Gravity Perception in Plants: From cell sensing to the biomechanical response – GRAP2

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.