Fatty acid incorporation in the biosynthesis of anacardic acids of geraniums

Abstract

Geraniums, Pelargonium xhortorum, have been shown to possess tall glandular trichomes which secrete anacardic acids, a viscous, sticky exudate which provides defence against small pest species. Pest-resistant geraniums possess primarily C 22- and C 24-unsaturated (ω-5) anacardic acids with lesser amounts of the C 22- and C 24-saturated side chain structures. Previous workers have suggested that anacardic acids are biosynthesized from fatty acids by addition of two carbon units. To investigate this hypothesis, [ 14C]lauric, -myristic, -palmitic, -stearic and -oleic acids were administered to leaves and floral buds of pest-resistant geraniums. Palmitate and stearate were shown to be the respective precursors for C 22 and C 24 anacardic acids with saturated side chains. A major portion of the applied [ 14C]stearate was degraded and subsequently incorporated into the C 22- and C 24-saturated and unsaturated (ω-5) anacardic acids indicating an indirect pathway of biosynthesis also exists. Therefore, the ω-5 anacardic acids must be the result of ω-5 fatty acid precursors rather than the desaturation of saturated anacardic acids. Data supports the hypothesis that anacardic acids are biosynthesized from saturated and ω-5 unsaturated C 16 and C 18 fatty acids by chain elongation through the addition of three acetate groups, which then undergo partial reduction and dehydration, followed by intramolecular aldol condensation, to give the aromatic ring in a manner similar to that proposed for the biosynthesis of salicylic acid.