Résumé
Most plant species establish mycorrhizal symbiosis, which represents a major biological phenomenonin terrestrial ecosystems. Improvement of host plant mineral nutrition appears as an essential functionof this interaction. The fungal partner absorbs mineral nutrients from the soil and transfers part ofthem to the host plant. While membrane transport systems play thereby a key role in the symbioticinteraction, very few of them are characterised on a molecular level. The objective of this project is toidentify such transport systems in the ectomycorrhizal fungus Hebeloma cylindrosporum and then tocharacterise their function in the exchange of solutes with the plant host. Hebeloma cylindrosporumwill possibly become an interesting model as genetic analysis are feasible (e.g. the whole cycle, fromspore to spore, can be achieved in vitro). Our first goal was to produce a cDNA library, and then toestablish, by systematic sequencing and in silico analysis, an EST database (Expressed Sequence Tag).We identified approximately 2700 transcripts after in silico clustering. This database and its analysis isaccessible to the scientific community via a web site. Bioinformatic analysis revealed a large numberof putative transport systems in this database. We focalised our interest on one gene encoding a Shakerpotassium channel named HcSKC (for H. cylindrosporum Shaker K+ Channel). After screening of agenomic DNA library from Hebeloma h1 strain, several copies of this gene were identified andanalysed regarding their structure. We show that multicopy HcSKC sequences (that hamperedexperimental and bioinformatic analyses) are typical of h1 strain, whereas other Hebeloma strains andspecies possess a single copy. Based on sequence analysis, HcSKC code for an outward channel, thatmight thus be involved in fungal K+ secretion towards the host plant. This hypothesis will be verifiedby functional expression in heterologous systems.