Airborne signals prime plants against insect herbivore attack

Abstract

Green leafy volatiles (GLV), six-carbon aldehydes, alcohols, and esters commonly emitted by plants in response to mechanical damage or herbivory, induced intact undamaged corn seedlings to rapidly produce jasmonic acid (JA) and emit sesquiterpenes. More importantly, corn seedlings previously exposed to GLV from neighboring plants produced significantly more JA and volatile sesquiterpenes when mechanically damaged and induced with caterpillar regurgitant than seedlings not exposed to GLV. The use of pure synthetic chemicals revealed that (Z)-3-hexenal, (Z)-3-hexen-1-ol, and (Z)-3-hexenyl acetate have nearly identical priming activity. Caterpillar-induced nocturnal volatiles, which are enriched in GLV, also exhibited a strong priming effect, inducing production of larger amounts of JA and release of greater quantities of volatile organic compounds after caterpillar regurgitant application. In contrast, GLV priming did not affect JA production induced by mechanical wounding alone. Thus, GLV specifically prime neighboring plants against impending herbivory by enhancing inducible chemical defense responses triggered during attack and may play a key role in plant-plant signaling and plant-insect interactions.

Fig. 1.

Effects of GLV, CIV, and pure C₆ compounds on JA production and the release of volatiles in intact corn plants. Error bars represent SD (n = 4). Data were analyzed for significance with t test (*, P ≤ 0.05; **, P ≤ 0.01). (A) Induction of JA in corn seedlings by GLV and 20 μg or 1 mg of (Z)-3-HAC in a 6-liter Plexiglas container. Corn plants were incubated for 0, 30, 60, and 180 min. Controls were treated the same way, except that no volatile compounds were added to the incubation chambers. ⋄, control; ▵, GLV-treated; □, ≤20 μg of (Z)-3-HAC; ×, 1 mg of (Z)-3-HAC. Twenty micrograms of (Z)-3-HAC corresponds to 30 nM maximum concentration in the gas phase, and 1 mg corresponds to 1.66 μM maximum concentration in the gas phase. Data points are connected by smoothed lines. (B) Black bars indicate levels of JA in intact corn plants after 30-min exposure to volatiles from synthetic compounds (Z)-3-HAL, (Z)-3-HOL, (Z)-3-HAC, or cis jasmone (20 μg each). White bars indicate JA levels in corn plants after 30-min exposure to CIV, crude regurgitant elicitor-induced volatiles (CREV), and control. Intact uninfested corn plants were used as a source of volatiles in the controls. (C) Induction of VOC after overnight exposure to GLV, synthetic C₆ compounds, CIV, or controls, as described above. The amounts of linalool, 4,8-dimethylnona-1,3,7-triene, β-caryophyllene, bergamotene, and β-farnesene emitted by the plants during the volatile collection period were measured and summed to obtain total volatiles.

Fig. 2.

Effects of pretreatment with GLV and pure chemicals on subsequent CRE- or wound-induced JA. Error bars represent SD (n = 4). Data for induction of JA by application of CRE (with or without previous exposure to GLV) were analyzed for significance with t test (*, P ≤ 0.05; **, P ≤ 0.01). (A) Induction of JA after application of CRE to GLV-pretreated intact corn plants. ⋄, GLV plus CRE; □, control plus CRE; ▵, control. Data points are connected by smoothed lines. (B) Effects of overnight exposure to synthetic chemicals on CRE-induced JA in intact corn plants. JA was quantified from leaf tissue 30 min after application of CRE. (C) Effects of GLV, CIV, and pure C₆ compounds on wound-induced JA. Corn plants were exposed to synthetic C₆ compounds, GLV, CIV, and the respective control for 15 h. One leaf then was wounded with a razor blade, and JA was quantified after 30 min.

Fig. 3.

Effects of pretreatment with GLV, pure chemicals, and CIV on subsequent CRE-induced volatiles. Error bars represent SD (n = 4). Data were analyzed for significance with t test (*, P ≤ 0.05; **, P ≤ 0.01). After induction with CRE, volatiles were collected during the period of maximum release rate (4-5 h after induction). Note that experiments were done with different batches of plants and CRE and at different times of the year. (A) Induction of volatiles by CRE in corn seedlings after overnight exposure to GLV. □, GLV plus CRE; ⋄, control plus CRE. Data points are connected by smoothed lines. (B) Induction of volatiles by CRE in corn seedlings after overnight exposure to CIV. □, CIV plus CRE; ⋄, control plus CRE. Data points are connected by smoothed lines. (C) Induction of volatiles by CRE in corn seedlings after overnight exposure to pure C₆ compounds.