On the role of gravity in shoot gravisensing

A plant accidentally put in a horizontal position rapidly bends and deforms to recover a vertical position. The ability of plants to feel gravity thus plays a key role in their development and adaptation to environmental changes (gravitropism and posture control). A crucial step in this gravisensing occurs in specific cells, the statocytes, which contain dense organites filled with starch granules (amyloplasts). The amyloplasts being denser than the surrounding intracellular fluid, they sediment at the bottom of the cell and are supposed to indicate the direction of gravity with respect to the cells (Morita 2010). However the mechanisms at work in statocytes and the link with the active bending of the plant at the macroscopic scale still need a better understanding (Moulia and Fournier 2009, Blancaflor 2015). In this study, we use an experimental approach to study gravitropic motions at the plant scale, and more specifically to investigate quantitatively the plant sensitivity to gravitropic stimuli and identify the sensed variable (e.g mechanical pressure by amyloplats, velocity or position of the amyloplasts in the cell ….). An original experimental setup called ``gravitron'' has been developed to investigate the response of plant shoots to changes in both gravity intensity and direction. The system is based on an instrumented rotating table allowing full kinematical tracking of the tropic mouvement. These records were then interpreted in term of gravisensitivity using the relevant dimensionless quantity introduced by recent quantitative studies on gravitropic control (Bastien et al. 2013, 2014