Identification of a novel genetic locus conferring virulence in the wheat tan spot pathogen Pyrenophora...

The ascomycete Pyrenophora tritici-repentis (Ptr) is the causal agent of tan spot, a common and economically important wheat disease worldwide. Three necrotrophic effectors (NEs), known as Ptr ToxA, Ptr ToxB, and Ptr ToxC, have been identified from the fungal pathogen as major virulence factors. The race 2 isolate 86-124 which produces Ptr ToxA is capable of causing disease on wheat lines that is insensitive to Ptr ToxA, suggesting the use of additional NEs during the infection. To identify new NE gene(s) from 86-124, we developed a biparental fungal population from a cross between this isolate and the race 5 isolate DW5 using genetically modified heterothallic strains. The fungal population was genotyped with SNP and SSR markers as well as the ToxA gene, the mating type genes, and six ToxB loci. Each progeny was phenotyped onto the hard red spring wheat line CDC-Osler, which is insensitive to both Ptr ToxA and Ptr ToxB, but is highly susceptible to 86-124. The constructed genetic map consisted of 11 linkage groups that corresponded to the 11 chromosomes (chr) of the Ptr reference genome. ToxA and mating type genes mapped to the expected positions. Five of the six ToxB copies were tightly linked, residing at the distal end of chr 11, while the sixth copy was localized to the distal end of chr 5. Composite interval mapping revealed a major QTL on the distal end of chr 2 conferring virulence toward CDC-Osler by 86-124. This locus was designated as VirOsler1. Genomic sequence alignment at the locus showed a region of approximately 900 kb at the end of chr 2 absent in DW5. The identification of VirOsler1 locus provides clear evidence that the wheat tan spot pathogen uses additional virulence factors that interact with unidentified host factors for disease susceptibility.