Symptoms of plants infected with some of the fungi studied in our department. Upper figure: Maize plant and ear (insert) infected by the biotrophic fungus Ustilago maydis. This smut fungus forms tumors on the above grounds parts of corn. Middle figure: Barley plants infected by Ustilago hordei. Kernels of infected plants are replaced by masses of dark brown smut spores. Lower figure: Barley root cortex cell colonized by the symbiotic endophyte Piriformospora indica. Upon penetration of the root, P. indica establishes a biotrophic interaction during which fungal cells are encased by the host plasma membrane. Later colonization stages are associated with death of cortex cells.
Department of Organismic Interactions
Research group leaders and research areas in the department are:
- Prof. Dr. Regine Kahmann (Director)
- Molecular phytopathology
- Dr. Gunther Döhlemann (Research Group Leader)
- Establishment of compatibility in biotrophic interactions
- Dr. Alga Zuccaro (Research Group Leader)
- Functional genomics and molecular biology of symbiotic fungi
Research in the department Organismic Interactions is focused on the mechanisms that enable fungi to colonize plants successfully and on the processes accounting for variations in host preference and fungal life styles. While the prime model for studies in the previous periods has been the fungus Ustilago maydis, a biotrophic pathogen that infects maize, the department has substantially broadened its focus during the reporting period. The smut fungus U. maydis is still the focus in the research groups of Gunther Doehlemann and Regine Kahmann, but now related smut fungi U. hordei, U. scitamiea and Sporisorium reilianum infecting other cereals and grasses occupy prominent roles for comparative studies. In addition, new research avenues are now followed by the research group of Alga Zuccaro who studies the symbi-otic root endophyte Piriformospora indica. This basidiomycete fungus benefits a broad range of plant species and displays a biphasic colonization strategy with an initial biotrophic phase followed by a cell death associated phase.
All these fungal systems (Figures) are genetically tractable, amenable to reverse genetics either relying on homologous recombination or RNAi and can build on completed genome sequences. To study the mechanisms that enable such diverse fungal species to enter and establish themselves in their respective hosts, all groups are using integrated approaches that rely on the combination of reverse genetics, transcriptomics, cell biology, biochemistry, comparative genomics and evolutionary history. What differs in the respective groups is how interesting candidate genes are identified, i.e. whether one relies on gene expression during plant colonization, positive selection, sequence conservation, presence-absence calls or other features of the genome like clustering and the presence of specific sequence signatures.
With respect to insights into how a fungus is able to colonize a plant and how the plant is reprogrammed to sustain fungal growth the U. maydis-maize system is clearly the most advanced. Prior to infection U. maydis cells are in contact with the plant surface. During this early phase of colonization chemical and physical surface cues are sensed and trigger infection related development. We have made significant progress in identify-ing the sensors for these cues and how they connect to the differentiation program. The challenge for the work to come lies in the elucidation of how these transmembrane proteins are activated and connect to the down-stream cascades. After plant penetration, U. maydis establishes a biotrophic relationship with its host in which the plant needs to stay alive. To achieve this, the pathogen is shielded from direct contact with the plant cytoplasm by a newly formed plant plasma membrane that completely encases the invading fungal hyphae. This establishes a tight interface (interaction zone) for the exchange of signals originating either from the fungus or from the host. In recent years we have uncovered many of the fungal molecules that are needed specifically for plant colonization. The majority of these molecules are secreted effectors that suppress plant defense responses at different stages of biotrophic development or repro-gram the metabolism of the host. We now know that such effectors are conserved in related pathogens but are absent from more distantly related pathogens and saprophytes. The challenge we are currently facing is to ascribe a function to these molecules which are mostly novel and rarely contain known structural features giving hints to their function, despite the fact that these mol-ecules have a key role in modulating host processes in the apoplast or inside plant cells if effectors are taken up after being secreted by fungal hyphae. The mechanism allowing effector uptake by host cells also remains to be elucidated. Additionally, we plan to address host specificity by making use of the genome sequences of relatives of U. maydis that parasitize different host plants. This latter aspect provides strong ties to research in the group of A. Zuccaro who studies how P. indica can suppress host defenses in a wide range of unrelated plants.