Apoplastic Transport through the Fungal Sheath of Pinus sylvestris/Suillus bovinus Ectomycorrhizae

Abstract

AbstractRoots of Pinus sylvestris L. were inoculated in vitro with the basidiomycete Suillus bovinus (Fr.) O. Kuntze. To investigate apoplastic transport in mycorrhizal and sterile roots of Pinus sylvestris, roots of intact plants were submerged for 20 h in 0.1% solutions of the fluorescent dye sulphorhodamine G (SR‐G) or for 6 h in 1.5% solutions of lanthanum nitrate. Samples treated with the dye were cryofixed, freeze‐dried or freeze‐substituted and embedded into Spurr's medium, maintaining strictly anhydrous conditions to prevent movement of the water‐soluble dye after cryofixation. Lanthanum‐treated roots were fixed in glutaraldehyde, post‐fixed in OsO4, dehydrated in a graded acetone series and embedded in Spurr's resin. The apoplastic distribution of the two tracers were examined either using fluorescence optics (sulphorhodamine) or with the electron microscope (La3+).The yellow‐green fluorescence of sulphorhodamine could be detected within the apoplast of the fungal sheath, the Hartig net and the host cortex, up to the endodermis. Electron‐dense lanthanum deposits were located in the fungal sheath, the Hartig net and in the root cortex. Greater deposition was detected within the matrix material, in which the hyphae of the mantle are embedded. The apoplastic distribution of the two tracers within the plant root did not indicate any significant qualitative differences between sterile and mycorrhizal rootlets. In contrast to reports by other authors, we conclude that the fungal sheath does provide an apoplastic pathway for water and ions at least in Pinus sylvestris/Suillus bovinus mycorrhizae. However, the mobility of charged molecules, particularly cations, may be limited by the fungal matrix.