Extensive tubular vacuole system in an arbuscular mycorrhizal fungus, Gigaspora margarita.

• A tubular vacuolar system is reported here for the first time in living hyphae of Gigaspora margarita, an arbuscular mycorrhizal fungus, during various phases in the development of a symbiotic relationship with onion (Allium cepa) seedlings. • Germ tubes, extraradical hyphae and intercellular hyphae were labeled with Oregon Green 488 carboxylic acid diacetate and observed by laser scanning confocal microscopy. Emphasis was placed on the relationship between the shape of vacuoles and the presence of cytoplasmic streaming. • In germ tubes, labeled vacuoles showed a variety of profiles, including spherical and tubular (< 0.5 µm diameter), with various compositions of these shapes along the length of the germ tubes. The tubular vacuoles rarely interconnected with spherical vacuoles and often formed longitudinally oriented, elongated bundles. The tubular vacuolar system appeared to be associated with cytoplasmic streaming, whereas spherical vacuoles were not. Tubular vacuoles were observed in all regions of the germ tubes and were also observed in both extraradical and intercellular hyphae. • The results question the hypothesis that discrete vacuoles may be involved in the translocation of polyphosphate along hyphae of arbuscular mycorrhizal fungi.