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International audience; 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.

International audience; Lodging is a major problem for herbaceous crops and young woody plants, causing significant yield losses. With climate change, the risk of severe storms during the growing season will increase. The risk of lodging will then be even greater.Resilience is one of the strategies evolved by plants to cope with this problem. Resilience is the ability of plants to recover their structures and functions back to their reference states after disturbance. Resilience to lodging involves active movements called tropisms, i.e. a redirection of organs in the direction of or opposite to the physical stimuli. Our study focuses on gravitropism and autotropism. A fine-tuned combination of these two tropisms is required for the postural control of plants.This relies on two perceptive abilities: i) graviperception (more exactly gravi-cline perception) for the perception of orientation with respect to gravity and ii) proprioception for the perception by the plant of its own curvature. Under the control of these perceptions, the plants activate motors that carry out movements. These motors can be differential elongation in primary growing zone and tension wood formation in secondary growing zone.To understand resilience to lodging, we therefore study the graviperceptive and proprioceptive sensitivities (with a greater focus on the latter, less well known), and the motors of the movements related to these tropisms. In particular, we test the relevance of a universal model of postural control called the AC model (Moulia et al. 2021)

Participation en visioconférenceAnimation scientifique de type Journal Club. Domaine: biologie (pluridisciplinaire). Organisé par Jean-Luc Morel pour les scientifiques intéressés par la recherche spatiale (CNES).; National audience

International audience; Gravitropic movements, which are essential in controlling plant shape and function, are driven by the sensing of organ inclination and organ curvature (proprioception [1]). Although crucial for understanding the growth dynamics and the final phenotype, disentangling these two sensitivities is challenging. In our two-step method, based on [2], the plant is first tilted to the horizontal to trigger tropic bending. Then, a clinostatic rotation around the horizontal axis is set to suspend graviception, and the stem straightens under pure proprioceptive driving. We implemented this method together with time-lapse image analysis using a novel version of the Interekt software [3]. We illustrate this automated pipeline on Arabidopsis WT and proprioception-deficient XI myosin mutants.[1] Moulia et al. 2021, https://doi.org/10.1126/science.abc6868[2] Okamoto et al. 2015, https://doi.org/10.1038/nplants.2015.31[3] Hartmann, Chauvet-Thiry et al. 2022, https://doi.org/10.1007/978-1-0716-1677-2_9

Croitre dans le vent : de la mécanoperception aux réponses de croissance de l'arbre. Journee Technique EchoPaysage "Arbres et Sécurité, de la santé des citadins à la gestion des risques de rupture"

Background and Aims Trees constantly experience wind, perceive resulting mechanical cues, and modify their growth and development accordingly. Previous studies have demonstrated that multiple bending treatments trigger ovalization of the stem and the formation of flexure wood in gymnosperms, but ovalization and flexure wood have rarely been studied in angiosperms, and none of the experiments conducted so far has used multidirectional bending treatments at controlled intensities. Assuming that bending involves tensile and compressive strain, we hypothesized that different local strains may generate specific growth and wood differentiation responses.• Methods Basal parts of young poplar stems were subjected to multiple transient controlled unidirectional bending treatments, which enabled a distinction to be made between the wood formed under tensile or compressive flexural strains. Local analyses of poplar stem responses were performed at growth, anatomical, biochemical and molecular levels. Moreover local wood properties were investigated.• Key Results Treated poplar stems developed significant cross-sectional ovalization. At the tissue level, some aspects of wood differentiation were similarly modulated in the compressed and stretched zones (vessel frequency and diameter of fibres without a G-layer), whereas other anatomical traits (vessel diameter, G-layer formation, diameter of fibres with a G-layer and microfibril angle) and the expression of fasciclin-encoding genes were differentially modulated in the two zones. Wood showed differential properties too.• Conclusions We propose new terminologies to distinguish the ‘flexure wood’ produced in response to multiple bidirectional bending treatments from wood produced under transient tensile strain (tensile flexure wood; TFW) or under transient compressive strain (compressive flexure wood; CFW). By highlighting similarities and differences between tension wood and TFW and by demonstrating that plants could have the ability to discriminate positive strains from negative strains, this work provides new insight into the mechanisms of mechanosensitivity in plants.

Graviception, Proprioception…vous connaissez ? Autant de mots improbables dont on ne suppose pas forcément l’existence et la signification, sauf si on a suivi les vœux du Centre début janvier. Explications ludiques et simples en BD, grâce à un dessinateur venu en immersion à l’UMR Piaf dans l’équipe MECA en fin d’année dernière.

Trees constantly experience wind, perceive resulting mechanical cues, and modify their growth and development accordingly. Previous studies in gymnosperms have demonstrated that multiple bendings mimicking the effect of wind trigger ovalization of the stem and the formation of a special type of reaction wood called flexure wood. Very few studies on ovalization and flexure wood relate to angiosperm trees, and all the experiments conducted so far have used multidirectional bendings of uncontrolled intensities. Assuming bending is composed of tensile and compressive strain, we hypothesized that different local strains may generate specific growth and wood differentiation responses. To assess this hypothesis, multiple quantified flexural strains were applied to young Populus stems. This was achieved using unidirectional bendings; thereby a given cell is experiencing always strains of the same sign (longitudinal compression or longitudinal tension only), allowing to distinguish the wood formed under tensile and compressive flexural strains. The different effects of bendings were characterized quantitatively studying radial growth, cell size and cell wall ultrastructure. Moreover, to gain a first molecular insight into how multiple bendings can modulate wood anatomical traits, we used a Q-PCR approach to investigate the expression of 4 target mechanosensitive genes known for their role in wood differentiation. In response to multiple unidirectional bendings, poplar stems developed a significant ovalization of their crosssection. At tissue level, some aspects of wood differentiation were similarly modulated by compressive and tensile strains (vessel frequency, diameter of fibres without G-layer), whereas other anatomical traits (vessel diameter, G-layer formation, diameter of fibres with G-layer, microfibril angle) and expression of fasciclin-encoding genes were differentially modulated by compressive and tensile strains. This work leads to the proposition of new terminologies to distinguish the “flexure wood” produced in response to multiple bidirectional bendings from wood produced under transient tensile strain (TSW) or under transient compressive strain (CSW). By highlighting similarities and differences between tension wood and TSW and by demonstrating that plants are able to discriminate positive strains from negative strains, this work brings new insights on the mechanisms of mechanosensitivity in plants.

Les arbres sont des organismes sensibles aux modifications de leur environnement mécanique. Ils sont d’ailleurs capables d’ajuster leur physiologie pour s’y acclimater. Mais tandis que les réponses des arbres à la gravité sont de mieux en mieux connues, peu d’études se sont intéressées à l’effet des flexions répétées dues au vent. Des résultats récents ont montré la capacité de différentes espèces feuillues à percevoir ces sollicitations mécaniques quotidiennes, puis à ajuster leur croissance en conséquence. En revanche, notre connaissance de leurs effets sur la production du bois tant en termes de quantité que de qualité reste naissante. Pourtant, les changements de régime de vents tels que prédits par les scenarii de changements climatiques (moins de vents chroniques et des évènements extrêmes de type tempête plus fréquents) sont soupçonnés de pouvoir mettre à mal la capacité d’acclimatation et par là même la stabilité mécanique des arbres face aux tempêtes. Il semble alors nécessaire d'acquérir une meilleure compréhension de l’effet des sollicitations mécaniques sur l'activité du tissu à l’origine du bois - le cambium - et sur la différenciation du bois.

Les arbres acclimatent en permanence leur développement aux conditions environnementales qu’ils subissent. Les tempêtes répétées de ces dernières années nous ont montré à quel point l’acclimatation aux stress mécaniques dus au vent était un processus biologique vital qui permet aux arbres de se maintenir pendant des dizaines d’années. Dans le cadre des changements climatiques, les régimes de vent vont être modifiés ; avec moins de vents chroniques et des évènements type tempête de plus en plus fréquents, posant ainsi la question cruciale de l’acclimatation mécanique des structures. Il est donc nécessaire de comprendre l’ensemble de ces mécanismes biologiques en vue de prévoir leur comportement devant de nouvelles conditions climatiques et d’aider les forestiers et sélectionneurs dans leurs choix forestiers. Le stress mécanique du au vent se traduit essentiellement par des flexions des organes, en particulier la tige. Nous avons donc mené une série d’expérimentations en serre où des flexions contrôlées, unidirectionnelles et répétées ont été conduites pendant plusieurs mois sur de jeunes peupliers. Le suivi de la croissance secondaire locale a permis de mettre en évidence une ovalisation importante des sections de tige que nous avons mise en relation avec le champ de déformations longitudinales imposé par la flexion macroscopique à travers un modèle de stimulation locale de l’activité cambiale. Les expressions moléculaires de gènes marqueurs de la mécanoperception ou liés à la croissance, ont aussi pu être mesurées localement. Nous avons donc mis en évidence une chaine de mécanismes allant de la flexion globale de la tige à la mise en action de mécanismes moléculaires et l’accroissement radial des tiges. Enfin, ces résultats sont analysés sous l’angle du bénéfice adaptatif pour le comportement mécanique de la tige face aux sollicitations du vent.

Les vents violents, tels qu’observés lors de tempêtes, représentent un risque majeurpour la stabilité mécanique des arbres et peuvent avoir un effet dévastateur sur les forêts. Les recherches des dernières décennies ont montré que les sollicitations récurrentes par les vents courants constitueraient un facteur d’acclimatation à des évènements extrêmes. Or, les scenarii envisagés dans le cadre des changements climatiques globaux prévoient une augmentation de la fréquence des tempêtes en Europe occidentale, tandis que la vitesse des vents courants diminuerait. Afin d’aider les propriétaires forestiers dans leurs choix de gestion, il apparaît pertinent de mieux évaluer la capacité future des arbres à acclimater leurs structures en réponse aux flexions répétées lesquelles constituent la composante majeur du vent. Cette acclimatation pourrait impliquer des modifications quantitatives et qualitatives de la production du bois. Il apparaît alors nécessaire d'acquérir une meilleure compréhension de l’effet des sollicitations mécaniques sur l'activité du tissu à l’origine du bois - le cambium - et sur la différenciation des cellules du bois.Ici, nous présentons une analyse des conséquences morphologiques et anatomiques de flexions unidirectionnelles, répétées et d’intensité contrôlée (mimant l’effet du vent) sur de jeunes tiges de peuplier. L’une des originalités de ce travail réside dans la prise en compte de la répartition spatiale inégale des variables physiques sur la section transversale de la tige.Tandis que la croissance secondaire semble ne pas dépendre pas du type de contrainte appliqué (tension ou compression des tissus), la différenciation du bois semble être modulée différentiellement selon les contraintes mécaniques. Le bois produit sous contraintes transitoires de tension semble présenter des similitudes avec le bois de tension produit chez les feuillus sous l’effet du stimulus gravité. L’acclimatation au vent du peuplier pourrait donc impliquer la mise en place d’un bois modifié (ou bois de réaction) permettant un ajustement de ses propriétés mécaniques. L’identification d’acteurs moléculaires impliqués dans ces réponses est en cours et pourrait à plus long terme aider à l’établissement de stratégies de sélection.

Bending device. The stem is transitory –10s back and forth– pushed against a plastic tube of known diameter; the stem is thus locally bent around the tube. Therefore, the stem is subjected to a quantified curvature in terms of strains. Locally, the applied strain is the product of the curvature of the central line and the stem radius (rstem). In the case of small curvature, the curvature of the central line is given by the inverse of the sum of the stem radius and of the radius of the plastic tube (rtube). (Modified from [4]). (PDF 747 kb)

International audience; In woody species, wood provides many functions such as nutrient transport, organic compounds storage and a mechanical support. Furthermore in human society, the wood is a source of raw material (paper industry), materials (construction, decoration ...) and energy. It seems now important to have a better control over wood production and quality and to gain a better understanding of how mechanical constraints modulate secondary growth and wood differentiation. In response to environmental stress, the plants acclimate and adjust their growth accordingly. External mechanical loadings, such as wind, lead to a decrease of primary growth, an increase of secondary growth, and modifications of stem mechanical properties and biomass reallocation to roots. Recent results showed that mechanical signals generate wood differentiation adjustments leading to the production of a wood presenting similarities with tension wood: the Flexure Wood. Here we present an analysis of the morphological and anatomical consequences of repeated bending on young poplar stems with a special emphasis on the effects of the uneven spatial distribution of physical factors over the stem cross-section. Stem anatomical observations revealed that recurrent bending stimuli modify cambial activity both quantitatively and qualitatively. Wood differentiation appears to be modulated according to the sign of mechanical strain (tension or compression). Genes responsible for such responses, among which a gene encoding a CLE-type peptide, are under investigation.

National audience

Mechanical signals are important factors controlling plants growth and development. External mechanical loadings, such as wind, lead to a decrease of primary growth, an increase of secondary growth, modifications of stem mechanical properties and biomass reallocation to roots (Telewski, 2006, Badel et al., 2015). Integrative approaches combining physical/modeling/kinematics at plant and organ levels demonstrated that strain-sensing (deformation) are involved in the process of mechanoperception (Moulia et al., 2011; Coutand et al., 2009). These results allowed the development of an integrative biomechanical model called S3m which postulates that mechanoperception is proportional to local strain and volume of the considered tissue (Moulia et al., 2011). In this study, we tested this biomechanical model at tissue level by studying the relation between local strain intensity, secondary growth and genes expression.

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L'une des principales sollicitations environnementales auxquelles les arbres sont confrontés sont les flexions répétées de la tige/du tronc générées par le vent. Alors que les tempêtes représentent un risque majeur pour la stabilité mécanique des arbres, les recherches des dernières décennies ont montré que les sollicitations récurrentes par les vents courants pourraient constituer un facteur d’acclimatation à des évènements extrêmes. Les modèles de changements climatiques globaux prévoient une augmentation de la fréquence des tempêtes sévères en Europe occidentale, tandis que la vitesse des vents courants diminuerait en période de végétation. Dans ce contexte, il apparaît alors pertinent de mieux comprendre comment les arbres seront ou non capables de d’acclimater leurs structures. Des résultats récents ont montré que cela implique leur capacité à percevoir les sollicitations mécaniques auxquelles ils sont soumis quotidiennement, puis à ajuster leur croissance et la différenciation de leurs cellules, avec la production d’un bois appelé « bois de flexion ». Ce bois, présentant des ressemblances avec le bois de tension à fibres G, reste pour l’heure peu caractérisé. Pour mieux contrôler la production et la qualité du bois dans ce contexte de changements climatiques, il est nécessaire d'acquérir une meilleure compréhension de l’effet des sollicitations mécaniques sur l'activité du cambium (production) et sur la différenciation du bois. Ici, nous présentons une analyse des conséquences morphologiques et anatomiques des flexions répétées sur de jeunes tiges de peuplier avec une prise en compte inédite de la répartition spatiale inégale des variables physiques sur la section transversale de la tige. Les observations menées à l’échelle anatomique confirment que l’activité du cambium est stimulée par les flexions. La croissance secondaire est proportionnelle à l’intensité des déformations locales mais ne dépend pas du type de contrainte appliqué (tension ou compression des tissus). En revanche, la différenciation du bois semble être modulée différentiellement selon les contraintes mécaniques (tension ou compression). On ne peut alors sans doute plus parler “du” mais “des” bois de flexion. Les acteurs moléculaires impliqués dans ces réponses, parmi lesquels un gène codant pour un peptide de type CLE (Clavata), sont à l’étude.

Equipe MECA Poster et résumé dans Proceedings; Mechanical signals are important factors that control plants growth and development. External mechanical loadings, such as wind, lead to a decrease of primary growth, an increase of secondary growth, modifications of stem mechanical properties and biomass reallocation to roots. Biomechanical studies on tomato and poplar demonstrated that tissue strains are sensed by plants (Coutand and Moulia 2000 ; Coutand, Martin et al., 2009). An integrative biomechanical model (S3m) has been developed. This model supposes that mecanoperception at competent cells scale is proportional to local strain and volume of the considered tissue (Moulia et al., 2011). In nature, mechanical stimuli do not occur as a single bending. In order to understand how plants acclimate to strain induced by wind, we have studied, on young poplars, the effect of successive bending separated at day scale, mimicking the alternation between windy or quiet weather. Our results show, as soon as a second bending was applied, a diminution of the secondary growth rate. Simultaneously, molecular responses to subsequent bending were observed. Trees acclimated rapidly to mechanical loadings and a desensitization period of a few days occurred after a single transitory bending (Martin et al., 2010). Stem anatomical observations showed that, in response to bending, cambium activity was locally impacted by the strain. The secondary growth rate was identically stimulated according to the strain level both in the stretched part and in the compressed part of the stem. However, wood differentiation appeared to be modulated according to the sign of mechanical loading (tension or compression).

The mechanical environment is a significant factor that controls tree growth. Growth response to mechanical stress inhibits primary growth, stimulates radial growth and produces acclimated wood. That results in trees designed at their proper limit of mechanical stability with a low margin. Several regionalized models under IPCC scenarios predict that winter storm frequency in Europ may increase, while chronic winds speed, that is responsible of growth acclimation, may decrease. As this biomechanical response is scarcely invoked when discussing forest vulnerability and acclimation to global change, assuming implicitly that other environmental effects are more significant, we will discuss its ecological relevancy, from observations of forest tree growth in natural both dried and windy conditions (Pinus halepensis in Spain) and models of tree mechanical stability related to tree size and allometry of growth. Then, wood responses to mechanical constraints include reaction and flexure woods, that differ from “normal” wood by their cellular and cell wall structure, with great consequences on wood properties. Wood acclimation to mechanical constraints should have feedback effects on carbon and water balance at the organ or whole tree level: for instance, reaction wood anatomy could induce a lower hydraulic conductivity that could result in lower stomatal conductance. We present some results from greenhouse experiments about how controlled mechanical stimuli modify local secondary growth rate and wood mechanical and hydraulical properties. We discuss how this acclimation of wood properties could change the margin of mechanical safety of trees, stating the whole complexity of the problem involving many feedback loops.

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Communication orale + résumé; In their natural environment, plants are continuously exposed to highly variable wind loads, and in particular to the days-to-week scale alternation of windy and quiet periods. In response to a single mechanical load, plants usually exhibit a dramatic growth response (thigmomorphogenesis) and genes involved are being characterized. However, molecular mechanisms involved in plant acclimation to recurring and successive mechanical loadings are not well characterized. More specifically how plants avoid over-responding in to continuously changing wind conditions is unknown. Through the analysis of the short-time effects of quantified stem bending on young poplars, we demonstrated the rapid induction of PtaZFP2 expression, a gene encoding a putative C2H2 zinc finger transcription factor. The PtaZFP2 transcripts accumulate 10 min after a single bending and the relative abundance of PtaZFP2 transcripts was linearly correlated with the amount of applied mechanical solicitation (Martin et al., 2009; Coutand et al., 2009). To test the effect of successive bending, young trees were submitted either to one transient bending per day for several days or to two bendings, 1–14 days apart. Our results indicate that both diameter growth and gene expression responses are reduced after several bendings. In particular, PtaZFP2 mRNA accumulated to a lesser extent after two bendings than after a single one. The minimum rest period between two successive loadings necessary to recover a response similar to that observed after a single bending, was 5 days. This response was observed for three other early mechano-responsive genes having different functions in the plant mechanosensing pathway, such as calcium signalling or wall modifications. These results clearly show a partial desensitization of plants to recurrent successive bendings, indicating a day-scale acclimation of sensitivity (accommodation) (Martin et al., 2010). Our objectives are now to identify molecular actors involved in such mechanism by studying the regulation of early responsive gene such as PtaZFP2 (Gourcilleau et al., 2011).

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Sucrose has been reported to play multiple roles in the winter biology of temperate woody species. However, no report on the molecular basis of sucrose transport in xylem tissue has yet been made. In the walnut tree, it is demonstrated that during the autumn-winter period, active absorption of sucrose from xylem vessels to parenchyma cells (sucrose influx) is much higher when samplings were taken shortly after a period of freezing temperatures. Here, the question of whether this increased sucrose influx mirrors a regulation of sucrose transporters in xylem tissue was tested. A putative sucrose transporter cDNA (JrSUT1: Juglans regia sucrose transporter 1) was isolated. Over the autumn-winter period, JrSUT1 transcripts and respective proteins were present in xylem parenchyma cells and highly detected when samplings were performed shortly after a freeze-thaw cycle. This up-regulation of JrSUT1 level was confirmed in controlled conditions and was not obtained in bark. Immunolocalization studies showed that JrSUT1 and plasma membrane H+-ATPase (JrAHA) were colocalized to vessel-associated cells (VACs), which control solute exchanges between parenchyma cells and xylem vessels. We propose that JrSUT1 could be involved in the retrieval of sucrose from xylem vessel. All these data are discussed with respect to the winter biology of the walnut tree.

Diplôme : Dr. d'Universite

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In perennial plants, freeze-thaw cycles during the winter months can induce the formation of air bubbles in xylem vessels, leading to changes in their hydraulic conductivity. Refilling of embolized xylem vessels requires an osmotic force that is created by the accumulation of soluble sugars in the vessels. Low water potential leads to water movement from the parenchyma cells into the xylem vessels. The water flux gives rise to a positive pressure essential for the recovery of xylem hydraulic conductivity. We investigated the possible role of plasma membrane aquaporins in winter embolism recovery in walnut (Juglans regia). First, we established that xylem parenchyma starch is converted to sucrose in the winter months. Then, from a xylem-derived cDNA library, we isolated two PIP2 aquaporin genes (JrPIP2,1 and JrPIP2,2) that encode nearly identical proteins. The water channel activity of the JrPIP2,1 protein was demonstrated by its expression in Xenopus laevis oocytes. The expression of the two PIP2 isoforms was investigated throughout the autumn-winter period. In the winter period, high levels of PIP2 mRNA and corresponding protein occurred simultaneously with the rise in sucrose. Furthermore, immunolocalization studies in the winter period show that PIP2 aquaporins were mainly localized in vessel-associated cells, which play a major role in controlling solute flux between parenchyma cells and xylem vessels. Taken together, our data suggest that PIP2 aquaporins could play a role in water transport between xylem parenchyma cells and embolized vessels.