A small secreted protein from Zymoseptoria tritici interacts with a wheat E3 ubiquitin to promote disease

Septoria Tritici Blotch, caused by the ascomycete fungus Zymoseptoria tritici, is a major threat to wheat production worldwide. The Z. tritici genome encodes many small, secreted proteins (ZtSSP) that likely play a key role in the successful colonisation of host tissues. However, few of these ZtSSPs have been functionally characterised for their role during infection. In this study, we identified and characterised a small, conserved cysteine-rich secreted effector from Zymoseptoria tritici which has homologues in other plant pathogens in the dothideomycetes. ZtSSP2 was expressed throughout Z. tritici infection in wheat with the highest levels observed early during infection. A yeast two-hybrid assay revealed an interaction between ZtSSP2 and wheat E3 ubiquitin ligase in yeast and this was further confirmed in planta using bimolecular fluorescence complementation, and co-immunoprecipitation. Down-regulation of this wheat E3 ligase using virus-induced gene silencing, increased the susceptibility of wheat to Septoria tritici blotch (STB). Together these results suggest that TaE3UBQ likely plays a role in plant immunity to defend against Z. tritici.