They can't run and they can't hide. But with those reactions denied to them, plants have evolved an extensive and varied repertoire for responding to threats to their well-being. Apart from the tolerance mechanisms discussed above by Schroeder and Kuhn, such responses can include calling upon insect allies to deal with pests that would otherwise damage or destroy the plant by eating it.

Christiane Schnee and colleagues have started to dissect one such signal system — that used by maize seedlings when they are attacked by caterpillars (Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0508027103; 2005). The plant signals consist of volatile chemicals, which in the case investigated by Schnee et al. attract female wasps of the species Cotesia marginiventris. The wasps lay their eggs in the caterpillars, with predictably unhappy results for the latter.

Maize emits a cocktail of volatile defence signals in response to an attack by herbivores, and the general difficulty in studying the process is identifying which constituent of these complex blends has which effect. From investigations of maize biosynthetic pathways, Schnee et al. first isolated an enzyme, a terpene synthase dubbed TPS10, that is responsible for producing most of the herbivore-induced volatiles.

Credit: T. TURLINGS

But that was only an initial step. To look into the biological effects of these terpenes, the authors used genetic engineering to insert the gene that encodes TPS10 into Arabidopsis, the standard lab plant for biologists. The transgenic Arabidopsis plants were then used in experiments in which female C. marginiventris could choose between the options offered in an ‘olfactometer’ (fresh air and untransformed Arabidopsis being the other choices).

The main result to emerge was that the wasps indeed showed a strong preference for the plant that produced the TPS-mediated terpene — but only after they had learned the association between the defence signal and the host by having previously laid eggs in the caterpillar host.

This approach, say Schnee et al., is an example of the value of using transgenic technology to study the effects of complex volatile compounds. Even when the compounds themselves or their constituents are not available, if the genes concerned have been identified, the ecological influences of these signals can nonetheless be investigated in genetically transformed plants.