The secret of how the Venus flytrap “remembers” when it captures prey

Scientists are continuing to tease out the mechanisms by which the Venus flytrap can tell when it has captured a tasty insect as prey as opposed to an inedible object (or just a false alarm). There is evidence that the carnivorous plant has something akin to a short-term “memory,” and a team of Japanese scientists has found evidence that the mechanism for this memory lies in changes in calcium concentrations in its leaves, according to a recent paper published in the journal Nature Plants.

The Venus flytrap attracts its prey with a pleasing fruity scent. When an insect lands on a leaf, it stimulates the highly sensitive trigger hairs that line the leaf. When the pressure becomes strong enough to bend those hairs, the plant will snap its leaves shut and trap the insect inside. Long cilia grab and hold the insect in place, much like fingers, as the plant begins to secrete digestive juices. The insect is digested slowly over five to 12 days, after which the trap reopens, releasing the dried-out husk of the insect into the wind.

Back in 2016, a team of German scientists discovered that the Venus flytrap can actually “count” the number of times something touches its hair-lined leaves—an ability that helps the plant distinguish between the presence of prey and a small nut or stone, or even a dead insect. The scientists zapped the leaves of test plants with mechano-electric pulses of different intensities and measured the responses. It turns out that the plant detects that first “action potential” but doesn’t snap shut right away, waiting until a second zap confirms the presence of actual prey, at which point the trap closes.

But the Venus flytrap doesn’t close all the way and produce digestive enzymes to consume the prey until the hairs are triggered three more times (for a total of five stimuli). The German scientists likened this behavior to performing a rudimentary cost-to-benefit analysis, in which the number of triggering stimuli help the Venus flytrap determine the size and nutritional content of any potential prey struggling in its maw and whether it’s worth the effort. If not, the trap will release whatever has been caught within 12 hours or so. (Another means by which the Venus flytrap tells the difference between an inedible object and actual prey is a special chitin receptor. Most insects have a chitin exoskeleton, so the plant will produce even more digestive enzymes in response to the presence of chitin.)

The implication is that the Venus flytrap must have some sort of short-term memory mechanism in order for that to work, since it has to “remember” the first stimulation long enough for the second stimulation to register. Past research has posited that shifts in the concentrations of calcium ions play a role, although the lack of any means to measure those concentrations, without damaging the leaf cells, prevented scientists from testing that theory.

That’s where this latest study comes in. The Japanese team figured out how to introduce a gene for a calcium sensor protein called GCaMP6, which glows green whenever it binds to calcium. That green fluorescence allowed the team to visually track the changes in calcium concentrations in response to stimulating the plant’s sensitive hairs with a needle.

“I tried so many experiments over two and a half years, but all failed,” said co-author Hiraku Suda, a graduate student at the National Institute for Basic Biology (NIBB) in Okazaki, Japan. “The Venus flytrap was such an attractive system that I did not give up. I finally noticed that foreign DNA integrated with high efficiency into the Venus flytrap grown in the dark. It was a small but indispensable clue.”

The results supported the hypothesis that the first stimulus triggers the release of calcium, but the concentration doesn’t reach the critical threshold that signals the trap to close without a second influx of calcium from a second stimulus. That second stimulus has to occur within 30 seconds, however, since the calcium concentrations decrease over time. If it takes longer than 30 seconds between the first and second stimuli, the trap won’t close. So the waxing and waning of calcium concentrations in the leaf cells really do seem to serve as a kind of short-term memory for the Venus flytrap.

The next step is to investigate the link between calcium concentrations and the plant’s electrical network that converts the movement of prey caught in the trap into small electrical charges that spread across the cells. Scientists already knew that there is a close association between calcium and those electrical signals in many plants, so it’s not that surprising that there would be a similar link in the Venus flytrap. What’s not clear is precisely how the two systems work together.

“This is the first step towards revealing the evolution of plant movement and carnivory, as well as the underlying mechanisms,” said co-author Mitsuyasu Hasebe, a professor and vice-director general of NIBB. “Many plants and animals have interesting but unexplored biological peculiarities.”

DOI: Nature Plants, 2020. 10.1038/s41477-020-00773-1  (About DOIs).

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