A Novel Enolic β-Ketoaldehyde Phytotoxin Produced by <i>Stemphylium botryosum</i> f. sp. lycopersici

A new phytotoxin, stemphyloxin I, C(21)H(32)O(5), was isolated from cultures of the pathogenic fungus Stemphylium botryosum f. sp. lycopersici. The toxin is a tricyclic compound possessing a most unusual beta-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 [(14)C]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 beta-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.