Role of the osaA Transcription Factor Gene in Development, Secondary Metabolism and Virulence in the Mycotoxigenic Fungus Aspergillus flavus

<i>Aspergillus flavus</i> colonizes oil-seed crops, contaminating them with aflatoxins; highly carcinogenic mycotoxins that cause severe health and economic losses. Genetic studies may reveal new targets for effective control strategies. Here, we characterized a putative WOPR transcription factor gene, <i>osaA</i>, in <i>A. flavus</i>. Our results revealed that <i>osaA</i> regulates conidiation and sclerotial formation. Importantly, deletion of <i>osaA</i> reduces aflatoxin B₁ production, while, unexpectedly, transcriptome analysis indicated upregulation of aflatoxin biosynthetic genes, suggesting post-transcriptional or cofactor-mediated regulation. Cyclopiazonic acid production also decreased in the absence of <i>osaA</i>. In addition, the <i>osaA</i> mutant exhibited upregulation of genes in the imizoquin and aspirochlorine clusters. Moreover, <i>osaA</i> is indispensable for normal seed colonization; deletion of <i>osaA</i> significantly reduced fungal burden in corn kernels. Aflatoxin content in seeds also decreased in the absence of <i>osaA</i>. Furthermore, deletion of <i>osaA</i> caused a reduction in cell-wall chitin content, as well as alterations in oxidative stress sensitivity, which could in part contribute to the observed reduction in pathogenicity. Additionally, promoter analysis of <i>osaA</i>-dependent genes indicated potential interactions with stress-responsive regulators, indicated by an enrichment in Sko1 and Cst6 binding motifs. Understanding the <i>osaA</i> regulatory scope provides insight into fungal biology and identifies potential targets for controlling aflatoxin contamination and pathogenicity.