When faced with danger, animals respond to stress in different ways. We humans advertise our internal state by producing pheromones to signal alert, our hands and armpits get sweaty and our heart rate skyrockets. These are involuntary signals that we are we perceive a risk. However we are unlikely to fight our opponent by simply raising our arms and hoping the smell would scare our adversary away.
Certain insects, on the contrary, are known to take this approach – by using chemical compounds to discourage their predators from attacking. The tobacco hornworm, Manduca sexta, relies on plant-derived toxins to gross predators out of attacking. The larvae of this species feed on tobacco plants (Nicotiana spp.) and exhale ingested nicotine to signal wolf spiders (Camptocosa parallela) to keep their distance. Nicotine is toxic for most animals, but hornworms can rely on the gene cytochrome P450 6B46 (CYP6B46) to metabolize it. Part of the nicotine then ends up in the larva’s hemolymph – its circulatory fluid, similar to our blood – from which it can be released into the air to deter predators. Indeed a deadly case of halitosis, but this clever mechanism allows the caterpillar to advertise its toxicity to predators before they take a bite.
Dr. Pavan Kumar and colleagues from the Max-Planck-Institute studied the molecular mechanisms of how hornworms recycle plant toxins for their own defense and the role of CYP6B46 in this process.
Researchers used transgenic plants of Nicotiana attenuate, engineered to produce dsRNA to silence CYP6B46 gene. Larvae feeding on the transgenic plants showed less expression of CYP6B46 and were less able to distribute ingested nicotine to hemolymph.
Spiders preferred CYP6B46-silenced larvae because they exhaled less nicotine due to its lower concentrations in the hemolymph.
“It’s really a story about how an insect that eats a plant co-opts the plant for its own defense,” coauthor Ian Baldwin declared to Live Science.
The use of plant-derived toxins against herbivores has also inspired some of the most modern insecticides. Surfactants are considered less human-toxic and cheaper alternatives to conventional insecticides. The most interesting aspect of this study, therefore, is its relevancy to agricultural practice. According to Dr. Baldwin the method used to silence the CYP6B46 gene in larvae, a procedure called Plant Mediated RNAi, “is the future of pest control for crop plants. Completely clean and targeted against only one type of pest”. It seems that one day we may well see plants protect themselves by regulating gene expression in their own predators.
References: Kumar P., Pandit S.S., Steppuhn A., Baldwin I.T. 2013. Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore defense. http://www.pnas.org/doi:10. 1073/pnas.1314848111.