The Aspergillus flavus homeobox gene, hbx1, is required for development and aflatoxin production

Authors: Cary, Jeffrey; HARRIS-COWARD, PAMELA - Retired ARS Employee; Scharfenstein, Leslie; Mack, Brian; Chang, Perng Kuang; Wei, Qijian - Mei Mei; Lebar, Matthew; Carter-Wientjes, Carol; Majumdar, Raj; MITRA, CHANDRANI - University Of South Carolina; BANERJEE, SOURAV - University Of South Carolina; CHANDA, ANINDYA - University Of South Carolina

Submitted to: Toxins
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2017
Publication Date: 10/12/2017
Citation: Cary, J.W., Harris-Coward, P.Y., Scharfenstein, L.L., Mack, B.M., Chang, P.-K., Wei, Q., Lebar, M.D., Carter-Wientjes, C.H., Majumdar, R., Mitra, C., Banerjee, S., Chanda, A. 2017. The Aspergillus flavus homeobox gene, hbx1, is required for development and aflatoxin production. Toxins. 9(10):315. https://doi.org/10.3390/toxins9100315.
DOI: https://doi.org/10.3390/toxins9100315

Interpretive Summary: This work describes experiments that have been conducted in an effort to better understand the genetic mechanisms that control aflatoxin production and growth and development in Aspergillus flavus. Aflatoxins are toxic and carcinogenic compounds often produced by the fungi, Aspergillus flavus during growth on crops such as corn, peanuts, cottonseed, and treenuts. Because of the potential health risks, aflatoxin contamination of food and feed crops is also of great economic importance to farmers who cannot sell their crops due to strict domestic and international regulatory guidelines with regards to aflatoxin contamination. We have succeeded identifying a gene, designated hbx1, from A. flavus that when inactivated stops the fungus from making aflatoxins and also structures known as conidia and sclerotia that help the fungus to spread and survive in the field under adverse conditions. The hbx1 gene is a class of genes known as homeobox transcription factors that are known to regulate development in fungi, animals and humans. In addition, this study showed that hbx1 controls the production of other secondary metabolites produced by A. flavus. This is the first report of a single gene from A. flavus that is required for production of conidia, sclerotia, and aflatoxins. Identification of hbx1 will add to our knowledge of regulation of growth and toxin production in A. flavus and this in turn will help in devising strategies for eliminating fungal toxin contamination of food and feed crops.

Technical Abstract: Homeobox proteins are a class of transcription factors that are well conserved in many eukaryotes including fungi. Homeobox proteins regulate the expression of target genes, especially those involved in development. Studies in several filamentous fungi have shown that homeobox genes are required for normal conidiogenesis and fruiting body formation. Here, we report on the identification of 8 homeobox (hbx) genes in the aflatoxin-producing ascomycete, Aspergillus flavus, and analysis of conidial and sclerotial production in hbx deletion mutants. Disruption of 7 of the 8 homeobox genes had little to no effect on fungal growth and development. However, disruption of the homeobox gene designated hbx1 (AFLA_069100) in both A. flavus CA14 and AF70 resulted in complete loss of conidial and sclerotial production as well as a production of aflatoxins B1 and B2. Gene expression analysis showed the inability of 'hbx1 mutants to produce conidia was related to down-regulation of the regulators of conidiophore development, brlA (bristle) and abaA (abacus). Both 'hbx1 mutants also demonstrated significant downregulation of the aflatoxin pathway-specific transcription factor gene, aflR, as well as biosynthetic genes aflD and aflM. Microscopic examination showed that the 'hbx1 mutants do not produce vesicles, metulae, phialides or conidia at the terminus of the conidiophore stipe. This is the first report of a gene in A. flavus that is required for conidial and sclerotial formation well as production of secondary metabolites including aflatoxins. These results also demonstrate for the first time the role of a fungal homeobox gene in sclerotial development and secondary metabolite production.