Abstract
Ectomycorrhizal fungi establish beneficial associations with trees, particularly in temperate and boreal forests. The extraradical mycelium of mycorrhizal fungi explores the soil and supplies nutrients to plant roots via soil-fungus-plant interfaces. In return, mycorrhizal plants supply carbon substrates to their root-associated fungi via efficient transport systems. The movement of these nutrients is mediated and regulated by several efficient membrane transport systems present in both fungi and plants. To gain a better understanding of the establishment and functioning of mycorrhizal symbiosis, the underlying molecular mechanisms need to be identified and the membrane transport involved characterised. The objectives of the PhD project included the analysis of (i) the impact of mycorrhizal symbiosis on plant adaptation to salt stress, (ii) the fungal transport systems likely to be involved in potassium nutrition in mycorrhizal plants, and (iii) the fungal membrane transporter most induced by mycorrhizae. The model for this study is the ectomycorrhizal association between Pinus pinaster and Hebeloma cylindrosporum. Interesting results were obtained showing that this beneficial association reduces the salt stress tolerance of young pine seedlings. Analyses of the fungal potassium transporters identified were completed, characterising the second Trk-type transporter as a selective potassium uptake system. Among the membrane transporters induced by mycorrhizae, a member of the CDF family was found to be up-regulated upon early contact with the host plant and characterised as a zinc transporter. As zinc is an essential micronutrient, necessary in sufficient quantities but toxic at high concentrations, fungal and ectomycorrhizal growth of pine was analysed as a function of external zinc. Summarising these results, two review articles are included in the PhD manuscript and three articles are being prepared for submission.