Trichothecenes and lipases are host-induced and secreted virulence factors of Fusarium graminearum
We investigate secreted virulence factors of F. graminearum, namely trichothecenes and lipases. The first gene of the trichothecene pathway, the trichodiene synthase (Tri5), was disrupted in different F. graminearum wild-type strains, producing high or medium amounts of deoxynivalenol (DON) or nivalenol. All disruption mutants ceased to produce trichothecenes and showed a similar reduced virulence on wheat but unchanged virulence to barley. Nivalenol proved to be a weak virulence factor to maize, whereas DON production displayed no influence on maize infection. Therefore, trichothecenes are host specific virulence factors. Mutants lacking DON colonized only the inoculated wheat spikelets. These spikelets showed heavy cell-wall thickening that seemed to prevent the mutant from growing through the rachis node into the rachis. No such symptoms were found in spikelets inoculated with the wild type. Disruption mutants of FGL1, a secreted lipase, showed similar symptoms on wheat spikelets as the Δtri5 mutants. In contrast to the trichothecene mutants, the lipase mutants were unable to colonize barley and maize, defining the fungal lipase a general virulence factor. In wheat, Δfgl1 mutants were able to colonize the inoculated spikelet but could not protrude into the rachis and the rest of the spike. The lipase-mutants produced a higher amount of DON compared to the wild type during initial infection of wheat spikelets, despite its inability to further colonize the spike. It seems that both virulence factors, lipases and trichothecenes, have to be present for an effective colonization of wheat.
To gain further knowledge of the circumstances that lead to mycotoxin production we created a mutant that expresses eGFP under control of the endogenous tri5 promoter as well as a fully functional tri5 gene. To help tracing the fungus on the plant dsRed under constitutive control of the gpd promoter was additionally transformed into the mutant. Now, we are able to monitor trichothecene biosynthesis induction on the plant with cellular resolution.
KeywordsFusarium Head Blight trichothecene tri5 induction eGFP
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