Even though they don’t have nerves, plants can still sense when something touches them and when it lets go, suggests a new study.
Scientists from Washington State University conducted experiments where they found that individual plant cells sent sluggish waves of calcium signals to other plant cells in response to the pressure of a very tiny glass rod and considerably faster waves when the pressure was relieved.
Although it has long been known that plants can respond to touch, this study demonstrates that when touch is begun and ended, plant cells produce various signals.
Michael Knoblauch, WSU biological sciences professor and senior author of the study in the journal Nature Plants, said, “It is quite surprising how finely sensitive plants cells are — that they can discriminate when something is touching them. They sense the pressure, and they sense the pressure drop when it is released. Surprisingly, plants can do this very differently than animals, without nerve cells and at a really fine level.”
Researchers used tobacco and thale cress plants that had been carefully cultivated to contain calcium sensors, a relatively new technique, in a series of 84 trials on 12 different plants. They used a micro-cantilever, a tiny glass rod approximately the size of a human hair, to lightly touch individual plant cells after examining parts of these plants under a microscope. Depending on the intensity and length of the contact, they saw a wide range of complex reactions, but it was obvious what the touch was and what it did when it was removed.
The scientists saw slow waves of calcium ions, known as cytosolic calcium, moving from one cell through the neighboring plant cells within 30 seconds of the applied touch and lasting for three to five minutes. When the touch was removed, an almost immediate set of faster waves appeared, but they quickly subsided.
According to the authors, the pressure change inside the cell is probably causing these waves. A mild touch will momentarily increase pressure inside a plant cell because, unlike animal cells with permeable membranes, plant cells have robust cellular walls that are difficult to penetrate.
The scientists used a tiny glass capillary pressure probe to test the pressure theory in a plant cell mechanically. Similar calcium waves to those produced by the onset and stop of touch were produced by changes in internal pressure.
Knoblauch said, “Humans and animals sense touch through sensory cells. The mechanism in plants appears to be via this increase or decrease of the internal cell pressure. And it doesn’t matter which cell it is. Humans may need nerve cells, but in plants, any cell on the surface can do this.”
“The current study was able to differentiate the calcium waves between touch and letting go, but how exactly the plant’s genes respond to those signals remains to be seen. Scientists can start to untangle that mystery with new technologies like the calcium sensors used in this study.”
“In future studies, we have to trigger the signal differently than before to know what signal, if touch or letting go, triggers downstream events.”