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 be tween stem and cap. Reorientation becomes visible as early as 2 h afte r displacing the fruiting body from the vertical to the horizontal pos ition. 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 extens ion. A model made of a bundle of interconnected balloons is used to de monstrate that a differential volume increase at one flank is sufficie nt 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 maj or, coordinated events: (i) gravisensing by the individual hyphae with in the transition zone, (ii) unidirectional signalling by means of a s oluble growth factor creating a vertical concentration gradient, and ( iii) translation of the concentration signal into elongation growth.