Pyrrocidine, a molecular off switch for fumonisin biosynthesis

Abstract

Sarocladium zeae is a fungal endophyte of maize and can be found co-inhabiting a single seed with Fusarium verticillioides, a major mycotoxigenic food safety threat. S. zeae produces pyrrocidines A and B that inhibit the growth of F. verticillioides and may limit its spread within the seed to locations lacking S. zeae. Although coinhabiting single seeds, the fungi are generally segregated in separate tissues. To understand F. verticillioides’ protective physiological response to pyrrocidines we sequenced the F. verticillioides transcriptome upon exposure to purified pyrrocidine A or B at sub-inhibitory concentrations. Through this work we identified a F. verticillioides locus FvABC3 (FVEG_11089) encoding a transporter critical for resistance to pyrrocidine. We also identified FvZBD1 (FVEG_00314), a gene directly adjacent to the fumonisin biosynthetic gene cluster that was induced several thousand-fold in response to pyrrocidines. FvZBD1 is postulated to act as a genetic repressor of fumonisin production since deletion of the gene resulted in orders of magnitude increase in fumonisin. Further, pyrrocidine acts, likely through FvZBD1, to shut off fumonisin biosynthesis. This suggests that S. zeae is able to hack the secondary metabolic program of a competitor fungus, perhaps as preemptive self-protection, in this case impacting a mycotoxin of central concern for food safety. The fungal food safety threat, Fusarium verticillioides—producer of the deadly fumonisin mycotoxins, is a seed borne pathogen of corn. Another fungus, Sarocladium zeae, also lives in corn seed. S. zeae produces compounds called pyrrocidines that inhibit the growth of F. verticillioides. Here we describe the F. verticillioides transcriptional response upon exposure to pyrrocidines. By making deletion mutants in highly upregulated genes we determined the gene crucial for F. verticillioides resistance to pyrrocidine. Another gene, which was the most highly upregulated, suppresses fumonisin production. Additionally, we found that pyrrocidine effectively eliminates fumonisin production in wild F. verticillioides. This provides potential strategies for development of biological and/or chemical control to eliminate fumonisin contamination of food and feed.