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Master; La croissance foliaire est un processus majeur dans le fonctionnement des plantes. Les feuilles jouent le rôle d’interface entre la plante et son environnement aérien et ont un intérêt tout particulier en agronomie. La croissance foliaire est dépendante de son environnement or le réchauffement climatique conduira à des périodes de sécheresse plus fréquentes et intenses. C’est dans cette optique que nous avons choisi d’effectuer une étude comparée de la morphogénèse aérienne chez 3 espèces de poacées face à une condition de sécheresse. Pour ce faire, 120 plantes de blé, fétuque élevée et de ray-grass anglais ont été étudiées lors d’une expérimentation en serre et exposées à une sécheresse à partir du stade feuille 7 ligulée. Des mesures d’allongement foliaire, de potentiel hydrique et osmotique ont été réalisées au niveau de la zone de croissance des feuilles. Un suivi de la dimension et de l’allongement de la zone de croissance a également été réalisé au cours de la sécheresse. Dans l’ensemble, les plantes exposées au traitement sécheresse ont vu leur vitesse d’allongement foliaire réduire tout comme la vitesse d’apparition de nouvelles feuilles quelle que soit l’espèce étudiée. Le potentiel hydrique des zones de croissance foliaire a peu varié chez les trois espèces en condition de sécheresse.

Engineering osmotolerant plants is a challenge for modern agriculture. An interaction between osmotic stress response and phosphate homeostasis has been reported in plants, but the identity of molecules involved in this interaction remains unknown. In this study we assessed the role of phytic acid (PA) in response to osmotic stress and/or phosphate deficiency in Arabidopsis thaliana. For this purpose, we used Arabidopsis lines (L7 and L9) expressing a bacterial beta-propeller phytase PHY-US417, and a mutant in inositol polyphosphate kinase 1 gene (ipk1-1), which were characterized by low PA content, 40% (L7 and L9) and 83% (ipk1-1) of the wild-type (WT) plants level. We show that the PHY-overexpressor lines have higher osmotolerance and lower sensitivity to abscisic acid than ipk1-1 and WT. Furthermore, PHY-overexpressors showed an increase by more than 50% in foliar ascorbic acid levels and antioxidant enzyme activities compared to ipk1-1 and WT plants. Finally, PHY-overexpressors are more tolerant to combined mannitol stresses and phosphate deficiency than WT plants. Overall, our results demonstrate that the modulation of PA improves plant growth under osmotic stress, likely via stimulation of enzymatic and non-enzymatic antioxidant systems, and that beside its regulatory role in phosphate homeostasis, PA may be also involved in fine tuning osmotic stress response in plants.

This study developed and used an advanced whole-plant grapevine model to unravel factors affecting water and carbon fluxes during fleshy fruit growth., The growth of fleshy fruits is still poorly understood as a result of the complex integration of water and solute fluxes, cell structural properties, and the regulation of whole plant source–sink relationships. To unravel the contribution of these processes to berry growth, a biophysical grape (Vitis vinifera L.) berry growth module was developed and integrated with a whole-plant functional–structural model, and was calibrated on two varieties, Cabernet Sauvignon and Sangiovese. The model captured well the variations in growth and sugar accumulation caused by environmental conditions, changes in leaf-to-fruit ratio, plant water status, and varietal differences, with obvious future application in predicting yield and maturity under a variety of production contexts and regional climates. Our analyses illustrated that grapevines strive to maintain proper ripening by partially compensating for a reduced source–sink ratio, and that under drought an enhanced berry sucrose uptake capacity can reverse berry shrinkage. Sensitivity analysis highlighted the importance of phloem hydraulic conductance, sugar uptake, and surface transpiration on growth, while suggesting that cell wall extensibility and the turgor threshold for cell expansion had minor effects. This study demonstrates that this integrated model is a useful tool in understanding the integration and relative importance of different processes in driving fleshy fruit growth.

COST Action FP1106 STReESS ; EU Life Programme (LIFE12 ENV/FI/000409 ; Ajut Marie Curie IF fellowship (No 659191) Phloem osmolality and its components are involved in basic cell metabolism, cell growth, and in various physiological processes including the ability of living cells to withstand drought and frost. Osmolality and sugar composition responses to environmental stresses have been extensively studied for leaves, but less for the secondary phloem of plant stems and branches. Leaf osmotic concentration and the share of pinitol and raffinose among soluble sugars increase with increasing drought or cold stress, and osmotic concentration is adjusted with osmoregulation. We hypothesize that similar responses occur in the secondary phloem of branches. We collected living bark samples from branches of adult Pinus sylvestris, Picea abies, Betula pendula and Populus tremula trees across Europe, from boreal Northern Finland to Mediterranean Portugal. In all studied species, the observed variation in phloem osmolality was mainly driven by variation in phloem water content, while tissue solute content was rather constant across regions. Osmoregulation, in which osmolality is controlled by variable tissue solute content, was stronger for Betula and Populus in comparison to the evergreen conifers. Osmolality was lowest in mid-latitude region, and from there increased by 37% toward northern Europe and 38% toward southern Europe due to low phloem water content in these regions. The ratio of raffinose to all soluble sugars was negligible at mid-latitudes and increased toward north and south, reflecting its role in cold and drought tolerance. For pinitol, another sugar known for contributing to stress tolerance, no such latitudinal pattern was observed. The proportion of sucrose was remarkably low and that of hexoses (i.e., glucose and fructose) high at mid-latitudes. The ratio of starch to all non-structural carbohydrates increased toward the northern latitudes in agreement with the build-up of osmotically inactive C reservoir that can be converted into soluble sugars during winter acclimation in these cold regions. Present results for the secondary phloem of trees suggest that adjustment with tissue water content plays an important role in osmolality dynamics. Furthermore, trees acclimated to dry and cold climate showed high phloem osmolality and raffinose proportion.

Résumé : Les phases concentrées de protéines sont au centre de nombreuses études visant à identifier et caractériser les interactions et transitions de phases mises en jeu, en utilisant le large corpus de connaissances acquis sur les phases concentrées de colloïdes. Ces phases concentrées de protéines possèdent en outre une grande importance dans des domaines aussi variés que l’industrie agroalimentaire, l’industrie pharmaceutique et la médecine. L’établissement d’équations d’état présentant la pression osmotique (Π) en fonction de la fraction volumique (Φ) est une méthode efficace de caractérisation des interactions entre les composants d’un système. Nous l’avons appliquée à des solutions de deux protéines globulaires, le lysozyme et l’ovalbumine, en balayant une gamme de fractions volumiques allant d’une phase diluée (Φ < 0,01) à une phase concentrée, solide (Φ > 0,62). Les équations d’état obtenues, couplées à d’autres techniques (SAXS, simulations numériques), ont permis de mettre en évidence un comportement très différent des deux protéines lors de la concentration et ont montré leur complexité en comparaison avec des colloïdes modèles. La mise en relation des équations d’état et du comportement interfacial de ces deux protéines a montré des points de convergence et permis de formuler une nouvelle hypothèse expliquant certaines observations portant sur l’adsorption des protéines à l’interface air-eau., Concentrated phases of proteins are the subject of numerous studies aiming at identifying and characterizing the interactions and phase transitions at play, using the large corpus of knowledge in the field of concentrated colloids. Those concentrated phases of proteins have, in addition, a great importance in various fields, such as food industry, pharmaceutical industry and medicine. The establishment of equations of state relating osmotic pressure (Π) and volume fraction (Φ) is an efficient way of characterization of the interactions between the components of a system. We applied this method to solutions of two globular proteins, lysozyme and ovalbumin, spanning volume fractions ranging from a dilute phase (Φ < 0,01) to a concentrated, solid phase (Φ > 0,62). The equations of state, coupled to other methods (SAXS, numerical simulations), enabled us to show that the two proteins carry a very different behavior when submitted to concentration and that their complexity is beyond that of colloids. Relating equations of state and interfacial behavior of these two proteins also showed points of convergence and enabled us to formulate a new hypothesis which explains some of the results obtained in the study of adsorption of proteins at the air-water interface.

When filtrating a colloidal dispersion, colloids accumulate at the membrane surface, thus forming a polarized layer that can ultimately turns into a deposit (= a "gel") in some extreme cases. Modeling such a filtration has always been a challenge. In recent developments, Bacchin et al. propose a model based on the analogy between permeation in a deposit and diffusion in a polarized layer [1]. The permeation flux is then given by the following equation, which is the analog of the classical Darcy's law: J = k(C)/μ . dΠ(C)/dx (eq.(1)), where x is the distance to the membrane, μ is the solvent viscosity, k(C) and Π(C) are the permeability and colloidal osmotic pressure at distance x, respectively. As for now, the model of Bacchin et al. has been used successfully to describe the filtration of impermeable and "hard" spherical objects like latex particles [2]: the idea is to measure the osmotic pressures Π(C) of the latex dispersions through osmotic stress experiments and to estimate the permeability k(C) using the well-known theoretical expressions of Happel. Knowing Π(C) and k(C), eq.(1) is then used to obtain important information like permeation fluxes or concentration profiles in the accumulated layer. The objective of our study is to adapt such a model to the filtration of more complex colloids, namely, deformable and permeable objects like colloidal microgels. Casein micelles, which represent 80% of the proteins in cow milk, are the perfect candidate for that since they are highly permeable and compressible protein aggregates [3]. Additionally, ultra- and microfiltration are widely used for the concentration of milk and there is a serious need for a model that is able to describe these operations. The osmotic pressure of casein micelle dispersions is now perfectly known [3]. So the main difficulty lies in determining k(C) since no theoretical expression is currently available for deformable and permeable objects. Our presentation focus on the different strategies we used to overcome this difficulty and on the significance of the k(C) values we managed to estimate either indirectly from filtration experiments or directly from osmotic stress experiments (Figs. 1 and 2). We then discuss on the ability for the resulting model to predict filtrations conducted in different modes (dead-end/cross-flow, constant pressure/constant flux).

Les objectifs de notre étude ont été de classer 17 génotypes de pommier issus d’une même descendance suivant des critères de physiologie foliaire, de croissance et d’allométrie dans des conditions de stress hydrique racinaire. Pour cela, 5 variables ont été mesurées : conductance stomatique, la transpiration, le potentiel hydrique foliaire, le potentiel osmotique foliaire, la croissance de plants et des relations d’allométrie foliaire. La conductance stomatique, le potentiel hydrique et le potentiel osmotique des plants stressés n’a pas permis de dissocier les génotypes. Par contre, la croissance s’est arrêtée en cas de stress de manière différente suivant les génotypes : un premier groupe de génotypes fut en arrêt de croissance et reprit sa croissance post stress beaucoup plus rapidement qu’un second groupe, qui semble moins réactif aux changements hydriques édaphiques. Aussi l’allométrie de la forme des feuilles n’est pas modifiée par le stress hydrique racinaire. Alors que la relation d’allométrie entre la conductance xylémienne pétiolaire et les caractéristiques foliaires n’est pas conservée lors du stress. Il serait intéressant de réitérer l’expérience avec plus de modalités hydriques différentes pour obtenir un échantillonnage de réactions au stress., This study is based on a genetic descent. The way was to classify 17 genotypes following criteria of leaf physiology, growth and allometry in root water stress. In this purpose, we measured the stomatic conductance, the transpiration, the hydric leaf potential, the osmotic leaf potential, the growth of trees and the relations of leaf allometry. On the one hand, the stomatic conductance, the hydric potential and the osmotic potential did not allow discriminating genotypes under stress. On the other hand, the growth stopped in case of stress in a different way following genotypes: a first group has stopped its growth faster and took back its growth faster too by going out of stress; and a second slower group in the hydric changes. So the allometry on the shape of sheets is not modified by hydric stress. While the relation of allometry enters the xylem conductance and the leaf characteristics is not maintained during the stress. It would be interesting to repeat the experience with more different hydric modalities to obtain a sampling of reactions to the stress.

An integrated physiological and proteomic approach was used to investigate the effects of high gellan gum concentration in the medium during maturation of somatic embryos (SE) of hybrid larch, by comparing embryos incubated in media with a high gellan gum concentration (8 g l(-1) ) and the standard concentration (4 g l(-1) ) after 1, 3, 6 and 8 weeks of maturation. Because of the reduced availability of water in the 8 g l(-1) medium, the cultured embryos had a lower osmotic water potential (Ψπ) and water contents, but higher dry weights (DWs), at 8 weeks compared with embryos cultured on the standard medium. The high gellan gum concentration induced a desiccation that is characteristic in zygotic embryo maturation. Total soluble proteins were extracted from SE with trichloroacetic acid (TCA)-acetone after 1 and 8 weeks of maturation on media with 4 and 8 g l(-1) of gellan gum, and separated by two-dimensional gel electrophoresis (2-DE) at pH 4-7. More than 1100 proteins were reproducibly detected on each gel. At 1 and 8 weeks respectively, the abundances of 62 and 49 spots detected in analyses of embryos matured at the two gellan gum concentrations, significantly differed. Among 62 significantly differing spots at 1 week of maturation, the corresponding proteins of 56 were reliably identified by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS), and were found to be mainly involved in 'carbohydrate metabolism', 'genetic information processing' or 'environmental information processing' according to kegg taxonomy. Both physiological parameters and the proteins identified suggested that the embryos were stressed when they were cultured on 4 g l(-1) of gellan gum.

Afin de prédire la pression de gonflement des argiles actives, différents modèles ont été proposés ces dernières années. Quelques modèles sont basés sur les propriétés géotechniques; ces modèles sont empiriques obtenus à partir d’approches statistiques. Les autres modèles reposent d’une part sur des concepts thermodynamiques, et d’autre part sur la théorie de la double couche diffuse de Gouy Chapman. Les caractéristiques de gonflement sont fortement influencées par la composition minéralogique et chimique des argiles. Le gonflement intercristallin étant considéré comme le processus principal responsable de l’augmentation de la pression de gonflement. Cette étude présente l’effet du potentiel osmotique sur le comportement au gonflement des argiles actives en utilisant des équations élastiques non linéaires basées sur l’approche de la surface d’état en contraintes nettes, la succion et la concentration du soluté chimique. Le modèle proposé est utilisé pour simuler la déformation observée par les argiles expansives. Il a été suggéré en particulier que la détermination de la rigidité du sol en fonction de la concentration chimique peut être déterminée par des approches théoriques. Un ensemble d’exercices a été présenté dans ce travail afin de montrer la capacité du modèle à prédire le comportement observé des différents types d’argiles.

Casein dispersions under osmotic stress: the impact of structure and mineral composition. 1st International Symposium on Minerals and Dairy Products

International audience

Osmoconditioning - controlled rehydration of seeds in a solution with low osmotic potential - has been shown to reinvigorate aged seeds. The present work aimed at investigating the effect of osmoconditioning on the germination of cryopreserved seeds of #Coffea arabica$, whose viability and vigour are drastically affected by cryopreservation. For cryopreservation, seeds were desiccated to 0.21 g H2O/g dw, cooled at 1°C/min to -50°C, then immersed rapidly in liquid nitrogen. After rapid rewarming, seeds were osmoconditioned for 1 to 6 weeks using solutions with osmotic potentials between -1 and -4 MPa. The time to produce half of the final percentage of normal seedlings, T50, was about three fold lower with osmoconditioned seeds than with non-osmoconditioned seeds (12-14 d vs 36 d). Moreover, after a 6-week osmoconditioning treatment with solutions with osmotic potential of -1 and -1.25 MPa, the percentage of seedlings recovered from cryopreserved seeds was 64-74%, against 13-16% only for cryopreserved seeds which were not osmoconditioned. (Résumé d'auteur)

In colloidal filtration, understanding the mechanisms responsible for deposit formation and determining the relations between filtration conditions and deposit characteristics are still major issues. Following the general - and now well admitted - description of colloidal filtration given by Bacchin et al. (2006, 2011), irreversible fouling occurs when a critical concentration in particles is reached at the membrane surface. At this concentration, the interactions between the particles get dominated by attractive forces and phase transition takes place from the liquid to the solid state (irreversible deposit – gel). In cross-flow filtration, this critical concentration is reached at a given flux (this is the well-known concept of critical flux) (Field et al., 1995); while it is reached at a critical filtered volume in dead-end filtrations conducted at constant flux (Bessière et al., 2005). Importantly, phase transition and deposit formation can also be interpreted in terms of a critical osmotic pressure crit, that can be viewed as the compressive pressure required to reach the critical concentration and go from the liquid to the solid state. The critical conditions for deposit formation, as well as the intrinsic characteristics of the deposit, are now well documented, especially for "hard" colloids like silica, latex, clays...; in opposition to "soft" colloids that can deform and even deswell when concentrated. As the "softness feature" might necessarily have an impact on the formation of a deposit and on its properties, it becomes essential to get new information about the behavior of soft colloids in filtration. It is all the more important that such soft objects (emulsions, microgels, protein aggregates...) are involved in an increasing number of industrial processes that use membrane operations.In this work, we focus on the behavior of one particular soft and sponge-like natural colloid, the casein micelle that makes up to 80% of the protein content of cow milk. For that purpose, specific dead-end filtration experiments were designed and performed, involving successive filtrations interspersed with rinse periods at zero transmembrane pressure P = 0). This methodology allowed us to precisely monitor the build-up -or not- of a deposit at the membrane surface, as well as to estimate the critical conditions at which the deposit is formed. A detailed analysis of the properties of the deposit is also proposed, based on the variation in its specific resistance (α) with the conditions of filtration.From the results obtained, we first demonstrate that deposit formation is intimately linked to the colloidal nature of the micelle, as no irreversible deposit is formed when caseins are not organized into micelles. The deposit, or gel, forms through intermicellar bonds that are created when the micelles are forced to come in contact with each other.We then discuss the critical conditions for deposit formation and we estimate the corresponding critical osmotic pressure at which colloidal phase transition occurs. Interestingly, the obtained value is a bit higher than the one measured in "static" osmotic stress experiments; which could be a direct effect of liquid flow between the particles. From our filtration experiments, we find that critical osmotic pressure crit Which corresponds to the compressive pressure that is required to reach the critical concentration and provokes phase transition, equals 31-35 kPa. Interestingly, this value is close to the one measured from "static" osmotic stress experiments. It is however sufficiently higher to postulate, among other assumptions, that the liquid flow between the particles and/or the kinetics of intermicellar bond formation have an effect on crit.We finally demonstrate that our results are consistent with a deposit made of closely-packed and connected casein micelles that are compressed during filtration, and that fully relax to their "native" initial dimensions when pressure is released (Figure 1). The deposit is found to be both compressible and relaxable; two properties that are related to the soft and elastic nature of the casein micelle alone. In most cases, the resistance to flow through the deposit is entirely governed by the internal porosity of the casein micelle; this porosity being directly associated to the degree of compression of the micelle body.

The inhibition of the initial growth by osmotic water-stress was studied using naked embryos of Quercus ilex and Quercus pubescens cultivated in the presence of polyethylene glycol 6000 at different concentrations. The inhibition of germination and that of the beginning of the radicle growth were observed for lower osmotic potentials with Quercus ilex compared to Quercus pubescens . This result suggests a better drought tolerance in the case of Quercus ilex. The physiological cellular characteristics such as the level of metabolic activity and/or the uptake capability might be affected by the osmotic potential and therefore considered as indirectly responsible for the differences in behaviour between the two species., Le blocage du développement initial en relation avec le potentiel osmotique du milieu a été étudié sur des embryons nus de Quercus ilex et Quercus pubescens cultivés dans différentes concentrations de polyéthylène glycol 6000 . L'inhibition de la germination et du début de croissance radiculaire intervient à des potentiels osmotiques inférieurs dans le cas du Quercus ilex par rapport au Quercus pubescens Ce résultat suggère une meilleure adaptation du chêne vert (Quercus ilex ) dans des conditions hydriques limitantes. Les caractéristiques physiologiques cellulaires telles que le degré de blocage des activités métaboliques et/ou le pouvoir d'absorption pourraient être affectées par la valeur du potentiel osmotique du milieu et, ainsi, intervenir dans cette différence de comportement entre les deux espèces., Vuillemin J. Ecophysiologie comparée du développement initial de Quercus pubescens Willd. et de Quercus ilex L. II. Germination et croissance radiculaire en conditions de stress hydrique. In: Ecologia mediterranea, tome 8 n°3, 1982. pp. 147-151.