Cellular graviperception in the basidiomycete Flammulina velutipes--can the nuclei serve as fungal statoliths?

In search for the cellular mechanisms of graviperception in basidiomycete fruit bodies, nuclear and vacuole motility was investigated in the fungus, Flammulina velutipes. In this organism, hyphal nuclei are closely linked with the actin cytoskeleton, which is involved in gravity perception. Active motion of the nuclei appears aligned with the axis of gravity, and is maintained after spatial reorientation of the cell. The vacuoles showed low motility not aligned with the gravity axis at all. The nuclear density was determined with 1.22 g/cm3. Calculation of the forces exerted by the nuclei shows that nuclear displacement in the submicrometer range already fulfills the physical minimum condition for a statolith. Based on these findings, a function of nuclei as statoliths in basidiomycete hyphae is proposed.