A new phytotoxin, stemphyloxin I, C₂₁H₃₂O₅, was isolated from cultures of the pathogenic fungus Stemphylium botryosum f. sp. lycopersici. The toxin is a tricyclic compound possessing a most unusual β-ketoaldehyde group. Injection of stemphyloxin I into a tomato leaflet caused unlimited necrotic spots and a loss of turgor, which at higher toxin concentration wilted the whole compound leaf. Visible symptoms could be observed at a toxin concentration as low as 2.7 micromolar. Stemphyloxin I is a nonspecific toxin. It exhibits a differential toxicity towards various plants, tomato and eggplant being the most sensitive. Incorporation of [¹⁴C]amino acids into proteins of exponentially growing tomato cell suspension was completely suppressed in the presence of 1 micromolar toxin. The toxin showed no significant difference in its inhibitory activity against green and white tomato cell cultures. The methoxy derivative of stemphyloxin I, in which the β-ketoaldehyde group is exclusively modified, showed a reduction of approximately 50 times in its inhibitory activity as compared to the toxin. The diacetate derivative conferred the same activity as stemphyloxin I.

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