Abstract
Gravity is a major abiotic cue for plant growth. However, little is known about the responses of plants to various patterns of gravi-stimulation, with apparent contradictions being observed between the dose-like responses recorded under transient stimuli in microgravity environments and the responses under steady-state inclinations recorded on earth. Of particular importance is how the gravitropic response of an organ is affected by the temporal dynamics of downstream processes in the signalling pathway, such as statolith motion in statocytes or the redistribution of auxin transporters. Here, we used a combination of experiments on the whole-plant scale and live-cell imaging techniques on wheat coleoptiles in centrifuge devices to investigate both the kinematics of shoot-bending induced by transient inclination, and the motion of the statoliths in response to cell inclination. Unlike previous observations in microgravity, the response of shoots to transient inclinations appears to be independent of the level of gravity, with a response time much longer than the duration of statolith sedimentation. This reveals the existence of a memory process in the gravitropic signalling pathway, independent of statolith dynamics. By combining this memory process with statolith motion, a mathematical model is built that unifies the different laws found in the literature and that predicts the early bending response of shoots to arbitrary gravi-stimulations.
Highlights
Plants have developed the ability to sense their inclination relative to the local vertical orientation as defined by the vector of gravitational acceleration g, and to adjust their shape a process called gravitropism.The main gravity sensor of plants is located in specialized cells called statocytes, in which starch-filled plastids are found (Moulia and Fournier, 2009; Morita, 2010)
Statoliths are denser than the surrounding cellular fluid and sediment at the bottom of the cell.When the plant is inclined, statoliths flow along the direction of gravity and trigger signalling processes, which induce the bending of the organ.A widely accepted step in this process is the creation of an auxin asymmetry between the upper and the lower part of the organ, which mediates differential growth and results in the bending of the plant.This asymmetric distribution, which is observed for shoots and roots (Harrison and Masson, 2008; Rakusová et al, 2011; Brunoud et al, 2012; Band et al, 2012), seems to be driven by the redistribution of auxin transporters (PIN proteins) along the statocyte membranes (Friml et al, 2002; Rakusová et al, 2011)
The first involves permanently inclining the base of the plant at different angles relative to gravity.The response, defined as the bending velocity, is found to vary with the sine of the initial inclination of the plant shoot, θinit, (for reviews see Sachs, 1887; Larsen, 1969; Iino et al, 1996; (Moulia and Fournier, 2009; Pouliquen et al, 2017).This relationship, known as the ‘sine law’, is at the origin of the ‘starchstatolith weight model’, which proposes that the sensor acts as a force sensor that is sensitive to the pressure exerted by weight of the statoliths on the lateral membrane of the statocyte (Perbal and Perbal, 1976; Galland, 2002; Leitz et al, 2009)
Summary
Plants have developed the ability to sense their inclination relative to the local vertical orientation as defined by the vector of gravitational acceleration g, and to adjust their shape a process called gravitropism.The main gravity sensor of plants is located in specialized cells called statocytes, in which starch-filled plastids (statoliths) are found (Moulia and Fournier, 2009; Morita, 2010). In a recent study, Chauvet et al (2016) tested this hypothesis using centrifuge experiments to disentangle the effect of the direction (i.e. the angle between the initial inclination of the plant shoot and the gravity vector) and the intensity of the gravity vector on the plant shoot response.They showed that the plant response is proportional to sin(θinit) and not to geff × sin(θinit). This insensitivity to the effective gravity, geff, reported for land angiosperms has been observed for characean green algae (Limbach, 2005). The gravity sensor in plants works as a clinometer and not as a force sensor, thereby falsifying the ‘statolith pressure hypothesis’ and the alternative ‘protoplast pressure hypothesis’ (for a more complete argument see Pouliquen et al, 2017).These observations support an alternative ‘position sensor hypothesis’, in which the relevant parameter triggering the signal pathway is the position of the statoliths within the statocytes (Strohm et al, 2014; Pouliquen et al, 2017)
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