Trophic exchanges between Hebeloma cylindrosporum and Pinus pinaster, characterisation of the molecular mechanisms responsible for the transfer of phosphate in ectomycorrhizal symbiosis
Thesis defended on 23 November 2015 - Montpellier University
Ectomycorrhizal symbiosis between the roots of permanent plants and soil fungi play an essential role in forest ecosystems. The availability of phosphorus (P) in inorganic form in these soils limits tree growth. The formation of ectomycorrhizal fungi generally improves the phosphate nutrition of the plants. Despite the great importance of ectomycorrhizal symbiosis, little is currently known about the mechanisms involved in the transport processes across the membrane between fungi and plant.
This study set out to determine the molecular mechanisms involved in the transfer of inorganic phosphate (Pi) within the soil-hypha-plant continuum by examining a model association between maritime pine (Pinus pinaster Soland in Ait.) and an ectomycorrhizal basidiomycete, Hebeloma cylindrosporum.
A study of the fungal genome showed that it had three Pi/H+ transporters (HcPT1.1, HcPT1.2 and HcPT2), of which HcPT1.1 and HcPT2 have already been characterised as H+/Pi transporters. These three transporters are potential candidates for ensuring both the influx of phosphate at the soil-fungus interface and the efflux at the fungus-plant interface. Immuno-localisation approaches localised these three proteins in the two influx sites (extraradical mycelium and fungal mantle) and the Pi efflux site (Hartig net). Transgenic lines of H.cylindrosporum over- and/or under-expressing the HcPT1.1 and HcPT2 genes were produced for association with the plant roots. The results obtained suggested that the HcPT1.1 and HcPT2 transporters not only play a role in absorbing Pi at the soil-fungus interface but HcPT1.1 may also be responsible for the uptake of Pi at Hartig net level, giving the fungus the possibility of recovering some of the phosphate released to the plant. The HcPT2 transporter may be involved in the Pi efflux through the apoplast to the plant. However, further study is required to understand the functioning and regulation processes allowing the HcPT2 transporter to be responsible for both the influx and efflux of phosphate through the fungus.