Racial Composition and Variability of the ToxA Gene in Geographically Distant Populations of Pyrenophora tritici-repentis

Pyrenophora tritici-repentis causing the tan spot of wheat produces specific necrotrophic effectors Ptr ToxA, Ptr ToxB, and Ptr ToxC, inducing necrosis and chlorosis on the leaves of susceptible varieties. On the basis of the ability of P. tritici-repentis strains to produce specific necrotrophic effectors or their combinations, eight races of the pathogen are distinguished. Monitoring the race composition of P. tritici-repentis populations is necessary to assess the evolutionary potential of the pathogen and develop a methodology for breeding wheat cultivars with long-term resistance. We have analyzed 179 monoconidial P. tritici-repentis strains from Kazakhstan and Russian populations in 2020–2022. The widespread distribution of races 2 and 4 was revealed, strains of which were present in each analyzed P. tritici-repentis population with a frequency of 2–36 and 7–82%, respectively. The dominance of avirulent race 4 was noted: the strains of this race accounted for 27% of all analyzed P. tritici-repentis strains. Molecular identification of the ToxA and ToxB genes, as well as toxb, a homolog of the ToxB gene, in 118 P. tritici-repentis strains from six populations revealed the presence of the ToxA gene in 69% of the analyzed strains. The ToxB gene was not detected in any strains, while the toxb gene was found sporadically and was identified in the genome of 18 P. tritici-repentis strains (9%), most of which were avirulent and belonged to race 4. In PCR with specific primers for the ToxA gene of ten P. tritici-repentis strains, a product of ~800 bp was amplified, which turned out to be significantly larger than expected. This was explained by the presence of an insertion in the amplified region of the ToxA gene. Such gene was called ToxAL. All P. tritici-repentis strains with the ToxAL variant were assigned to races 4 and 5, which do not form the necrotrophic effector Ptr ToxA. The structure of the ToxAL gene and its protein product is the subject of further research.