Edited by May R. Berenbaum, University of Illinois at Urbana–Champaign, Urbana, IL, and approved December 9, 2013 (received for review August 6, 2013)
When hornworm (Manduca sexta) larvae feed on Nicotiana attenuata plants in native habitats, more disappear at night when they feed on transgenic, nicotine-free N. attenuata plants because wolf spiders (Camptocosa parallela) selectively prey on nicotine-free larvae. When larvae consume nicotine-replete plants, their midgut-expressed cytochrome P450 6B46 (CYP6B46) is upregulated. The larvae in which CYP6B46 is silenced by plant-mediated RNAi excrete more of their ingested nicotine. Silencing CYP6B46 impairs the mechanisms of passing ingested nicotine from the midgut to the hemolymph to be exhaled from the spiracles during spider attack. Spiders are deterred by this nicotine-rich halitosis. Thus, the CYP6B46-silenced larvae exhale less nicotine and become ready spider prey, demonstrating that CYP6B46 functions to repurpose the normally excreted nicotine for defense.
Manduca sexta (Ms) larvae are known to efficiently excrete ingested nicotine when feeding on their nicotine-producing native hostplant, Nicotiana attenuata. Here we describe how ingested nicotine is co-opted for larval defense by a unique mechanism. Plant-mediated RNAi was used to silence a midgut-expressed, nicotine-induced cytochrome P450 6B46 (CYP6B46) in larvae consuming transgenic N. attenuata plants producing MsCYP6B46 dsRNA. These and transgenic nicotine-deficient plants were planted into native habitats to study the phenotypes of larvae feeding on these plants and the behavior of their predators. The attack-behavior of a native wolf spider (Camptocosa parallela), a major nocturnal predator, provided the key to understanding MsCYP6B46’s function: spiders clearly preferred CYP6B46-silenced larvae, just as they had preferred larvae fed nicotine-deficient plants. MsCYP6B46 redirects a small amount (0.65%) of ingested nicotine from the midgut into hemolymph, from which nicotine is exhaled through the spiracles as an antispider signal. CYP6B46-silenced larvae were more susceptible to spider-attack because they exhaled less nicotine because of lower hemolymph nicotine concentrations. CYP6B46-silenced larvae were impaired in distributing ingested nicotine from midgut to hemolymph, but not in the clearing of hemolymph nicotine or in the exhalation of nicotine from hemolymph. MsCYP6B46 could be a component of a previously hypothesized pump that converts nicotine to a short-lived, transportable, metabolite. Other predators, big-eyed bugs, and antlion larvae were insensitive to this defense. Thus, chemical defenses, too toxic to sequester, can be repurposed for defensive functions through respiration as a form of defensive halitosis, and predators can assist the functional elucidation of herbivore genes.
↵1P.K. and S.S.P. contributed equally to this work.
↵2Present address: Molecular Ecology, Dahlem Centre of Plant Sciences, Institute of Biology, Freie Universität Berlin, 12163 Berlin, Germany.
This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2013.
Author contributions: P.K., S.S.P., A.S., and I.T.B. designed research; P.K., S.S.P., A.S., and I.T.B. performed research; P.K., S.S.P., A.S., and I.T.B. analyzed data; and P.K., S.S.P., A.S., and I.T.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1314848111/-/DCSupplemental.
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