Signal transduction pathways in the fungal wheat pathogen Fusarium graminearum

The filamentous ascomycete Fusarium graminearum is a highly organ specific pathogen that resides on small grain cereals like rice, wheat, and maize. Once the host plants are infected, crops yielded from them are unsuited for food and feed production. This is mainly due to massive mycotoxin accumulation but also due to reductions in grain development. The most devastating mycotoxins produced by F. graminearum are deoxynivalenol (DON) and zearalenone (ZEA). The signaling cascades that are necessary for pathogenicity and mycotoxin production as well as the external stimuli that trigger those cascades are yet not fully understood. Aim of this thesis was to analyse a complete signaling cascade starting at membrane-bound receptors down to a transcriptional response. Transmembrane receptors (TMRs) are specialized integral membrane proteins that perceive external signals and communicate them to intracellular heterotrimeric G‐protein‐signaling cascades. Thus, TMRs play a pivotal role in the adaptation of entities to environmental stresses. In this study I present a comprehensive functional characterization of seven TMRs of Fusarium graminearum. They are involved in numerous physiological functions and different signaling cascades in F. graminearum. It was found that TMR FGSG_01861 is involved in stress tolerance towards oxidative, osmotic, fungicide, temperature and cell wall stress. Two TMRs (FGSG_03023 and FGSG_02655) play roles in pathogenicity, DON production, lipase secretion and sexual reproduction (the latter only FGSG_02655). Another TMR (FGSG_05006) was necessary for utilization of poor carbon sources and the intracellular level of the second messenger cAMP. Three other TMRs (FGSG_07716, FGSG_05239 and FGSG_09693) did not provoke any obvious phenotype. Acting downstream in the signaling cascade there are several transcription factors and mitogen-activated protein kinases (MAPK). MAPK signaling is a ubiquitous and well preserved regulation system for nearly all developmental processes throughout all eukaryotic organisms in response to several external stimuli like biotic or abiotic stresses or hormones. As shown in this study, the stress-activated MAP-kinase FgOS-2 (S. cerevisiae HOG1p) as a central regulator in the life cycle of F. graminearum. It is involved in nearly all developmental processes, such as perithecia formation, oxidative and osmotic stress tolerance, fungicide resistance, virulence, metabolism of reactive oxygen species (ROS) and secondary