Pathway and sink activity for translocation of 14C absorbed as amino acids in the Pisolithus extraradical mycelium of ectomycorrhizal Pinus thunbergii seedlings

To characterize the pathway and sink activity for the translocation of amino acids and their metabolites in the extraradical mycelium (ERM) of ectomycorrhizal (ECM) symbiosis, we locally labeled a part of the ERM of Pinus thunbergii seedlings colonized by a Pisolithus isolate with [14C(U)]-l-alanine (14C-Ala) or [14C(U)]-l-glutamine (14C-Gln) and followed the 14C distribution in the ERM by autoradiography. After 14C-Ala or 14C-Gln labeling, 14C was mainly distributed in a bundle of rhizomorphs extending from the labeled part towards ECM root tips of the host, but not in thick roots or the aboveground part of the seedling. When 14C-Ala was added after seedling removal, no 14C bundle appeared in the remaining ERM. These results indicate that 14C amino acids and their metabolites are translocated unidirectionally towards the ECM root tips, where strong sink activity for amino acids is localised. 14C was also distributed diffusely over a broad region in ERM surrounding the labeled part. This result suggests that 14C amino acids and their metabolites are also translocated multidirectionally in the ERM by sink activity distributed diffusely all over the whole ERM.