“Wait, how do plants know things? Do they know things? They can’t, right?”
My third-grader’s brow furrowed as she ran scenarios over in her head. Plants aren’t the same as animals. They don’t have muscles to move with. They don’t have eyes like we do, and no brain to do any thinking. Nonetheless, they are known to follow the Sun, open and close flowers, and even react to the sound of predators eating their neighbors’ leaves. How was any of this possible without the anatomical gear we depend on to do any of those jobs?
It’s a tough question, and we don’t know all those answers yet. The sound sensory in particular is quite odd, but fortunately, some very reactive plants have helped botanists figure out how a plant can sense and respond to stimuli without nerve cells and a brain to do so. Because carnivorous plants like the Venus flytrap (Dionaea muscipula) and sundew plants (Drosera) have to actively trap their prey, they need to operate in a time-frame that matches the critters they want to catch. To do that, the plants are relying on so-called trigger hairs that essentially give the plant a way to “sense” when something has touched it.
As the titular fly lands on the attractively-scented “lobe” at the end of a leaf, it’s likely to bump into one or more trigger hairs. Unlike your nerve cells, those hairs don’t report back to any brain to trigger further activity. Instead, they create an action potential, which is an electrical charge that builds up in the trigger cell, eventually reaching a threshold where it gets discharged to the next cell, and the next, and the next. Eventually, this signal reaches the plant’s version of a muscle cell, which either expands or contracts to change the water pressure along the joint of the mouth-shaped lobe, causing it to close shut on the bug. With further stimulation of those trigger hairs, the flytrap will start to excrete digestive enzymes to it can actually eat its prey.
Turning off the trigger hairs
Even though trigger hairs aren’t exact matches for animal nerve cells, they’re close enough that they can be used to study the effects of the general anesthetics we use on animals. Even though those drugs are used every day to numb and temporarily paralyze people in surgery, we don’t know exactly how they do it. By using them on reactive plants like Venus flytraps and the “shy plant” Mimosa pudica, researchers are getting closer to understanding the exact cellular mechanisms that make modern surgery possible.
The answer seems to go back to the idea of action potentials, and how they get started in the first place. In both the plants trigger cells and animal nerves, a charge is built up on the outside of the cell membrane or wall. In the membrane are openings called ion channels that open and close when specific molecules are present to unlock them, a bit like a key opening a gate. Charged molecules, ions, can then move into the cell, helping it accumulate a larger charge, until eventually it triggers the release of an electrical charge to kick off another cell.
Paralyzing the plants
When carnivorous plants were subjected to anesthetics like diethyl ether, their trigger cells became unresponsive. The shy plant didn’t curl its leaves. Flytraps didn’t close after being poked. More importantly, no charge was detected at the plants’ trigger cells, indicating that the ion channels weren’t opening, heading off any action potential before it started. A second test with the roots of a mustard plant, Arabidopsis thaliana, found that the lipids, or fat proteins, in the cell membrane were being disrupted, helping researchers narrow their focus even further.
This wasn’t done to numb Venus flytraps, of course. They seem to be quite good at regulating their own activity already; they only close if multiple hairs have been touched, don’t usually close down on their pollinators, and reserve digestive enzymes for when they really have lunch in their clutches. Instead, this work may help us understand and then design better anesthetics for humans and other animals, taking some of the trial and error out of how we temporarily stop each other from sensing the world around us.
Source: We can make plants pass out—with the same drugs that mysteriously knock us out by Beth Mole, Ars Technica