The fungal bladders of the endocyanosisGeosiphon pyriforme, aGlomus-related fungus: cell wall permeability indicates a limiting pore radius of only 0.5 nm

Abstract

Geosiphon pyriforme, a consortium of aGlomiw-like fungus andNostoc spp., forms syncytial, up to 2 mm long bladders accommodating the endosymbiotic cyanobacteria. The bladders are bordered by an elastic cell wall and have a turgor of about 0.6 MPa, as measured by piercing them with oil filled microcapillaries within different osmolarities of sorbitol. In the presence of certain organic osmolytes in the surrounding medium, the bladders collapsed, i.e., showed cytorrhysis. We studied systematically the cytorrhytic effectivity of the diverse osmolytes in relation to their hydrodynamic molecule radii by a solute-exclusion method with living bladders and those which have been extracted by different methods. The results suggest that the cell wall of the bladders has an unusually small limiting pore size thus representing an effective diffusion barrier for glucose and is virtually impermeable for sucrose for at least 8 h. The pore radii of the cell wall are estimated to be about 0.5 nm. Na2CO3 extraction, frequently used to partially extract pectic substances from plant cell walls, strongly increases wall permeability. Electron microscopic observations show an electron-dense outer cell wall layer, perhaps responsible for the low permeability. The finding that the cell wall of theGeosiphon bladders represents an effective osmotic barrier provides not only new insights into the cell physiology ofGeosiphon but may also contribute more generally to a better understanding of the mechanisms of selectivity of transport across the cell walls of AM fungi.