Flammulina as a model system for fungal graviresponses.

Gravitropic bending of fruiting bodies of Flammulina velutipes (Curtis) Karst. is based on the differential growth of the transition zone between stem and cap. Reorientation becomes visible as early as 2 h after displacing the fruiting body from the vertical to the horizontal position. It is preceded by a preferential accumulation of microvesicles within the hyphae on the lower side of the transition zone and related to an increase in the vacuolar compartment required for hyphal extension. A model made of a bundle of interconnected balloons is used to demonstrate that a differential volume increase at one flank is sufficient to bend the entire structure in the opposite direction. Gravitropic raising of intact stems or segments derived from the transition zone requires positional information which can be accomplished by three major, coordinated events: (i) gravisensing by the individual hyphae within the transition zone, (ii) unidirectional signalling by means of a soluble growth factor creating a vertical concentration gradient, and (iii) translation of the concentration signal into elongation growth.