Plants have shown the ability to sense when they are touched and when the touch is released, even without the presence of nerves, according to a study led by Washington State University. The research demonstrated that individual plant cells responded to touch by sending slow waves of calcium signals to other cells. When the pressure of the touch was released, the cells sent much more rapid waves. While scientists were already aware that plants could respond to touch, this study reveals that plant cells transmit different signals when touch is initiated and when it ends.
The study, published in the journal Nature Plants, involved a series of experiments on 12 plants, including thale cress and specially bred tobacco plants containing calcium sensors. Researchers used a microscopic glass rod, similar in size to a human hair, to apply a gentle touch to individual plant cells. They observed complex responses depending on the force and duration of the touch, but the distinction between the touch and its removal was evident.
Within 30 seconds of the touch, the researchers observed slow waves of calcium ions, known as cytosolic calcium, moving from the touched cell to neighboring plant cells. These waves lasted approximately three to five minutes. Upon the removal of the touch, a rapid set of waves occurred, dissipating within a minute.
The scientists believe that these waves are likely a result of the change in pressure inside the plant cells. Unlike animal cells, plant cells have sturdy cellular walls that are not easily penetrated. Therefore, even a light touch temporarily increases the pressure in a plant cell.
To test this theory, the researchers mechanically manipulated the pressure inside a plant cell using a small glass capillary pressure probe. They observed similar calcium waves triggered by the start and stop of the touch, confirming the role of pressure in generating these responses.
According to Michael Knoblauch, the senior author of the study and a biological sciences professor at WSU, humans and animals sense touch through sensory cells, whereas plants appear to sense touch through the alteration of internal cell pressure. This mechanism is not limited to specific cells, as any cell on the plant’s surface can carry out this function.
Previous studies have demonstrated that when a plant is bitten by a pest like a caterpillar, it initiates defensive responses, such as the release of chemicals that make the leaves less appealing or even toxic to the pest. Another study revealed that brushing a plant triggers calcium waves that activate various genes.
While this study successfully differentiated the calcium waves associated with touch and release, the exact response of the plant’s genes to these signals remains unclear. However, Knoblauch highlighted that with advancements in technologies such as calcium sensors used in this research, scientists can further investigate this mystery and better understand how genes are influenced by these signals.
The study received support from grants by the National Science Foundation. The research team comprised scientists from the Technical University of Denmark, Ludwig Maximilian Universitaet Muenchen, and Westfaelische Wilhelms-Universitaet Muenster in Germany, as well as the University of Wisconsin-Madison and Washington State University.
