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In the phytopathogenic fungus Ustilago maydis, plant infection is initiated by fusion of two compatible haploid sporidia. Pathogenic development is controlled by two distinct mating type loci, a and b. The biallelic a locus encodes a pheromone/receptor system that is required for cell recognition and cell fusion. After cell fusion the multiallelic b-mating type locus, encoding two homeodomain transcription factors, is essentiell for the establishment of a stable dikaryon and subsequent pathogenesis. Furthermore, the sexual development of U. maydis is strictly dependent on its host plant. In this thesis, the early b-dependent processes on the leaf surface were analyzed by DNA-microarrays. The expression profile of a non-pathogenic strain was compared to a pathogenic strain, harbouring an active b-heterodimer. In total 327 genes were identified, including genes encoding proteins with putative functions in plant cell wall degredation or transcriptional regulation.
One of the on planta b-depenently induced genes, biz1, encodes a putative C2H2-zinc finger transcription factor. Deletion of biz1 leads to drastically reduced appressoria formation; hyphae are still able to penetrate the plant surface, however, invasive growth and in planta proliferation was not observed. Thus biz1 seems not only to play a role during appressoria formation, but particularly during plant colonization. Further DNA-microarray analysis revealed that biz1 is sufficient and necessary for the regulation of 19 genes on planta. Systematic deletion analysis lead to the identification of pst1 and pst2, both encoding for putative Ustilago maydis specific secreted proteins. Combined deletion of both genes results in a phenotype that resembles the phenotype of the biz1 deletion. The pst-double deletion strain is able to penetrate the plant surface, but arrests its growth directly after penetration and is not able to proliferate in planta. Pst1 and Pst2 seem to exert their main function like Biz1 in post penetration processes.