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SUMMARY: Culture filtrates of Trichothecium roseum contain two heat-stable substances that inhibit infection with plant viruses. One is trichothecin, an antifungal substance with the molecular formula C19H24O5; it sometimes visibly damages bean leaves. The other was isolated as a polysaccharide, [α] D 19° = −33° it contains 1·1–1·4% nitrogen, yields 60–70% reducing sugars (as glucose) on acidic hydrolysis, and the predominant (45%) component sugar is d-galactose. The polysaccharide does not combine with tobacco mosaic virus in vitro.
The extent to which infection is inhibited depends on the species of the host plant but not on the identity of the virus. Trichothecin, like ribonuclease, is relatively more effective with beans than with Nicotiana glutinosa, whereas the polysaccharide and two derivatives of trichothecin (trichothecolone and acetyltrichothecolone) affect N. glutinosa more than beans. Trichothecin inhibits infection when sprayed over leaves a day after they have been inoculated with viruses, but is ineffective when applied 2 days before. The polysaccharide inhibits when sprayed over leaves before inoculation but not after. It is suggested that inhibitors act by temporarily altering the metabolism of leaf cells so that introduced virus particles cannot multiply and are inactivated.