Vibrio gazogenes inhibits aflatoxin production through downregulation of aflatoxin biosynthetic genes in Aspergillus flavus

Authors: Kandel, Shyam; JESMIN, RUBAIYA - University Of South Carolina; Mack, Brian; Majumdar, Raj; Gilbert, Matthew; Cary, Jeffrey; Lebar, Matthew; GUMMADIDALA, PHANI - University Of South Carolina; CALVO, ANA - Northern Illinois University; Rajasekaran, Kanniah - Rajah; CHANDA, ANINDYA - University Of South Carolina

Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2022
Publication Date: 1/14/2022
Citation: Kandel, S.L., Jesmin, R., Mack, B.M., Majumdar, R., Gilbert, M.K., Cary, J.W., Lebar, M.D., Gummadidala, P.M., Calvo, A.M., Rajasekaran, K., Chanda, A. 2022. Vibrio gazogenes inhibits aflatoxin production through downregulation of aflatoxin biosynthetic genes in Aspergillus flavus. PhytoFrontiers. 2(3):218-229. https://doi.org/10.1094/PHYTOFR-09-21-0067-R.
DOI: https://doi.org/10.1094/PHYTOFR-09-21-0067-R

Interpretive Summary: This work describes experiments that have been conducted ito better understand the mechanisms by which a bacterium is capable of inhibiting aflatoxin production in the fungus, Aspergillus (A.) flavus. Aflatoxins are toxic and carcinogenic compounds often produced by the fungus, 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. Here we show that placing the bacterium, Vibrio (V.) gazogenes, in the growth medium of A. flavus results in an almost complete inhibition (>99%) of aflatoxin biosynthesis. Analysis of the expression of genes in A. flavus upon exposure to V. gazogenes indicated that the genes present in the aflatoxin biosynthetic gene cluster are downregulated. Microscopy suggested that V. gazogenes-dependent aflatoxin inhibition was associated with internalization of bacterial cell materials but not intact bacteria, into specialized cell compartments in the fungus. Identification of the mechanism by which V. gazogenes inhibits aflatoxin production 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 thus ensuring a safe and secure food and feed supply to consumers.

Technical Abstract: Aspergillus flavus is an opportunistic pathogen of oilseed crops such as maize, peanut, cottonseed, and tree nuts. During seed colonization of the host plant, the fungus produces carcinogenic secondary metabolites known as aflatoxins, which not only reduces the value of the produce but also is a health hazard to humans and animals. Previously, we observed inhibition of A. flavus growth and aflatoxin biosynthesis upon exposure to the non-pathogenic, marine bacterium, Vibrio gazogenes (denoted here as Vg). However, the biochemical and molecular mechanisms responsible for this phenomenon remains unclear. In this study, we used an RNA sequencing to examine the transcriptional profiles of untreated A. flavus (control samples) with A. flavus treated with both live Vg and heat-inactivated Vg (denoted here as dead Vg). Fungal growth, total accumulated aflatoxins, and expression profiles of genes constituting secondary metabolite biosynthetic gene clusters were determined at 24, 30, and 40 h after treatment. Significant reduction in total aflatoxins were detected in Vg-treated samples as compared to control samples at 40 h. Smaller fungal dry weight was observed in both live and dead Vg treated samples as compared to controls however the difference was statistically insignificant (P-value <0.05) across all time points. Higher numbers of fungal genes were significantly up- or down regulated (log2 fold change > 1 and padj. < 0.05) in A. flavus using live Vg compared to dead bacteria at all time points. At 40 h, several aflatoxin biosynthesis related genes were significantly downregulated upon treatment with live or dead Vg, however, no significant changes in gene expression were detected in other secondary metabolic pathways. Significantly enriched gene ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways involved aflatoxin biosynthesis were associated with downregulated genes (live or dead Vg treated versus control) at 40 h, especially in live Vg treatments. Our results show that Vg specifically inhibited aflatoxin biosynthesis by strongly repressing several aflatoxin biosynthetic cluster genes.