A multichemical defense mechanism of bitter oliveOlea europaea (oleaceae) : Is oleuropein a phytoalexin precursor?

Olea europaea (Oleaceae) is resistant in nature to insect and microbe attack. Two types of chemical protection were found in the foliage. One type is the bitterseco-iridoid glycosides oleuropein (1) and ligstroside (2); The other is a physical barrier of crystalline oleanolic acid (4) that coats the leaf surface. Theseco-iridoid glycosides were isolated using two different countercurrent chromatographies: rotation locular countercurrent chromatography (RLCC) and droplet countercurrent chromatography (DCCC). The dimethyl ester (III) was shown to be an artifact. This is the first isolation of ligstroside fromO. europaea. In an antimicrobial test by the paper disk method againstBacillus subtilis, Saccharomyces cerevisiae, andEscherichia coli, compounds I, II, and III inhibited a growth ofB. subtilis at pH 7. Similar tests under the influence of β-glucosidase suggest an aglycone of oleuropein, either the hemiacetal (i) or the possible enal-aldehyde (ii), could be the active intermediate. This intermediate could be produced rapidly in response to microorganism invasion. Oleuropein producing such a postinfection active intermediate could be referred to as a phytoalexin precursor.