Comparison of anacardic acid biosynthetic capability between insect-resistant and-susceptible geraniums

The garden geranium (Pelargonium xhortorum) has been shown to secrete anacardic acids in the form of a viscous sticky exudate from tall glandular trichomes, and this exudate provides a sticky trap defense against small pest species. The anacardic acids from genetically related pest-resistant and -susceptible plants have been characterized, and resistance has been shown to depend upon the presence of5 unsaturated anacardic acids. In this study, the biosynthesis of these anacardic acids was comparatively investigated by incubating [¹⁴C]methyl palmìtate, margarate, stearate, oleate and linoleate on floral buds of resistant and susceptible plants. In addition, the incorporation of [¹⁴C]valine, -isoleucine, and -leucine into anacardic acids was also studied. Nineteen anacardic acids were quantitated utilizing an improved HPLC technique. Fatty acids and, to a much lesser extent, amino acids were incorporated into anacardic acids. There are at least two pathways of biosynthesis operating: direct elongation, and -oxidation with reincorporation of the [¹⁴C]acetate, the latter being more prevalent in the resistant plant. The amino acids were processed into branched chain anacardic acids, isoleucine being the precursor of the anteiso compounds, and valine the iso branched ones. The major difference between resistant and susceptible plants was the ability of resistant plants, but not the susceptible plants, to synthesize5 unsaturated anacardic acids. Both types of plants were capable of directly incorporating¹⁴C-labeled fatty acid methy esters into anacardic acids regardless of the plant's normal anacardic acid composition, thus bypassing the plant's tightly controlled regulation of the chemical structures of anacardic acids. No evidence was found for5 desaturation of saturated anacardic acids. A revised biosynthesis scheme is presented.