About ten passes with a soft brush are enough. In less than a minute, inside a small plant of Arabidopsis thalianaa chain of proteins is activated. If the contact is repeated over time, this response can also modify the growth of the plant.
An international team led by Lund University has reconstructed one of the main mechanisms by which plants respond to physical contact. The study published on Nature Communications has identified a chain of proteins called MAP kinases that come into action within about 60 seconds of touch and contributes to modifying the activity of many genes.
It was already known that wind, rain, wounds or an animal touching a leaf could provoke a response in plants. For over 25 years it has also been known that contact activates some MAP kinases, proteins that transmit signals within cells. However, an important step was missing: better understanding how that first stimulus is connected to the changes that occur immediately afterwards.
Ten brush strokes and the plant begins to respond
The experiment is very simple. The researchers used seedlings of Arabidopsis thaliana 12 days old and touched them about ten times with a soft brush. For the plant, however, those brush strokes are far from irrelevant. Within about a minute, MPK3 and MPK6, two of the proteins involved in the chain discovered by the researchers, are activated. At the same time, many other proteins present in cells also begin to change.
After just a minute the researchers identified 160 points of the proteins that had already undergone a chemical modification. After three minutes they were 321 and after ten minutes 380. Considering the entire period observed, the changes affected 488 points. Two proteins, called MKK4 and MKK5, appear to play a particularly important role. When the researchers altered its functioning, a significant part of the normal response to touch was reduced.
After 22 minutes the activity of more than two thousand genes changes
The answer also continues at the gene level. Twenty-two minutes after the brush was passed, 2,032 genes had changed their activity in normal plants. Comparing these results with previous experiments, the team then identified 1,168 genes that appear to be part of the plants’ more immediate response to contact.
Many depend precisely on the MKK4 and MKK5 proteins. When these did not function normally, 905 genes reacted differently than in other plants. For 779 of these the activation was weaker. A remarkable amount of movement for something that, looking at the vase from the outside, looks like just a brushstroke. The mechanism identified, however, does not explain every response. That leaves at least 215 genes that appear to react via other pathways yet to be identified.
If the touching continues, the growth also changes
The researchers then wanted to understand whether this very rapid response had visible consequences over time. To do this, they touched some plants grown in the soil twice a day for 14 days. After two weeks, the surface area occupied by leaves had been reduced by about 38% in normal plants compared to plants that had not been touched.
In plants in which the first part of the signaling chain had been altered, the reduction stopped at around 20%. In those with MKK4 and MKK5 protein problems it was about 28%. Repeated contact also had effects on flowering. Normal plants touched every day started flowering about three days later. In some modified plants the delay seemed smaller, but the difference was not clear enough to rule out the possibility that it was due to chance.
The ability of plants to modify their development in response to physical stimuli is called thigmomorphogenesis. In nature, brush strokes are replaced by something much more common: wind, rain, animals, insects and any other contact capable of folding or damaging fabrics.
The first touch sensor is still missing
The study therefore reconstructs an important part of the mechanism, but the beginning of the story is still missing. Something has to sense the pressure on the leaf first and start the signal chain. The researchers hypothesize that proteins present on the cell membrane or other sensors capable of reacting to physical stress may be involved. What exactly this first sensor is, however, is not yet known.
The study also analyzed jasmonic acid, a plant hormone involved in responses to injury and other forms of stress. Within the first ten minutes of the touch, the plants unable to produce it showed a protein response very similar to that of normal plants. It means that most of the very first signals observed follow at least partly a different path.
Lund University, in presenting the results, also reminds us that we are still in the field of basic research. The experiments were mainly conducted on Arabidopsis and under controlled conditions. To understand whether this knowledge can also be used in agriculture, specific studies will be needed on crops and directly in the fields. For now we know a little more about what happens when we touch a leaf. We almost don’t notice it. After a minute, the plant has already set in motion hundreds of signals.
