Allelic diversity of the Zymoseptoria tritici effector Zt-11 leads to the loss of interactions with small, secreted proteins from wheat

Pathogen secreted effector proteins play a key role in host-pathogen interactions, however the evolutionary and structural mechanisms underlying their diversification remain poorly understood. Previously, Zt-11 a Zymoseptoria tritici pathogen effector was found to interact with small, secreted proteins from wheat (TaSRTRG6, TaSSP6 and TaSSP7). Deletion of Zt-11 delayed Septoria tritici blotch (STB) disease development in wheat. Here, we investigate the diversity of Zt-11 in 168 field isolates which revealed high allelic diversity in Zt-11, three distinct Zt-11 haplotype groups and signatures of positive selection. Structural predictions of these isoforms and the wheat host interacting protein TaSRTRG6, exhibit high structural confidence (pLDDT >80). Whereas the wheat interactors TaSSP6 and TaSSP7 are intrinsically disordered proteins with no reliable structural model. Molecular docking and yeast two-hybrid assays revealed haplotype-specific binding between Zt-11 and TaSRTRG6, with some isoforms showing loss of interaction in vivo. Finally, I1:H1 which no longer interact with small, secreted wheat proteins (TaSRTRG6, TaSSP6 and TaSSP7) also displayed increased disease symptoms during wheat infection assays. Together these findings suggest that Zt-11 undergoes evolution through positive selection and structural adaptation enabling evasion from wheat host small, secreted proteins.