Nutrient exhaustion was thought to be the single most powerful general stimulus for the initiation of reproduction in fungi. Adams and Timberlake (1990) questioned whether conidiation development could be induced in the presence of excess nutrients. Adopting the recombinant DNA methodologies, they constructed strains of A. nidulans in which the promoter gene (p) from the alcohol dehydrogenase gene (alcA) was fused to conidiation regulatory genes (brlA or abaA) and critically tested the nutrient exhaustion hypothesis of conidiation. The genotype of constructed gene fusion strains is denoted as alc(p) :: brlA abaA and alcA(p) :: abaA. The fusion strain germlings were grown as a homogeneous mycelium in submerged cultures containing a carbon source. After three hours, the cultures were shifted to media containing threonine or ethanol to force the induction of the brlA or abaA gene. The forced expression of the conidiation-specific regulatory genes brlA or abaA led to a generalized metabolic shutdown as indicated by the reduction in total protein and RNA and to a loss in the ability to take up nutrients from the medium. Vegetative growth ceased and conidia were formed at the ends of hyphae, although without the formation of vesicle, metula and phialide (Adams et al., 1988; Adams and Timberlake, 1990). The ability to induce conidiation by direct activation of the brlA gene suggested that brlA mediates the development switch from the polarized growth of stalk cell to budding growth leading to conidium formation. It was inferred that reproduction is a genetically programmed event in development that is triggered by an internal signal which shuts down the genes involved in nutrient uptake and activates the conidiation pathway by activating the expression of brlA alone. The fungus must somehow sense the external environment and transduce the stimulus (signal) across the cell membrane into the intra-cellular environment.
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