Gramillin A and B: Cyclic Lipopeptides Identified as the Nonribosomal Biosynthetic Products of Fusarium graminearum
Bahadoor, A., Brauer, E.K., Bosnich, W., Schneiderman, D., Johnston, A., Aubin, Y., Blackwell, B., Melanson, J.E., Harris, L.J. (2018). Gramillin A and B: Cyclic Lipopeptides Identified as the Nonribosomal Biosynthetic Products of Fusarium graminearum. Journal of the American Chemical Society, [online] 140(48), 16783-16791. http://dx.doi.org/10.1021/jacs.8b10017
Plain language summary
The fungal pathogen Fusarium graminearum possesses an exceptional flexibility to adapt to different plant hosts and environments. Fusarium graminearum is the major cause of the crop diseases fusarium head blight (wheat, barley and oats) and gibberella ear rot (corn). Contaminating grain with the regulated mycotoxin deoxynivalenol during plant infection, this fungus poses a constant threat to cereal crops and food safety around the world. We are seeking to discover what other toxins this fungus may make to promote plant infection. We observed that there was a cluster of fungal genes that was preferentially expressed during Fusarium infection on corn, but only briefly expressed on barley and wheat. This led us to identify a Fusarium graminearum gene that is required to make two cyclic molecules, named gramillin A and B. The gramillins are small lipopeptides that act as potent toxins in certain plant hosts. Low amounts of purified gramillins cause cell death within hours in corn leaves but have no effect on wheat leaves. Gramillins can help the fungus cause disease in maize, but not wheat, an example of a host-specific infection strategy of a serious plant pathogen. We are now seeking to understand why this toxin works in certain plants but not others, to help us identify new anti-Fusarium defences.
The virulence and broad host range of Fusarium graminearum is associated with its ability to secrete an arsenal of phytotoxic secondary metabolites, including the regulated mycotoxins belonging to the deoxynivalenol family. The TRI genes responsible for the biosynthesis of deoxynivalenol and related compounds are usually expressed during fungal infection. However, the F. graminearum genome harbors an array of unexplored biosynthetic gene clusters that are also co-induced with the TRI genes, including the nonribosomal peptide synthetase 8 (NRPS8) gene cluster. Here, we identify two bicyclic lipopeptides, gramillin A (1) and B (2), as the biosynthetic end products of NRPS8. Structural elucidation by high-resolution LC-MS and NMR, including 1 H- 15 N- 13 C HNCO and HNCA on isotopically enriched compounds, revealed that the gramillins possess a fused bicyclic structure with ring closure of the main peptide macrocycle occurring via an anhydride bond. Through targeted gene disruption, we characterized the GRA1 biosynthetic gene and its transcription factor GRA2 in the NRPS8 gene cluster. Further, we show that the gramillins are produced in planta on maize silks, promoting fungal virulence on maize but have no discernible effect on wheat head infection. Leaf infiltration of the gramillins induces cell death in maize, but not in wheat. Our results show that F. graminearum deploys the gramillins as a virulence agent in maize, but not in wheat, thus displaying host-specific adaptation.