Newswise — A Washington State University-led study has discovered that plants, despite lacking nerves, possess the ability to perceive the touch of an object and discern its release.
In a series of trials, single plant cells exhibited a reaction to the touch of an exceedingly delicate glass rod, transmitting unhurried waves of calcium signals to neighboring plant cells. Conversely, upon the release of this pressure, they emitted considerably swifter waves. While it has been acknowledged by scientists that plants possess the ability to react to touch, this research demonstrates that plant cells convey distinct signals upon the commencement and cessation of touch.
"It is remarkably astounding how exquisitely perceptive plant cells are—being able to discern when they are being touched. They have the ability to sense the applied pressure, and when it is alleviated, they detect the subsequent decrease in pressure," expressed Michael Knoblauch, senior author of the study and a professor of biological sciences at WSU. The findings were published in the journal Nature Plants. "It is astonishing that plants can accomplish this in a distinct manner from animals, without the presence of nerve cells, and with such remarkable precision."
Using a groundbreaking technology involving calcium sensors, Knoblauch and his team executed a series of 84 experiments on 12 plants, utilizing thale cress and specially bred tobacco plants. Under a microscope, fragments of these plants were positioned, and individual plant cells were delicately touched using a micro-cantilever—an incredibly minute glass rod comparable in size to a human hair. The researchers observed a multitude of intricate reactions, contingent upon the intensity and duration of the touch, but the disparity between the touch itself and its subsequent removal was evident.
Within a timeframe of 30 seconds following the application of touch to a cell, the scientists observed the propagation of unhurried waves of calcium ions, known as cytosolic calcium, traversing from that specific cell to the neighboring plant cells. These waves persisted for approximately three to five minutes. Conversely, when the touch was withdrawn, an almost instantaneous succession of swifter waves emerged, dissipating within a minute.
The researchers propose that these waves are likely a result of alterations in the internal pressure of the plant cell. In contrast to animal cells, which have permeable membranes, plant cells possess robust cellular walls that are not easily penetrated. Therefore, even a gentle touch can lead to a transient increase in pressure within a plant cell.
To mechanistically investigate the pressure theory, the researchers conducted experiments involving the insertion of a minuscule glass capillary pressure probe into a plant cell. By incrementally increasing and decreasing the pressure within the cell, they observed comparable calcium waves being triggered, akin to the patterns observed during the initiation and cessation of a touch. These findings further supported the notion that changes in cellular pressure play a crucial role in the generation of calcium waves within plant cells.
"Human and animal touch perception relies on sensory cells. In plants, however, it seems that the mechanism operates through fluctuations in internal cell pressure," explained Knoblauch. "Interestingly, this ability is not limited to specific cells. In humans, nerve cells are necessary for touch perception, but in plants, any cell on the surface is capable of fulfilling this function."
Earlier investigations have provided evidence that when a plant leaf is bitten by a pest such as a caterpillar, it can trigger defensive reactions within the plant. These responses may involve the release of chemicals that render the leaves unappetizing or even toxic to the pest. Additionally, a prior study demonstrated that the act of brushing a plant stimulates calcium waves, which, in turn, activate various genes in the plant. These findings highlight the intricate and dynamic nature of plant responses to external stimuli, including both biotic (such as pests) and abiotic factors.
While the present study successfully distinguished the calcium waves associated with touch initiation and release, the specific manner in which the plant's genes respond to these signals remains unclear. However, with the aid of advanced technologies like the calcium sensors employed in this research, scientists are now better equipped to unravel this mystery. Knoblauch expressed optimism that these emerging tools can facilitate a deeper understanding of how plant genes precisely react to the touch-related signals, paving the way for further discoveries in this field of study.
"In future investigations, it will be crucial to explore alternative methods of triggering the signal, distinct from the approaches employed thus far. This will enable us to discern which specific signal, whether it is touch or the act of letting go, initiates the subsequent downstream events," Knoblauch emphasized. By experimenting with different stimuli and observing the resulting gene responses, researchers can gain deeper insights into the intricate mechanisms underlying plant perception and response to touch.
This study was supported by grants from the National Science Foundation. The international team included researchers from the Technical University of Denmark; Ludwig Maximilian Universitaet Muenchen and Westfaelische Wilhelms-Universitaet Muenster in Germany; and University of Wisconsin-Madison as well as WSU.
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