Investigating fungal cellular processes involved in early colonisation of wheat by Zymoseptoria tritici

Abstract

The fungal pathogen Zymoseptoria tritici causes the most economically important disease of wheat in Europe. Despite recent advances in our understanding of its molecular host-pathogen interaction, fundamental questions remain about the cellular processes underlying plant colonisation by this fungus. The work presented in this thesis uses both reverse and forward genetic techniques to further understand the molecular determinants of Z. tritici virulence. To address questions about the source of nutrients utilised by Z. tritici to support symptomless colonisation of the leaf, this thesis explores cellular pathways utilised by other plant pathogenic fungi to access stored macromolecules. While autophagy is crucial for the infection-related development of many fungal plant pathogens, this study reveals that autophagy is dispensable for Z. tritici pathogenicity, and points towards a potential autophagy-independent function of ZtATG8 in virulence. The mitochondrial fatty acid β-oxidation pathway was however found to support the switch to hyphal growth on the leaf surface, providing strong evidence that catabolism of stored lipids is required for early host invasion by Z. tritici. Forward genetic investigation identified enzymes within the cell wall integrity (CWI) and cyclic adenosine monophosphate (cAMP) signalling pathways as playing a key role in Z. tritici virulence. In planta transcriptomic analysis revealed that the CWI pathway regulates the expression of infection-related secreted proteins, including the characterised LysM effectors required for host defence evasion, suggesting that Z. tritici may co-regulate virulence gene expression with the response to cell wall perturbation. Findings presented here also suggest that cAMP signalling regulates transcription during the switch to necrotrophic growth, providing insights into the elusive mechanisms controlling this infection cycle transition. Finally, genomic and transcriptomic analysis of a spontaneous Z. tritici mutant revealed the potential function of the light responsive transcription factor white collar 1 in controlling Z. tritici morphological development and infection. These novel findings advance our understanding of the cellular pathways contributing to Z. tritici infection and inform the development of future strategies to control this devastating pathogen.