ARS | Publication request: Inhibition of Aflatoxin Biosynthesis by Gallic Acid

Submitted to: Aflatoxin Workshop
Publication Type: Abstract
Publication Acceptance Date: July 1, 2003
Publication Date: October 31, 2003
Citation: Cary, J.W., Harris, P.Y., Molyneux, R.J., Mahoney, N.E. 2003. Inhibition of aflatoxin biosynthesis by gallic acid. Proceedings of the 16th Annual Aflatoxin Elimination Workshop, October 13-15, 2003, Savannah, Georgia. p. 40.

Technical Abstract: Aflatoxin (AF) biosynthesis is inhibited by gallic acid present in tree nuts. During invasion by Aspergillus flavus, fungal tannase releases gallic acid (GA) from the hydolyzable tannins present in the pellicle of walnut and the hull of pistachio. Previous studies have shown that GA content of the seed coat or hull correlated inversely with the ability of the fungus to produce AF. The mechanism by which GA inhibits AF biosynthesis is not clear. Using a molecular biological approach, we have shown that growth of A. flavus in the presence of GA results in the inhibition of transcription of AF biosynthetic genes. Growth of A. flavus on solid medium supplemented with 0.2% GA or 0.4% tannic acid resulted in 95% or greater inhibition of AF production but did not affect fungal growth. The inhibition appears to be at the level of transcription of AF biosynthetic genes as GA at 0.25% was shown to almost completely inhibit expression of nor1 and ver1 genes and only slightly reduced the levels of aflR. However, the transcriptional level of a housekeeping gene, nmt1, was not affected. Due to the fact that aflR was still being expressed in the presence of GA, we postulate that another transcriptional activation factor, that interacts with aflR and is required for efficient transcription of AF biosynthetic genes, may be down-regulated by a signaling pathway involved in the oxidative stress response. It is known that GA is an antioxidant and it may be suppressing mechanisms in the fungus that regulate responses to oxidative stress which has been shown to activate fungal AF biosynthesis. If this proves true, then potential control strategies should focus on increasing the tannin content in seed of susceptible crops or identify other factors that can interfere with oxidative stress signaling pathways involved in regulation of AF biosynthesis.