Résumé
Ectomycorrhizal (EcM) symbiosis improves plant nutrition and water absorption, due to a better exploration of the soil and an efficient translo-cation of poorly available nutrients in forest ecosystems. Among other major nutrients as nitrogen and phosphate (Pi), potassium (K + ) as themost abundant cation in plant cells is involved in various physiological processes and in plant salt tolerance. We have shown 1 improvement of K+nutrition by EcM symbiosis under K + shortage using the model couple Pinus pinaster and Hebeloma cylindrosporum. Regarding the symbiotictransportome 2 , we contributed to identify and characterize fungal transport systems involved in uptake of nutrients from the soil and in theirtransfer towards the plant root at the symbiotic fungus-plant interface, the Hartig net. Fungal K + transporters and channels 1-5 are suggestedto absorb K+ from the soil (Trk & HAK) and to release K+ in the Hartig net via plant root cells (Shaker-like & TOK). Presence of three HcTOK6(Two-pore Outward K +) members of a fungi-specific channel family indicates different roles in K+ nutrition and in symbiosis. Interestingly, one ofthese TOK members was induced by symbiotic association with the pine. Moreover, fungal Pi transporter have been characterized and foundto be regulated on the transcriptional and proteomic level (MYCOTRANS project). Remarkably, transcriptomic data7 from H. cylindrosporumcultured alone or in symbiosis with mycorrhizal P. pinaster revealed, that a CDF (Cation Diffusion Facilitator) member, HcZnT2, was the mosthighly induced fungal membrane transporter. Mycorrhizal fungi modulate the transfer of macro- and micronutrients, among them Zn 8 , to thehost plant. We demonstrated the ability of HcZnT2 to transport Zn by functional yeast complementation. Excitingly, we showed that HcZnT2 isupregulated in symbiotic interaction, even by a short presence of the host plant without the need of physical contact, indicating gene inductionduring pre-symbiotic signalling. Thus, HcZnT2 is suggested to play a key role in Zn homeostasis and regulation during the establishment andfunctioning of the EcM symbiosis.