Bait, not reward: CO2-enriched Nepenthes pitchers secrete toxic nectar

Abstract

Nepenthes pitchers are leaf-evolved biological traps holding high levels of CO2 within them. Extrafloral nectar (EFN) secreted by these pitchers has long been regarded as the major reward to visiting arthropods, but its chemical constituents and their role in prey capture are least explored. Here we demonstrate Nepenthes EFN as a sugar (glucose-fructose-sucrose) mix with high C:N ratio, minimal amino acids, proteins, and vitamin C. Nepenthes khasiana peristome and lid EFNs displayed strong acetylcholinesterase (AChE) inhibition; the naphthoquinone derivative, (+)-isoshinanolone, has been identified as the AChE inhibitor. Plumbagin, the major volatile naphthoquinone in Nepenthes, also showed strong AChE inhibition. Direct EFN- and (+)-isoshinanolone-feeding bioassays demonstrated symptoms of cholinergic toxicity in ants. We testify that Nepenthes EFN is a toxic bait which hinders neuronal activity in visiting arthropods. These unique traps adopt various deceptive strategies for prey capture, and our discovery abolishes the notion that Nepenthes EFN is a reward to visiting ants and other arthropods. Moreover, our findings infer elevated CO2 within their pitchers as the key factor influencing the growth, metabolism, herbivory, and carnivory in Nepenthes. HighlightNepenthes extrafloral nectar has high content of carbohydrates and minimal nitrogenous metabolites. It is laced with (+)-isoshinanolone, an acetylcholinesterase inhibitor, and acts as a toxic bait, aiding prey capture.