Inside the Plant That Eats Its Enemies

How does a plant digest its prey? What molecular processes aid digestion? A group of German scientists worked to solve the mysteries of the Venus flytrap.

A potential meal stimulates the touch-sensitive trigger hairs inside a Venus flytrap. Credit: Soenke Scherzer

A potential meal stimulates the touch-sensitive trigger hairs inside a Venus flytrap. Credit: Soenke Scherzer

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Venus flytraps are botanical carnivores. Despite their unorthodox diets, they don’t have special genes, say German scientists. Instead, the plants rewired existing mechanisms and copied gene expressions from other parts.

Carnivorous plants’ appetite for meat makes them seem animal-like. Since they live in poor soils, they stave off malnutrition by catching insects. Sundews brandish glue-covered arms while pitcher plants and Venus flytraps fashion traps.

Each green leaf of the Venus flytrap serves many purposes. Like any other normal leaf, it photosynthesises, while the leaf tip is a trap and stomach. The ingenious contraption is no more than a pair of lobes that close like a metal jaw trap used by poachers to snare mammals. The exposed surface is an insect magnet – coloured red like a flower and sweet smelling like a fruit.

Plants move slowly. But the Venus flytrap snaps shut in a fraction of a second, although it has no nerves or muscles. The finger-like growths on the margins of each lobe mesh together, like the teeth of a metal trap, preventing its prey from escaping.

How does a plant digest its prey? What molecular processes aid digestion? A group of German scientists worked to solve the mysteries of the Venus flytrap.

In a study published earlier this year, they discovered the plant can count. Tiny hairs that are sensitive to touch cover the lobes’ surface. When an insect lands, these hairs send an electrical pulse that alerts the plant. But it doesn’t react. If the prey moves and triggers the hairs again within 20 seconds, the trap slams shut. The delayed reaction is an energy saver. Without it, the plant would trap falling leaves, seeds, or any wind-blown debris. Should the insect trigger more than five times, the plant closes the trap tighter creating a hermetic seal. The more the insect struggles, the more digestive enzymes the plant secretes.

In the latest study, the team led by Professor Rainer Hedrich of the University of Würzburg, Germany, investigated molecular processes, examined gene expression, and compared the entire transcriptome, including flower, root and petiole.

Caption: Venus flytraps have fascinated biologists for centuries, however, the molecular underpinnings of their carnivorous lifestyle remain largely unknown. In a study published online today in Genome Research, researchers characterised gene expression, protein secretion, and ultrastructural changes during stimulation of Venus flytraps and discover that common plant defence systems, which typically protect plants from being eaten, are also used by Venus flytraps for insect feeding. Credit: Soenke Scherzer

The researchers confirmed the flytrap is a modified leaf. But they found the active genes of 37,000 digestive glands in the trap are similar to roots. After all, the glands and roots perform the same function: absorbing nutrients.

“If one considers that glands take up prey-derived nutrients, this overlap is not surprising at all,” Hedrich told The Wire, “but of course, we were excited to corroborate our hypothesis with strong molecular evidence.”

The glands have three layers. Cells of the two outer layers excrete digestive enzymes while the inner third layer is densely packed with oil bodies. These supply the energy needed by the enzyme-producing layers.

When the trap snares an insect, it turns into a stomach. The plant’s glands exude a concoction of chemical compounds. Insects have tough external skeletons made of chitin. Digestive enzymes from the glands, including chitinase to digest chitin, suffuse the trap.

The plant also secretes molecules to ‘taste’ and identify what it has caught and estimate its nutritive value. This helps the plant decide the quantity and cocktail of enzymes to digest its prey efficiently. In the lab, when the researchers dropped chitin on Venus flytraps already in digestion mode, the plants cranked up production of chitinase almost a thousand-fold.

The thale cress of the mustard family, a common non-carnivorous plant, is a well-researched plant subject. When insects feed on it, the plant activates a hormone called jasmonate that renders the plant indigestible or produces toxins in its leaves. “Contact with chitin normally means danger for a plant – [it signals the presence of] insects that will eat it,” Hedrich explained in a press release.

Instead of blocking insects’ digestion, jasmonate stimulates the flytrap’s digestive enzymes. In the case of the Venus flytrap, “defence has simply turned into offence – the plant no longer needs to protect itself from the enemy but eats it,” says Hedrich. “The scientific community has long been speculating that carnivory might have evolved from ancient defence pathways, but we were the first to support this assumption with extensive molecular data.”

When plants come under attack, they give up on the infested part, letting it wither and die. However, carnivorous plants suppress this defence mechanism called cell death since they have no use for it.

“The plant recognises the invader, entraps it, secretes digestive enzymes just like a stomach and resorbs nutrients like animal intestine or plant roots,” says Hedrich.

The most astonishing discovery of the study is the plant didn’t invent any specific genes for its carnivorous diet. “We expected to find something like a carnivory-signature in our transcriptomic data,” says the professor, “but the Venus flytrap simply seems to have refined existing, evolutionary-proved programs commonly operating in root nutrient uptake and plant defence. Therefore, plants very likely did not invent carnivory from scratch. Carnivory is a result of smart adaptations of preexisting pathways – and that´s evolution!”

Venus flytraps make trapping insects seem easy, but it’s an expensive lifestyle. Only plants with no other choice, like those living in poor soils, resort to it. The researchers plan to study other carnivorous plants to understand their adaptations.

The study was published in the journal Genome Research on May 4, 2016.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.