Investigation on TOX2 cluster genes of the fungal wheat pathogen Zymoseptoria tritici

Zymoseptoria tritici, the causal agent of Septoria tritici blotch (STB), is an economically damaging foliar disease of wheat and a potential agent threatening global food security. Gaining insight into secondary metabolites (SMs) is needed to extend our knowledge on the biology of Z. tritici. Here, the well-studied HC-toxin, a virulence factor in the necrotrophic fungal pathogen Cochliobolus carbonum, was chosen to test the hypothesis, that Z. tritici is equipped with the TOX2 cluster genes, a locus involved in the biosynthesis of the HC-toxin. We additionally assume that Z. tritici could generate this SM during the necrotrophic phase by killing the living cells, leading to the emergence of STB symptoms. We combined a bioinformatic analysis with liquid chromatography-mass spectrometry (LC-MS) technique to investigate our claim. We found that all genes presented in the TOX2 cluster existed in Z. tritici IPO323, but the degree of identity between C. carbonum and Z. tritici is not high, ranging between 30 and 48%. Moreover, expression analysis showed that transcript levels of selected TOX2 genes are particularly high at the necrotrophic phase. We used the LC/MS technique to trace the HC-toxin in the culture filtrate of Z. tritici isolates. Our analysis indicated that the selected isolates were unable to produce the HC-toxin in vitro. This report is the first investigating the HC-toxin production by Z. tritici.