Culmorin inhibits detoxification of the mycotoxin deoxynivalenol by plant UDP-glucosyltransferases

The Fusarium metabolite culmorin (CUL) frequently co-occurs with the mycotoxin deoxynivalenol (DON) on cereals. While DON is recognized as a major Fusarium virulence factor on plants, the function of CUL is still unclear. In this study, we show that CUL-deficient F. graminearum mutants created by deletion of the terpene synthase gene CLM1 are less aggressive on wheat than the wild-type, and this is accompanied by increased DON-3-glucoside/DON ratios in infected wheat ears. In root elongation assays with wheat and Brachypodium distachyon, CUL had no effect alone but significantly increased the toxicity of DON. Analysis of roots treated with DON and/or CUL further indicated that both wheat and B. distachyon are able to glucosylate CUL and that its presence impedes DON-glucosylation in both species. We identified two B. distachyon UDP-glucosyltransferases (UGTs) able to glucosylate CUL, and further investigated the effects of CUL on the kinetics of validated DON-glucosylating plant UGTs (BdUGT5g03300, HvUGT13248, OsUGT79). This suggested that CUL inhibits DON-glucosylation either by serving as a competitive substrate with DON or by unproductive binding. BdUGT5g03300 in particular was strongly inhibited by CUL and even its glucosides. Our results indicate that CUL contributes to Fusarium virulence by weakening plant defences related to UGT-catalysed DON-detoxification. As even CUL-glucosides are potentially inhibitory to UGTs, this implies a complex synergy of CUL with DON.