THE FUNGAL BLADDERS OF THE ENDOCYANOSIS GEOSIPHON PYRIFORME, A GLOMUS-RELATED FUNGUS - CELL-WALL PERMEABILITY INDICATES A LIMITING PORE RADIUS OF ONLY 0.5 NM

Geosiphon pyriforme, a consortium of a Glomus-like fungus and Nostoc s pp., forms syncytial, up to 2 mm long bladders accommodating the endos ymbiotic cyanobacteria. The bladders are bordered by an elastic cell w all and have a turgor of about 0.6 MPa, as measured by piercing them w ith oil filled microcapillaries within different osmolarities of sorbi tol. In the presence of certain organic osmolytes in the surrounding m edium, the bladders collapsed, i.e., showed cytorrhysis. We studied sy stematically the cytorrhytic effectivity of the diverse osmolytes in r elation to their hydrodynamic molecule radii by a solute-exclusion met hod with living bladders and those which have been extracted by differ ent methods. The results suggest that the cell wall of the bladders ha s an unusually small limiting pore size thus representing an effective diffusion barrier for glucose and is virtually impermeable for sucros e for at least 8 h. The pore radii of the cell wall are estimated to b e about 0.5 nm. Na2CO3 extraction, frequently used to partially extrac t pectic substances from plant cell walls, strongly increases wall per meability. Electron microscopic observations show an electron-dense ou ter cell wall layer, perhaps responsible for the low permeability. The finding that the cell wall of the Geosiphon bladders represents an ef fective osmotic barrier provides not only new insights into the cell p hysiology of Geosiphon but may also contribute more generally to a bet ter understanding of the mechanisms of selectivity of transport across the cell walls of AM fungi.