Communication in the tripartite Sebacinalean root symbiosis: the role of endomycotic bacteria

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The overall goal of this collaborative project is to elucidate the role of endomycotic bacteria association with Sebacinalean fungi. Due to their beneficial impact on plants, highly diverse mycorrhizal fungi of the order Sebacinales have a high ecological and agricultural significance. Piriformospora indica, the most intensively studied Sebacinalean species, contains the Alphaproteobacterium Rhizobium radiobacter PABac-DSM. The stability of the tripartite association, the vertical transmission of bacteria during asexual fungal reproduction and fungal plant colonization, were shown (Sharma et al. 2008). Further screening for endomycotic bacteria in other species of the Sebacina vermifera complex of the Sebacinales identified three species-specific associations with Paenibacillus, Acinetobacter, and Rhodococcus species. Based on the finding that endobacteria form intricate tripartite association with Sebacinalean fungi and the plant hosts we hypothesize that bacteria are at least partly required for the beneficial outcome of the symbiosis. This view is supported by the fact that the free-living R. radiobacter mediates plant growth promotion and induced resistance when applied to barley roots reminiscent to activities observed after inoculation with P. indica. Significance of endobacteria is also inferred by the fact that Sebacinalean fungi could not be cured from bacteria so far.To further elucidate the molecular basis of the interaction of P. indica and endobacteria and the role of the endobacteria in the tripartite association, we will employ genetic, molecular, biochemical, biophysical, and structural methods - based on the now accessible genomes of P. indica and R. radiobacter. In cooperation with F. Schröder, BTI, Ithaca, we will use 2-dimentional NMR spectroscopy-based metabolomics combined with HPLC-MS analysis to identify small molecules involved in those processes released either by the fungi or bacteria. In addition to elucidating the mechanisms of the tripartite association in the P. indica-Rhizobium model, we will identify endomycotic bacteria in the Sebacinales P. williamsii and Sebacina vermifera MAFF305838, characterize their interaction with the fungal host, determine the stability of the association, and characterize their contribution to the specific tripartite interaction. Furthermore, we will conduct experiments under field test conditions to assess the agronomic potential of endobacteria.Due to the partly complementary expertise of Kämpfer, Kogel, and Schröder labs we expect a high synergism facilitating efforts to elucidate the role of endobacteria in the Sebacinalean symbiosis. Fundamental research on mechanism of the bacterial-fungal-plant interaction will eventually lead to a deeper understanding of the symbiosis and how it exerts beneficial activities on crop plants. Results from this project will enable us to assess the agronomic potential of endomycotic bacteria and/or their metabolites for plant protection.