Fungal adhesion pad formation and penetration of root cuticle in early stage mycorrhizas ofPicea abies andLaccaria amethystea

Summary Primary events during the establishment of the fungus-root symbiosis in ectomycorrhizas are still little understood. No attention has been paid so far to the adhesion of hyphae to the root cuticle and penetration of this barrier, although the importance of the cuticle has been shown for pathogen-plant interactions. Early developmental stages of in vitro mycorrhization ofLaccaria amethystea onPicea abies after short periods of incubation in growth chambers under elevated CO₂ concentrations were studied by light and transmission electron microscopy. No structural changes in mycorrhization related to elevated CO₂ were found, but fine roots and mycorrhizas developed faster. Adhesion pad formation was observed at hyphal tips in contact with the root cuticle. The adhesion pad was connected to the outer cell wall layer of the hypha and reacted positively to the Swift reaction for cysteine rich proteins. Although the reaction cannot be considered as totally specific, findings are discussed in respect to hydrophobins, which have recently been found to be expressed during early steps in ectomycorrhizal development. The root cuticle was dissolved and penetrated by fungal tips of the fingerlike branching mycelium attached to the root surface. The findings are compared with well documented pathogenic fungus-plant interactions at the cuticle. The possibility of restriction of hyphal attack to that part of the cuticle covering cell junctions is discussed.