Part1_Herbivores: Plants can defend themselves against insect harbivores by attracting the natural enemies of the herbivores. After herbivore attack, maize (Zea mays) seedlings emit a complex volatile blend dominated by sesquiterpene hydrocarbons. These volatiles attract females of Cotesia marginiventris, a hymenopteran parasitoid, that have already associated the odor with an initial oviposition experience, and they can therefore be used by these wasps as a cue for locating their lepidopteran hosts. Parasitized lepidopteran larvae forage less and have a shorter lifetime compared to non-parasitized ones and thus parasitization benefits the plant by reducing the levels of herbivory.
Other sesquiterpenes are produced constitutively in young maize plants and released together with the induced ones after herbivore damage. We compared the attractiveness of the induced and constitutive sesquiterpene volatiles to the parasitoids after associative learning. We used transgenic Arabidopsis thaliana plants expressing maize sesquiterpene synthases as odor sources in olfactometer bioassays with C. marginiventris females.

Part2_Pathogens: Plant terpenes have been frequently shown to have an anti-fungal activity when tested in vitro, but studies leading to these conclusion have been mainly conducted applying concentrated plant extracts to fungal cultures and they therefore do not model the way volatile terpenes are emitted from intact plants. The availability of transgenic A. thaliana plants overexpressing maize sesquiterpene synthases allowed us to investigate the effect of plant terpenes on fungi at natural concentrations and using a natural delivery system.
We infected sesquiterpene-producing and wild-type A. thaliana plants with a spore suspension of the necrotrophic fungus Alternaria brassicicola. The fungal biomass in the leaves was quantified after the development of the symptoms. Quantification was performed by extraction of chitin and chitosan from leaf material, followed by acidic monomerization and HPLC analysis.

Results/Conclusions

Part1_Herbivores: The wasps were more attracted to a combination of the herbivore-induced and constitutive blend than to the single blends. This result indicates a synergistic action of the two volatile blends, suggesting that constitutive as well as induced volatiles may contribute to the specificity of indirect defense synonomes.

Part2_Pathogens: Fungal biomass was reduced in the sesquiterpene-producing lines tested in comparison to wild-type A. thaliana, indicating that maize sesquiterpenes may have a direct negative effect on the growth of the fungus. Since terpenes are released constitutively in many different plant species, this may be a widespread, previously unappreciated way for plants to defend themselves against fungal pathogens.