Résumé
The vast majority of plants is associated with beneficial fungi capable to form new organs on the root system called mycorrhizas in which the plant supplies sugars to the fungus. According to the host plant and the fungal species, one can define three main types of mycorrhizas that are (i) arbuscular mycorrhizas (AM) formed on the roots of most land plants and aseptate fungi belonging to the Glomeromycotina subphylum, (ii) ectomycorrhizas (ECM) between roots from trees and shrubs and fungi belonging to the Ascomycota and Basidiomycota phyla and (iii) ericoid mycorrhiza (ERM) between roots from Ericaceae plants and mycorrhizal fungi belonging to the Ascomycota phyla. Despite large morphological and anatomical differences, all mycorrhizal root systems comprise (i) a hyphal net exploring the soil external to the root and (ii) a common space inside the root where the exchanges with the plant take place. Also, it is well known that mycorrhizal fungi improve the growth of their hosts by providing growth-limiting nutrients and particularly inorganic phosphate (Pi). In this keynote, first I will briefly describe the different mechanisms enabling the fungi to to get more Pi from the soil solution and how P is transported from the external hyphae towards the mycorrhizal roots, emphasizing on the putative role of polyphosphate metabolism. Then, I will report on our knowledge regarding the role of fungal P transporters in the uptake and the release of Pi from the fungal cytosol towards the common apoplast where fungal and plant membranes are opposite, without any direct contact between them. I will describe in more detail our knowledge on the putative role of HcPT2, a H+:Pi transporter we identified in the ectomycorrhizal fungus Hebeloma cylindrosporum, in Pi unloading in ECM formed with its host plant, Pinus pinaster. However, despite this case study, it is still difficult to generalize to all types of mycorrhizal associations as shown by the exploration of available fungal genomes which vary greatly regarding their repertory of P transporters.