In the model fungus Aspergillus nidulans the velvet proteins of VeA and VelB form a complex with LaeA to coordinate sexual development and secondary metabolite production in response to darkness. VelB also forms a complex with VosA, another velvet protein, to repress asexual conidiation in the dark. Aspergillus flavus is a major producer of the potent carcinogenic secondary metabolites, aflatoxins. In nature, A. flavus reproduces and disseminates asexually; only recently has sexual reproduction been demonstrated under laboratory conditions. In the present study, we examined the roles of A. flavus veA, velB and velC on conidiation and aflatoxin production by gene knockout, complementation and overexpression approaches. We also determined expression of selected aflatoxin genes in the cross-complemented overexpression strains by qRT-PCR. Knockout of veA and velBseverely impaired conidiation in the dark but not under light. Both types of mutants were unable to produce aflatoxins irrespective of illumination conditions. In contrast, velC knockout mutants developed and produced aflatoxins normally. Complementation of the veA and velB knockout strains with the respective gene remediated the observed defects. Overexpression of veA in the velB knockout strain remediated the defect in conidiation but not the lack of aflatoxin production in the dark although the aflatoxin pathway regulatory gene aflR was expressed at levels comparable to that of the wild-type strain. Overexpression of velB in the veA knockout strain failed to restore normal conidiation and aflatoxin production; aflR was expressed at levels about 10 to 40% that of the wild-type strain. VeA and VelB of A. flavus function differently from those of A. nidulans and are required for asexual conidiation in the dark. Concerted interactions of VeA and VelB with LaeA and other factors are critical for aflatoxin biosynthesis.
Full conference title:
- ASM 112th (2012)