Revealing the time scale hierarchy that controls the tropic response of shoots to gravi-stimulations

Gravity is a major abiotic cue for plant growth. Yet, little is known on the response of plants to various pattern of gravistimulation, with apparent contradiction between dose-like responses observed under transient stimuli in microgravity environments and response under steady inclination observed on earth. Of particular importance is how the gravitropic response of the organ is affected by the temporal dynamics of downstream processes in the signalling pathway, like statoliths motion in statocytes or auxin transporter redistribution. Here, we use a combination of plant scale experiments and live cell imaging techniques on centrifuge devices, to investigate on the same organ both the kinematics of shoots bending induced by transient inclinations, and the motion of the statoliths in response to cell inclination. Unlike previous observations in microgravity, the response of shoots to transient inclinations appears independent of the gravity level, with a time scale much longer than the statolith sedimentation time. This reveals the existence of a new memory process in the gravitropic signalling pathway, independent of statoliths dynamics. By combining this memory process with statoliths motion, a mathematical model is built that unifies the different laws found in literature and predicts the early bending response of shoots to arbitrary gravistimulations