Geosiphon pyriforme, an Endocytosymbiosis Between Fungus and Cyanobacteria, and its Meaning as a Model System for Arbuscular Mycorrhizal Research

Abstract

Geosiphon pyriforme (Kütz.) v. Wettstein is a coenocytic soil fungus and until now the only known example of a fungus living in endocytobi-otic association with a cyanobacterium, i.e. with Nostoc punctiforme. The symbiotic nature of the system was first recognized by F.v. Wettstein (1915), who described it as a symbiosis between a heterotrophic siphonal chlorophyceaen alga and Nostoc. The fungal nature of the macrosymbiont was recognized by Knapp (1933). The fungus lives together with the cyanobacterium on the surface and in the upper layer of wet soils poor in inorganic nutrients, particularly in phosphate. When a fungal hypha comes into contact with free-living Nostoc cells, the latter are incorporated by the fungus at the hyphal tip, which thereafter swells and forms a unicellular “bladder”, about 1–2mm in size and appearing on the soil surface (Fig. 1). Inside this bladder the cyanobacteria are physiologically active and dividing. Life history, ultra-structure and physiological activity of the system will be described in this chapter.