NEW TRICKS OF AN OLD ENEMY: ISOLATES OF FUSARIUM GRAMINEAR UM PRODUCE A TYPE A TRICHOTHECENE MYCOTOXIN

摘要

The ubiquitous filamentous fungus Fusarium graminearum causes the important disease Fusarium head blight on various species of cereals, leading to contamination of grains with mycotoxins. In a survey of F. graminearum (sensu stricto) on wheat in NorthAmerica several novel strains were isolated, which produced none of the known trichothecene mycotoxins despite causing normal disease symptoms. In rice cultures a new trichothecene mycotoxin (named NX-2) was characterized by liquid-chromatography-tandem-mass spectrometry. NMR measurements identified NX-2 as 3alpha-acetoxy-7alpha, 15-dihydroxy-12,13-epoxytrichothec-9-ene. Compared to the well-known 3-acetyl-deoxynivalenol it lacks the keto group at C-8 and hence is a type A trichothecene. Wheat ears inoculated with the isolated strains revealed a ten-fold higher contamination with its deacetylated form, named NX-3, (up to 540 mg kg~(-1)) compared to NX-2. The toxicities of the novel mycotoxins were evaluated utilizing two in vitro translation assays and the alga Chlamydomonas reinhardtii. NX-3 inhibits protein biosynthesis to almost the same extent as the prominent mycotoxin deoxynivalenol, while NX-2 is far less toxic, similar to 3-acetyl-deoxynivalenol. Genetic analysis revealed a different TRIl allele in the N-isolates which was verified to be responsible for the difference in hydroxylation at C-8.The occurrence of isolates producing the new toxin raises the question whether such strains have a selective advantage, and in the worst case can counteract progress made by plant breeders in the last decade. We will discuss the hypothesis that production of a toxin with an acetylated C3-OH may be a response of the fungus to circumvent inactivation by glycosylation, while lacking the keto-group may prevent glutathione-mediated detoxification. Population genetic studies to determine whether the frequency of NX-producers is changing seem highly warranted.