Contrary to what happens in humans, plants use gravitational perception to distinguish their angle of inclination while not being destabilised by weight and acceleration forces. In a study that has just been published in Scientific Reports, researchers from INRA, CNRS and the Université Blaise Pascal demonstrated that while plants sway considerably in the wind, they do not experience such movement as a loss in vertical positioning. This research opens the door to promising applications in agricultural sciences and biomimetics, such as for the development of miniaturised position sensors.
Like us, most plants stay in an upwards direction. It has long been known that humans control their posture using gravity perception through our inner ears and especially the otolithic organs, a set of small ‘stones’ floating in viscous fluid in with mechanoreceptor hair cells.
In statocytes, starch-filled organelles called statoliths (in dark purple) settle under the influence of gravity as a sensing mechanism perceives their position. This movement informs the cells about their inclination with respect to gravity. © INRA, Nicole Brunel (PIAF, équipe MECA) Plants also have a system to perceive their vertical positioning, but it is very tiny. A pile of small starch-filled organelles called statoliths settle in specialised cells called statocytes found all along a plant’s stem (see illustration opposite). However, one difference caught the researchers’ attention: if humans are shaken or subjected to centrifugal force, we lose our sense of vertical positioning. We have known since Einstein that a local observer (an organism, a cell) can distinguish inertial gravitational acceleration, such as that caused by shaking or centrifugal force. Our inner ear is very sensitive to force intensity.
However, plants are often moved by the wind without becoming confused about which way is up. How are they able to prevent their ‘heads spinning’, like us’ This was the question asked by a group of mechanobiologists from INRA and the Université Blaise Pascal and physicists from CNRS. The researchers created a growth chamber within a centrifugal device on two rotational axes, similar to those used to train astronauts, and closely followed the movements of plants as they straightened their positions (see figure below). By analysing several hundred plants belonging to four species representative of major flowering crops (wheat, lentils, sunflowers and Arabidopsis), they showed that contrary to our inner ears, plants are able to perceive their angle of inclination without being affected by the intensity of gravitational or inertial forces to which they are subjected. This means that plants can be blown around in strong wind without confusing this movement with a sustainable loss in vertical positioning. The experimental setup was combined with a microscope to follow the movements of the statoliths within cells in real time and unravel the cellular and molecular phenomena that allow plants to carry out this perceptive feat.
Over the past few years, the researchers have shown that like us, plants are able to control their positions by combining a sense of vertical placement and the configuration of their ‘bodies’ using proprioception1. We now know that they can tell up from down even when they’ve been disturbed. These remarkable results open the door to two types of applications. In the field of agricultural sciences, it may be possible to improve plants’ ability to straighten up after a strong storm, a problem responsible for nearly 10% of grain yield losses worldwide. The second application is in biomimetics: by drawing inspiration from plants’ statocyte cells, better and smaller position sensors can be designed.
Figure of the experimental setup used in the study: Plants are placed in a growth chamber (60x16x3) mounted on a centrifuge with two rotational axes on which a camera is installed to film the movements of the plants as they straighten up. The rotation of the centrifuge around the vertical axis allows for various levels of acceleration and centrifugal force, while the slow horizontal rotation of the plants compensates for the perception of gravity by the statocytes (clinostat effect). The inclination of each plant can be adjusted. © INRA, H Chauvet ,O Pouliquen, Y Forterre, V Legué,
1. Les végétaux ont le sens de la rectitude ! : presse.inra.fr/Ressources/Communiques-de-presse/rectitude_arbres - Renaud Bastien, Tomas Bohr, Bruno Moulia and Stéphane Douady, A unifying model of shoot gravitropism reveals proprioception as a central feature of posture control in plants - PNAS, 11 December 2012.
Hugo Chauvet, Olivier Pouliquen, Yoël Forterre, Valérie Legué & Bruno Moulia. Inclination not force is sensed by plants during shoot gravitropism. Scientific Reports, 14 October 2016. 10.1038/srep35431 - hal-01381357