Structural and functional interactions between extraradical mycelia of ectomycorrhizal Pisolithus isolates

Extraradical mycelia from different ectomycorrhizal (ECM) roots coexist and interact under the forest floor. We investigated structural connections of conspecific mycelia and translocation of carbon and phosphorus between the same or different genets. Paired ECM Pinus thunbergii seedlings colonized by the same or different Pisolithus isolates were grown side by side in a rhizobox as their mycelia contacted each other. 14CO2 or 33P-phosphoric acid was fed to leaves or a spot on the mycelium in one of the paired seedlings. Time-course distributions of 14C and 33P were visualized using a digital autoradiographic technique with imaging plates. Hyphal connections were observed between mycelia of the same Pisolithus isolate near the contact site, but hyphae did not connect between different isolates. 14C and 33P were translocated between mycelia of the same isolate. In 33P-fed mycelia, accumulation of 33P from the feeding spot toward the host ECM roots was observed. No 14C and 33P translocation occurred between mycelia of different isolates. These results provide direct evidence that contact and hyphal connection between mycelia of the same ECM isolate can cause nutrient translocation. The ecological significance of contact between extraradical mycelia is discussed.