Comment la cellulose et les microtubules contribuent à la croissance des cellules et des organes végétaux : une analyse temporelle et spatiale multi-échelles

Growth of living organisms originates from molecular events. These events add up, and generate cellular behaviors that together produce morphogenesis. In my PhD, I studied how such microscopic events contribute to plant morphogenesis. More specifically, in a first set of projects, we investigated the dynamics of the microtubule cytoskeleton in Arabidopsis pavement cells by quantifying distribution of microtubule orientation and tip dynamics at subcellular scale. Disruption of microtubule patterning was predicted to lead to mechanical failure of cells, but we highlighted compensation mechanisms that prevented it. In my main project, we investigated the role of cellulose synthase guidance along microtubules via a protein called CELLULOSE-SYNTHASE INTERACTIVE 1 (CSI1). In the absence of CSI1, cellulose synthases follow previously deposited cellulose microfibrils, which should lead to increased alignment. Contrary to the current growth model where cellulose alignment is associated with growth anisotropy, we found that this did not lead to an increase in cell elongation and even led to a decrease of organ elongation in Arabidopsis sepal. Sepals also presented less anisotropic mechanical properties and displayed cells with a snake-like shape. At a multicellular scale, we found that spatial consistency of growth direction was impaired in csi1, which explains the reduced length of its organs, the reduced mechanical anisotropy of organs, and the defects in cell shape. Our results contribute to bridging knowledge between scales, from the cytoskeleton, to the cell wall and to plant morphogenesis.La croissance des organismes vivants est le résultat d'événements moléculaires. Ces événements s'additionnent et génèrent des comportements cellulaires qui, ensemble, produisent la morphogenèse. J'ai étudié comment ces événements microscopiques contribuent à la morphogenèse des plantes. D’une part, nous avons étudié la dynamique des microtubules dans les cellules de pavage d'Arabidopsis, en quantifiant la distribution d’orientation des microtubules et la dynamique de leurs extrémités à échelle subcellulaire. La perturbation de l'orientation des microtubules est censée entraîner une défaillance mécanique des cellules, mais nous avons mis en évidence des mécanismes de compensation qui l'empêchent. Dans mon projet principal, nous avons étudié le rôle du guidage des complexes synthétisant la cellulose le long des microtubules par une protéine appelée CELLULOSE-SYNTHASE INTERACTIVE 1 (CSI1). En l'absence de CSI1, les complexes synthétisant la cellulose suivent les fibres de cellulose précédemment déposées, ce qui devrait conduire à un alignement accru. Contrairement au modèle selon lequel l’alignement de la cellulose augmente l’anisotropie de croissance, nous avons constaté que cela n’augmente pas l'élongation cellulaire et entraîne même une diminution de l’élongation du sépale d'Arabidopsis. Ces sépales présentent également des propriétés mécaniques moins anisotropes et des cellules ondulant en forme de serpent. À l'échelle multicellulaire, nous avons constaté que la cohérence spatiale de la direction de la croissance est altérée chez csi1, ce qui explique la longueur réduite de ses organes, l'anisotropie mécanique réduite des organes et les défauts de forme des cellules. Nos résultats contribuent à éclairer le lien entre échelles spatiales lors de la morphogenèse végétale

Comment la cellulose et les microtubules contribuent à la croissance des cellules et des organes végétaux : une analyse temporelle et spatiale multi-échelles

Growth of living organisms originates from molecular events. These events add up, and generate cellular behaviors that together produce morphogenesis. In my PhD, I studied how such microscopic events contribute to plant morphogenesis. More specifically, in a first set of projects, we investigated the dynamics of the microtubule cytoskeleton in Arabidopsis pavement cells by quantifying distribution of microtubule orientation and tip dynamics at subcellular scale. Disruption of microtubule patterning was predicted to lead to mechanical failure of cells, but we highlighted compensation mechanisms that prevented it. In my main project, we investigated the role of cellulose synthase guidance along microtubules via a protein called CELLULOSE-SYNTHASE INTERACTIVE 1 (CSI1). In the absence of CSI1, cellulose synthases follow previously deposited cellulose microfibrils, which should lead to increased alignment. Contrary to the current growth model where cellulose alignment is associated with growth anisotropy, we found that this did not lead to an increase in cell elongation and even led to a decrease of organ elongation in Arabidopsis sepal. Sepals also presented less anisotropic mechanical properties and displayed cells with a snake-like shape. At a multicellular scale, we found that spatial consistency of growth direction was impaired in csi1, which explains the reduced length of its organs, the reduced mechanical anisotropy of organs, and the defects in cell shape. Our results contribute to bridging knowledge between scales, from the cytoskeleton, to the cell wall and to plant morphogenesis.La croissance des organismes vivants est le résultat d'événements moléculaires. Ces événements s'additionnent et génèrent des comportements cellulaires qui, ensemble, produisent la morphogenèse. J'ai étudié comment ces événements microscopiques contribuent à la morphogenèse des plantes. D’une part, nous avons étudié la dynamique des microtubules dans les cellules de pavage d'Arabidopsis, en quantifiant la distribution d’orientation des microtubules et la dynamique de leurs extrémités à échelle subcellulaire. La perturbation de l'orientation des microtubules est censée entraîner une défaillance mécanique des cellules, mais nous avons mis en évidence des mécanismes de compensation qui l'empêchent. Dans mon projet principal, nous avons étudié le rôle du guidage des complexes synthétisant la cellulose le long des microtubules par une protéine appelée CELLULOSE-SYNTHASE INTERACTIVE 1 (CSI1). En l'absence de CSI1, les complexes synthétisant la cellulose suivent les fibres de cellulose précédemment déposées, ce qui devrait conduire à un alignement accru. Contrairement au modèle selon lequel l’alignement de la cellulose augmente l’anisotropie de croissance, nous avons constaté que cela n’augmente pas l'élongation cellulaire et entraîne même une diminution de l’élongation du sépale d'Arabidopsis. Ces sépales présentent également des propriétés mécaniques moins anisotropes et des cellules ondulant en forme de serpent. À l'échelle multicellulaire, nous avons constaté que la cohérence spatiale de la direction de la croissance est altérée chez csi1, ce qui explique la longueur réduite de ses organes, l'anisotropie mécanique réduite des organes et les défauts de forme des cellules. Nos résultats contribuent à éclairer le lien entre échelles spatiales lors de la morphogenèse végétale

Expression of cell-wall related genes is highly variable and correlates with sepal morphology

Control of organ morphology is a fundamental feature of living organisms. There is, however, observable variation in organ size and shape within a given genotype. Taking the sepal of Arabidopsis as a model, we investigated whether we can use variability of gene expression alongside variability of organ morphology to identify gene regulatory networks potentially involved in organ size and shape determination. We produced a dataset composed of morphological parameters and genome-wide transcriptome obtained from 27 individual sepals from wild-type plants with nearly identical genetic backgrounds, environment, and developmental stage. Sepals exhibited appreciable variability in both morphology and transcriptome, with response to stimulus genes and cell-wall related genes displaying high variability in expression. We additionally identified five modules of co-expressed genes which correlated significantly with morphology, revealing biologically relevant gene regulatory networks. Interestingly, cell-wall related genes were overrepresented in two of the top three modules. Overall, our work highlights the benefit of using coupled variation in gene expression and phenotype in wild-type plants to shed light on the mechanisms underlying organ size and shape determination. Although causality between gene expression and sepal morphology has not been established, our approach opens the way to informed analysis for mutant characterization and functional studies

Spatial consistency of cell growth direction during organ morphogenesis requires CELLULOSE SYNTHASE INTERACTIVE1

International audienceExtracellular matrices contain fibril-like polymers often organized in parallel arrays. Although their role in morphogenesis has been long recognized, it remains unclear how the subcellular control of fibril synthesis translates into organ shape. We address this question using the Arabidopsis sepal as a model organ. In plants, cell growth is restrained by the cell wall (extracellular matrix). Cellulose microfibrils are the main load-bearing wall component, thought to channel growth perpendicularly to their main orientation. Given the key function of CELLULOSE SYNTHASE INTERACTIVE1 (CSI1) in guidance of cellulose synthesis, we investigate the role of CSI1 in sepal morphogenesis. We observe that sepals from csi1 mutants are shorter, although their newest cellulose microfibrils are more aligned compared to wild-type. Surprisingly, cell growth anisotropy is similar in csi1 and wild-type plants. We resolve this apparent paradox by showing that CSI1 is required for spatial consistency of growth direction across the sepal

Robustness of organ morphology is associated with modules of co-expressed genes related to plant cell wall

Reproducibility in organ size and shape is a fundamental trait of living organisms. Themechanisms underlying such robustness remain, however, to be elucidated. Taking the sepal ofArabidopsis as a model, we investigated whether variability of gene expression plays a role invariation of organ morphology. To address this question, we produced a dataset composed ofboth transcriptomic and morphological information obtained from 27 individual sepals fromwild-type plants. Although nearly identical in their genetic background, environment, anddevelopmental stage, these sepals exhibited appreciable variability in both morphology andtranscriptome. We identified modules of co-expressed genes in sepals, three of whichcorrelated significantly with morphology, revealing biologically relevant gene regulatorynetworks. Interestingly, cell-wall related genes were overrepresented in two of these threemodules. Additionally, we found that highly variable genes were unexpectedly enriched incell-wall related processes. We then analyzed sepal morphology from 16 cell-wall mutants andfound that the more a gene is expressed in wild-type, the more variable the morphology of thecorresponding mutant. Altogether, our work unravels the contribution of cell-wall related genesto the robustness of sepal morphology. More generally, we propose that canalizing traits duringdevelopment could rely on the modulation of highly expressed genes

Puzzle cell shape emerges from the interaction of growth with mechanical constraints

Abstract The puzzle-shaped cells found in the shoot epidermis of many plant species are a fascinating example of complex cell shapes. Because biological form often follows function, the unique shape of these cells suggests that they must serve some adaptive purpose for the plant. We previously proposed that these intricate shapes provide an effective strategy for reducing mechanical stress on the cell wall when epidermal cells undergo growth in more than one direction. Here we analyze a large selection of living and paleo plant species and find that the ability to make puzzle cells is a shared feature across all plant species, although their presence can be hidden as it varies depending on the organ, developmental stage, and environmental conditions. Computational modeling of Arabidopsis and maize epidermal cells revealed that presence and patterning of lobes is a dynamic process that is intricately linked to the growth history and environmental context of the plant organ. Conversely, disrupted lobeyness in mutants or with drug treatments affects plant development and leads to compensatory strategies. We propose that the mechanism underlying the formation of puzzle-shaped cells is likely conserved among higher plants and is a response to a developmental constraint driven by growth and mechanical stress