Extremely flexible infection programs in a fungal plant pathogen
Authors: Haueisen Janine , Möller Mareike , Eschenbrenner Christoph J. , Grandaubert Jonathan , Seybold Heike , Adamiak Holger , Stukenbrock Eva H. 
Published: December 2017

Filamentous plant pathogens exhibit extraordinary levels of genomic variability that is proposed to facilitate rapid adaptation to changing host environments. However, the impact of genomic variation on phenotypic differentiation in pathogen populations is largely unknown. Here, we address the extent of variability in infection phenotypes of the hemibiotrophic wheat pathogen Zymoseptoria tritici by studying three field isolates collected in Denmark, Iran, and the Netherlands. These three isolates differ extensively in genome structure and gene content, but produce similar disease symptoms in the same susceptible wheat cultivar. Using advanced confocal microscopy, staining of reactive oxygen species, and comparative analyses of infection stage-specific RNA-seq data, we demonstrate considerable variation in the temporal and spatial course of infection of the three isolates. Based on microscopic observation, we determined four core infection stages: establishment, biotrophic growth, lifestyle transition, and necrotrophic growth and asexual reproduction. Comparative analyses of the fungal transcriptomes, sequenced for every infection stage, revealed that the gene expression profiles of the isolates differed significantly, and 20% of the genes are differentially expressed between the three isolates during infection. The genes exhibiting isolate-specific expression patterns are enriched in genes encoding effector candidates that are small, secreted, cysteine-rich proteins and putative virulence determinants. Moreover, the differentially expressed genes were located significantly closer to transposable elements, which are enriched for the heterochromatin-associated histone marks H3K9me3 and H3K27me3 on the accessory chromosomes. This observation indicates that transposable elements and epigenetic regulation contribute to the infection-associated transcriptional variation between the isolates. Our findings illustrate how high genetic diversity in a pathogen population can result in highly differentiated infection and expression phenotypes that can support rapid adaptation in changing environments. Furthermore, our study reveals an exceptionally high extent of plasticity in the infection program of an important wheat pathogen and shows a substantial redundancy in infection-related gene expression.

Author summary

Zymoseptoria tritici is a pathogen that infects wheat and induces necrosis in leaf tissue. Z. tritici field populations exhibit high levels of genetic diversity, and here we addressed the consequences of this diversity on infection phenotypes. We conducted a detailed comparison of the infection processes of three Z. tritici isolates collected in Denmark, the Netherlands, and Iran. We inoculated leaves of a susceptible wheat cultivar and monitored development of disease symptoms and infection structures in leaf tissue by confocal microscopy. The three isolates exhibited highly differentiated spatial and temporal patterns of infection, although quantitative disease was similar. Furthermore, more than 20% of the genes were differentially expressed in the three isolates during wheat infection. Variation in gene expression is particularly associated with transposable elements, suggesting a role of epigenetic regulation in transcriptional variation among the three isolates. Finally, we find that genes encoding putative virulence determinants were enriched among the differentially expressed genes, suggesting that each of the three Z. tritici isolates utilizes different strategies to manipulate host defenses. Our results emphasize that phenotypic diversity plays an important role in pathogen populations and should be considered when developing crop protection strategies.