How a Venus flytrap knows to snap shut

Few plants capture our imagination like the otherworldly Venus flytrap. Its snap-trap leaves clamp down on small insects in as fast as 1/10 of a second. That’s unexpected behavior among the staid plants we’re familiar with! Even now, scientists continue to learn new things about these enigmatic plants, like, for instance, how they know when to snap shut.

How does a Venus flytrap close up?

A Venus flytrap leaf has two lobes connected at a hinge on its stalk. Each lobe secretes nectar to attract insects. And each lobe is lined with cilia hairs that will later seem like prison bars to some unfortunate prey.

The lobes themselves are curved out and stretched open, like a clam. Each inner lobe has three sensory hairs. When something bumps against those hairs – whether it’s a raindrop, debris in the wind, or a visiting insect – that mechanical stimulus is converted into electrical signals in cells at the base of the hair. Those electrical signals then spread across the leaf.

Two images; left, sensory hairs in inner lobes of open Venus flytrap; right, closed lobes with a trapped insect.
The image on the left shows the sensory hairs on the leaf lobes of a Venus flytrap leaf. These sensory hairs alert the plant to the presence of potential prey. Image via National Institute for Basic Biology.

An insect crawling on a lobe will most likely bump against the sensory hairs several times. But all it takes are two bumps within 30 seconds of each other for the lobes to clamp shut, imprisoning the prey.

So, how does the Venus flytrap keep track of the time interval between these bumps? Scientists had long suspected that calcium ions were somehow involved.

This was definitively confirmed in October 2020, when researchers at the National Institute of Basic Biology in Okazaki, Japan, published a study in the peer-reviewed journal Nature Plants showing that changes in calcium concentrations inside leaf cells helped the plant keep time.

The researchers did this by genetically modifying Venus flytraps to emit green fluorescence when calcium ions were present in the leaf cells. When sensory hairs were first stimulated, the scientists were able to directly observe the increased concentration of calcium ions as a green glow. That concentration would gradually drop. Then, a second stimulus raised the calcium concentration again. When a certain threshold was reached in the calcium concentration, the trap snapped shut. But this threshold could only be reached if the two stimuli occurred within 30 seconds.

Needle prodding inside an open Venus flytrap which closes on it.
This video clip shows green fluorescence lighting up the Venus flytrap leaf when a sensor hair on its lobe is stimulated by a needle. Video via National Institute for Basic Biology.

But the two-bumps-within-30-second rule doesn’t always apply. Scientists at the University of Zurich in Switzerland showed that even slow-moving animals, like insect larvae and snails, can wind up in the Venus flytrap’s clutches.

Using precise micro-robots and very sensitive sensors, they studied the amount and duration of force on sensory hairs that was needed to trigger a trap to shut. That data was used to create a mathematical model of how the plant reacted to stimuli from prey of different sizes. Their model showed a scenario – that they were later able to verify experimentally – where a single slow touch could create the two electrical signals needed to shut the trap. So, snails and caterpillars are just as likely to end up as a meal as fast-moving ants and flies.

Small mechanism rolling on track inside Venus flytrap, with graphs below animated image.
In this video clip, a small robot is used to apply a highly precise force to a sensor hair on a Venus flytrap. Microscope cameras record the position of the needle and reaction of the leaf to the mechanical stimuli. Video via Jan T. Burri / ETH Zurich.

What happens when an insect is captured?

Once an insect is captured, the trap does not close completely. If the insect is small enough, it can still crawl out. That’s the plant’s way of ensuring that its prey is large enough to be worth the effort for the next stage: digestion. So, if the struggling bug is big enough, the leaf will then close even tighter around it and start releasing digestive liquids to break down the prey for absorption. If you’d like to learn more about it, check out this wonderful article by science writer Ed Yong in The Atlantic. There’s also a nice overview of Venus flytraps and how to grow them at the Botanical Society of America website.

Venus flytrap with closed leaves. An insect is visible as a silhouette in one. One withered leaf.
A Venus flytrap plant with several closed leaves. After a leaf has been used several times to capture prey, it dies. Image via Shireen Gonzaga/ Flickr.

Dionaea muscipula, the Venus flytrap, is a rare plant in the wild, found only in the coastal bogs of North Carolina and South Carolina. They’re increasing hard to find and have been proposed for listing as an endangered species. The biggest problem they face is habitat loss, not just from development but also from a lack of forest fires that they need to clear out competing plants. Plant poaching is also a major reason for their decline in the wild. If you want to grow a Venus flytrap, they are easily available at reputable nurseries.

A pot of Venus flytrap plants. The spiky lobes are green with a red inner surface.
Venus flytrap in a pot at the Carnivorous Plant Nursery in Maryland. Image via Shireen Gonzaga / Flickr.

Why do Venus flytraps go after small insects and spiders?

Like other carnivorous plants, the evolution of Venus flytraps in nutrient-poor habitats led to a unique adaptation: trapping live animals for additional sustenance. A few other carnivorous plants have developed movements to trap live food, like the waterwheel. But none of them have the dramatic trapping effect of the Venus flytrap, which inspired Audrey II, the star of “Little Shop of Horrors.”

Bottom line: Venus flytraps have small sensory hairs in their leaf lobes that alert them to potential prey, causing the snap-trap lobes to dramatically clamp down on small insects and spiders.

Source: Calcium dynamics during trap closure visualized in transgenic Venus flytrap

Source: A single touch can provide sufficient mechanical stimulation to trigger Venus flytrap closure

Via National Institute for Basic Biology

Via University of Zurich

May 30, 2021

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