Mechanisms of infection and response of the fungal wheat pathogen Zymoseptoria tritici during compatible, incompatible and non-host interactions
Zymoseptoria tritici is responsible for Septoria tritici blotch, a disease causing significant annual yield losses in wheat. To investigate infection phase-specific gene expression in the pathogen, we analyzed gene expression during infection of susceptible (Taichung 29) and resistant (Veranopolis and Israel 493) wheat cultivars, plus the non-host species barley at 1, 3, 6, 10, 17 and 23 days post inoculation (DPI). There were dramatic differences in pathogen gene expression at 10 DPI in the susceptible compared to both resistant interactions. The most pronounced differences in pathogen gene expression were observed at 3 DPI in both the susceptible and resistant host interactions compared to the non-host interaction. Thirty-one putative effectors showed early expression during the susceptible interaction compared to the non-host interaction, and six effectors were selected for subcellular localization studies. Using Agrobacterium-mediated transient expression in Nicotiana benthamiana, subcellular localization assays revealed that two candidate effectors, Mycgr3109710 and Mycgr394290, localized to putative mobile cytosolic bodies when expressed without their signal peptides, suggesting potential roles in intracellular signaling or host gene regulation. When expressed with their native signal peptides, four candidate effectors localized to the cytosol, while one effector, Mycgr3107904, did not accumulate to detectable levels, as shown by immunoblot analysis, indicating degradation. Comparison of pathogen gene expression in the susceptible host to expression in the resistant hosts, allowed us to identify genes that are expressed during the transition from biotrophic to necrotrophic growth at 10 DPI. Comparison of pathogen gene expression in resistant and susceptible hosts, versus in the non-host barley, allowed us to identify genes involved in initial colonization and host recognition. In addition, our study contributes to understanding candidate effectors that are activated early during infection and may play a role in the initial suppression of plant immunity, making them strong candidates for functional characterization.