CHARACTERIZATION OF THE GEOSIPHON-PYRIFORME SYMBIOSOME BY AFFINITY TECHNIQUES - CONFOCAL LASER-SCANNING MICROSCOPY (CLSM) AND ELECTRON-MICROSCOPY

Geosiphon pyriforme represents a photoautotrophic endosymbiosis of a G lomus-like fungus with the cyanobacterium Nostoc punctiforme. The fung us forms unicellular bladders of up to 2 mm in length and 0.5 mm in di ameter growing on the soil surface and harboring the endosymbiotic Nos toc filaments. The cyanobacteria are located in a compartment (the sym biosome) bordered by a host membrane. The space between this symbiosom e membrane (SM) and the Nostoc cell wall is filled with an about 30-40 nm thick layer of amorphous material, which is present also in the re gions of the symbiosome where no Nostoc filaments are located. At thes e sites the amorphous material consists of a 20-30 nm thick layer sepa rating the SM. The region between the SM and the cyanobacterium is def ined as symbiosome space (SS). Fungal bladders, hyphae and free living Nostoc were analyzed by affinity techniques as well as the material o ccurring in the SS. FITC-coupled lectins with sugar specificity to, al pha-D-mannosyl/alpha-D-glucosyl (Con A), N-acetyl-beta-D-glucosamine o ligomers (WGA), alpha-L-fucosyl (UEA-I), beta-D-galactosyl (RCA-120), alpha-D-galactosyl (BS-I-B-4), N-acetyl-alpha-D-galactosamine (HPA), a nd sialic acid (EEL) residues were tested. WGA binding and calcofluor white staining demonstrated that the bladder wall as well as the SS co ntain fibrillar chitin. Of the other lectins only Con A clearly labele d the symbiosome. On the contrary, the lectin binding properties of th e slime produced by free living Nostoc-colonies indicate the presence of mannose, fucose, GalNAc, sialic acid, and galactose, while chitin o r GlucNAc-oligomers could not be detected, The symbiosome was also inv estigated electron microscopically. WGA-gold binding confirmed the pre sence of chitin, while a slight PATAg reaction indicated some polysacc haridic molecules within the SS. Our results show that the amorphous m aterial within the SS contains molecules typical of the fungal cell wa ll and suggest that the SM is related to the fungal plasma membrane. T he applied lectins all bind to the hyphal surface, indicating a high m olecular complexity. Mannosyl, beta-galactosyl, and sialic acid residu es are strongly exposed at the outer cell wall layer, whereas GlucNAc, GalNAc, and alpha-galactosyl residues seem to be present in smaller a mounts. The symbiotic interface established between the fungus and Nos toc in Geosiphon shows many similarities to that occurring between fun gi and root cells in arbuscular mycorrhizas.