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The goal of Dr. Kahmann's studies is to elucidate how eukaryotic pathogens, in her case the fungal parasite Ustilago maydis, manage to colonize plants. Dr. Kahmann's laboratory has chosen U. maydis as a model for their investigations as this fungus can cause disease on all above ground parts of the maize plant in short time and is fully amenable for genetic and reverse genetic analysis. U. maydis causes smut disease, a wide-spread disease associated with the formation of large plant tumors in which the fungus proliferates and differentiates spores. U. maydis is a biotrophic pathogen that depends on living plant cells to complete its sexual life cycle. Related smut fungi like Ustilago hordei, Ustilago scitaminea and Sporisorium reilianum are important cereal pathogens, but in these systems disease symptoms develop only in the male or female inflorescence. In all smut fungi pathogenic development is initiated by the filamentous dikaryon that is generated by mating of compatible haploid strains. On the leaf surface, the dikaryon forms infection structures which allow direct penetration of the cuticle, presumably aided by lytic enzymes. During penetration the plasma membrane of the host invaginates and surrounds the infection hyphae, shielding the fungus from direct contact with the host cytoplasm. Although hyphae traverse plant cells, there are only limited host defense responses and the infected plant tissue remains alive until late in the infection process when fungal proliferation occurs mostly in the apoplast. The lab has shown that initial plant defense responses, presumably triggered by fungal PAMPs, are actively suppressed by U. maydis during colonization. In recent years they have demonstrated that this is accomplished by secreted protein effector molecules that are mostly novel and exist only in related smut fungi. These secreted effector proteins either have a function in the tight interaction zone between fungal hyphae and host plasma membrane or are translocated to host cells. Effectors taken up by plant cells are involved in reprogramming the metabolism of the host to prepare it for fungal invasion. Their current work is focused on the early phase of fungal development on the leaf surface, and here they are particularly interested to elucidate how the leaf surface is sensed and how these cues trigger fungal differentiation. The second focus of the lab's work concerns the elucidation how and where secreted effectors function and how they shape the biotrophic interaction. Here they heavily rely on comparative genomics, reverse genetics and a combination of biochemical, molecular and cell biological approaches.
For more information, please visit: http://www.mpi-marburg.mpg.de/kahmann/research.html