Characterization of a bZIP Transcription Factor ZipD in Aspergillus flavus

Aspergillus flavus produces aflatoxin B1, a Group 1 carcinogen that causes severe economic and agricultural losses and can lead to invasive aspergillosis in humans. During its growth, A. flavus develops hyphae that form a continuous mycelium, which is associated with host colonization and toxin production. Transcription factors are considered regulators of fungal development and pathogenicity. Among these families, bZIP proteins are associated with growth, metabolism, and stress tolerance. In A. fumigatus, the bZIP transcription factor ZipD regulates calcium signaling pathways and cell wall integrity. However, the ortholog of zipD in A. flavus has not been characterized. To address this gap, we generated a deletion mutant and a complemented strain of zipD in A. flavus. The zipD deletion mutant showed reduced colony growth and conidia production, but increased sclerotia formation. The ΔzipD strain exhibited hypersensitivity to osmotic stress-related agents, cell wall stress-related agents, and high calcium concentrations. Additionally, ΔzipD showed reduced conidial colonization in pathogenicity assays using corn, soybean, and soybean brick compared to the wild-type strain. All of these phenotypes were restored in the complemented strain. These results suggest that zipD functions as a regulator of growth, developmental processes, and pathogenic traits in A. flavus.