Summary: A phytotoxic extract of (cherry isolate c28) was shown to contain phytotoxic protein associated with bacteriophage-specific lipopolysaccharide. This and similar extracts from two other pathogenic pseudomonads produced a silvery effect when infiltrated at 100 μg ml (14·4 mu;g protein ml) into leaves of some plant species. The effect was similar to that caused by infection in cherry. In contrast to the fungal toxin, the bacterial extracts were not translocated in treated plants. Wax-embedded sections of cherry leaves silvered either by the fungus or by bacterial extracts showed optically active cells when stained with toluidine blue. Different plant species differentiated between extracts of different pseudomonads by their reaction to treatment. The hypersensitive response of tobacco to heterologous pathogenic bacteria was suppressed by prior treatment with extracts of cherry-type and saprophytic bacteria. Similar phytotoxic material isolated from culture filtrates of the cherry isolate produced the silvery effect in bean leaves when infiltrated at 10 μg ml (5 μg protein ml). The silvering agents (molecular weight in excess of 25 × 10 by gel filtration) were dissociated into smaller units by electrophoresis in sodium dodecyl sulphate-polyacrylamide gel. The activities described were destroyed by protein-inactivating treatments.


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