Symplastic Continuity During Hartig Net Formation in Norway Spruce Ectomycorrhizae

Summary Pits with plasmodesmata were observed by scanning and transmission electron microscopy in cortical cells of uninfected short roots, grown in vitro, and ectomycorrhizal roots grown in vitro and in vivo, of Norway Spruce, Picea abies (L.) Karst. The ectomycorrhizal fungus, Piloderma bicolor (Peck) Jülich, while forming a Hartig net during mycorrhiza synthesis in vitro, did not separate the host cell walls in the pit zones. In natural spruce ectomycorrhizae with P. bicolor and an unidentified ascomycete, the cortex symplast remained continuous by means of frequent pits with plasmodesmata. The tanniferous cells bordering the fungal mantle were never pitted. The symplastic continuity, which has not been previously demonstrated in the cortical cells of the host of ectomycorrhizae, is essential for the exchange of substrates between the fungus and the host.