Mitogen-activated protein kinases (MAPKs) are essential components of fungal signaling pathways involved in different developmental processes and are required for host plant infection. Mycosphaerella graminicola, the causal agent of Septoria tritici leaf blotch (STB) of wheat, has three MAPK pathways that are all required for infection (MgFus3, MgHog1, MgSlt2; Cousin et al., 2006; Mehrabi et al., 2006a, Mehrabi et al., 2006b). We showed that Mgfus3 null mutants are nonpathogenic on intact wheat leaves (paint brush inoculation), but highly-reduced in pathogenicity when infiltrated into leaf tissues by syringe injection (reduced necrosis, low number of pycnidia). This suggests that MgFUS3 is involved infungal penetration, host colonization and pycnidia formation. Mghog1 null mutants have pathogenicity defects similar to Mgfus3 null mutants. This result highlights that the role of HOG1 in pathogenicity on plants differs among fungi (Segmüller et al., 2007). Mgslt2 null mutants are fully non-pathogenic on inoculated wheat leaves either by paint brushinoculation or injection. This phenotype is unusual among slt2 null mutants from other fungi. In addition, we haveidentified defects in Mgslt2 null mutants such as reduced mycelium hydrophobicity and complete lack of melanisationthat were not frequently reported in other fungal slt2 up to now, while classical defects such as hypersensitivityto calcofluor was not observed in Mgslt2 mutants. Therefore, M. graminicola SLT2 pathway has likely evolved tocontrol gene networks in part different from those of other fungal SLT2 pathways. To identify these networks using transcriptomics, we have constructed strains expressing an active MAPKK allele either under its native promoter or under the control of the nitrate reductase promoter. These strains allow the induction of the SLT2 pathway in controlled conditions. Differential genome wide expression analyses are currently performed to identify genes whose expression requires an active SLT2 MAPK. The role of these genes in development and infection will be studied by reverse genetics.