Comparative Analyses of Compatible and Incompatible Host-Pathogen Interactions Provide Insight into Divergent Host Specialization of Closely Related Pathogens

Abstract

Host-pathogen co-evolutionary dynamics drive constant changes in plant pathogens to thrive in their plant host. Factors that determine host specificity are diverse and range from molecular and morphological strategies to metabolic and reproductive adaptations. We applied an experimental approach and conducted comparative microscopy, transcriptome analyses, and functional analyses of selected pathogen traits to identify determinants of host specificity in an important wheat pathogen. We included three closely related fungal pathogens, Zymoseptoria tritici, Z. pseudotritici, and Z. ardabiliae, that establish compatible and incompatible interactions with wheat. Although infections of the incompatible species induce plant defenses during invasion of stomatal openings, we found a conserved early-infection program among the three species whereby only 9.2% of the 8,885 orthologous genes are significantly differentially expressed during initial infection. The genes upregulated in Z. tritici likely reflect specialization to wheat, whereas upregulated genes in the incompatible interaction may reflect processes to counteract cellular stress associated with plant defenses. We selected nine candidate genes encoding putative effectors and host-specificity determinants in Z. tritici and deleted these to study their functional relevance. Despite the particular expression patterns of the nine genes, only two mutants were impaired in virulence. We further expressed the Z. tritici proteins in Nicotiana benthamiana to investigate protein function and assess cell death reaction. Hereby, we identify three effectors with cell-death-inducing properties. From the functional analyses, we conclude that the successful infection of Z. tritici in wheat relies on an extensive redundancy of virulence determinants.