The plant pathogenic fungus Aspergillus flavus produces several types of mycotoxins. The most well known are the carcinogenic compounds called aflatoxins. In addition, A. flavus produces cyclopiazonic acid and aflatrem mycotoxins, contributing to the toxicity of A. flavus infected crops. Cyclopiazonic acid is a specific inhibitor of calcium-dependent ATPase in the sarcoplasmic reticulum that results in altered cellular Ca++ levels. Aflatrem is a potent tremorgenic mycotoxin known to lead to neurological disorders. Previously we showed that a gene called veA controls aflatoxin and sclerotial production in A. parasiticus. In this study in A. flavus, we show that the veA homolog in A. flavus not only is necessary for the production of aflatoxins B1 and B2 and sclerotia, but also regulates the synthesis of the mycotoxins cyclopiazonic acid and aflatrem. The A. flavus DeltaveA mutant was completely blocked in the production of aflatrem and showed greater than twofold decrease in cyclopiazonic acid production. The genes involved in the synthesis of cyclopiazonic acid are unknown; however, the aflatrem gene cluster has been characterized. Northern hybridization analysis showed that veA is required for expression of the A. flavus aflatrem genes atmC, atmG, and atmM. This is the first report of a regulatory gene governing the production of cyclopiazonic acid and aflatrem mycotoxins.

Secondary metabolism and development are linked in Aspergillus through the conserved regulatory velvet complex composed of VeA, VelB, and LaeA. The founding member of the velvet complex, VeA, shuttles between the cytoplasm and nucleus in response to alterations in light. Here we describe a new interaction partner of VeA identified through a reverse genetics screen looking for LaeA-like methyltransferases in Aspergillus nidulans. One of the putative LaeA-like methyltransferases identified, LlmF, is a negative regulator of sterigmatocystin production and sexual development. LlmF interacts directly with VeA and the repressive function of LlmF is mediated by influencing the localization of VeA, as over-expression of llmF decreases the nuclear to cytoplasmic ratio of VeA while deletion of llmF results in an increased nuclear accumulation of VeA. We show that the methyltransferase domain of LlmF is required for function; however, LlmF does not directly methylate VeA in vitro. This study identifies a new interaction partner for VeA and highlights the importance of cellular compartmentalization of VeA for regulation of development and secondary metabolism.

Environmental stimuli trigger an adaptative cellular response to optimize the probability of survival and proliferation. In eukaryotic organisms from mammals to fungi osmotic stress, mainly through the action of the high osmolarity glycerol (HOG) pathway, leads to a response necessary for adapting and surviving hyperosmotic environments. In this review we show that the osmoadaptative response is conserved but not identical in different fungi. The osmoadaptative response system is also intimately linked to morphogenesis in filamentous fungi, including mycotoxin producers. Previous studies indicate that the response to osmotic stress is also coupled to the biosynthesis of natural products, including mycotoxins.

The aflatoxin-producing fungus Aspergillus flavus is a causal agent of preharvest contamination of food commodities such as oil seed crops worldwide. Peanut, corn and cottonseed are among the oil seeds that are susceptible to aflatoxin contamination during invasion of these crops by A. flavus. Contamination of agricultural commodities with aflatoxins can result in serious economic hardships to producers and adverse health impacts in both humans and domestic animals. It is therefore of great importance to determine ways to control A. flavus dissemination, survival and toxin formation. In this study we demonstrate the role of the veA regulatory gene in the contamination of peanut, corn and cotton by A. flavus. Virulence of A. flavus on peanut and corn seeds was reduced in the absence of the veA gene product. Generation of air-borne asexual spores was reduced and production of aflatoxin and sclerotia in peanut seeds, viable or non-viable, or in viable corn seed was completely blocked when infected with the A. flavus veA mutant ( veA). In planta inoculation of cotton bolls also showed that conidiation was decreased in bolls inoculated with the veA strain and spread of the veA strain to seed in locules adjacent to the inoculated locule was less than observed with the wild-type veA strain. As observed in peanut and corn, no aflatoxin was produced in seed harvested from cotton bolls inoculated with the veA strain while aflatoxin was present in seed from wild-type veA inoculated bolls.

Survival of fungal species depends on the ability of these organisms to respond to environmental stresses. Osmotic stress or high levels of reactive oxygen species (ROS) can cause stress in fungi resulting in growth inhibition. Both eukaryotic and prokaryotic cells have developed numerous mechanisms to counteract and survive the stress in the presence of ROS. In many fungi, the HOG signaling pathway is crucial for the oxidative stress response as well as for osmotic stress response. This study revealed that while the osmotic stress response is only slightly affected by the master regulator veA, this gene, also known to control morphological development and secondary metabolism in numerous fungal species, has a profound effect on the oxidative stress response in the aflatoxin-producing fungus Aspergillus flavus. We found that the expression of A. flavus homolog genes involved in the HOG signaling pathway is regulated by veA. Deletion of veA resulted in a reduction in transcription levels of oxidative stress response genes after exposure to hydrogen peroxide. Furthermore, analyses of the effect of VeA on the promoters of cat1 and trxB indicate that the presence of VeA alters DNA-protein complex formation. This is particularly notable in the cat1 promoter, where the absence of VeA results in abnormally stronger complex formation with reduced cat1 expression and more sensitivity to ROS in a veA deletion mutant, suggesting that VeA might prevent binding of negative transcription regulators to the cat1 promoter. Our study also revealed that veA positively influences the expression of the transcription factor gene atfB and that normal formation of DNA-protein complexes in the cat1 promoter is dependent on AtfB.