The induction of sporulation in mould fungi

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Abstract
Sporulation of Penicillium griseofulvum is rapidly induced in submerged conditions when mycelium is placed in a medium containing a high concentration of glucose but without assimilable nitrogen. External Mg$^{2+}$, K$^{+}$, PO$_{4}^{3-}$ are required for sporulation. In these conditions the induction of sporulation is completed in 3 to 4 h, before there is any sign of conidiophore formation. The high concentration of glucose required for sporing can be effectively replaced by an equivalent concentration of non-assimilable sugars (sorbose, D-arabinose) provided a small amount of glucose is present to supply the energy requirements of the fungus. High concentrations of salts are not effective and tend to inhibit sporing. Induction of sporing in submerged conditions is inhibited by certain concentrations of ascorbate, Hg$^{2+}$, cetyltrimethylammonium bromide and 3,4-dichlorphenylserine which do not affect vegetative growth. The most powerful stimulus to sporulation in Penicillia and other fungi comes from emergence of mycelium from submerged into aerial conditions. Experiments with P. griseofulvum and P. chrysogenum show that emergence of the mycelium causes a sharp change in physiological behaviour which is manifest before any morphogenetic changes are apparent. Aerial mycelium assimilates NH$_{4}^{+}$ in the complete absence of external carbohydrate, whereas submerged mycelium will not assimilate NH$_{4}^{+}$ in these conditions even when fully aerated. The aerial stimulus is not associated with the supply of O$_{2}$ nor with concentration of CO$_{2}$ nor with water loss from the mycelium. Evidence from three independent methods of investigation points to the occurrence of an abrupt change in the physical condition of the cell surface when mycelium emerges from a liquid medium into the air. Each method indicates a physical change which takes place within a few seconds of emergence, which increases slowly with time in aerial conditions, and which, like aerial sporulation itself, is reversed or delayed by re-wetting. It is considered that the primary aerial stimulus to sporulation is associated with some physical change at the cell surface caused by the abrupt formation of an air/water interface.

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