Diffuse extension of hyphae in stipes of Agaricus bisporus may be based on a unique wall structure

Hyphal walls of mycelium and fruit-body stipes from Agaricus bisporus were compared at the ultrastructural level. Polarized-light microscopy revealed very weak anisotropy in untreated and alkali-extracted wall preparations from elongating and non-elongating stipe hyphae but not in those from substrate hyphae. The differences in anisotropy corresponded to differences in wall texture. In elongating hyphae of the mushroom stipe, glucosaminoglycan chains were transversely oriented and not generally organized in distinct chitin microfibrils, whereas walls from substrate hyphae showed randomly oriented microfibrils embedded in an amorphous matrix. In contrast to substrate hyphae, intact walls from elongating stipe hyphae axially contracted upon treatment with exo-(1 → 3)-β-gIucanase while fragmented walls completely disintegrated, leaving glucosaminoglycan in the form of scattered fibrils and amorphous material. A tentative model is advanced explaining diffuse extension growth of the wall by creep of the polymers in the wall due to continuous breakage and reformation of hydrogen bonds among the glucan chains and passive re-orientation of the glucosaminoglycan chains in a transverse direction.