Ghent researchers uncover dirty insect-plant wars


Biologists from Ghent University have identified a gene that allows insects to repel predators with one of the deadliest poisons on Earth

The two-spotted spider mite (Tetranychus urticae) feeding on a plant

“Herbivores detonate cyanide bomb”

The chemical compound cyanide has been a popular poisoning agent throughout the centuries. One of the deadliest poisons on Earth, it was, for instance, used in the Nazi gas chambers under the brand name Zyklon B during the Second World War, and was also the preferred suicide method of German officers and commanders. Adolf Hitler, Erwin Rommel, Heinrich Himmler and Hermann Göring … they all died after swallowing a cyanide capsule.

But use of cyanide as a deadly weapon or chemical warfare in general is not just the prerogative of humans.

Several plants, among them crops like barley, apples, peanuts and almonds, can produce cyanide. The poison is formed in their leaves, stems, fruits, seeds, roots and tubers. The amounts of the substance produced are not harmful to larger animals or to humans. For small herbivores, however, they are highly poisonous.

But there are some exceptions. Butterflies, moths and mites share the enviable quality of being fully resistant to cyanide. They seem to be equipped with some sort of “detoxification apparatus” that converts the deadly compound into harmless molecules.

A team of biologists from Ghent University has now identified the gene responsible for this surprising immunity to cyanide, and their findings have shed light on the mysterious practice of chemical warfare between plants and insects.

Cyanide bomb

Ghent biologist Thomas Van Leeuwen says the release of cyanide by plants typically takes place in two phases. “First, plants synthesise and accumulate a non-toxic compound by adding sugars to certain amino acids,” he explains. “Although these special cyanide sugars can already deter herbivores through their vile taste, they are not yet toxic.”

Plants need to protect themselves from their own poison

- Researcher Thomas Van Leeuwen

In a second phase, when a herbivore begins to nibble on a cyanide-producing plant, the production of enzymes that releases cyanide from the special sugars is triggered. “So in a way, it’s the attacker himself that detonates the cyanide bomb,” Van Leeuwen says.

The resistance of butterflies, moths and mites to the cyanide is caused by a unique metabolic reaction in their tiny bodies that sees the poison converted into a harmless amino-acid-based product. In a recent experiment, Van Leeuwen and his colleagues identified the gene that enables this reaction – the beta-cyanoalanine synthase – or BCS – gene.

But on examining theirs findings, the researchers furrowed their brows. The gene looked familiar. The scientists realised it had been spotted before, but never in animals. In fact, the BCS gene is a typically bacterial gene and is often found in the Methylobacteria genus.

“This is a kind of bacteria commonly found on plants,” Van Leeuwen explains. “They are also resistant to cyanide, as are many plants, because they need to protect themselves from their own poison. It seems that at some point in evolutionary history, these three cyanide-resistant insects must have hijacked this gene.”

Evolutionary stories

A remarkable evolutionary story unfolded before the eyes of the Ghent scientists. Over the course of time, butterflies, moths and mites had apparently developed a gene that allowed them to thrive on plants poisonous to their plant-eating rivals, like snails, centipedes and aphids. But there was more.

It’s amazing to see how butterflies use the same mechanism as plants to deter their enemies

- Thomas Van Leeuwen

In the case of the butterflies, the insects not only proved to be resistant to the cyanide released by their host plant, they themselves produced the poison to repel their own predators.

“If the plant they are feeding on produces cyanide, butterflies can stock the cyanide sugars in special tissues,” Van Leeuwen says. “If the host plant doesn’t produce cyanide, however, they can still synthesise the sugars they need from amino acids.” In both cases, butterflies degrade the sugars to subsequently release toxic cyanide when a predator attacks.

“In the meantime, these predators have learned not to attack butterflies anymore, as they could literally sit rather heavy on the stomach,” the biologist explains. “It’s really amazing to see how butterflies use the same mechanism as plants to deter their enemies. For moths and mites, it’s still unknown if they use the same strategy.”

Van Leeuwen admits that this kind of research is quite fundamental, so without particular uses in mind. But he stresses that it can yield applicable knowledge – in biotechnology, for example.

“This work is mainly a remarkable evolutionary story,” the researcher says. “But our group works hard to indeed use this knowledge to devise better, sustainable crop-protection tools. Searching and uncovering the protagonists in insect-plant interactions can be of major importance in the biotech industry of tomorrow.”

Photo by G San Martin