The passion vine, the flowery source of passion fruit, is almost invulnerable to insects: Whenever one starts munching on its leaves, the plant releases a deadly cyanide gas. But in this week's issue of Nature, ecologists report that the larvae of one butterfly have developed ways to disarm the plant, perhaps even using cyanide's chemical precursors as food. The researchers hope the discovery will lead to new ways of making some poisonous plants safer for human and animal consumption.
Over 3000 plant species produce cyanogens, compounds that are converted into cyanide by specialized enzymes also produced by the plant. This reaction usually takes place inside a herbivore's gut, says Helene Engler, a chemical ecologist at the University of Texas, Austin. Among the plants are almonds, lima beans, and cassava, a starchy root that provides a major source of calories for people living in tropical areas such as sub-Saharan Africa. With a balanced diet, the body can detoxify small amounts of cyanide; but when certain proteins are lacking, cassava consumption in particular causes health problems, such as goitres and blindness.
That's why Engler and her colleagues wanted to find out how the larvae of Heliconius sara, a brightly colored butterfly of South and Central American rainforests, could maintain a strict diet of passion vine leaves, even though the plant's hydrogen cyanide gas stifles other insects' respiration.
To find out, Engler and her colleagues extracted cyanogens from the plants and recently emerged butterflies and determined their chemical structures using nuclear magnetic resonance and mass spectroscopy. In the butterflies, they found a plant cyanogen whose nitrile (CN-) group had been replaced with a thiol (SH-) group--a preemptive strike that prevents the release of cyanide. Engler suspects the butterflies may use the nitrile group to spruce up their own armamentarium--they are known to produce a chemically different class of cyanogens of their own--or they may use the valuable nitrogen for their growth. If researchers can identify the enzymes that do the trick, perhaps those can be built into human foodstuffs to make them less toxic, she adds.
The larvae's ability to prevent cyanide production is "unique," says David Jones, a chemical ecologist at the University of Florida, Gainesville. But Jones is skeptical of plans to disarm cyanogenic plants through biotechnology, because it would leave the plants vulnerable to pests. "If you remove the cyanogenic potential of a plant, it opens it up to herbivores of all types," he says.