How plants know when they are touched

Photo: Pixabay/Alexas_Fotos

We think of plants as insensate organisms, yet this perception is being increasingly challenged by scientists. In the latest finding experts at Washington State University have uncovered how plants can feel being touched despite lacking any nerve cells.

Although it has been known that plants can feel when they are touched, the researchers examined how individual plant cells responded when a tiny glass rod, like an insect’s leg, was lightly pressed against them. They did so by sending slow waves of calcium signals to other plant cells.

When the pressure was released, however, the cells sent much more rapid waves, the scientists report in a study in Nature Plants.

“It is quite surprising how finely sensitive plants cells are: that they can discriminate when something is touching them. They sense the pressure, and when it is released, they sense the drop in pressure,” notes Michael Knoblauch, a professor of biological sciences who was an author of the paper.

“It’s surprising that plants can do this in a very different way than animals without nerve cells and at a really fine level,” he adds.

Knoblauch and his colleagues conducted experiments on 12 thale cress and tobacco plants, which had been specially bred to include calcium sensors. They then placed pieces of the plants under a microscope and applied a slight touch to individual plant cells with a micro-cantilever, a glass rod the size of a human hair.

The plants demonstrated many complex responses depending on the force and duration of the touch, the scientists say. Within 30 seconds of being touched, the affected cells released slow waves of calcium ions, called cytosolic calcium, to adjacent plant cells for about three to five minutes. When the touch ceased, the cells responded with more rapid waves that ended within a minute.

The scientists believe these waves change in pressure inside the cell. “Unlike animal cells with permeable membranes, plant cells also have strong cellular walls that cannot be easily breached, so just a light touch will temporarily increase pressure in a plant cell,” they explain.

This finding reinforces earlier research that showed that when they are being chewed by pests like caterpillars the leaves of plants can initiate a defensive response such as by releasing chemicals that make leaves less tasty or even toxic to pests.

“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,” Knoblauch says. “And it doesn’t matter which cell it is. We humans may need nerve cells, but in plants any cell on the surface can do this.”