factor (Ca-EGF) and galacturonan-binding domains, all of which are necessary for function.  Further analysis regarding expression and function is ongoing and will be presented.  This work shows that necrotrophic pathogens such as S. nodorum can hijack different classes of genes and pathways to cause disease.