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Poplar plantations, widely used for the production of woody biomass, might be at high risk from the climate change-induced increase in the frequency of drought periods. Therefore, selecting improved genotypes, which are highly productive but with a high water use efficiency (WUE), is becoming a major target. The use of automated weighing systems in controlled environments facilitates the estimation of cumulated water loss and whole plant transpiration efficiency (TE). Differences in TE and leaf level intrinsic WUE as well as the contribution of underlying ecophysiological traits were determined in three contrasting P. nigra genotypes. Strong differences in TE among the selected genotypes were congruent with differences in leaf level intrinsic WUE. Our data show that a high total leaf area was overcompensated by a low per leaf area transpiration rate, leading to higher TE in highly productive genotypes originating from cool locations. Nocturnal water loss was relatively low but contributed to variations in TE among genotypes. In response to drought, leaf level WUE increased but not TE, suggesting that carbon losses due to whole plant respiration could offset the drought-induced increase in intrinsic WUE.

Element content and expression of genes of interest on single cell types, such as stomata, provide valuable insights into their specific physiology, improving our understanding of leaf gas exchange regulation. We investigated how far differences in stomatal conductance (g(s)) can be ascribed to changes in guard cells functioning in amphistomateous leaves. g(s) was measured during the day on both leaf sides, on well-watered and drought-stressed trees (two Populus euramericana Moench and two Populus nigra L. genotypes). In parallel, guard cells were dissected for element content and gene expressions analyses. Both were strongly arranged according to genotype, and drought had the lowest impact overall. Normalizing the data by genotype highlighted a structure on the basis of leaf sides and time of day both for element content and gene expression. Guard cells magnesium, phosphorus, and chlorine were the most abundant on the abaxial side in the morning, where g(s) was at the highest. In contrast, genes encoding H+-ATPase and aquaporins were usually more abundant in the afternoon, whereas genes encoding Ca2+-vacuolar antiporters, K+ channels, and ABA-related genes were in general more abundant on the adaxial side. Our work highlights the unique physiology of each leaf side and their analogous rhythmicity through the day.

Distinct cellular processes trigger root growth regulation: cell elongation rate drives rapid growth response to environment while cell division rate determines the growth capacity of individual roots., Plant organ growth results from cell production and cell expansion. Deciphering the contribution of each of these processes to growth rate is an important issue in developmental biology. Here, we investigated the cellular processes governing root elongation rate, considering two sources of variation: genotype and disturbance by chemicals (NaCl, polyethylene glycol, H2O2, abscisic acid). Exploiting the adventitious rooting capacity of the Populus genus, and using time-lapse imaging under infrared-light, particle image velocimetry, histological analysis, and kinematics, we quantified the cellular processes involved in root growth variation, and analysed the covariation patterns between growth parameters. The rate of cell production by the root apical meristem and the number of dividing cells were estimated in vivo without destructive measurement. We found that the rate of cell division contributed more to the variation in cell production rate than the number of dividing cells. Regardless of the source of variation, the length of the elongation zone was the best proxy for growth rate, summarizing rates of cell production and cell elongation into a single parameter. Our results demonstrate that cell production rate is the main driver of growth rate, whereas elemental elongation rate is a key driver of short-term growth adjustments.

International audience; The adaptive capacity of long-lived organisms such as trees to the predicted climate changes, including severe and successive drought episodes, will depend on the presence of genetic diversity and phenotypic plasticity. Here, the involvement of epigenetic mechanisms in phenotypic plasticity toward soil water availability was examined in Populus×euramericana. This work aimed at characterizing (i) the transcriptome plasticity, (ii) the genome-wide plasticity of DNA methylation, and (iii) the function of genes affected by a drought-rewatering cycle in the shoot apical meristem. Using microarray chips, differentially expressed genes (DEGs) and differentially methylated regions (DMRs) were identified for each water regime. The rewatering condition was associated with the highest variations of both gene expression and DNA methylation. Changes in methylation were observed particularly in the body of expressed genes and to a lesser extent in transposable elements. Together, DEGs and DMRs were significantly enriched in genes related to phytohormone metabolism or signaling pathways. Altogether, shoot apical meristem responses to changes in water availability involved coordinated variations in DNA methylation, as well as in gene expression, with a specific targeting of genes involved in hormone pathways, a factor that may enable phenotypic plasticity.

Summary A fast and in vivo approach to assess the dynamics of root apical meristem length., Understanding how cell division and cell elongation influence organ growth and development is a long-standing issue in plant biology. In plant roots, most of the cell divisions occur in a short and specialized region, the root apical meristem (RAM). Although RAM activity has been suggested to be of high importance to understand how roots grow and how the cell cycle is regulated, few experimental and numeric data are currently available. The characterization of the RAM is difficult and essentially based upon cell length measurements through destructive and time-consuming microscopy approaches. Here, a new non-invasive method is described that couples infrared light imaging and kinematic analyses and that allows in vivo measurements of the RAM length. This study provides a detailed description of the RAM activity, especially in terms of cell flux and cell division rate. We focused on roots of hydroponic grown poplars and confirmed our method on maize roots. How the RAM affects root growth rate is studied by taking advantage of the high inter-individual variability of poplar root growth. An osmotic stress was applied and did not significantly affect the RAM length, highlighting its homeostasis in short to middle-term responses. The methodology described here simplifies a lot experimental procedures, allows an increase in the number of individuals that can be taken into account in experiments, and means new experiments can be formulated that allow temporal monitoring of the RAM length.

International audience; Phenotyping the dynamics of root responses to environmental cues is necessary to understand plant acclimation to their environment. Continuous monitoring of root growth is challenging because roots normally grow belowground and are very sensitive to their growth environment. This protocol combines infrared imaging with hydroponic cultivation for kinematic analyses. It allows continuous imaging at fine spatiotemporal resolution and disturbs roots minimally. Examples are provided of how the procedure and materials can be adapted for 3D monitoring and of how environmental stress may be manipulated for experimental purposes.

Wood is a renewable resource that can be employed for the production of second generation biofuels by enzymatic saccharification and subsequent fermentation. Knowledge on how the saccharification potential is affected by genotype-related variation of wood traits and drought is scarce. Here, we used three Populus nigra L. genotypes from habitats differing in water availability to (i) investigate the relationships between wood anatomy, lignin content and saccharification and (ii) identify genes and co-expressed gene clusters related to genotype and drought-induced variation in wood traits and saccharification potential. The three poplar genotypes differed in wood anatomy, lignin content and saccharification potential. Drought resulted in reduced cambial activity, decreased vessel and fiber lumina, and increased the saccharification potential. The saccharification potential was unrelated to lignin content as well as to most wood anatomical traits. RNA sequencing of the developing xylem revealed that 1.5% of the analyzed genes were differentially expressed in response to drought, while 67% differed among the genotypes. Weighted gene correlation network analysis identified modules of co-expressed genes correlated with saccharification potential. These modules were enriched in gene ontology terms related to cell wall polysaccharide biosynthesis and modification and vesicle transport, but not to lignin biosynthesis. Among the most strongly saccharification-correlated genes, those with regulatory functions, especially kinases, were prominent. We further identified transcription factors whose transcript abundances differed among genotypes, and which were co-regulated with genes for biosynthesis and modifications of hemicelluloses and pectin. Overall, our study suggests that the regulation of pectin and hemicellulose metabolism is a promising target for improving wood quality of second generation bioenergy crops. The causal relationship of the identified genes and pathways with saccharification potential needs to be validated in further experiments.

Highlight Maximal axial pressures exerted by freely growing roots are restricted by root buckling but are increased when root lateral bracing is provided., Strong regions and physical barriers in soils may slow root elongation, leading to reduced water and nutrient uptake and decreased yield. In this study, the biomechanical responses of roots to axial mechanical forces were assessed by combining 3D live imaging, kinematics and a novel mechanical sensor. This system quantified Young’s elastic modulus of intact poplar roots (32MPa), a rapid

Highlight Osmotic stress rapidly induces strong transcriptome responses within the root apex and interferes with growth through a deep remodeling of hormonal status rather than by repressing the machinery of expansive growth., Molecular regulation of growth must include spatial and temporal coupling of cell production and cell expansion. The underlying mechanisms, especially under environmental challenge, remain obscure. Spatial patterns of cell processes make the root apex well suited to deciphering stress signaling pathways, and to investigating both processes. Kinematics and RNA-sequencing were used to analyze the immediate growth response of hydroponically grown Populus nigra cuttings submitted to osmotic stress. About 7400 genes and unannotated transcriptionally active regions were differentially expressed between the division and elongation zones. Following the onset of stress, growth decreased sharply, probably due to mechanical effects, before recovering partially. Stress impaired cell expansion over the apex, progressively shortened the elongation zone, and reduced the cell production rate. Changes in gene expression revealed that growth reduction was mediated by a shift in hormone homeostasis. Osmotic stress rapidly elicited auxin, ethylene, and abscisic acid. When growth restabilized, transcriptome remodeling became complex and zone specific, with the deployment of hormone signaling cascades, transcriptional regulators, and stress-responsive genes. Most transcriptional regulations fit growth reduction, but stress also promoted expression of some growth effectors, including aquaporins and expansins. Together, osmotic stress interfered with growth by activating regulatory proteins rather than by repressing the machinery of expansive growth.

Phenotypic plasticity is considered as an important mechanism for plants to cope with environmental challenges. Leaf growth is one of the first macroscopic processes to be impacted by modification of soil water availability. In this study, we intended to analyze and compare plasticity at different scales. We examined the differential effect of water regime (optimal, moderate water deprivation and recovery) on growth and on the expression of candidate genes in leaves of different growth stages. Candidates were selected to assess components of growth response: abscisic acid signaling, water transport, cell wall modification and stomatal development signaling network. At the tree scale, the four studied poplar hybrids responded similarly to water regime. Meanwhile, leaf growth response was under genotype × environment interaction. Patterns of candidate gene expression enriched our knowledge about their functionality in poplars. For most candidates, transcript levels were strongly structured according to leaf growth performance while response to water regime was clearly dependent on genotype. The use of an index of plasticity revealed that the magnitude of the response was higher for gene expression than for macroscopic traits. In addition, the ranking of poplar genotypes for macroscopic traits well paralleled the one for gene expression.

Root development is under the control of hormonal, metabolic, and environmental cues that can act on genetically-controlled developmental programmes and thus affect the plasticity of root architecture. These processes involve not only the five `classical' plant hormones, but also other growth regulators, such as polyamines. The present review emphasises the importance of polyamines in the different aspects of root development: primary root growth and lateral and adventitious root formation. Free (agmatine, putrescine, spermidine, spermine), conjugated (such as hydroxycinnamate conjugates) and macromolecule-bound polyamines are reported to be present in root systems. Modifications of their endogenous levels by inhibitor treatment, by mutation, by gene manipulation, or by exogenous treatment can have drastic effects on root development and subsequent architecture. These effects may be related to the involvement of polyamines in the control of cell division and differentiation, which plays an important role in the root apex and during lateral and adventitious root formation. The exact mechanisms of action remain to be elucidated, but accumulating evidence in plant and animal cells supports the idea that, besides biophysical effects on membranes and nucleic acids, polyamines interact with protein kinases and transcription factors and are thus involved in signal transduction pathways. The high flexibility of polyamine metabolism in response to environmental stress and the metabolic link between polyamine and ethylene synthesis strongly suggest that polyamines may play a role in environmentally-induced plasticity of root development. Moreover, polyamines may be implicated in the establishment of biotic interactions between roots and rhizospheric micro-organisms.

Polyamine involvement in root development at low temperature was studied in seedlings of Pringlea antiscorbutica R. Br. This unique endemic cruciferous species from the subantarctic zone is subjected to strong environmental constraints and shows high polyamine contents. In the present study, free polyamine levels were modified by inhibitors of polyamine biosynthesis (D-arginine, difluoromethylornithine, cyclohexylammonium, and methylglyoxal-bis-guanylhydrazone) and variations of the endogenous pools were compared to changes in root growth. The arginine decarboxylase pathway, rather than that of ornithine decarboxylase, seemed to play a major role in polyamine synthesis in Pringlea antiscorbutica seedlings. Root, but not shoot, phenotypes were greatly affected by these treatments, which modified polyamine endogenous levels according to their expected effects. A positive correlation was found between agmatine level and growth rate of the primary root. Spermidine and spermine contents also showed positive correlations with primary root growth whereas the putrescine level showed neutral or negative effects on this trait. Free polyamines were therefore found to be differentially involved in the phenotypic plasticity of root architecture. A comparison of developmental effects and physiological concentrations suggested that agmatine and spermine in particular may play a significant role in the control of root development.

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BackgroundComparative genomics has emerged as a promising means of unravelling the molecular networks underlying complex traits such as drought tolerance. Here we assess the genotype-dependent component of the drought-induced transcriptome response in two poplar genotypes differing in drought tolerance. Drought-induced responses were analysed in leaves and root apices and were compared with available transcriptome data from otherPopulusspecies.ResultsUsing a multi-species designed microarray, a genomic DNA-based selection of probesets provided an unambiguous between-genotype comparison. Analyses of functional group enrichment enabled the extraction of processes physiologically relevant to drought response. The drought-driven changes in gene expression occurring in root apices were consistent across treatments and genotypes. For mature leaves, the transcriptome response varied weakly but in accordance with the duration of water deficit. A differential clustering algorithm revealed similar and divergent gene co-expression patterns among the two genotypes. Since moderate stress levels induced similar physiological responses in both genotypes, the genotype-dependent transcriptional responses could be considered as intrinsic divergences in genome functioning. Our meta-analysis detected several candidate genes and processes that are differentially regulated in root and leaf, potentially under developmental control, and preferentially involved in early and long-term responses to drought.ConclusionsIn poplar, the well-known drought-induced activation of sensing and signalling cascades was specific to the early response in leaves but was found to be general in root apices. Comparing our results to what is known in arabidopsis, we found that transcriptional remodelling included signalling and a response to energy deficit in roots in parallel with transcriptional indices of hampered assimilation in leaves, particularly in the drought-sensitive poplar genotype.

Background Technological advances have enabled the accurate quantification of gene expression, even within single cell types. While transcriptome analyses are routinely performed, most experimental designs only provide snapshots of gene expression. Molecular mechanisms underlying cell fate or positional signalling have been revealed through these discontinuous datasets. However, in developing multicellular structures, temporal and spatial cues, known to directly influence transcriptional networks, get entangled as the cells are displaced and expand. Access to an unbiased view of the spatiotemporal regulation of gene expression occurring during development requires a specific framework that properly quantifies the rate of change of a property in a moving and expanding element, such as a cell or an organ segment. Results We show how the rate of change in gene expression can be quantified by combining kinematics and real-time polymerase chain reaction data in a mechanistic model which considers any organ as a continuum. This framework was applied in order to assess the developmental regulation of the two reference genes Actin11 and Elongation Factor 1-β in the apex of poplar root. The growth field was determined by time-lapse photography and transcript density was obtained at high spatial resolution. The net accumulation rates of the transcripts of the two genes were found to display highly contrasted developmental profiles. Actin11 showed pulses of up and down regulation in the accelerating and decelerating parts of the growth zone while the dynamic of EF1β were much slower. This framework provides key information about gene regulation in a developing organ, such as the location, the duration and the intensity of gene induction/repression. Conclusions We demonstrated that gene expression patterns can be monitored using the continuity equation without using mutants or reporter constructions. Given the rise of imaging technologies, this framework in our view opens a new way to dissect the molecular basis of growth regulation, even in non-model species or complex structures.

Background Renewed interest in plant × environment interactions has risen in the post-genomic era. In this context, high-throughput phenotyping platforms have been developed to create reproducible environmental scenarios in which the phenotypic responses of multiple genotypes can be analysed in a reproducible way. These platforms benefit hugely from the development of suitable databases for storage, sharing and analysis of the large amount of data collected. In the model plant Arabidopsis thaliana, most databases available to the scientific community contain data related to genetic and molecular biology and are characterised by an inadequacy in the description of plant developmental stages and experimental metadata such as environmental conditions. Our goal was to develop a comprehensive information system for sharing of the data collected in PHENOPSIS, an automated platform for Arabidopsis thaliana phenotyping, with the scientific community. Description PHENOPSIS DB is a publicly available (URL: http://bioweb.supagro.inra.fr/phenopsis/) information system developed for storage, browsing and sharing of online data generated by the PHENOPSIS platform and offline data collected by experimenters and experimental metadata. It provides modules coupled to a Web interface for (i) the visualisation of environmental data of an experiment, (ii) the visualisation and statistical analysis of phenotypic data, and (iii) the analysis of Arabidopsis thaliana plant images. Conclusions Firstly, data stored in the PHENOPSIS DB are of interest to the Arabidopsis thaliana community, particularly in allowing phenotypic meta-analyses directly linked to environmental conditions on which publications are still scarce. Secondly, data or image analysis modules can be downloaded from the Web interface for direct usage or as the basis for modifications according to new requirements. Finally, the structure of PHENOPSIS DB provides a useful template for the development of other similar databases related to genotype × environment interactions.

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International audience; Arabidopsis thaliana (L.) Heynh. is used as a model plant in many research projects. However, few models simulate its growth at the whole-plant scale. The present study describes the first model of Arabidopsis growth integrating organogenesis, morphogenesis and carbon-partitioning processes for aerial and subterranean parts of the plant throughout its development. The objective was to analyse competition among sinks as they emerge from patterns of plant structural development. The model was adapted from the GreenLab model and was used to estimate organ sink strengths by optimisation against biomass measurements. Dry biomass production was calculated by a radiation use efficiency-based approach. Organogenesis processes were parameterised based on experimental data. The potential of this model for growth analysis was assessed using the Columbia ecotype, which was grown in standard environmental conditions. Three phases were observed in the overall time course of trophic competition within the plant. In the vegetative phase, no competition was observed. In the reproductive phase, competition increased with a strong increase when lateral inflorescences developed. Roots and internodes and structures bearing siliques were strong sinks and had a similar impact on competition. The application of the GreenLab model to the growth analysis of A. thaliana provides new insights into source–sink relationships as functions of phenology and morphogenesis.

International audience; This study investigated the relationships between root structure and anatomy and whole-plant functioning in herbaceous species. Fourteen annual and perennial species representative of a Mediterranean old-field succession were grown in monocultures in a common-garden experiment. Whole-plant functioning was assessed by inherent relative growth rate (RGR max), measured in standardized conditions, and maximum height ( Hmax). Root tissue density (TMD r), considered as a major component of root structure, was measured on roots harvested within in-growth cores. Anatomical characteristics were analysed on cross-sectional areas (CSA). TMD r was correlated positively with Hmax and negatively with RGR max. Root CSA explained interspecific variation in Hmax but not that in TMD r and RGR max. Root xylem CSA and xylem proportion in root CSA were positively correlated with TMD r and Hmax and negatively with RGR max. Mean xylem vessel CSA did not account for variations in TMD r , Hmax and RGR max. These results suggested that RGR max and Hmax are constrained by opposite root structural and anatomical traits, which have potential links with hydraulic conductance, support and longevity.

International audience; In its simplest definition, a trait is a surrogate of organismal performance, and this meaning of the term has been used by evolutionists for a long time. Over the last three decades, developments in community and ecosystem ecology have forced the concept of trait beyond these original boundaries, and trait-based approaches are now widely used in studies ranging from the level of organisms to that of ecosystems. Despite some attempts to fix the terminology, especially in plant ecology, there is currently a high degree of confusion in the use, not only of the term ‘‘trait’’ itself, but also in the underlying concepts it refers to. We therefore give an unambiguous definition of plant trait, with a particular emphasis on functional trait. A hierarchical perspective is proposed, extending the ‘‘performance paradigm’’ to plant ecology. ‘‘Functional traits’’ are defined as morpho-physiophenological traits which impact fitness indirectly via their effects on growth, reproduction and survival, the three components of individual performance. We finally present an integrative framework explaining how changes in trait values due to environmental variations are translated into organismal performance, and how these changes may influence processes at higher organizational levels. We argue that this can be achieved by developing ‘‘integration functions’’ which can be grouped into functional response (community level) and effect (ecosystem level) algorithms.

International audience; Although the existence of phenological impact on biomass partitioning in the plant is known for many species, it is difficult to quantify this effect and to unravel it from the complex functional processes that interact during plant growth. This work explores the variations of biomass allocated to fruits according to simple changes in the topological and phenological development of Arabidopsis thaliana plants. Four plants of the same genotype (ecotype Columbia) were grown in controlled conditions in growth chamber. Their topological differences were studied using the functional-structural model GreenLab. It showed that when fitting the four plants with a single set of parameters, but each plant being given its own topological structure, half of the biomass variability can be reproduced.

International audience; Arginine decarboxylase (ADC) is a key enzyme involved in the synthesis of polyamines, which have been implicated in developmental processes and stress responses in higher plants. An ancestral ADC gene appears to have been duplicated at the origin of the Brassicaceae family, thus yielding two paralogues in the derived taxa. ADC gene structure was investigated in Pringlea antiscorbutica R. Br., a geographically isolated Brassicaceae species that is endemic from the subantarctic region. P. antiscorbutica exhibits several biochemical and physiological adaptations related to this cold and harsh environment, including high levels of polyamines, which is unusual in higher plants, and especially high levels of agmatine, the product of the ADC-catalysed reaction. Various ADC clones were obtained from P. antiscorbutica. Sequence and phylogenetic analysis showed that all these clones fitted with the presence of two paralogues, PaADC1 and PaADC2, in P. antiscorbutica. Expression of these ADC paralogues was analyzed in P. antiscorbutica during vegetative development and response to stress. Whereas PaADC2 was expressed at both seedling and mature stages, PaADC1 transcripts were hardly detected during early development and were significantly expressed in mature plants. Moreover, PaADC2, but not PaADC1, expression was up-regulated in response to chilling and salt stress at seedling stage. Analysis of 5Vregulatory regions of these ADC genes revealed several differences in putative cis-regulatory elements, which could be associated with specific expression patterns. These results were compared to ADC paralogue expression in Arabidopsis thaliana and are discussed in the evolutionary context of genetic diversity resulting from gene duplication.

Pringlea antiscorbutica, which is the sole endemic crucifer in the subantarctic zone, undergoes seedling development in a harsh and cold environment. Since, at the mature stage, this species exhibits several adaptations linked to cold tolerance such as high polyamine levels, potential adaptations and polyamine response were investigated in seedlings. In order to assess the specificity of responses, P. antiscorbutica was compared with Arabidopsis thaliana, which is characterized by a life cycle preventing cold exposure at seedling stage. P. antiscorbutica and A. thaliana seedlings were found to have strikingly contrasted responses to temperature changes and to mineral nutrition. Whereas A. thaliana seedlings showed the typical growth arrest of chilling-sensitive plants, P. antiscorbutica seedlings showed optimal root growth at low temperature (5/10 degrees C) and temperate conditions caused the early arrest of root growth. Cold tolerance was associated with increased levels of polyamines or with maintenance of high levels of polyamines. Comparison of both species showed that polyamine levels could be a significant marker of chilling tolerance in seedlings. Treatments with varying mineral supply showed a positive relationship between root growth rate and variations of agmatine and putrescine endogenous contents in roots of P. antiscorbutica. This may be the first demonstration that, even under conditions of accumulation induced by environmental stress, polyamine levels can still be correlated with developmental processes. Com parison of mineral supply and temperature effects strongly indicated a trade-off of polyamine involvement between development and response to stress.

International audience; In plants, polyamines can generally be synthesized by the ornithine decarboxylase and arginine decarboxylase pathways. However, the model plant Arabidopsis thaliana appears to possess only the arginine decarboxylase pathway. As two paralogous ARGININE DECARBOXYLASE (ADC) genes are present in Arabidopsis, we investigated differential expression and potential differences of promoter activity during seedling development and under specific stress conditions.Promoter activities were studied in stable homozygotic transformants harbouring promoter::reporter gene fusions.Under temperate conditions, ADC2 promoter activity was strongly associated with seed germination, root and leaf development, whereas ADC1 promoter activity was low during vegetative development. Light, sucrose and ethylene were shown to be important regulators of ADC2 promoter activity. By contrast, in roots and leaves of plantlets subjected to chilling treatment the ADC1 paralogue showed high promoter activity whereas ADC2 promoter activity was considerably decreased.In situations of seed germination, root development and response to chilling, the modifications of promoter activities were associated with changes in mRNA levels, emphasizing the involvement of transcriptional regulation in ARGININE DECARBOXYLASE gene expression

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Diplôme : Dr. d'Université; Cette étude porte sur le contrôle moléculaire de l’expansion cellulaire dans l’apex de la racine du peuplier. L’étude a été focalisée sur la régulation de l’expression des aquaporines TIP1s sous deux niveaux de déficit hydrique. Un cadre conceptuel, combinant analyse de la densité de transcrits (PCR quantitative) à haute résolution spatiale et un formalisme de la mécanique des fluides, a été établi pour décrire la régulation de l’expression des gènes dans le temps et l’espace, le long d’apex. Deux états de croissance contrastés ont été étudiés : un où la croissance de l'apex de la racine est rétablie (après 3 jours de stress modéré, 80 mosmol kg-1, 100 g L-1 de polyéthylène glycol PEG à 3500 g mol-1) et un où elle est réduite (après 3 jours de stress fort, 250 mosmol kg-1, 200 g L-1 de PEG). L'analyse cinématique a révélé une sensibilité différentielle de la vitesse d’allongement relatif (REGR) selon le niveau de stress et la position le long de l’apex. Au niveau moléculaire, nous montrons que lorsque la croissance est réduite, l’expression de la plupart des TIP1s est induite précocement dans le temps et l’espace, générant un décalage des pics de densité de transcrits dans les premiers millimètres de l’apex, zone où la croissance des cellules est maintenue. Ce décalage était accompagné, pour certaines TIP1s, d’une augmentation de la densité de transcrits. Cependant les niveaux d’induction/répression n’étaient jamais plus élevés que dans la condition témoin, soulignant qu’une forte densité de transcrits n’est pas forcément synonyme d’une forte induction de l’expression. En effet le temps d’induction, rarement pris en compte, a un impact fort sur l’accumulation des transcrits. Parmi les patrons de densité de transcrits, nous avons détecté que seul celui de PtTIP1;4 coïncidait avec le profil de REGR quel que soit le traitement. De plus, la régulation de son expression apparait suivre un patron temporel similaire dans tous les traitements, bien que la taille de la zone de croissance varie. Ces deux arguments (profil de densité et régulation d’expression) soutiennent l’hypothèse d’un lien entre cette aquaporine et l’expansion cellulaire. Focalisé sur la combinaison de deux processus dynamiques, l’expansion cellulaire et l’expression des gènes, ma thèse a mis en évidence que les conclusions tirées de l’analyse de ces processus sont influencées par la façon dont le temps, l’espace et l’âge sont considérés.