Upon insect herbivory, many plants emit elevated levels of volatile organic compounds.1 These herbivory-induced plant volatiles play an important role in prey/host searching behavior of parasitoids and predators of the attacking herbivores.2–4 Insect-induced plant volatiles are often chemically complex comprising a myriad of compounds derived from multiple biochemical pathways.5 They also appear to be variable in quality and quantity among plant species. Even different cultivars of the same plant species may display significant variations in composition of herbivory-induced volatiles.6–9 For some plant species, volatile production and emission can also be affected by the type of insects that attack host plants.10 While the chemistry of herbivory-induced plant volatiles and their emission have been relatively well studied, the complexity and variability of the molecular and genomic basis underlying production of herbivory-induced volatiles that mediate indirect defense across different plant species are little understood.11

A number of genes involved in the biosynthesis of herbivory-induced plant volatiles have been isolated and characterized from a number of plant species.12–18 In light of the large number of volatile compounds being produced, however, our knowledge on the sets of key genes for volatile biogenesis in most plant species is still lacking. Identifying a presumably large number of genes involved in making herbivory-induced plant volatiles could be a daunting task. Nonetheless, the increasing availability of novel genomic information and tools for an expanding list of plant species can be leveraged to accelerate the identification of key volatile-producing genes.

Recently, we established rice as a model for studying the molecular and genomic basis of indirect defense.19 When damaged by fall armyworm (Spodoptera frugiperda, FAW) caterpillars, rice plants become highly attractive to females of parasitic wasp Cotesia marginiventris. Targeted metabolic profiling using a headspace technique coupled with Gas chromatography-Mass spectrometry determined that the rice volatiles potentially responsible for attracting parasitic wasps are a mixture of approximately 30 compounds mainly derived from three biochemical pathways: the terpenoid pathway, the fatty-acid degradation pathway and the shikimate pathway.

A number of herbivory-induced volatiles have been characterized for their individual contrition to indirect defense in behavioral set-ups. These include methyl salicylate,20 linalool,21,22 and Z-3-hexenol.22,23 S-Linalool is the most abundant volatile emitted from FAW-damaged rice plants. We therefore elected to test whether the synthetic linalool could function in attracting C. marginiventris wasps. Racemic linalool was applied to intact rice plants and had an emission rate similar to that of linalool emitted from FAW-damaged rice plants. Two choice Y-tube bioassays showed that racemic linalool could significantly attract parasitic wasps (Fig. 1). The contribution of two enantiomeric isomers of linalool in attracting C. marginiventris still needs to be determined. Nonetheless, the effectiveness of linalool in attracting different types of parasitoids and predators in a number of studies in the context of indirect defense21,22 suggests that linalool may act as a general attractant, as that has been demonstrated for Z-3-hexenol.23 Recently, genetic engineering was used to manipulate production and emission of herbivory-induced plant volatiles for understanding the relevance of individual or a group of volatiles in indirect defense. By overexpressing terpene synthase (TPS) genes in Arabidopsis, constitutive emission of novel terpenes in Arabidopsis plants were shown to attract predators24 and parasitoids.18 Whether there are synergistic effects among these individual volatiles in attracting the natural enemies of the attacking herbivores, however, remains to be determined.

To identify candidate genes for volatile biosynthesis, genome-level transcript profiling using microarray was performed to compare gene expression changes in control rice plants vs. FAW-damaged rice plants. Among the approximately 200 genes identified to be significantly upregulated by FAW herbivory, 18 of them encode enzymes potentially directly involved in the production of insect-induced volatiles. Further biochemical analysis identified three TPS genes that are responsible for making the majority of the FAW-induced volatile terpenoids in rice.19 We are continuing to characterize the additional candidate genes. Some of them, for example, a rice salicylic acid methyltransferase (SAMT) gene for making FAW-induced methyl salicylate, have been biochemically verified (Zhao N and Chen F, unpublished). In addition to the identification of key genes that encode enzymes catalyzing the final step in the formation of a specific compound, the study in rice also provides novel information on regulation of volatile production. For example, the upstream pathways of the terpenoid metabolism were found to be upregulated.19

The study in rice demonstrates that it is possible to identify most, if not all, key genes for making insect-induced volatiles by combining volatile profiling, gene expression profiling and biochemical characterization. In the future, a systems biology approach that would also incorporate proteomics data will deepen our understanding of the metabolic network as well as the regulatory network that govern the biosynthesis of herbivory-induced volatiles for indirect defense. With the advent of new sequencing technologies and subsequently the availability of sequences of more plant genomes, the strategy used in the rice study can be employed to identify genes of volatile biosynthesis in other plant species. The identified key volatile producing genes in rice may serve as a useful reference for comparison of the genomic basis of volatile-mediated indirect defense across a range of plant species, especially in monocots. Such comparative studies may lead to important novel insights into the evolution of this important defense trait.