The phosphate transporter HcPT2, first candidate for phosphate efflux in ectomycorhizal symbiosis?

In forest ecosystems, the availability of phosphorus as inorganic P (Pi) limits tree growth. To overcome this deficiency, trees are associated with fungi through new organs called ectomycorrhizae (ECM). Despite the importance of phosphate nutrition, the mechanisms involved in the unidirectional transfer of P between the fungus and the plant are still unknown. Therefore, the identification of these mechanisms essential for symbiosis is a major issue. To understand better these mechanisms, we used a model association between the ECM basidiomycete Hebeloma cylindrosporum and the maritime Pine, Pinus pinaster. The fungal genome contains three transporters of Pi (HcPT1.1, HcPT1.2 and HcPT2) which have been already characterized as H+:Pi carriers.We hypothesized that one of these carriers could be responsible for both influx and efflux of Pi respectively from the soil to the fungus and from the fungus to the plant. Our first results suggest that HcPT2 would be the best candidate to fill both roles. Immunolocalization showed that the protein is located in (i) the extraradical hyphae and the fungal sheath involved in Pi uptake and (ii) the Hartig net, ie the interface between fungal and host cells. We also produced overexpressing and knock-down transgenic H. cylindrosporum strains to study specifically the role of HcPT2 in (i) the P efflux from the fungus measured in vitro and (ii) the P nutrition of the host plant grown at low and high P levels. Finally, if our hypothesis is confirmed, the main question will be to define how this transporter is regulated to provide both influx and efflux of phosphate in ECM symbiosis.