A biochemical, evolutionary and genetic model of glandular trichome-mediated small pest resistance in Pelargonium xhortorum

Inheritance of biochemical components of a pest-resistance phenomenon in Pelargonium xhortorum was examined by chromatographic analysis of glandular trichome exudate. {dollar}Delta{dollar}11/{dollar}omega{dollar}5-desaturated fatty acids are precursors to the {dollar}omega{dollar}5-anacardic acids characteristic of exudate from resistant genotypes. Presence of {dollar}Delta{dollar}11({dollar}omega{dollar}5)-desaturation was inherited as a dominant allele at a single locus, {dollar}underlinerm{lcub}fadDelta 11{rcub}{dollar} (fatty acid desaturase {dollar}underline{lcub}rmDelta 11{rcub}{dollar}). Two complementary epistatic loci conditioned inheritance of anacardic acid acyl chain length. Inbred parents had similar anacardic acid acyl chain length composition, while exudate from F{dollar}sb1{dollar} progeny contained elevated levels of shorter chain anacardic acids. Segregation analysis of F{dollar}sb2{dollar} and backcross progenies indicated that digenic complementary epistasis explained this phenomenon. Inheritance of glandular trichome density was conditioned by co-dominant alleles at a small number of loci. Based on a single locus, each allele contributes about 45 trichomes.; Selected F{dollar}sb2{dollar} progeny with either resistant (high {dollar}omega{dollar}5-anacardic acids) or susceptible (low) exudate chemistry, and either high or low glandular trichome density, were examined by leaf disk bioassay. Two-spotted spider mite fecundity and survival were strongly and negatively affected by the presence of {dollar}omega{dollar}5-anacardic acids in the exudate, even at low trichome densities.; {dollar}omega{dollar}5-anacardic acids were assayed in vitro for potential inhibition of prostaglandin endoperoxide synthase (PES) and lipoxygenase (LOX). Concentrations as low as 11.1 {dollar}mu{dollar}M inhibited PES and LOX. Differential methylation of carboxyl and phenol groups indicated that anacardic acid-mediated inhibition of PES and LOX may occur by different mechanisms.; Anacardic acid composition and glandular trichome distribution of 21 Pelargonium species, 39 diploid, and 33 tetraploid cultivars of P. xhortorum was examined. Only three species of section Ciconium (acetosum, frutetorum, inquinans) produced anacardic acids. Only frutetorum and inquinans contained {dollar}omega{dollar}5-compounds. Exudate from all P. xhortorum cultivars contained {dollar}omega{dollar}5-anacardic acids; however, wide variation in composition was observed.; Fatty acid composition of leaves, pedicels, sepals, seeds, and petals of resistant and susceptible P. xhortorum inbreds was examined. Only glandular trichomes from resistant lines contained {dollar}omega{dollar}5-fatty acids. No other tissue of either genotype contained significant amounts of {dollar}omega{dollar}5-fatty acids. Neutral lipids and galactolipids from glandular trichomes of resistant genotypes contained {dollar}omega{dollar}5-fatty acids, while phospholipids contained no {dollar}omega{dollar}5-fatty acids.