Plants can distinguish when touch starts and stops
VIDEO: A MICROSCOPIC VIEW OF WHAT HAPPENS WHEN A SINGLE CELL OF A THALE CRESS PLANT IS TOUCHED BY A FINE GLASS ROD. WHEN THE TOUCH IS FIRST APPLIED (01:33) THE CELL SENDS A SLOW CALCIUM SIGNAL WAVE TO OTHER CELLS. WHEN THE TOUCH IS RELEASED, A FASTER WAVE IS CREATED (06:53). view more
CREDIT: WASHINGTON STATE UNIVERSITY, COPYRIGHT NATURE PLANTS
PULLMAN, Wash. – Even without nerves, plants can sense when something touches them and when it lets go, a Washington State University-led study has found.
In a set of experiments, individual plant cells responded to the touch of a very fine glass rod by sending slow waves of calcium signals to other plant cells, and when that pressure was released, they sent much more rapid waves. While scientists have known that plants can respond to touch, this study shows that plant cells send different signals when touch is initiated and ended.
“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,” said Michael Knoblauch, WSU biological sciences professor and senior author of the study in the journal Nature Plants. “It’s surprising that plants can do this in a very different way than animals, without nerve cells and at a really fine level.”
Knoblauch and his colleagues conducted a set of 84 experiments on 12 plants using thale cress and tobacco plants that had been specially bred to include calcium sensors, a relatively new technology. After placing pieces of these plants under a microscope, they applied a slight touch to individual plant cells with a micro-cantilever, essentially a tiny glass rod about the size of a human hair. They saw many complex responses depending on the force and duration of the touch, but the difference between the touch and its removal was clear.
Within 30 seconds of the applied touch to a cell, the researchers saw slow waves of calcium ions, called cytosolic calcium, travelling from that cell through the adjacent plant cells, lasting about three to five minutes. Removal of the touch showed an almost instant set of more rapid waves that dissipated within a minute.
The authors believe these waves are likely due to the 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.
The researchers tested the pressure theory mechanically by inserting a tiny glass capillary pressure probe into a plant cell. Increasing and decreasing pressure inside the cell resulted in similar calcium waves elicited by the start and stop of a touch.
“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,” said Knoblauch. “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.”
Previous research has shown that when a pest like a caterpillar bites a plant leaf, it can initiate the plant’s defensive responses such as the release of chemicals that make leaves less tasty or even toxic to the pest. An earlier study also revealed that brushing a plant triggers calcium waves that activate different genes.
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. With new technologies like the calcium sensors used in this study, scientists can start to untangle that mystery, Knoblauch said.
“In future studies, we have to trigger the signal in a different way than has been done before to know what signal, if touch or letting go, triggers downstream events,” he said.
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.
JOURNAL
Nature Plants
ARTICLE TITLE
Pavement cells distinguish touch from letting go
Light conveyed by the signal transmitting molecule sucrose controls growth of plant roots
Plant growth is driven by light and supplied with energy through photosynthesis by green leaves. It is the same for roots that grow in the dark – they receive the products of photosynthesis, in particular sucrose, i.e. sugar, via the central transportation pathways of phloem. Dr. Stefan Kircher and Prof. Dr. Peter Schopfer from the University of Freiburg’s Faculty of Biology have now shown in experiments using the model plant Arabidopsis thaliana (thale cress) that the sucrose not only guarantees the supply of carbohydrates to the roots, it also acts as a signal transmitter for the formation of light-dependent root architecture. It does this in two ways: firstly, sucrose directly guides elongation of the primary root. Secondly, the sucrose that is transported to the tip of the root then regulates the production of the plant hormone auxin. This hormone drives the rate of formation of new lateral roots, which along with elongation of the primary root is synchronised by the joint signal transmitter. “This enables the root growth to adapt to the current photosynthesis performance of the leaves as light and other environmental conditions change, for example on the change from day to night,” says Kircher.
Experimental evidence
To demonstrate that the sucrose produced through photosynthesis is the decisive signal transmitter, Kircher and Schopfer placed the plants in a room with light but with no carbon dioxide (CO2) in the air, thus making photosynthesis impossible. The outcome was that no more lateral roots were formed. This result was confirmed by another experiment in which the two biologists treated either the leaves or the roots in the dark with a solution of sucrose. In both approaches, lateral roots developed the same as in control plants which were exposed to light. “These results show that the production of sucrose in leaves is necessary for the formation of lateral roots. And it confirms the hypothesis that sucrose acts as a signal transmitter for light stimuli,” says Kircher.
Activation of auxin biosynthesis by sucrose signal
In earlier studies, researchers had already shown that the auxin produced in the roots from the amino acid tryptophan drives the rate of development of new lateral roots. Kircher and Schopfer have now shown how sucrose triggers this process. To do this, they placed the plants in a dark room for two days and carried out various experiments to discover their influence on the formation of lateral roots. Administering tryptophan to the roots at the same time as treating the leaves with sucrose had the greatest effect. By contrast, tryptophan had little effect if it was applied to the leaves or without sucrose at the roots. “These observations confirm that the sucrose produced through photosynthesis serves as a trigger for the synthesis of auxin,” says Kircher.
Summary:
- Original publication: Kircher, Stefan, Schopfer, Peter: Photosynthetic sucrose drives the lateral root clock in Arabidopsis seedlings. In: Current Biology 33, June 2023 https://doi.org/10.1016/j.cub.2023.04.061
- Stefan Kircher and Peter Schopfer research at the Department of Molecular Plant Physiology in the University of Freiburg’s Faculty of Biology.
- The project was funded by the German Research Foundation (DFG project no. 261041208).
Contact:
Office of University and Science Communications
University of Freiburg
Tel.: 0761/203-4302
email: kommunikation@zv.uni-freiburg.de
JOURNAL
Current Biology
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