In the INRA greenhouse, sheltered from the icy cold outside, biologist Bruno Mulia points out two groups of young trees in pots. Samples of the first were bent three times a day for five months for a few seconds; members of the second group were not injured. In the end, everything is vertical. Except that the first ones have a thick, oval-shaped stem in the area that has undergone curvature. The latter have a round stem, thinner and more flexible. This experiment, carried out in Clermont-Ferrand (Puy-de-Dôme), is a very simple introduction to a subject of research that is not taken for granted: plant biomechanics.
“When faced with limitations, they resort to sophisticated responses, either to compensate for them or to better counter future bad consequences. an experience”, explains the researcher. The specialty of the Piaf division (Integrative Physics and Physiology of Trees in a Variable Environment), to which it belongs, is to investigate this response – trees in particular – to physical and environmental aggressions: wind, heat, light, drought… “We are studying vulnerability to various stresses, how plants change their physiology to adapt to them, the consequences for their growth … in vitro thislive, from the genome to the whole plant and throughout the annual cycle”, says Bruno Mulia. First of all, it is desirable to completely forget the received idea. Just because plants seek light doesn’t mean they grow vertically. But because of gravity. At INRA, an experiment demonstrates this “gravitropism”: a plant placed in the center of a sphere, on which light of the same intensity falls from all directions, grows perfectly straight! Still, in nature it happens that a strong wind strongly and for a long time tilts a plant or tree. How does he manage to recover? He is simply able to perceive his orientation in relation to the direction of gravity! And this is due to a special type of cells, statocytes, present in numerous copies throughout the stem or trunk.
Such a small hourglass filled with grains of starch acts like a mini-gyroscope. Depending on the inclination of the plant, grains are deposited in one place or another in the cell (see infographic). Thus, the plant accurately perceives the degree of its inclination. The mechanism, as Piaf’s team experimentally showed in 2016, is completely original: it is not the weight of the accumulated grains that comes into play, but the contacts between them and the cell wall; the more contacts, the greater the perceived attraction.
Math demo
However, by bending the plants and observing the kinematics of their straightening, the Clermont researchers realized that the statocyte was not acting alone. They first confirmed it with a… mathematical demonstration! After modeling how the plant controls its movements, they entrusted their equations to Thomas Bohr, a physicist at the Technical University of Denmark. After several months of work, he got a startling result: a plant cannot straighten up if it only has data on its tilt. Piaf’s researchers concluded from this that he must mobilize another ability hitherto unknown in the plant kingdom: proprioception.
In the INRA greenhouse, sheltered from the icy cold outside, biologist Bruno Mulia points out two groups of young trees in pots. Samples of the first were bent three times a day for five months for a few seconds; members of the second group were not injured. In the end, everything is vertical. Except that the first ones have a thick, oval-shaped stem in the area that has undergone curvature. The latter have a round, thinner and more flexible stem. This experiment, carried out in Clermont-Ferrand (Puy-de-Dôme), is a very simple introduction to a subject of research that is not taken for granted: plant biomechanics.
“When faced with limitations, they resort to sophisticated responses, either to compensate for them or to better counter future bad consequences. an experience”, explains the researcher. The specialty of the Piaf division (Integrative Physics and Physiology of Trees in a Variable Environment), to which it belongs, is to investigate this response – trees in particular – to physical and environmental aggressions: wind, heat, light, drought… “We are studying vulnerability to various stresses, how plants change their physiology to adapt to them, the consequences for their growth … in vitro thislive, from the genome to the whole plant and throughout the annual cycle”, says Bruno Mulia. First of all, it is desirable to completely forget the received idea. Just because plants seek light doesn’t mean they grow vertically. But because of gravity. At INRA, an experiment demonstrates this “gravitropism”: a plant placed in the center of a sphere, on which light of the same intensity falls from all directions, grows perfectly straight! Still, in nature it happens that a strong wind strongly and for a long time tilts a plant or tree. How does he manage to recover? He is simply able to perceive his orientation in relation to the direction of gravity! And this is due to a special type of cells, statocytes, present in numerous copies throughout the stem or trunk.
Such a small hourglass filled with grains of starch acts like a mini-gyroscope. According to the slope of the plant, grains are deposited in one place or another in the cell. Thus, the plant accurately perceives the degree of its inclination. The mechanism, as Piaf’s team experimentally showed in 2016, is completely original: it is not the weight of the accumulated grains that comes into play, but the contacts between them and the cell wall; the more contacts, the greater the perceived attraction.
Math demo
However, by bending the plants and observing the kinematics of their straightening, the Clermont researchers realized that the statocyte was not acting alone. They first confirmed it with a… mathematical demonstration! After modeling how the plant controls its movements, they entrusted their equations to Thomas Bohr, a physicist at the Technical University of Denmark. After several months of work, he got a startling result: a plant cannot straighten up if it only has data on its tilt. Piaf’s researchers concluded from this that he must mobilize another ability hitherto unknown in the plant kingdom: proprioception.
In humans, this is a “sixth sense”, this is the perception of the shape and position of our body, transmitted by our muscles and tendons. A “background task” provided by the cerebellum that allows us to get up and walk. So the Clermont researchers created a model that combines gravitropism and proprioception, which predicts extension movements after flexion. “We ran experiments on eleven plants, representing the entire angiosperm lineage, and every time we found the predicted straightening. So it’s a generic model.”Bruno Mulia explains.
Since plants don’t have a brain, what is their proprioception based on? Keishi Okamoto’s team in Kyoto, Japan, found the answer. She discovered other specialized cells, “not yet named, but may be called ‘propriocytes'”Imagine Bruno Mulia. These cells, as it were, allow the plant to perceive not its slope, but its curvature.
Principle: Inside the cell is a cytoskeleton made up of filaments made up of two substances, actin and myosin, which are also present in our muscle fibers. When the plant is bent, its cells are stretched out and the cables are tighter than when it is straight. She feels this tension. Knowing its slope and curvature, the plant can carry out the process of its straightening. In green parts that are not lignified (i.e., not turned into wood), this occurs due to differentiated growth: cells on one side elongate more than on the other. On the other hand, wood does not grow in a longitudinal direction, but in a radial direction, which can be seen by observing the rings on a cut of a tree. In this case, the cellulose fibers are drawn in more or less according to the configuration measured by statocytes and “propriocytes”.
All these reorientation mechanisms are very slow: from a hundred days for a very young tree to… almost a century for a large one. And they only work when the plant is tilted by a storm. But more often than not, trees tilt under the influence of the wind for a few seconds before returning to an upright position simply because of their resilience. It is this phenomenon that is being modeled in the INRA greenhouse. So why, in this case, the trunks of the trees of the first group have a thick oval cross-section? Because plants, when faced with repeated constraints, change their physiology. With every gust of wind, real or simulated, a certain mechanism is triggered. A mechanism that could turn on “propriocytes” – although the researchers are not sure yet – and induce the production of molecules that send a signal to growing areas of the plant. “Then we observe that the diameter expands in the direction of the bend. Thus, the stem is strengthened. This is an adaptive advantage for the plant, which becomes more resistant to wind over time,” emphasizes Eric Badel, a researcher from the Piaf division. This explains why, for example, in windy regions, trees develop a wider trunk. But they are also able to reduce their height thanks to the hydraulic system (see box opposite).
Roses are folded three times a day
The results of Piaf’s team have nothing to do with laboratory curiosities. Applications in agronomy are significant! In particular, this is due to counteracting the phenomenon of “lodging”: fields of grain, tumbled down by the wind or very heavy rains, become impossible to harvest by machines. Thus, up to 20% of world production is unusable! Hence the importance of genetic selection of plants most capable of recovery. Another area of research: strengthening the resistance of some species by their moderate mechanical impacts. For example roses. In a greenhouse sheltered from the wind, these shrubs grow tall thin stems that can eventually break under the weight of the flowers. Until now, treatment has consisted of prescribing growth hormone inhibitors. The INRA centers in Angers and Clermont-Ferrand are conducting joint experiments with the new system. Principle: Wind-simulating rods cause the rods to bend three to four times a day, thereby increasing their strength. Roses just have to behave…
interrupted growth
Why are trees in windy regions often small? Clermont researchers believe they have found the answer. “Recently, we have demonstrated that a kink creates an overpressure that propagates through the plant’s conductive sap network.” explains Eric Badel, a researcher in Piaf’s department. This excess pressure reaches the ends of the plant, where it sends a signal to the cells. The plant then suppresses its growth, thereby offering less resistance to the wind.