Résumé
It is well known that ectomycorrhizal (ECM) fungi improve the growth of their hosts by providing growth-limiting nutrients and particularly inorganic phosphate (Pi). However, the molecular players driving the Pi allocation from the fungus to the host are largely unknown thus far. In the ECM association between Pinus pinaster and Hebeloma cylindrosporum, we hypothesized that one of the three Pi transporters identified in the fungal genome (named HcPT1.1, HcPT1.2 and HcPT2) could be more specifically involved in Pi unloading in the Hartig net. To address this hypothesis, we combined microscopy, physiology and fungal genetic approaches to unravel the role of each Pi transporter. Immuno-localization of Pi transporter proteins showed that only HcPT1.1 and HcPT2 are localized in the Hartig net, indicating that these two proteins could play a role in Pi efflux in the ECM. We produced transgenic lines, either down- (RNAi) or over- (OE) expressing one of the two proteins. We used them to measure in vitro Pi fluxes as well as P accumulation in young P. pinaster seedlings. Our experiments showed that only OE-PT2 lines were able to increase in vitro Pi effluxes and shoot P accumulation of pine seedlings whereas the OE-PT1.1 lines increased only root P accumulation. Taken as a whole, these results support that the HcPT2 Pi transporter is a key component of the fungal Pi unloading machinery in the Hartig net of ECM.