The Aspergillus flavus Homeobox Gene, hbx1, Is Required for Development and Aflatoxin Production

Abstract

Homeobox proteins, a class of well conserved transcription factors, regulate the expression of targeted genes, especially those involved in development. In filamentous fungi, homeobox genes are required for normal conidiogenesis and fruiting body formation. In the present study, we identified eight homeobox (<i>hbx</i>) genes in the aflatoxin-producing ascomycete, <i>Aspergillus flavus</i>, and determined their respective role in growth, conidiation and sclerotial production. Disruption of seven of the eight genes had little to no effect on fungal growth and development. However, disruption of the homeobox gene AFLA_069100, designated as <i>hbx1</i>, in two morphologically different <i>A. flavus</i> strains, CA14 and AF70, resulted in complete loss of production of conidia and sclerotia as well as aflatoxins B₁ and B₂, cyclopiazonic acid and aflatrem. Microscopic examination showed that the Δ<i>hbx1</i> mutants did not produce conidiophores. The inability of Δ<i>hbx1</i> mutants to produce conidia was related to downregulation of <i>brlA</i> (bristle) and <i>abaA</i> (abacus), regulatory genes for conidiophore development. These mutants also had significant downregulation of the aflatoxin pathway biosynthetic genes <i>aflC</i>, <i>aflD</i>, <i>aflM</i> and the cluster-specific regulatory gene, <i>aflR</i>. Our results demonstrate that <i>hbx1</i> not only plays a significant role in controlling <i>A. flavus</i> development but is also critical for the production of secondary metabolites, such as aflatoxins.