Y VENTURA¹, A LOHBERGER¹, A LOHBERGER², S BINDSCHEDLER<sup>1

1</sup>Laboratory of Microbiology, University of Neuchâtel, Neuchâtel, Switzerland, ²Laboratory of Biogeosciences, IDYST, University of Lausanne, Lausanne, Switzerland

Fungi are involved in many type of interactions with other living organisms. An example of those are bacterial-fungal interactions (BFI), which are essential for soil functioning. In some cases, the presence of endobacteria inside fungal cells affects the fitness and pathogenicity of fungi interacting with plants. However, in these studies mostly the function of the endosymbiotic bacterium has been considered, while the role of the fungal host is still poorly understood. In this study we analyse the interaction between the ectomycorrhizal and saprophytic fungus Morchella crassipes and Massilia sp., an endobacterium isolated from this fungal species. The first aim of this study was to establish the type of interaction (i.e. positive or negative) between these two organisms. We hypothesized that Massilia sp. helps its host to cope with stressing environmental conditions. In order to address this hypothesis, the wild type fungus was cured by an antibiotic treatment to remove endobacterial populations. A co-culture was then performed in order to reintroduce the bacterium (wild type and GFP labelled) into the fungus. The effect of curing the fungus on fitness will be measured by comparing fungal biomass. Finally, the cured fungus with the reintroduced bacterium will be cultured under different varying growth conditions and the presence of the endobacterium will be assessed by qPCR of the 16S rRNA gene. We expect observing that the physiology of the wild type fungus is re-established by the reintroduction of its endobacterium. However, under extreme abiotic conditions for the fungus, we expect the endobacterium to be released in the medium. Based on these expected results and initial observations so far, the interaction between the fungus M. crassipes and its endobacterium Massilia sp. seems to be a positive symbiotic interaction according to Koch’s postulates. However, the fungus seems not to need its endobacterium under all environmental situations, suggesting that this interaction follows a trade-off suiting both organisms. This study represents an essential step towards understanding the close relationship existing between a fungus and its endobacterial partner.