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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 anti- fungal substance with the molecular formula C,,H,,O, ; it sometimes visibly damages bean leaves. The other was isolated as a polysaccharide, (a)y = - 33" ; it contains 1-1-1.4 yo nitrogen, yields 60-70 yo reducing sugars (as glucose) on acidic hydrolysis, and the predominant (45 yo) component sugar is D-galactose. The polysaccharide does not combine with tobacco mosaic virus in Witro. 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. The infectivity of plant virus solutions can be inhibited by extracts from many different kinds of organism. The responsible substances mostly remain unidentified, but it is known that inhibitors can have widely different chemical compositions. For example, among the inhibitors most studied are the two pancreatic enzymes, trypsin (Caldwell, 1933 ; Stanley, 1934) and ribonuclease (Loring, 1942), a polysaccharide from yeast (Takahashi, 1942, 1946), and a glycoprotein from Phytolaccu esculentu (Kassanis & Kleczkowski, 1948). Weight for weight, these substances have different activities, but all share some features. They do not obviously damage the host plants; infectivity is inhibited immediately they are mixed with the virus solutions, and it can be restored partly or wholly by diluting the mixtures or removing the inhibitor. Whether inhibitors act by affecting the viruses or the host plants has been much debated, but remains undecided. Stanley (1934) concluded that trypsin was most likely affecting the plant, but he could find no evidence that the enzyme and virus combined in vitro. Not only trypsin (Bawden & Pirie, 1936; Hills & Vinson, 1938; Kleczkowski, 1944) but also the more powerful in- hibitors, ribonuclease (Loring, 1942; Kleczkowski, 1946) and the phytolacca glycoprotein (Kassanis & Kleczkowski, 1948), are now known to combine with plant viruses when mixed with them at pH values around neutrality. This provides a ready explanation for inhibition, because the combination might block specific groups on the viruses that need to combine with receptors in the host cell as a first step in infection. However, as Kassanis & Kleczkowski

The Nature and Behaviour of Inhibitors of Plant Viruses produced by Trichothecium roseum Link