Identifizierung von Pflanzensignalen, die Differenzierung von Ustilago maydis auf der Blattoberfläche induzieren

After fusion of two haploid cells of compatible mating type, the smut fungus Ustilago maydis forms a dikaryon and initiates its pathogenic development. The cell-cell recognition is based on a pheromone/receptor system. The pheromone signal is transmitted via a MAP kinase modul, leading to the activation of Prf1, an essential regulator of sexual and pathogenic development. On the surface of Zea mays, the dikaryon switches to filamentous growth, differentiates an appressorium and penetrates the host. In this thesis the plant signals required for filament formation and appressorium development in U. maydis were characterized. Using a solopathogenic strain that is able differentiate without a mating partner, we show that hydroxy-fatty acids stimulate filament formation under in vitro conditions by induction of pheromone gene expression. This leads to the activation of the MAP-kinase module. These filaments were morphologically indistinguishable from conjugation tubes. Hydrophobicity also induced formation of filaments and these resemble the dikaryotic filaments formed on the plant surface. The integration of both plant derived stimuli was dependent on the MAP kinase modul, the presence of the MAPK Crk1 and the transcription factor Prf1. With the help of a marker gene that is specifically expressed in the tip cell of hyphae forming appressoria, it was shown that hydrophobicity in vitro is essential for appressorium formation. Hydroxy-fatty acids on the hydrophobic surface had an enhancing effect on the rate of appressorium formation. These results show that the early phase of communication between U. maydis and its host plant maize involves the integration of two distinct stimuli. To obtain insight into which genes were induced after exposure to hydrophobicity and hydroxy-fatty acids, a transcriptome analysis was performed using the in vitro system. A significant share of the induced genes coded for putative secreted proteins, among those were proteins with a role in fungal cell wall modification, components of the cell wall, proteins with enzymatic function and proteins without homology to known proteins in the databases. The latter proteins resembled putative effectors for the interaction with the host. This indicates that already during the development on the plant surface a molecular dialog between U. maydis and its host is initiated.