Gravitropic bending of fruiting bodies--a model based on hyphal gravisensing and cooperativity

Gravitropic bending of the winter mushroom Flammulina velutipes is achieved by differential growth of the apical part of the stem, the transition zone. Ultrastructural analysis revealed that bending is due to the relaxation of tissue tensions at the lower flank of the stem where hyphal extension growth is promoted in contrast to the upper flank. Extension of lower flank hyphae is preceded by a conspicuous accumulation of microvesicles in the cytosol and their subsequent fusion with the vacuolar compartment, leading to a large volume increase. The hypothesis is put forward that all hyphae in the transition zone are capable of gravisensing. It is derived from experiments with transition zone segments, which exhibit negative gravitropic response independent from their origin within the stem. A model is presented which connects individual gravisensing of the hyphae with a cooperative response within the stem or small segments of the stem. An essential step is the transmission of positional information, by each hypha with respect to the gravitational vector, to the surroundings. The existence of a soluble growth regulator, which is enriched at the lower flank of the stem, is discussed. A gradient could be formed which precedes the gradient of microvesicle formation, and thereby determines the change of growth direction.