The Aspergillus flavus rtfA gene regulates plant and animal pathogenesis and secondary metabolism

Authors: LOHMAR, JESSICA - Northern Illinois University; PUEL, OLIVIER - University Of Toulouse; Cary, Jeffrey; CALVO, ANA - Northern Illinois University

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2018
Publication Date: 3/6/2019
Citation: Lohmar, J.M., Puel, O., Cary, J.W., Calvo, A.M. 2019. The Aspergillus flavus rtfA gene regulates plant and animal pathogenesis and secondary metabolism. Applied and Environmental Microbiology. 85(6):e02446-18. https://doi.org/10.1128/AEM.02446-18.
DOI: https://doi.org/10.1128/AEM.02446-18

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 previously identified a gene, designated rtfA, from A. flavus that when inactivated significantly reduces the ability of the fungus to infect both plants and insects as well as produce aflatoxins. In the current study the rtfA gene was shown to be needed by the fungus in order to successfully infect either peanut seeds or insect larvae. This is because rtfA controls expression of a number of genes required for A. flavus pathogenicity including adhesion to surfaces, production of enzymes that degrade host tissues, cell wall composition and resistance to stress. This study sldo showed that rtfA controls the production of a number of other potent compounds produced by A. flavus in addition to aflatoxins. The study of RtfA has added to our knowledge of regulation of virulence 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: Aspergillus flavus is a fungal opportunistic plant and human pathogen and producer of deadly mycotoxins including aflatoxin B1 (AFB1). In an effort to discover new genetic targets against A. flavus infections, in this study we examined the possible role of the rtfA gene in pathogenicity of this fungus. rtfA encodes a putative RNA-Pol II transcription elongation factor, previously characterized in Saccharomycese cerevisiae, Aspergillus nidulans, Aspergillus fumigatus, where it was shown to regulate several important cellular processes, including morphogenesis, secondary metabolism. In addition, an initial study in A. flavus indicated that rtfA also influences development and production of AFB1, however its effect on virulence is unknown. The current study revealed that the rtfA gene is indispensable for normal pathogenicity in plants when using peanut seed as infection model, and in animals, as shown in the Galleria mellonella infection model. Interestingly, rtfA positively regulate several processes known to be necessary for successful fungal invasion and colonization of host tissue, such as adhesion to surfaces; protease and lipase activity, proper cell wall composition and integrity; and resistance to oxidative stress. In addition, metabolomics analysis revealed that A. flavus rtfA effects the production of several secondary metabolites present in different biosynthetic pathways of such as those corresponding to AFB1, aflatrem, paxilline, leporins, aflavinines, ditryptophenaline, and aspirochlorine, supporting that rtfA is a global regulator of secondary metabolism. Heterologous complementation of an A. flavus deletion rtfA strain with rtfA homologs from A. nidulans or S. cerevisiae did not fully rescue wild type phenotype, indicating that these rtfA homologs are only partially conserved between these three species.