Carnivorous plants, such as the Venus flytrap (Dionaea muscipula), depend on an animal diet when grown in nutrient-poor soils. They sense the arrival of insects with the aid of sensitive trigger hairs on the inner surfaces of their traps.
The Venus flytrap (Dionaea muscipula). Image credit: Michal Klajban / CC BY-SA 3.0.
Now, a team of biologists led by Dr. Rainer Hedrich of Universität Würzburg has looked closely at exactly how Venus flytraps decide when to keep their traps shut and begin producing their prey-decomposing cocktail of enzymes. The surprising answer is: they count.
“The Venus flytrap, can count how often it has been touched by an insect visiting its capture organ in order to trap and consume the animal prey,” said Dr. Hedrich, who is the senior author on a study in the journal Current Biology.
To find out whether Venus flytraps record touches, the scientists fooled the plants into thinking they’d landed an insect by applying increasing numbers of mechano-electric stimuli to their trap and monitoring their responses.
“In order to investigate the Venus flytrap’s signaling cascade involved in the prey capturing and digestion, we monitored the electrical activity of a prey-catching flytrap via surface electrode measurements,” they explained. “For these experiments we used crickets (Acheta domesticus) 6-12 mm in length and with an average weight of 23.8 mg.”
“It is well known that only two stimuli are sufficient for provoking fast trap closure, thereby capturing the insect. Once trapped, the still-moving victim continues to activate these mechanosensors, prolonging the electrical stimulation for many hours.”
At this stage, the plant begins to produce a special touch hormone called jasmonate. After five triggers, glands on the inner surface of the trap also produce digestive enzymes and transporters to take up nutrients.
This input also allows the plant to scale its production of costly ingredients to the size of the meal.
“The number of action potentials informs the plant about the size and nutrient content of the struggling prey. This allows the Venus flytrap to balance the cost and benefit of hunting,” Dr. Hedrich said.
Interestingly, the plants show a particularly marked increase in production of a transporter that allows them to take up sodium.
It’s not clear exactly what the salt does for the plant, but Dr. Hedrich and co-authors suggest that it may have something to do with how Venus flytraps maintain the right balance of water inside their cell walls.
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Jennifer Böhm et al. The Venus Flytrap Dionaea muscipula Counts Prey-Induced Action Potentials to Induce Sodium Uptake. Current Biology, published online January 21, 2016; doi: 10.1016/j.cub.2015.11.057
