Search

International audience; Flax (Linum usitatissimum) lodging is an issue of great interest for industrial producers due to its economic impact; despite a strong varietal selection over around one century, this plant remains sensitive to lodging which represents a main technico-economic issue. To better understand lodging effects at the cell wall and stem scale, the cell wall composition dynamics during cell wall development and after a 90 degrees tilt bending stress is reported. Deep-UltraViolet (DUV) fluorescence emission dynamics recorded at the Synchrotron SOLEIL-DISCO beamline by multichannel autofluorescence imaging is addressed for five cellular wall types of flax stems after an artificially induced gravitropic reaction. The quantitative fluorescent profile intensities were computed after image analysis, and compared to the control flax stems, we reported a systematically higher average intensity fluorescence (probability >95%) for the 90 degrees tilted plants. Moreover, the average stem fluorescence intensities were significantly different among the 3 developmental stages, with the youngest stage (VS) exhibiting on average 30% and 20% less fluorescence than the medium (FG) and mature (M) stages, respectively. The flax stem response to tilt impacted the xylem cellular type, while the bast fibres were arguably less affected by the protein, and hydroxycinnamate contents. A complementary investigation was carried out on bast fibres by infrared microspectroscopy to explore the polysaccharide components not detected in DUV fluorescence, and significant modifications were monitored.

Quantitative measurements of plant gravitropic response are challenging. Differences in growth rates between species and environmental conditions make it difficult to compare the intrinsic gravitropic responses of different plants. In addition, the bending movement associated with gravitropism is competing with the tendency of plants to grow straight, through a mechanism called proprioception (ability to sense its own shape). Disentangling these two tendencies is not trivial. Here, we use a combination of modeling, experiment and image analysis to estimate the intrinsic gravitropic and proprioceptive sensitivities of stems, using Arabidopsis as an example.

Quantitative measurements of plant gravitropic response are challenging. Differences in growth rates between species and environmental conditions make it difficult to compare the intrinsic gravitropic responses of different plants. In addition, the bending movement associated with gravitropism is competing with the tendency of plants to grow straight, through a mechanism called proprioception (ability to sense its own shape). Disentangling these two tendencies is not trivial. Here, we use a combination of modeling, experiment and image analysis to estimate the intrinsic gravitropic and proprioceptive sensitivities of stems, using Arabidopsis as an example.

Flax lodging is an issue of great interest for producers due to its economic impact. To better understand its effects at the cell wall and stem scale, new knowledge regarding the cell wall composition dynamics during cell wall development and after a 90° tilt bending stress is reported. Deep-Ultra Violet fluorescence emission (DUV) dynamics recorded at the Synchrotron SOLEIL-DISCO beamline by multichannel autofluorescence imaging is reported for five cellular wall types of flax stems after an artificially induced gravitropic reaction. Three flax growth development stages, namely, the vegetative stage (VS), the fast growth (FG) and the mature stage (MS), were selected in normal plants, referred to as the control plants, or in gravitropic-induced response plants, referred to as 90° tilted plants.

International audience; This work presents a dynamic view of the enzymatic degradation of maize cell walls, and sheds new light on the recalcitrance of hot water pretreated maize stem internodes. Infra-red microspectrometry, mass spectrometry, fluorescence recovery after photobleaching and fluorescence imaging were combined to investigate enzymatic hydrolysis at the cell scale. Depending on their polymer composition and organisation, cell types exhibits different extent and rate of enzymatic degradation. Enzymes act sequentially from the cell walls rich in accessible cellulose to the most recalcitrant cells. This phenomenon can be linked to the heterogeneous distribution of enzymes in the liquid medium and the adsorption/desorption mechanisms that differ with the type of cell.

The colonization of the atmosphere by land plants was a major evolutionary step. The mechanisms that allow for vertical growth through air and the establishment and control of a stable erect habit are just starting to be understood. A key mechanism was found to be continuous posture control to counterbalance the mechanical and developmental challenges of maintaining a growing upright structure. An interdisciplinary systems biology approach was invaluable in understanding the underlying principles and in designing pertinent experiments. Since this discovery previously held views of gravitropic perception had to be reexamined and this has led to the description of proprioception in plants. In this review, we take a purposefully pedagogical approach to present the dynamics involved from the cellular to whole-plant level. We show how the textbook model of how plants sense gravitational force has been replaced by a model of position sensing, a clinometer mechanism that involves both passive avalanches and active motion of statoliths, granular starch-filled plastids, in statocytes. Moreover, there is a transmission of information between statocytes and other specialized cells that sense the degree of organ curvature and reset asymmetric growth to straighten and realign the structure. We give an overview of how plants have used the interplay of active posture control and elastic sagging to generate a whole range of spatial displays during their life cycles. Finally, a position-integrating mechanism has been discovered that prevents directional plant growth from being disrupted by wind-induced oscillations.

Experiments on shoot gravitropism at the plant and cell scale reveal the existence of a memory process and provide a unifying framework for predicting the bending response of shoots to arbitrary gravi-stimulations., 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.

The Bayanbulak Grassland, Tianshan, P. R. China, is located in an intramontane sedimentary basin where meandering and braided gravel-bed rivers coexist under the same climatic and geological settings. We report and compare measurements of the discharge, width, depth, slope and grain size of individual threads from these braided and meandering rivers. Both types of threads share statistically indistinguishable regime relations. Their depths and slopes compare well with the threshold theory, but they are wider than predicted by this theory. These findings are reminiscent of previous observations from similar gravel-bed rivers. Using the scaling laws of the threshold theory, we detrend our data with respect to discharge to produce a homogeneous statistical ensemble of width, depth and slope measurements. The statistical distributions of these dimensionless quantities are similar for braided and meandering threads. This suggests that a braided river is a collection of intertwined threads, which individually resemble those of meandering rivers. Given the environmental conditions in Bayanbulak, we furthermore hypothesize that bedload transport causes the threads to be wider than predicted by the threshold theory.

During root progression in soil, root cap cells are the first to encounter obstacles. The root cap is known to sense environmental cues, making it a relevant candidate for a mechanosensing site. An original two-layer medium was developed in order to study root responses to growth medium strength and the importance of the root cap in the establishment of these responses. Root growth and trajectory of primary roots of Arabidopsis thaliana seedlings were investigated using in vivo image analysis. After contact with the harder layer, the root either penetrated it or underwent rapid curvature, enabling reorientation of the root primary growth. The role of the root cap in tip reorientation was investigated by analyzing the responses of Arabidopsis mutant roots with altered caps. The primary root of fez-2 mutant lines, which has fewer root cap cell layers than wild-type roots, showed impaired penetration ability. Conversely, smb-3 roots of mutant lines, which display a higher number of root cap cells, showed enhanced penetration abilities. This work highlights that alterations in root cap shape and size affect the root responses to medium strength.HighlightThe analysis of the growth and orientation of Arabidopsis thaliana mutant roots affected in root cap size and shape showed that properly formed root cap is required to trigger the root responses to medium strength.AbbreviationsCOLcolumella;LRCLateral Root Cap;SISharpness Index;SMBSOMBRERO.

Gravity is a permanent environmental signal guiding plant growth and development. Gravity sensing in plants starts with the displacement of starch-filled plastids called statoliths, ultimately leading to auxin redistribution and organ curvature. While the involvement in gravity sensing of several actors such as calcium is known, the effect of statolith displacement on calcium changes remains enigmatic. Microgravity is a unique environmental condition offering the opportunity to decipher this link. In this study, roots of Brassica napus were grown aboard the International Space Station (ISS) either in microgravity or in a centrifuge simulating Earth gravity. The impact of short simulated gravity onset and removal was measured on statolith positioning and intracellular free calcium was assessed using pyroantimonate precipitates as cytosolic calcium markers. Our findings show that a ten-minute onset or removal of gravity induces very low statolith displacement, but which is, nevertheless, associated with an increase of the number of pyroantimonate precipitates. These results highlight that a change in the cytosolic calcium distribution is triggered in absence of a significant statolith displacement.

Perception d'un stimulus gravitropique transitoire chez les plantes. GDR PhyP - Biophysique et biomécanique des plantes

Gravity perception plays a key role in how plants develop and adapt to environmental changes. However, more than a century after the pioneering work of Darwin, little is known on the sensing mechanism. Using a centrifugal device combined with growth kinematics imaging, we show that shoot gravitropic responses to steady levels of gravity in four representative angiosperm species is independent of gravity intensity. All gravitropic responses tested are dependent only on the angle of inclination from the direction of gravity. We thus demonstrate that shoot gravitropism is stimulated by sensing inclination not gravitational force or acceleration as previously believed. This contrasts with the otolith system in the internal ear of vertebrates and explains the robustness of the control of growth direction by plants despite perturbations like wind shaking. Our results will help retarget the search for the molecular mechanism linking shifting statoliths to signal transduction.

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)

The Bayanbulak Grassland, Tianshan, China is located in an intramountane sedimentary basin where meandering and braided gravel-bed streams coexist under the same climatic and geological settings. We report on measurements of their discharge, width, depth, slope and grain size. Based on this data set, we compare the morphology of individual threads from braided and meandering streams. Both types of threads share statistically indistinguishable regime relations. Their depths and slopes compare well with the threshold theory, but they are wider than predicted by this theory. These findings are reminiscent of previous observations from similar gravel-bed streams. Using the scaling laws of the threshold theory, we detrend our data with respect to discharge to produce a homogeneous statistical ensemble of width, depth and slope measurements. The statistical distributions of these dimensionless quantities are similar for braided and meandering streams. This suggests that a braided river is a collection of intertwined channels, which individually resemble isolated streams. Given the environmental conditions in Bayanbulak, we furthermore hypothesize that bedload transport causes the channels to be wider than predicted by the threshold theory.

We report on a new set of measurements on the morphology of braided and meandering threads of the Kosi megafan, North Bihar, India. All threads develop on a uniform sandy sediment and under a similar climate. The data set is composed of the width, depth, water discharge and grain size of 51 threads. Downstream slopes and sinuosity are also available. Using this data set, we show that braided and meandering threads share common hydraulic geometries. We then use the threshold theory to explain why the aspect ratio of threads is almost naturally detrended, and rescale the data according to this theory. As expected, the rescaled dimensions of braided and meandering threads are weakly correlated to water discharge. We propose that the large dispersion observed, which is common to meandering and braided threads, is the signature of sediment transport, vegetation or cohesion effects.

International audience; Secondary flow cells are commonly observed in straight laboratory channels, where they are often associated with duct corners. Here, we present velocity measurements acquired with an acoustic Doppler current profiler in a straight reach of the Seine river (France). We show that a remarkably regular series of stationary flow cells spans across the entire channel. They are arranged in pairs of counter-rotating vortices aligned with the primary flow. Their existence away from the river banks contradicts the usual interpretation of these secondary flow structures, which invokes the influence of boundaries. Based on these measurements, we use a depth-averaged model to evaluate the momentum transfer by these structures, and find that it is comparable with the classical turbulent transfer.