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Functional genomics and molecular biology of symbiotic fungi

Research in our group focus on the mechanisms that enable symbiotic fungi to colonize plants successfully and on the processes accounting for variations in host preferences and fungal lifestyles. The prime models for our studies are the root endophyte Piriformospora indica (Basidiomycota, Sebacinales), and the orchid mycorrhizal Sebacina vermifera, two biotrophic symbiont that colonizes the root epidermal and cortex cells of a broad range of plant species, including the dicot model plant Arabidopsis thaliana and the agriculturally important monocot Hordeum vulgare. Beside their ability to establish a long-lasting beneficial relationship with their hosts, these symbionts are able to gain nutrients by degrading dead root material saprotrophically. The dual lifestyle of these endophytes is also evident during the mutualistic fungal development in roots where they display a biphasic colonization strategy. Initial root cell invasions are biotrophic where colonized host cells maintain membrane integrity and invasive hyphae remain enveloped by the host plasma membrane. Later, the fungus is found more often in dead or dying host cells, especially in the root cortex of barley. Colonization at later stages was shown to be reduced by overexpression of the negative cell death regulator BAX INHIBITOR-1 (BI-1) in barley and to be mediated by an endoplasmic reticulum stress-triggered caspase-dependent cell death in Arabidopsis. Although a defined switch to necrotrophy with massive cell death and tissue maceration is missing and instead beneficial effects for the hosts are present, this strategy of colonizing plants resembles that of hemibiotrophic fungi, straddling the divide of saprotrophy, necrotrophy and mutualism.
Primary interest is to find answers to the following specific questions by means of functional analyses and comparative genomics and transcriptomics:

  • Which mechanisms allow root endophytes to suppress host defense in a wide range of unrelated plants? Do root endophytes use effector-like molecules in the form of small secreted proteins in order to manipulate different host plants and/or are there other mechanisms like shielding itself in place?
  • Which signals mediate the switch from the biotrophic to the cell death associated phase? And is this latter phase actively triggered by the fungus?
  • What are the basic events involved in the transition from saprotrophic to symbiotic lifestyles?