Infection phenotypes and host specialization of closely related Zymoseptoria pathogens

Host specialization allows pathogens to exploit host tissues as niche for growth and reproduction. Adapted pathogens are able to infect while non-adapted species are blocked. Host specialization involves a diversity of lifecycle traits. Chapter 1 reviews strategies and structures filamentous pathogens use to colonize and exploit their hosts. In Chapter 2, we address the impact of high genomic variation on host infection phenotypes of Z. tritici by studying three isolates that cause similar quantitative disease. By combining disease monitoring, ROS staining, confocal microscopy, and infection stage-specific RNA-seq, we show an unexpectedly high extent of plasticity in the Z. tritici infection program. The three isolates showed highly differentiated spatial and temporal infection development whereby more than 20% of the genes were differentially expressed between the isolates. In Chapter 3, we address host specialization in the closely related fungal plant pathogens Z. tritici, Zymoseptoria pseudotritici, and Zymoseptoria ardabiliae. The early infection program of the three species is conserved to a large extent. We found conserved initial infection development and similar gene expression profiles in the adapted and non-adapted Zymoseptoria species. We identified a small number of differentially expressed orthologous genes that contain candidates for host-specific effectors and avirulence genes. Little is known about the biology and host-pathogen interactions of the Z. tritici sister species Z. pseudotritici and Z. ardabiliae. Chapter 4 describes the experiments we performed to study the infection biology of both fungi. We conducted plant infection experiments on 20 host species and genotypes in vivo and in vitro. Further, we tested the influence of leaf age and light stress on Zymoseptoria infections. In vivo, host infections were blocked early after infection. In vitro and on compromised leaves, asexual pycnidia were formed by Z. pseudotritici and Z. ardabiliae.


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