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Abstract:
The fungal wheat pathogen Zymoseptoria tritici is the causal agent of Septoria tritici Blotch (STB), a major threat to wheat production
worldwide. Infection begins with a symptom-free and presumably biotrophic phase of fungal growth followed by a lifestyle switch to
necrotrophic growth. We aimed to elucidate the nature of the symptom-free infection phase and the putative immune suppression during
biotrophic fungal growth. In addition to wheat cultivars of varying STB susceptibility, we used different pathovars of Pseudomonas syringae
bacteria to study the physiological responses and the extent of immune suppression in wheat caused by Z. tritici. During the biotrophic infection,
we observed that non-adapted P. syringae strains were able to co-infect wheat. This effect was not limited to the fungal infection site but extends
to adjacent and systemic leaf areas. In contrast, after Z. tritici infection of wheat cultivars with STB resistance genes, we observed systemic
acquired resistance also towards adapted P. syringae strains. Using a plant metabolomics approach, we could confirm the fungus-mediated local
and systemic resistance suppression during colonization of susceptible wheat. Our findings suggest that (I) virulent Z. tritici infections of wheat
cause systemic acquired susceptibility allowing colonization by non-adapted phytopathogens and (II) the fungal infection impacts leaf
microbiome dynamics.
