In the corn smut fungus Ustilago maydis, sexual development is initiated by fusion of two haploid sporidia, generating a filamentous dikaryon that is capable to infect the plant. To get insight into the processes that precede plant infection, we performed microarray analysis of U. maydis cells grown on the plant surface. Two of the genes specifically induced in a pathogenic strain are a C2H2 zinc finger transcription factor and a homeodomain transcription factor named biz1 and hdp2, respectively. Whereas Δhpd2 strains are completely blocked in appressoria formation, Δbiz1 cells are severely reduced in their ability to form appressoria and to penetrate the plant. Hdp2 seems to be required for expression of most genes induced on the plant surface. However, Biz1 regulates about 30% of all genes induced on the plant surface; for 19 of these genes, Biz1 is both required and sufficient for induction. Systematic deletion analysis of these genes led to the identification of pst1 and pst2, encoding potentially secreted U. maydis specific proteins. Δpst1/pst2 cells are still able to penetrate the plant surface, but subsequently fail to invade and colonize the plant, resembling the biz1 deletion phenotype. In contrast to wild type strains, for both Δbiz1 and Δpst1/pst2 strains reactive oxygen species (ROS) can be detected at site of penetration, suggesting a function in suppression of plant defence for Pst1/Pst2. Further microarray analysis revealed that 76 of the biz1-dependent genes are induced during various stages of pathogenic development. This data suggest that Biz1 is not only a regulator of genes required for plant penetration, but also for genes with impact on pathogenicity at later stages. Furthermore, eight additional binuclear zinc transcription factors (Cys6Zn2) were identified to be induced specifically in planta. Currently we are performing deletion mutants of those transcription factors and analyzing their role during pathogenic development.