The plants use that information to speed digestion, the scientists found, BBC News reports.
Conducting a study, the scientists recorded the impulses generated by these hairs and measured changes within the plant. They found that two touches triggers hormone increases within the Venus flytrap; five taps cause the production of digestive enzymes.
Their findings appear in Current Biology.
It’s been established for some time that two touches are the trigger for the plants to snap shut, Wired reports. This prevents the Venus flytrap, (Dionaea muscipula), from unnecessarily expending energy on things like raindrops or stiff breezes. Once a hormone is triggered by two touches, the plant snaps shut in a mere tenth of a second.
“The number of action potentials informs the plant about the size and nutrient content of the struggling prey,” said Rainer Hedrich, lead author in the University of Würzburg study. “This allows the flytrap to balance the cost and benefit of hunting.”
In earlier research the team discovered the two-touch trigger. They decided to test this further by studying the plant’s reaction when they fed it crickets. So they stimulated the trigger up to 60 times to see what would happen.
Once the trigger was touched a second time, the insect was sealed inside the plant’s maw, and thus began the production of jasmonic acid, a hormone that aids the plant in capturing prey. A third touch triggers the production of digestive enzymes. When the trigger hairs are touched a fifth time, digestion begins in earnest and sodium is sucked from the insect, Wired reports.
How do the trigger hairs actually work?
Once these hairs have been bumped that second crucial time, calcium ions are triggered that send an electrical impulse through the lobe-like leaves, much in the same way that these impulses travel through our own nerves, Ed Yong reports in The Atlantic. Next, the plant flushes water into its leaves and they rapidly change shape from being bent outwards to being bent inwards. Thus, the trap swings shut.
Meanwhile, the still-living insect, most often a fly, struggles, striking the trigger hairs increasingly often and sending off more electrical impulses — about one per minute The third impulse raises calcium levels even higher, and that’s when the plant produces jasmonic acid, or jasmonate. Many plants use jasmonate as a touch hormone that’s usually released by wounds and injuries. It helps the plant repair and defend itself.
However, in the Venus fly trap, this hormone does double-duty because it promotes meat-eating. It does this by priming the gland cells within the trap to begin producing digestive enzymes, which they do once a fifth electrical impulse is detected.
Remarkably, the plant can even calibrate supply and demand of these enzymes. A large fly struggles more determinedly, striking more trigger hairs, and this sets off more electrical impulses. The plant steps up production of jasmonate, and secretes proportionally more digestive enzymes.
In about six or seven hours, the trap hermetically seals itself and fills with fluid. Deprived of oxygen, the fly suffocates. The fluid inside the trap turns extremely acidic, and meat-disintegrating enzymes infiltrate. This turns the trap into what Hedrich calls a “green stomach.”
That “green stomach” takes several days to digest the fly, and the plant keeps up the barrage of acid and enzymes by tasting its meal. The fly is no longer setting off electrical impulses, but in as-yet-unpublished work, Hedrich demonstrates that chemical sensors also line the trap. These sensors detect the chitin in the fly’s shell and the substances in its blood. As long as the plant can taste something, it keeps digesting, Yong notes.
Then it starts the job of absorbing, acting much like our intestines do. Those same five electrical impulses that fire up the production of digestive enzymes also active transporter enzymes, which absorb the sodium freed from the disintegrating fly.
It isn’t really known what salt does for the plant, but researchers think it may have something to do with helping Venus flytraps maintain the proper balance of water within their cell walls, Phys.org notes.
Hedrich and colleagues are in the process of sequencing the Venus flytrap genome. The scientists suspect that within those sequences lie additional clues about the plants’ sensory system and the chemistry it needs to support carnivory. Additionally, Hedrich and his colleagues hope to see how those traits evolved over time.
Considered vulnerable by the International Union for Conservation of Nature and Natural Resources. Venus flytraps are native to North America, mainly North- and South Carolina.
