Structure-activity relationship of chemical defenses from the freshwater plant Micranthemum umbrosum

Vascular plants produce a variety of molecules of phenylpropanoid biosynthetic origin, including lignoids. Recent investigations indicated that in freshwater plants, some of these natural products function as chemical defenses against generalist consumers such as crayfish. Certain structural features are shared among several of these anti-herbivore compounds, including phenolic, methoxy, methylenedioxy, and lactone functional groups. To test the relative importance of various functional groups in contributing to the feeding deterrence of phenylpropanoid-based natural products, we compared the feeding behavior of crayfish offered artificial diets containing analogs of elemicin (1) and beta-apopicropodophyllin (2), chemical defenses of the freshwater macrophyte Micranthemum umbrosum. Both allyl and methoxy moieties of 1 contributed to feeding deterrence. Disruption of the lactone moiety of 2 reduced its deterrence. Finally, feeding assays testing effects of 1 and 2 at multiple concentrations established that these two natural products interact additively in deterring crayfish feeding.