Transformation of a Ptr ToxB Nonproducing Isolate of Pyrenophora tritici‐repentis with the ToxB Gene Confers Pathogenicity on a Toxin‐Sensitive Wheat Genotype

Abstract

ABSTRACTThe causal agent of tan spot of wheat, Pyrenophora tritici‐repentis, is a necrotrophic ascomycete that produces multiple host‐selective toxins (HSTs) responsible for the development of destructive foliar lesions. Ptr ToxB, a small proteinaceous HST produced by some races of P. tritici‐repentis, causes extensive chlorosis on toxin‐sensitive wheat genotypes. To investigate the role of Ptr ToxB in the pathogenicity and virulence of P. tritici‐repentis, the open reading frame of the encoding gene, ToxB, was cloned from a wild‐type race 5 isolate of the fungus and transferred into a Ptr ToxB nonproducing isolate of race 2. Ectopic integration of ToxB was confirmed by Southern blotting analysis with a digoxigenin (DIG)‐labeled DNA probe, while the abundance of ToxB transcript in culture was evaluated by quantitative reverse transcriptase polymerase chain reaction (qRT‐PCR). The production and secretion of Ptr ToxB were assessed by Western blotting analysis of the transformed fungal strains. The pathogenicity of the transformants was tested by inoculation on Ptr ToxB‐sensitive and insensitive wheat genotypes. The Ptr ToxB‐producing transformants were able to induce chlorosis in a host‐specific manner only on the toxin‐sensitive host genotype, resulting in significantly higher levels of disease than those caused by the nontransformed isolate. The extent of chlorosis was well correlated with the amount of Ptr ToxB produced. These results demonstrate that the acquisition of Ptr ToxB‐producing ability is a sufficient condition for pathogenicity in P. tritici‐repentis and confirm the role of this toxin in mediating the compatibility between the fungus and different genotypes of its wheat host.