Plants gravisensing: a granular flow problem?

Gravity perception by plants plays a key role in their development and adaptation to environmental change (gravitropism), from the moment a shoot grows upward after germination to the control of the final posture (Moulia & Fournier 2009). A crucial step in this gravisensing occurs in specific cells, the statocytes, which contain small grains of starch called the statoliths (Morita 2010). The grains being denser than the surrounding intracellular fluid, they sediment, and give the direction of gravity. When the plant is inclined, a redistribution of auxin transporters (the plant growth hormone) is triggerred and the resulting auxin gradients causes an asymmetric growth and a global bending of the organ. However, more than a century after the pioneering work of Darwin and Sachs, little is known about the sensing mechanism in statocytes and the link between the dynamics of the statoliths and the macroscopic gravitropic response.  In this project we address this issue by performing experiments both at the macroscopic plant scale (study of the gravitropic response for a wide range of inclination and gravity intensity) and at the cellular level (flowing behavior of the statolith in the statocytes). We also design biomimetic systems (PDMS microcells filled with colloidal or non-Brownian particles) to separate physical mechanisms (granular behavior, Brownian motion) from biological ones (e.g. cytoskeleton activity).