The goal of the project is to study in an interdisciplinary approach involving physicists and biologists the plant response to gravity. A first proposed approach concerns macroscopic experiments at the plant scale: the goal is to obtain precise and systematic measurement of the response of plants to different levels of gravtiy and different inclination. The second part of the project concern the microscopic scale. The goal is to observe under a microscope in a horizontal position the dynamics of the statoliths, which are organelles rich in starches. They are considered as the gravity sensors, their sedimentation in the cells giving the direction of gravity. The final stage of the project concerns an attempt to integrate the results of macro and micro experiences in a theoretical model that would predict the response of a plant to gravity changes.
Macroscopic scale experiments : An experimental setup was made in Clermont Ferrand in order to study in different species. The response of plants to a continuous tilt at different gravity levels g were obtained for wheat coleoptile, lentil, sunflower, Arabidopsis, covering the phylogenetic tree. These measures show an independence of the response with g, enabling us to say that the gravity sensor is a tilt sensor and not a pressure sensor.
Microscopic scale experiments : Experiments were carried out in Marseille to observe the dynamics of statoliths on wheat coleoptile . Initial results are encouraging and have shown that the statoliths do not behave as a simple granular medium but are agitated probably by the activity of the cytoskeleton. These observations allowed to estimate the time of avalanches at the sudden tilting of the cells.
- experiment imposing transient inclinaison to link the macorscopic response to the avalanches dynamics of the statoliths.
- Finalization of experiences under clinostat. The use of clinostat raises many questions and the influence of the slow and continuous rotation is ultimately unclear and poorly documented in the literature. We plan in the near future clinostat experiments at the cell scale to observe the dynamics of statoliths.
1. A Unified Model of Shoot Tropism in Plants: Photo-, Gravi-and Propio-ception. PLOS Comp. Bio. (2015) 1004037.
2. D. Lopez, K. Tocquard, J.-S. Venisse, V. Legué et P. Roeckel-Drevet, 2014. Gravity sensing, a largely misunderstood trigger of plant orientated growth. Frontiers in Plant Science, 5, 610. doi: 10.3389/fpls.2014.00610.
Gravity perception by plants plays a key role in their development and adaptation to environmental change (gravitropism), from the direction of seed germination to the control of the final posture. A crucial step in this gravisensing occurs in specific cells, the statocytes, which contain small grains of starch, the statoliths. The grains being denser than the surrounding intracellular fluid, they sediment, and give the direction of gravity. Despite many studies on the subject, the mechanisms at work in statocytes and the link with the active bending of the plant at the macroscopic scale are still poorly understood. Our project at the frontier of biology, mechanics and physics aims at studying the gravisensing response at the two scales, the plant scale and the cell scale, trying to understand the mechanisms involved in the remarkable sensitivity of plants to gravity.
The first part of the project focuses on the macroscopic level and aims at precisely and systematically characterizing the response of shoots to changes in gravity direction and/or intensity. The experiments are based on an original setup called “gravitron” combining a rotation table with a plant cultivation module and a vital imaging of growth kinematics, so to obtain a quantitative phenotyping on a large number of plants. The “gravitron” has been developed thanks to a CNRS project "Interface physique, biologie et chimie: soutien à la prise de risque 2010-2012”. Thanks to the centrifugal force, the response function of the plants over a large range of inclination angles/and gravity intensity (“g levels”) is obtained and the gravisensitivity is estimated. Using this device, striking preliminary results have been obtained showing that the response of the plants to steady g levels is independent of the gravity intensity , depending only on the inclination. This preliminary observation of a gravi-sensing independent of gravity has to be confirmed on different species, different mutant and need to be investigated in more details by studying the response to transient stimuli and the influence of drugs affecting the cytoskeleton.
The second step of the project focuses on the cellular level , specifically on the motion of the statoliths when the cell is inclined. This motion is far from being a simple sedimentation and involves cytoskeletal activity. We will quantify the motion statistics and analyzed these results in connection with the recent development of the physics of granular suspensions and active gels. The strength of our project relies on the possibility of observing the motion of the statoliths on the same species and using the same stimuli or treatments as the one studied at the macroscopic scale on the gravitron. Our recent observation that the gravity intensity does not play a role in the macroscopic response suggests that the position of the statoliths may be the relevant parameter, rather than the pressure they exert on the membrane, or their motion as assumed in some studies. Our aim is to test this hypothesis by carrying out the study at the cell level.
The last and ultimate step is to link the microscopic motion of the statoliths to the macroscopic bending response of the plants in theoretical models. Simple approaches will be first developed based on the description of the sedimentation of the statoliths coupled with models of active bending of beams. More integrative model taking into account the whole plant structure and the mechanics of growing tissues will be developed in a second step.
The major originality of our project lies in the interdisciplinary approach proposed, largely inspired by our physics and mechanics background. Indeed, the use and the test of dimensionless numbers to characterize the gravitropic response, the analysis of statoliths motions in terms of sedimentation in an active gel, and the development of integrative theoretical models are approaches that have been never developed in the domain.
Monsieur Olivier Pouliquen (Centre National de la Recherche Scientifique Délégation Provence et Corse _ laboratoire IUSTI UMR7343)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
umr 547 Piaf UMR 547 Physique et Physiologie Intégrée des Arbres fruitiers et Forestiiers
CNRS DR 12 _ IUSTI Centre National de la Recherche Scientifique Délégation Provence et Corse _ laboratoire IUSTI UMR7343
Help of the ANR 369,234 euros
Beginning and duration of the scientific project: January 2014 - 48 Months