Characterization of MST7, a MAPK kinase gene essential for appressorium formation in Magnaporthe grisea

Many fungal pathogens invade plants by forming specialized infection structures called appressoria. In the rice blast fungus Magnaporthe grisea, the PMK1 MAP kinase is essential for appressorium formation and invasive growth. I have identified the MEK (MST7) and MEK kinase (MST11) genes that are responsible for the activation of PMK1. The mst7 and mst11 mutants are nonpathogenic and fail to form appressoria. Although expressing a dominant active MST7 allele results in the activation of Pmk1 in the absence of Mst11 and improper regulation of appressorium formation, the direct interaction between Mst7 and Pmk1 is not observed in yeast two-hybrid assays. Thus, it is not clear how Mst7 transmits the upstream signals to Pmk1. Mst7 contains a well-conserved putative MAPK-docking site at its N-terminus. Deletion of this docking site has no obvious effect on the expression of MST7 but blocks appressorium formation and plant infection. The kinase activity of Mst7 is not affected by the deletion of this docking site, but Mst7Delta12-20 fails to activate Pmk1 in M. grisea. In both co-immunoprecipitation and bimolecular fluorescence complementation (BiFC) assays, the direct interaction between Mst7 and Pmk1 is detected only during appressorium formation. These observations indicate that the MAPK-docking site of Mst7 is essential for its association and activation of downstream Pmk1 and the Mst7-Pmk1 interaction is enhanced during appressorium formation.