Potential Roles of Secondary Metabolite Production in Environmental Oxidative Stress Responses Revealed in the Aspergillus flavus Transcriptome

The contamination of agricultural crops with aflatoxins is exacerbated in the dry and hot season worldwide, and poses a serious food security concern, particularly in developing countries. Drought stress results in the accumulation of reactive oxygen species (ROS) in the tissues of plants. Previously, we have shown that ROS stimulate the production of aflatoxin by Aspergillus flavus in vitro, and that aflatoxin production is correlated with fungal isolates that have greater oxidative stress tolerance. In order to investigate the role of aflatoxin in oxidative stress responses, we sequenced the transcriptomes of both toxigenic and atoxigenic isolates of A. flavus in aflatoxin conducive and non-conducive media amended with increasing levels of H₂O₂. Isolates which previously exhibited greater oxidative stress tolerance exhibited fewer differentially expressed genes than those with less tolerance demonstrating a less vigorous response to oxidative stress. In addition, mechanisms related to the production of three secondary metabolites, aflatoxin, aflatrem, and kojic acid, were regulated in response to increasing stress. Aflatoxin and aflatrem genes were expressed only in the conducive medium while kojic acid genes were expressed to a greater extent in the non-conducive medium. Also, monooxygenase genes were among the most commonly regulated genes including those involved in aflatoxin and aflatrem production. Therefore, the biosynthesis of these mycotoxins may consume ROS in order to alleviate oxidative stress in A. flavus. Kojic acid, as a bidentate iron chelator, may also alleviate oxidative stress by inhibiting Fenton-derived ROS production in order to alleviate ROS in aflatoxin non-conducive media. Continuing biochemical and proteomic analyses will examine these mechanisms in greater detail.