The role of two component signalling in the regulation of stress responses and virulence in the fungal phytopathogen, Zymoseptoria tritici

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The fungal pathogen Zymoseptoria tritici is the causative agent of Septoria tritici blotch (STB), a disease that represents a major threat to wheat production and food security worldwide. Recently, the development of a range of molecular tools for Z. tritici has provided an opportunity to decipher the pathways that regulate its pathogenicity and inform new strategies for its control. The identification of such strategies is important due to the increasing incidence of fungicide resistant strains which threaten sustainable agriculture and food production. The ability to mount effective responses to host-imposed environmental stresses has been shown to be an important virulence attribute in a variety of plant and human fungal pathogens. Central to these responses are evolutionarily conserved stress activated MAP kinase (SAPK) pathways. In Z. tritici deletion of the Hog1 SAP kinase results in sensitivity to a range of environmental stresses and loss of virulence. However, the genes controlled by the SAPK pathway and the mechanisms by which is regulated have yet to be determined. Importantly, fungi are distinct as they use 'two component' phosphorelay systems to sense and transmit specific stress signals to SAPK modules. The prototypical fungal two component system was characterised in the budding yeast, S. cerevisiae and consists of a histidine kinase Sln1, a phosphorelay protein Ypd1 and a fungal-specific response regulator protein Ssk1. Analysis of the Z. tritici genome sequence has revealed that there are multiple genes encoding histidine kinases but single homologues of Ypd1 and Ssk1. Therefore, the aim of this project is to determine the role of ZtYpd1 and ZtSsk1 in the stress response and virulence of Z. tritici. Using methodology that is already established in the lab, strains carrying deletions in these genes will be constructed. Based on analyses of model fungi, it would be predicted that loss of ZtYpd1 would result in constitutive SAPK activation whereas loss of ZtSsk1 would prevent SAPK activation in response to specific signals. Therefore, these knockout strains will allow the effect of perturbing two component signalling on stress resistance to be determined. SAPK pathway activation (ZtHog1 phosphorylation) will monitored in order to identify which stress signals are relayed via the two component system and transcript profiling will be employed to reveal the impact of ZtYpd1 and ZtSsk1 on stress-responsive gene expression. Ultimately, wheat infection assays using the knockout strains will be used to assess the contribution of the two component system to the virulence of Z. tritici.