Structural characterization of highly branched glucan sheath from Ceriporiopsis subvermispora

Wood rotting basidiomycetes produce extracellular mucilaginous sheaths interfacing fungal hyphae and plant biomass. While the versatility of these fungal sheaths has been addressed, sheaths generated by selective white-rot fungi remain poorly understood. To fill this gap, the sheath produced by the basidiomycete Ceriporiopsis subvermispora, which degrades lignin while inflicting limited cellulose damage, was analyzed in this study. Fluorescence and transmission electron microscopy revealed that the sheath formed three days after inoculation into a beech wood slice on an agar plate and was embedded at the interface between fungal hyphae and wood cell walls. The sheath's chemical structure was evaluated from fungus cultures in a liquid medium containing [U-¹³C₆]-d-glucose and beech wood slices. Compositional analysis, methylation analysis, and ¹³C NMR demonstrated that the sheath mainly consisted of a comb-like β-1,6-glucopyranose residue-branched β-1,3-glucan, which is advantageous to retain water and extracellular secondary metabolites.