Characteristics of the geographically distant populations of Pyrenophora tritici-repentis in terms of virulence and ToxA and ToxB toxin-forming genes
Keywords:Pyrenophora tritici-repentis, population, necrorotropic effector, ToxA, ToxB, necrosis, wheat
A collection of 183 monoconidial isolates of the causal agent of wheat tan spot Pyrenophora tritici-repentis was created. It is represented by 7 populations of different geographical origins isolated in 2017 and 2018: two “southern” populations from Krasnodar and South Kazakhstan (Almaty), two “northern” populations – from Finland and the Northwest and West Siberian (Northern Kazakhstan, Chelyabinsk and Omsk) regions of Russian Federation. The composition of races in populations and the presence of the ToxA and ToxB genes in the pathogen isolates were determined by PCR. ToxB gene was not detected in the isolates tested. All southern population isolates possessed ToxA gene (ToxA+). However, the results of virulence assays and genetic tests did not coincide: 14 % of the isolates from the South Kazakhstan population and 61 % of the Krasnodar population did not induce necrosis on the leaves of Glenlea, i.e. they were nec?, and did not possess the necrosis inducing toxin Ptr ToxA and other necrosis toxins. The opposite situation was observed in the “northern” and Western Siberian populations of the pathogen with the ToxA+ fraction varying from 5.5 % to 66 %, but the quote of nec+ isolates was significantly higher than that of ToxA+ isolates. This finding indicates the dominance of isolates producing necrosis-inducing toxin(s) other than Ptr ToxA in these populations. Our work shows that the P. tritici-repentis-wheat pathosystem is not restricted to the interactions of three necrotrophic effectors (ToxA, ToxC, ToxB) and three susceptibility genes (Tsn1, Tsc1, Tsc2) and probably involves other additional effectors and susceptibility genes or other types of parasite-host relationships. Additionaly, the isolates with impaired ToxA gene expression prevail in the southern populations of the pathogen. The maintenance of ToxA+ nec- isolates in the southern populations indicates the existence of additional functions of the ToxA gene, important for the competitiveness of the fungal isolates.