Deletion of the Aspergillus flavus Orthologue of A. nidulans fluG Reduces Conidiation and Promotes Production of Sclerotia but Does Not Abolish Aflatoxin Biosynthesis

Abstract

The fluG gene is a member of a family of genes required for conidiation and sterigmatocystin production in Aspergillus nidulans. We examined the role of the Aspergillus flavus fluG orthologue in asexual development and aflatoxin biosynthesis. Deletion of fluG in A. flavus yielded strains with an approximately 3-fold reduction in conidiation but a 30-fold increase in sclerotial formation when grown on potato dextrose agar in the dark. The concurrent developmental changes suggest that A. flavus FluG exerts opposite effects on a mutual signaling pathway for both processes. The altered conidial development was in part attributable to delayed expression of brlA, a gene controlling conidiophore formation. Unlike the loss of sterigmatocystin production by A. nidulans fluG deletion strains, aflatoxin biosynthesis was not affected by the fluG deletion in A. flavus. In A. nidulans, FluG was recently found to be involved in the formation of dehydroaustinol, a component of a diffusible signal of conidiation. Coculturing experiments did not show a similar diffusible meroterpenoid secondary metabolite produced by A. flavus. These results suggest that the function of fluG and the signaling pathways related to conidiation are different in the two related aspergilli.A spergillus nidulans has been the model system for studying asexual conidiation at the molecular level. The genes brlA (bristle) (1), abaA (abacus) (39), and wetA (wet-white conidia) (29) are expressed sequentially and form the central developmental pathway of conidiation (17). BrlA is a transcription factor that mediates the budding growth of conidiophores. Mutations in brlA prevent vesicle formation, yielding stalks that grow indeterminately and that are unable to bear conidia (1). BrlA activates the expression of abaA, a gene that controls phialide differentiation (39). AbaA further mediates transcriptional activation of wetA (31) and also expression of brlA (3, 4). Efforts to elucidate early events leading to activation of conidiation in A. nidulans have identified six genes, fluG (fluffy) (2) and flbA to flbE (fluffy with low brlA expression), that are required for normal activation of brlA (42). Mutations of these genes result in proliferation of undifferentiated vegetative hyphae that produce fluffy cotton-like colonies. A. nidulans fluG is not transcribed in dormant conidia and is significantly downregulated during early vegetative growth; its expression before and after induction of conidiation does not change significantly (2, 7). A positive role of fluG in brlA activation has been suggested in A. nidulans, although the genetic relationship of brlA and fluG is not well defined. A. nidulans fluG mutants do not produce the secondary metabolite sterigmatocystin (20), the penultimate precursor of the carcinogenic aflatoxins. Because contact of the wild-type A. nidulans strain with the fluG null mutant remediates phenotypic defects (26), it was postulated that FluG synthesizes a diffusible factor that initiates conidiation. The effector molecules have recently ...

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