SUMMARY Fusarium graminearum is the main causative agent of Fusarium head blight on small grain cereals and of ear rot on maize. The disease leads to dramatic yield losses and to an accumulation of mycotoxins. The most dominant F. graminearum mycotoxins are the trichothecenes, with deoxynivalenol and nivalenol being the most prevalent derivatives. To investigate the involvement of trichothecenes in the virulence of the pathogen, the gene coding for the initial enzyme of the trichothecene pathway was disrupted in three field isolates, differing in chemotype and in virulence. From each isolate three individual disruption mutants were tested for their virulence on wheat, barley and maize. Despite the different initial virulence of the three wild-type progenitor strains on wheat, all disruption mutants caused disease symptoms on the inoculated spikelet, but the symptoms did not spread into other spikelets. On barley, the trichothecene deficient mutants showed no significant difference compared to the wild-type strains: all were equally aggressive. On maize, mutants derived from the NIV-producing strain caused less disease than their wild-type progenitor strain, while mutants derived from DON-producing strains caused the same level of disease as their progenitor strains. These data demonstrate that trichothecenes influence the virulence of F. graminearum in a highly complex manner, which is strongly host as well as moderately chemotype specific.

The presence of the tetracyclic diterpene 16alpha-hydroxykaurane (16alpha-hydroxy-ent-kaurane, C20H34O, CAS 5524-17-4) was detected in sterile cell cultures of the moss Physcomitrella patens (Hedw.) B.S.G. using gas chromatography and mass spectrometry. 16alpha-hydroxykaurane was found to be a major lipid compound in P. patens, with an estimated intracellular concentration of up to 0.84 mmol/l and an extracellular concentration of up to 9.3 micromol/l. The overall content of 16alpha-hydroxykaurane (in milligrams) produced per culture reached 0.37-fold that of chlorophyll a+b. In agar cultures with low air exchange, 16alpha-hydroxykaurane forms needle-like crystals on tissue and on the inner surface of the culture vessels, indicating that it is being released into the atmosphere. Solid phase microextraction confirmed the air-bound release of 16alpha-hydroxykaurane. To our knowledge this is the first report on the release of a plant-derived tetracyclic diterpene into the air.

Fusarium graminearum causes important diseases of small-grain cereals and maize and produces several mycotoxins. Among them, deoxynivalenol (DON) and zearalenone (ZEA) can accumulate in feedstuffs and foods to health-threatening levels. Although DON is important for fungal virulence in wheat, disease severity in the field does not correlate with mycotoxin concentrations. We compared gene expression and mycotoxin production of lipase-deficient mutants (Dfgl1), strongly reduced in virulence, and the respective wild-type isolate. Dfgl1 mutants exhibited up-regulated DON production during wheat head infection. On isolated wheat kernels, DON was only produced in low quantities, but higher in wild-type than in Dfgl1 mutants. In contrast, neither wild-type nor Dfgl1 mutants produced ZEA during wheat head infection. However, ZEA was clearly detectable on wheat kernels. Here, Dfgl1 mutants revealed a dramatically enhanced ZEA production. We could correlate the altered amounts of DON and ZEA directly with the expression of the toxin-related genes Tri5 for DON and PKS4 and PKS13 for ZEA. Based on Tri5 expression and the infection pattern of the wild-type and Dfgl1 mutants, we suggest that the transition zone of rachilla and rachis is important in the induction of DON synthesis. Gene expression studies indicate an involvement of the lipase FGL1 in regulation of 8 PKS genes and ZEA production.Abbreviations: FHB -Fusarium head blight; DON -deoxynivalenol; ZEA -zearalenone; PKS -polyketide synthase; Dpi -days post-inoculation

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