A dynamic continuum of pleiomorphic tubules and vacuoles in growing hyphae of a fungus

The vacuole system in growing hyphal tips of Pisolithus tinctorius is a dynamic continuum of vacuoles and extensible tubular elements. The system varies from a tubular reticulum with few vacuoles across a spectrum of intermediate forms to clusters of vacuoles with few tubules. Spherical vacuoles interconnected in clusters are situated at intervals along the hyphal tip and are transiently linked by tubules that extend from a vacuole in one cluster and fuse with that of another. Extension and retraction of the tubules is independent of cytoplasmic streaming, can occur in either direction, and covers distances as great as 60 μm. The tubules pulsate and peristalsis-like movements transfer globules of material along them between the vacuoles in different clusters. The tubules also generate vacuoles. The tubular system has the potential for intracellular transport of solutes in the hyphal tips without concomitant transfer of large amounts of membrane. This contrasts with models of intracellular transport via vesicles, where the ratio of membrane transferred to internal content is very much higher. The system has many features in common with tubular endosomal and lysosomal systems in cultured animal cells.