The products of a single maize sesquiterpene synthase form a volatile defense signal that attracts natural enemies of maize herbivores

Abstract

Plants can defend themselves against herbivores by attracting natural enemies of the herbivores. The cues for attraction are often complex mixtures of herbivore-induced plant volatiles, making it difficult to demonstrate the role of specific compounds. After herbivory by lepidopteran larvae, maize releases a mixture of volatiles that is highly attractive to females of various parasitic wasp species. We identified the terpene synthase TPS10 that forms (E)-beta-farnesene, (E)-alpha-bergamotene, and other herbivory-induced sesquiterpene hydrocarbons from the substrate farnesyl diphosphate. The corresponding gene is expressed in response to herbivore attack and is regulated at the transcript level. Overexpression of tps10 in Arabidopsis thaliana resulted in plants emitting high quantities of TPS10 sesquiterpene products identical to those released by maize. Using these transgenic Arabidopsis plants as odor sources in olfactometer assays showed that females of the parasitoid Cotesia marginiventris learn to exploit the TPS10 sesquiterpenes to locate their lepidopteran hosts after prior exposure to these volatiles in association with hosts. This dissection of the herbivore-induced volatile blend demonstrates that a single gene such as tps10 can be sufficient to mediate the indirect defense of maize against herbivore attack.

Fig. 1.

The sesquiterpene hydrocarbons emitted by maize after herbivore damage are formed by the terpene synthase TPS10. Volatile sesquiterpene hydrocarbons released by intact (A) and herbivore-damaged (B) 2-week-old maize plants were collected by headspace trapping. (C) TPS10 was overexpressed in Escherichia coli, and products were collected after incubation with farnesyl diphosphate as substrate. FID, flame ionization detector. The sesquiterpenes identified are as follows: 1, α-copaene; 2, (E)-β-caryophyllene; 3, (E)-α-bergamotene; 4, sesquisabinene A; 5, (E)-β-farnesene; 6, germacrene D; 7, zingiberene; 8: α-muurolene; 9, β-bisabolene; 10, δ-cadinene; and 11, sesquiphellandrene. (D) The structures of the major sesquiterpene hydrocarbons are shown.

Fig. 2.

High transcript levels of tps10 are found only in herbivore-damaged maize leaves. RNA was isolated from plants of the cultivar B73 that were 2 weeks old, except for the mature leaf and husk samples, which were isolated from plants after anthesis. (Upper) The RNA was hybridized with radiolabeled tps10 cDNA, washed, and analyzed with a PhosphorImager. (Lower) Ethidium bromide-stained agarose gel with 28S rRNA as a control for equal RNA loading.

Fig. 3.

Arabidopsis transformed with tps10 emitted ≈2.1 μg per plant per h of the TPS10 sesquiterpene products. The ORF of tps10 was cloned into the vector pBIN420 and transformed into Arabidopsis. The volatiles from transgenic plants (A) and control plants (B) (transformed with the same vector lacking any insert) were collected. The compounds are as follows: 3, (E)-α-bergamotene; 4, sesquisabinene A; 5, (E)-β-farnesene; 7, zingiberene; 9, β-bisabolene; and 11, sesquiphellandrene.

Fig. 4.

Responses of the parasitic wasp C. marginiventris to transgenic Arabidopsis that emits TPS10 sesquiterpenes. Transgenic, TPS10 sesquiterpene-releasing Arabidopsis and wild-type plants were placed in two arms of a six-arm olfactometer (A) to test the attraction of parasitoid females (B). Three groups of parasitoids were tested (C), including naive wasps, wasps with a previous oviposition experience in a host larva in the presence of transgenic Arabidopsis emitting TPS10 sesquiterpenes, and wasps with a previous oviposition experience in a host larva feeding on maize. The parasitoids were tested in groups of six. **, significant preference (P < 0.01) for the odor of the transgenic Arabidopsis.