The bZIP-type transcription factor FlbB regulates distinct morphogenetic stages of colony formation in Aspergillus nidulans.

Conidiophore formation in Aspergillus nidulans involves a developmental programme in which vegetative hyphae give rise to an ordered succession of differentiated cells: foot cell, stalk, vesicle, metulae, phialides and conidia. The developmental transition requires factors that are expressed in vegetative hyphae that activate the expression of the main regulator of conidiation, BrlA. One such element is the bZIP-type transcription factor FlbB. We found that flbB(-) mutants show defective branching patterns and are susceptible to autolysis under high sorbitol or sucrose concentrations, revealing a role in vegetative growth. In addition, FlbB plays a role in conidiophore initiation, as its upregulation reduces conidiophore vesicle swelling and generates a reduced number of metulae. FlbB was located at the tip of growing metulae, following a similar pattern as described in vegetative hyphae. In wild-type strains, the transition from metulae to phialides could be reversed to generate vegetative hyphae, indicating the existence of a specific control point at this stage of conidiophore formation. The combined evidence points to FlbB as a key factor in the transition to asexual development, playing a role at various control points in which the process could be reversed.