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.
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.