Does herbivore diversity increase induced phytochemical diversity? Testing the pathway co-activation hypothesis
The mechanistic link between multiple herbivory and phytochemical diversity as an emergent plant trait is poorly understood. Because alternative, interacting biosynthetic pathways enable high specificity of induced plant responses to different herbivores, we hypothesized that multi-species herbivory increases phytochemical diversity by simultaneously activating mutually-antagonistic response pathways. We tested this hypothesis using a dual-herbivory experiment with phloem-feeding aphids and leaf-chewing caterpillars on cabbage seedlings, combined with untargeted metabolomics. To isolate plant responses to herbivore identities from responses to damage levels, we equalized herbivory pressures across treatments. The underlying mechanism was tested using similar applications of the respective defense phytohormones. Phytochemical richness increased under dual-herbivory relative to single-species herbivory. Phytochemical evenness (2D diversity) increased from caterpillar-only to dual-herbivory. Phytohormone applications produced similar effects, supporting the hypothesized mechanism. Analyses of glucosinolate levels and whole-metabolome shifts were consistent with prevalent, but not exclusive, antagonistic JA-SA crosstalk, explaining the increased phytochemical evenness under combined exposures. The demonstrated general mechanistic basis suggests a broad positive relationship between phytochemical and herbivore diversity across plant-herbivore systems. Using a theoretical simulation model, we extend our two-species hypothesis to the community level. The ecological effects of herbivore-induced plasticity in phytochemical diversity may ultimately cascade through communities and food web interactions.