Patrick Guilfoile: When tobacco breath can save a life
Some insects that feed on toxic plants accumulate the toxins and use that as a defense against predators.
Nicotine is a toxin produced by tobacco plants, which, at high doses, causes muscle paralysis in almost all animals, including humans.
Yet tobacco hornworm caterpillars feed on tobacco, and are immune to the effects of nicotine, as they efficiently metabolize and excrete the toxin.
Some predators (wasps and ants) avoid tobacco hornworm caterpillars that feed on plants containing high levels of nicotine.
But exactly how the caterpillar uses the toxin to deter predators was unclear. Other insects accumulate the toxins in out-of-the way places in their bodies, so they are not affected, but a predator feeding on the insect would ingest the toxin.
However, tobacco hornworms don’t accumulate nicotine, so they must use it in some other way to defend themselves.
To understand how tobacco hornworms use nicotine to protect themselves, researchers used two strategies.
In one set of experiments, they genetically engineered tobacco plants to produce different levels of nicotine, and then allowed the caterpillars to feed on the different tobacco strains.
They found that the caterpillars that fed on the low-nicotine tobacco were more likely to be eaten by a predatory spider.
Next, they tried to decipher exactly how the caterpillar used nicotine to protect itself.
They identified a protein in the gut of the caterpillar that appeared to channel some of the nicotine to its bloodstream.
They engineered tobacco plants again, this time with a chemical that inactivated the protein pump. They found that caterpillars with the deactivated pump were much more susceptible to spiders, and they had much lower levels of nicotine in their blood. But what was the connection between nicotine in the blood and deterring spiders? If the spider started devouring a caterpillar, it would be too late for the caterpillar if the spider found out partway through the meal that this caterpillar was toxic. So maybe the nicotine was excreted from the blood in a way that warned the spider to back off.
The researchers analyzed the air surrounding normal caterpillars and those with the nicotine pump turned off. They found that the normal caterpillars had much higher nicotine levels surrounding them, as compared with the pump-impaired caterpillars. Caterpillars breathe using small tubes (the equivalent of their mouth). Researchers attached small, nicotine-absorbing tubes to these mouths, and measured the nicotine levels in the caterpillar’s breath. They found that if the blood had high levels of nicotine, so did the caterpillar’s breath. Apparently, the nicotine being exhaled by the caterpillars gave them bad breath, which dissuaded spiders from feeding on them. So, at least for tobacco hornworm caterpillars, tobacco breath may save their lives.
More information is available in the article: “Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore defense by Pavan Kumar and others. Proceedings of the National Academy of Science, USA. www.pnas.org/cgi/doi/10.1073/pnas.1314848111
PATRICK GUILFOILE has a doctorate in bacteriology and is the associate vice president at Bemidji State University.