Within-plant signaling by volatiles leads to induction and priming of an indirect plant defense in nature

Abstract

Plants respond to herbivore attack with the release of volatile organic compounds (VOCs), which can attract predatory arthropods and/or repel herbivores and thus serve as a means of defense against herbivores. Such VOCs might also be perceived by neighboring plants to adjust their defensive phenotype according to the present risk of attack. We exposed lima bean plants at their natural growing site to volatiles of beetle-damaged conspecific shoots. This reduced herbivore damage and increased the growth rate of the exposed plants. To investigate whether VOCs also can serve in signaling processes within the same individual plant we focused on undamaged "receiver" leaves that were either exposed or not exposed to VOCs released by induced "emitter" leaves. Extrafloral nectar secretion by receiver leaves increased when they were exposed to VOCs of induced emitters of neighboring plants or of the same shoot, yet not when VOCs were removed from the system. Extrafloral nectar attracts predatory arthropods and represents an induced defense mechanism. The volatiles also primed extrafloral nectar secretion to show an augmented response to subsequent damage. Herbivore-induced VOCs elicit a defensive response in undamaged plants (or parts of plants) under natural conditions, and they function as external signal for within-plant communication, thus serving also a physiological role in the systemic response of a plant to local damage.

Fig. 1.

Protection of volatile-exposed plants in the field (experiment 1). (Left) Representative gas chromatic profiles of headspaces of tendrils exposed to herbivore-induced emitter tendrils (VOCs) and to undamaged emitters (C). (Right) Development of leaf number, herbivory (percent missing leaf area), and percentage of living shoot tips (means ± SE) during the experiment (August 26 until September 15). Asterisks indicate significant differences (P < 0.05 according to Wilcoxon pair test) between C and VOC tendrils. See SI Fig. 5 for detailed results of headspace analyses and Table 1 for identity of volatile compounds.

Fig. 2.

Induction and priming of EFN secretion by volatiles (experiment 2). (Left) The experimental setup with receiver tendrils being exposed (B_f) or not exposed (D_f) to VOCs of artificially induced emitter tendrils (A_f and C_f) and the respective GC profiles are displayed. (Center) EFN secretion (in micrograms of soluble solids secreted per gram of leaf dry mass and 24 h +SE) of different leaf age classes (leaves 1–3, leaves 4 and 5, and leaves 6 and 7) on day 1. (Right) Change in EFN secretion on day 2 relative to day 1 (a value of +2 indicating a 2-fold-higher secretion on day 2 than on day 1). Asterisks indicate significant (∗∗∗, P < 0.001; ∗∗, P < 0.01; ∗, P < 0.05) differences in EFN secretion on day 2 as compared with day 1 as tested by paired t tests within each leaf age class and treatment (n.s., not significant; n.d., not determined). Bars marked by different letters within the same leaf age group are significantly different (P < 0.05; LSD post hoc analysis conducted on effects of treatment separately for each leaf group after univariate ANOVA). See SI Fig. 6 for detailed results of headspace analyses and Table 1 for identity of volatile compounds.

Fig. 3.

Within-plant signaling by volatiles (experiment 3). (Upper) The experimental setup (A_p, redirection of VOCs released by the induced leaves 4 and 5 to untreated leaves 1–3; B_p, removing VOCs from the plant; C_p, gas flow unaffected; D_p, air from uninduced leaves 4 and 5 redirected to leaves 1–3) and representative GC profiles. The resulting rates in EFN secretion (in micrograms of soluble solids secreted per gram of leaf dry mass and 24 h ± SE) are displayed separately for leaves 1–3 and leaves 4 and 5 and separately for plants whose leaves 4 and 5 were induced by beetles or artificially. See SI Fig. 7 for detailed results of headspace analyses, Table 1 for identity of volatile compounds, and Table 2 for results of ANOVA of EFN secretion rates. Treatments marked by different letters are significantly different (P < 0.05; LSD post hoc analysis on effects of treatment separately for each leaf group after univariate ANOVA).