Mycorrhizal symbioses in epiphytic orchids ; Symbioses mycorhiziennes des orchidées épiphytes

Almost 90% of terrestrial plants associate with mutualistic fungi in their roots, forming symbiotic associations called mycorrhizas. Mycorrhizas have a nutritional role for both partners: fungi improve the hydro- mineral nutrition of their host and obtain in return sugars from the plant photosynthesis. Orchids form mycorrhizal symbioses with a polyphyletic group of fungi in the Basidiomycota whose ecology is far from being well understood. Orchids have the particularity to produce microscopic, dust-like seeds which do not contain reserve and therefore need the presence of a compatible fungus to germinate. Thus, the orchid-mycorrhizal fungus interaction is of particular interest since the distribution of the fungal partners can influence those of their plant hosts, and conversely.These symbioses have been mainly investigated in temperate, terrestrial ecosystems. Yet, the majority of orchid species are epiphytic, meaning that they germinate and root on the bark of tropical trees. Their mycorrhizal fungi colonize the surrounding bark, but their ecology needs further investigations.This thesis aims to better understand the relationships between epiphytic orchids and their fungal partners. For this purpose, we studied fungal communities in the roots of epiphytic orchids and in the surrounding bark by metabarcoding approaches (i.e., amplification and sequencing of microbial communities) and isolation of fungal strains. We studied several species of epiphytic orchids thriving in tropical rainforests of Brazil and of La Réunion. We address the following questions: what is the distribution of mycorrhizal fungi on the bark of tropical trees? Can the fungal partners mediate indirect plant-plant interactions, forming epiphytic mycorrhizal networks? Can the mycorrhizal fungi explain the spatial distribution of a given orchid species?Finally, the analytic tools and methods are applied to the study of the cultivated vanilla in order to describe, for the first time, its mycorrhizal communities in La Réunion island. ; Près de ...