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Effect of exogenous H 2 O 2 and catalase was tested in liquid cultures of the deoxynivalenol and 15-acetyldeoxynivalenol-producing fungus Fusarium graminearum. Accordingly to previous results, H 2 O 2 supplementation of the culture medium leads to increased toxin production. This study indicates that this event seems to be linked to a general up regulation of genes involved in the deoxynivalenol and 15-acetyldeoxynivalenol biosynthesis pathway, commonly named Tri genes. In catalase-treated cultures, toxin accumulation is reduced, and Tri genes expression is significantly down regulated. Furthermore, kinetics of expression of several Tri genes is proposed in relation to toxin accumulation. Biological meanings of these findings are discussed.

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Liquid cultures of Fusarium graminearum were supplemented with H2O2 or other oxidative compounds. The accumulation kinetics of the resulting trichothecenes were monitored. At non-lethal concentrations, the H2O2 treatments modulated toxin accumulation, dependent on the method of supplementation. When H2O2 was added at the same time as the inoculation, higher levels of toxins accumulated 30 days later. Conversely, adding H2O2 2 or 7 days after inoculation had little effect. When H2O2 was added daily over the course of the culture, the accumulation of trichothecenes was rapidly and strongly enhanced. The fungus may adapt to oxidative stress when the first exposure to H2O2 occurs at the beginning of the culture course. The highest toxin levels were measured when the H2O2 was added daily. The importance of the first hours of culture was confirmed: pre-treating conidia with H2O2 does not affect their germination kinetics but leads to a reduction in the yield of trichothecenes 40 days later. The H2O2 regulation of this trichothecene accumulation may be specific, as paraquat, another pro-oxidant compound, inhibits their production. Since H2O2 is a major component of the oxidative burst occurring in pathogen/host interactions, these data support the theory that trichothecenes may act as virulence factors.

The impact of five phenolic acids (ferulic, coumaric, caffeic, syringic, and p-hydroxybenzoic acids) on fungal growth and type B trichothecene production by four strains of Fusarium graminearum was investigated. All five phenolic acids inhibited growth but the degree of inhibition varied between strains. Our results suggested that the more lipophilic phenolic acids are, the higher is the effect they have on growth. Toxin accumulation in phenolic acid-supplemented liquid glucose, yeast extract, and peptone cultures was enhanced in the presence of ferulic and coumaric acids but was reduced in the presence of p-hydroxybenzoic acid. This modulation was shown to correlate with a regulation of TRI5 transcription. In this study, addition of phenolic acids with greater antioxidant properties resulted in a higher toxin accumulation, indicating that the modulation of toxin accumulation may be linked to the antioxidant properties of the phenolic acids. These data suggest that, in planta, different compositions in phenolic acids of kernels from various cultivars may reflect different degrees of sensitivity to "mycotoxinogenesis."