Aerial growth in Neurospora crassa: Characterization of an experimental model system

Liquid-grown Neurospora crassa mycelia, when filtered and exposed to the air, rapidly formed aerial hyphae that conidiated in a synchronous manner. The air-exposed mycelial mat could be separated into two different layers: an upper layer of adherent mycelia and a lower layer of loose mycelia. Adhesion of the hyphae occurred in less than 0.5 h and the hyphae were characteristically arranged in interconnecting bundles. This upper layer produced the aerial hyphae that could be seen after 3 h exposure to air. Aerial hyphae grew during the next 9 h, conidiophores were formed after about 9 h exposure to air, and at 12 h loose conidia were obtained. In both layers, profuse septation occurred during the first hours of incubation, vacuolation increased, and the number of nuclei diminished with time. Aerial hyphae were also septated and vacuolation was observed at their bases; numerous nuclei were seen in the aerial hyphae and conidiophores. The lower layer also contributed to the formation of aerial hyphae in an indirect manner. The lower layer was metabolically much less active than the upper layer with regard to incorporation of labeled amino acids into both hyphae and macromolecules. Translocation of label from the lower layer to the upper layer and to the aerial hyphae was detected. Ammonium, nitrate, and glutamine inhibited, whereas sucrose stimulated aerial growth.