It was long been noted that secondary metabolism is associated with fungal development. In Aspergillus nidulans , conidiation and mycotoxin production are linked by a G protein signaling pathway. Also in A. nidulans , cleistothecial development and mycotoxin production are controlled by a gene called veA . Here we report the characterization of a veA ortholog in the aflatoxin-producing fungus A. parasiticus . Cleistothecia are not produced by Aspergillus parasiticus ; instead, this fungus produces spherical structures called sclerotia that allow for survival under adverse conditions. Deletion of veA from A. parasiticus resulted in the blockage of sclerotial formation as well as a blockage in the production of aflatoxin intermediates. Our results indicate that A. parasiticus veA is required for the expression of aflR and aflJ , which regulate the activation of the aflatoxin gene cluster. In addition to these findings, we observed that deletion of veA reduced conidiation both on the culture medium and on peanut seed. The fact that veA is necessary for conidiation, production of resistant structures, and aflatoxin biosynthesis makes veA a good candidate gene to control aflatoxin biosynthesis or fungal development and in this way to greatly decrease its devastating impact on health and the economy.

Secondary metabolism is commonly associated with morphological development in microorganisms, including fungi. We found that veA , a gene previously shown to control the Aspergillus nidulans sexual/asexual developmental ratio in response to light, also controls secondary metabolism. Specifically, veA regulates the expression of genes implicated in the synthesis of the mycotoxin sterigmatocystin and the antibiotic penicillin. veA is necessary for the expression of the transcription factor aflR , which activates the gene cluster that leads to the production of sterigmatocystin. veA is also necessary for penicillin production. Our results indicated that although veA represses the transcription of the isopenicillin synthetase gene ipnA , it is necessary for the expression of acvA , the key gene in the first step of penicillin biosynthesis, encoding the delta-( l -alpha-aminoadipyl)- l -cysteinyl- d -valine synthetase. With respect to the mechanism of veA in directing morphological development, veA has little effect on the expression of the known sexual transcription factors nsdD and steA . However, we found that veA regulates the expression of the asexual transcription factor brlA by modulating the α/β transcript ratio that controls conidiation.

Summary: The veA gene is a light‐dependent regulator governing development and secondary metabolism in Aspergillus nidulans. We have identified a putative bipartite nuclear localization signal (NLS) motif in the A. nidulans VeA amino acid sequence and demonstrated its functionality when expressed in yeast. Furthermore, migration of VeA to the nucleus was dependent on the importin α. This bipartite NLS is also functional when VeA is expressed in A. nidulans. Interestingly, we found that VeA migration to the nucleus is light‐dependent. While in the dark VeA is located mainly in the nuclei, under light VeA is found abundantly in the cytoplasm. The VeA1 mutant protein (lacking the first 36 amino acids at the N‐terminus) was found predominantly in the cytoplasm independent of illumination. This indicates that the truncated bipartite NLS in VeA1 is not functional and fails to respond to light. These results might explain the lack of the morphological light‐dependent response in strains carrying the veA1 allele. We also evaluated the effect of light on production of the mycotoxin sterigmatocystin in a veA wild‐type and the veA1 mutant strains and found that the highest amount of toxin was produced by the veA+ strain growing in the dark, condition favouring accumulation of VeA in the nucleus.

Fungal secondary metabolism and morphological development have been shown to be intimately associated at the genetic level. Much of the literature has focused on the co-regulation of secondary metabolite production (e.g. sterigmatocystin and aflatoxin in Aspergillus nidulans and Aspergillus flavus, respectively) with conidiation or formation of sexual fruiting bodies. However, many of these genetic links also control sclerotial production. Sclerotia are resistant structures produced by a number of fungal genera. They also represent the principal source of primary inoculum for some phytopathogenic fungi. In nature, higher plants often concentrate secondary metabolites in reproductive structures as a means of defense against herbivores and insects. By analogy, fungi also sequester a number of secondary metabolites in sclerotia that act as a chemical defense system against fungivorous predators. These include antiinsectant compounds such as tetramic acids, indole diterpenoids, pyridones and diketopiperazines. This chapter will focus on the molecular mechanisms governing production of secondary metabolites and the role they play in sclerotial development and fungal ecology, with particular emphasis on Aspergillus species. The global regulatory proteins VeA and LaeA, components of the velvet nuclear protein complex, serve as virulence factors and control both development and secondary metabolite production in many Aspergillus species. We will discuss a number of VeA and LaeA-regulated secondary metabolic gene clusters in A. flavus that are postulated to be involved in sclerotial morphogenesis and chemical defense. The presence of multiple regulatory factors that control secondary metabolism and sclerotial formation suggests that fungi have evolved these complex regulatory mechanisms as a means to rapidly adapt chemical responses to protect sclerotia from predators, competitors and other environmental stresses.