Résumé
A major role of mutualistic interactions between plant roots and soil fungi is the improvement of plant nutrition by an equitable exchange of nutrients leading also to tolerance of environmental stress. Ectomycorrhizal symbiosis established between woody plants and soil fungi, widespread in forest ecosystems, is crucial for the plant partner to efficiently take up poorly available nutrients. Physiological studies as well as recent genome sequencing projects (Kohler et al. (2015) Nature Genet 47: 410-415) and transcriptome analyses (Doré et al. (2015) New Phytol 208: 1169-1187; Doré et al. (2017) Environ Microbiol 19:1338-1354) have allowed progress towards the identification and characterization of the symbiotic transportome (Garcia et al. (2016) Trends in Plant Science 21: 937-950). One of the major nutrients is potassium (K+ ), the most abundant cation in plant cells. We have shown improvement of potassium nutrition (Garcia et al. (2014) New Phytol 201: 951-960) under K+ shortage conditions by ectomycorrhizal symbiosis using two model species from European forests, Pinus pinaster and Hebeloma cylindrosporum. Questions are raised to identify the transport systems involved in the uptake of nutrients from the soil and in their transfer towards the plant at the symbiotic fungus-plant interface, called Hartig net. In the case of potassium (Garcia and Zimmermann (2014) Front Plant Sci 5: 337), we have identified two types of K+ transporters, Trk and HAK, as candidates to perform K+ uptake from the soil by the fungal extraradical hyphae, and two types of K+ channels, Shaker-like and TOK, that may release K+ by the hyphae of the Hartig net into the plant apoplasm. We have studied the three TOK (Two-pore Outward K+) channels identified in the genome of H. cylindrosporum, a channel family specific for fungi initially described in yeast (Ketchum et al. (1995) Nature 376: 690-695). These three TOK channels from the ectomycorrhizal fungus H. cylindrosporum belonging to two different subfamilies have been functionally characterized and localized (Carmen Guerrero-Galán et al. (2018) Env Microbiol , in press). Finally, we have analyzed whether these K+ channels might play specific roles within the fungus and within the symbiosis.