Expression of the Fusarium graminearum terpenome and involvement of the endoplasmic reticulum-derived toxisome

Authors: FLYNN, CHRISTOPHER - University Of Minnesota; Broz, Karen; JONKERS, WILFRIED - University Of Minnesota; SCHMIDT-DANNERT, CLAUDIA - University Of Minnesota; Kistler, Harold

Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2018
Publication Date: 3/18/2019
Citation: Flynn, C.M., Broz, K.L., Jonkers, W., Schmidt-Dannert, C., Kistler, H.C. 2019. Expression of the Fusarium graminearum terpenome and involvement of the endoplasmic reticulum-derived toxisome. Fungal Genetics and Biology. 124:78-87. https://doi.org/10.1016/j.fgb.2019.01.006.
DOI: https://doi.org/10.1016/j.fgb.2019.01.006

Interpretive Summary: A common mold called Fusarium graminearum contaminates wheat and barley grain with harmful metabolites called mycotoxins. Mycotoxins are important risks to the health of humans and livestock. We seek to understand how mycotoxins are produced in grain in order to develop measures to minimize grain contamination. This manuscript describes the biochemical requirements and cellular location of enzymes which assemble two types of mycotoxins. This information is important for understanding how fungal metabolites are produced and how mycotoxins accumulate in grain and the environment. The study concludes that mycotoxin synthesis requires particular protein structures that may be targeted for reduction of toxin synthesis. This information will be helpful to develop novel strategies to control contamination of grain with these toxins.

Technical Abstract: The sesquiterpenoid deoxynivalenol (DON) is an important trichothecene mycotoxin produced by the cereal pathogen Fusarium graminearum. DON is synthesized in specialized subcellular structures called toxisomes. The first step in DON synthesis is catalyzed by the sesquiterpene synthase (STS), Tri5 (trichodiene synthase), resulting in the cyclization of farnesyl diphosphate (FPP) to produce the sesquiterpene trichodiene. Tri5 is one of eight putative STSs in the F. graminearum genome. To better understand the F. graminearum terpenome, the volatile and soluble fractions of fungal cultures were sampled. Stringent regulation of sesquiterpene accumulation was observed. When grown in trichothecene induction medium, the fungus produces trichothecenes as well as several volatile non-trichothecene related sesquiterpenes, whereas no volatile terpenes were detected when grown in non-inducing medium. Surprisingly, a 'tri5 deletion strain grown in inducing conditions not only ceased accumulation of trichothecenes, but also failed to produce the non-trichothecene related sesquiterpenes. To test whether Tri5 from F. graminearum may be a promiscuous STS directly producing all observed sesquiterpenes, Tri5 was cloned and expressed in E. coli and shown to produce primarily trichodiene in addition to minor, related cyclization products. Therefore, while Tri5 expression in F. graminearum is necessary for non-trichothecene sesquiterpene biosynthesis, direct catalysis by Tri5 does not explain the sesquiterpene deficient phenotype observed in the 'tri5 strain. To test whether Tri5 protein, separate from its enzymatic activity, may be required for non-trichothecene synthesis, the Tri5 locus was replaced with an enzymatically inactive, but structurally unaffected tri5N225D S229T allele. This allele restores non-trichothecene synthesis but not trichothecene synthesis. The tri5N225D S229T allele also restores toxisome structure which is lacking in the 'tri5 deletion strain. Our results indicate that the Tri5 protein, but not its enzymatic activity, is also required for the synthesis of non-trichothecene related sesquiterpenes and the formation of toxisomes. Toxisomes thus not only may be important for DON synthesis, but also for the synthesis of other sesquiterpene mycotoxins such as culmorin by F. graminearum.