Defense-inducing volatiles: in search of the active motif

Abstract

Herbivore-induced volatile organic compounds (VOCs) are widely appreciated as an indirect defense mechanism since carnivorous arthropods use VOCs as cues for host localization and then attack herbivores. Another function of VOCs is plant-plant signaling. That VOCs elicit defensive responses in neighboring plants has been reported from various species, and different compounds have been found to be active. In order to search for a structural motif that characterizes active VOCs, we used lima bean (Phaseolus lunatus), which responds to VOCs released from damaged plants with an increased secretion of extrafloral nectar (EFN). We exposed lima bean to (Z)-3-hexenyl acetate, a substance naturally released from damaged lima bean and known to induce EFN secretion, and to several structurally related compounds. (E)-3-hexenyl acetate, (E)-2-hexenyl acetate, 5-hexenyl acetate, (Z)-3-hexenylisovalerate, and (Z)-3-hexenylbutyrate all elicited significant increases in EFN secretion, demonstrating that neither the (Z)-configuration nor the position of the double-bond nor the size of the acid moiety are critical for the EFN-inducing effect. Our result is not consistent with previous concepts that postulate reactive electrophile species (Michael-acceptor-systems) for defense-induction in Arabidopsis. Instead, we postulate that physicochemical processes, including interactions with odorant binding proteins and resulting in changes in transmembrane potentials, can underlie VOCs-mediated signaling processes.

Fig. 1

EFN secretion in μg soluble solids secreted per gram leaf dry mass and per 24 h is depicted. Plants were exposed for 24 h to (A) (Z)-3-hexenyl acetate, (B) (E)-3-hexenyl acetate, (C) (E)-2-hexenyl acetate, (D) 5-hexenyl acetate, (E) (Z)-3-hexenyl isovalerate, (F) (Z)-3-hexenyl butyrate, (G) 2-ethylhexanol, (H) (E)-2-hexanal, (I) decanal, and (K) nonanal dissolved in Lanolin paste. Plants to which pure Lanolin paste had been applied served as controls (LC). Lower and upper whiskers represent the 5% and the 95% percentile, lower and upper margins of boxes the 25% and the 75% percentile, the lines within boxes indicate medians. Different letters appearing above boxes mark treatment effects that differ significantly from each other (P < 0.05 according to LSD posthoc analysis), and structures of compounds leading to a significant induction of EFN secretion as compared to the control are graphically presented. LogP values of all compounds were calculated with ChemDraw Ultra 6.0 and are given under the structures (compounds A–F) and in the insert (G–K)