Synergistic interactions between small molecules provide a new, but growing, framework that accounts for the fact that plant secondary metabolites always occur in mixtures and that singular compounds rarely demonstrate potent biological activity. A growing body of evidence suggests that this extra dimension of chemical complexity plays a substantial role in a plant’s ability to defend itself against an array of enemies through enhanced potency. Synergistic plant defenses are broadly defined as effects of multiple compounds that are greater than expected based on projected additive values of each individual compound. In the study systems investigated for synergies, there has been consistent evidence that synergy affects generalists and specialist herbivores differently, where generalists experience toxic effects, while specialists experience more subtle effects through reduced digestibility and nutrient uptake, and impaired immune response. Despite the strong motivation to fully understand the synergistic effects found in natural mixtures, studies of synergistic activity have been largely limited due to the labor intensive empirical screening of synthesized mixtures or exploring the activity of native mixtures one species at a time. How should we test for synergistic effects of plant defenses and what new methods can we use to explore phytochemical mixtures?
Results/Conclusions
Synergistic interactions occur across the angiosperm phylogeny and in many of the major classes of chemical defense. As studies of mixtures increase, widespread synergy may explain why phytochemical mixtures are the rule and not the exception and is likely responsible for what some have termed “redundancy” in chemical defenses. The synergistic effects of multiple plant secondary metabolites on upper trophic levels constitute an underexplored but potentially widespread component of coevolution and ecological interactions. Advances in organic spectroscopy and statistical treatments of spectral data will allow us to investigate phytochemical mixtures, identify unique compounds and address previously unanswered questions.