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No description available.

A reaction-diffusion system, which can be considered as a generalised Schnakenberg-like model, is studied mathematically in 1D and 2D. This system models an initiation process within a root-hair cell which involves biochemical interactions of the G-proteins, known as Rho of Plants, or ROPs. These proteins attach to the cell membrane prompting a localised patch which, in consequence, induces cell wall softening and subsequently hair growth. This model assumes that the auxin provided is the key catalyst. Auxin is a plant hormone which is known to because of many different features in plant morphogenesis. Also, this hormone is experimentally known to enter the cell including a spatially dependent gradient.. Numerical bifurcation analysis is carried on in order to explore solutions which resemble all features that the G-proteins and auxin arc known to cause. The main bifurcation parameters arc taken to be the overall auxin rate, and the cell length. The analysis is backed up by full numerical simulations and asymptotic analysis using semi-strong interaction theory. The asymptotics not only provides existence of solutions and explains numerical properties, but also sheds light on transition mechanisms via the theory of competition instability and transverse instability of homoclinic stripes. The analytical results are found to agree favourably with numerical simulations, and to give further explanation of the agreement between the model and biological data for different scenarios. From a mathematical point of view, pattern formation of non-homogeneous reaction-diffusion systems is a subject that is not yet well understood. However, upon using the theory of semi-strong interactions, light is shed on the dynamics and instabilities that. spatially dependent coefficients bring about. As a consequence, transitions between different spot-like patterns and the dynamics of their location can be explored and theoretically explained.

Plants respond to environmental signals by following growth responses termed tropisms. like gravity, light and touch. Root gravitropism, for example, is the response by which the primary root orients its growth according to the gravity vector. There is a lot of genetic and molecular evidence that support the crucial role of the phytohormone auxin during root gravitropism. For example mutations in genes' encoding aux in synthesis, signalling and transport proteins resulted in agravitropic or partially agravitropic phenotypes. Two novel auxin biosensors, DII-VENUS and DR5:VENUS had been used to monitor auxin abundance and auxin response respectively in mutants affecting the above mentioned aux in processes, Results reflected the powers of these 2 reporters combined as they \,,,'ere able to reflect a tight relation between the root tip bending and the formation of an auxin gradient following a 90° gravistimulus, They were also able to uncover unexpected effects on auxin homeostasis in arj7arf19 that were validated by gas chromatography- based studies. Despite the wealth of information describing a lot of auxin mutants, there are a lot of genes that had not been characterised yet. So, a transcriptomics approach was followed to discover some of these genes, Results showed thai over 500 genes were observed to be differentially expressed following a gravity stimulus over an 8 hours time course. Many of these genes were shown to be expressed in an ARF7/ARFI9 dependant manner. Functional studies identified several mutants which had slower rate of bending following a 90° gravistimulus: AT5G47540 (similar to M025 protein; unknown function), AT2G27660 (Del -domain containing protein, zinc ion binding: unknown function) and AT2G25900 (ATCTH; Transcription factor containing a CCCH-type zinc ion binding domain). Although a lot of work has been done on root gravitropism in A. Ihaliana, the natural variation within this model system was not explored a lot. So, the Gravitropic Set Angle (GSA) - or the angle at which the root normally grows with respect to the gravity vector - for the Arabidopsis ecotype, from the Cape Verde islands (Cvi) was studied in comparison with the laboratory strain Landsberg erecta (Ler-l). The GSA phenotype in the Cvi ecotype shows a skewing of the primary root to the left at an angle of about 90°, Using Near Isogenic Lines (NILs), this root trait was shown to be controlled by a Cvi-0 region on chromosome 2. This region was mapped to a region between 11. 16 Mb and 11.28 Mb.

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Ma thèse s'inscrit dans le cadre du projet européen ENERGYPOPLAR, dont l'objectif est d'améliorer la production de biomasse d'hybrides de Peuplier dans un système de sylviculture intensive. La propagation clonale, utilisant des boutures, est un outil approprié dans la production rapide. Une étape clé de cette propagation clonale est la formation de racines adventives. Ainsi, j'ai analysé la capacité à enraciner de plusieurs génotypes de peuplier sélectionnés. Cette étude suggère que certains génotypes présentent des facilités à enraciner par rapport à d'autres. Le principal objectif de la thèse a été d'identifier des gènes clefs de la rhizogenèse adventive du peuplier, plus précisément le rôle des facteurs de transcription (FT). J'ai débuté cette étude, par une analyse transcriptomique globale à différents stades de la formation de la racine adventive de P. trichocarpa. Durant le développement de la racine adventive, des transcrits des familles PIN, GH3, AUX/IAA, AUX/LAX et ARF sont régulés suggérant l'implication des voies de signalisation de l'auxine. De plus, cette analyse a révélé la régulation de l'expression de la famille de facteurs de transcription AP2/ERF, particulièrement le membre AIL1. Des études phénotypiques des lignées transgéniques de Peuplier (35S::AIL1 et RNAi), indiquent une variation dans la formation et dans le nombre de racines adventives. Dans le but de décrypter le réseau de gènes associé à l'expression de PtAIL1, j'ai réalisé une analyse transcriptomique des plantes transgéniques et du sauvage. Cette analyse a permis de suggérer des cibles potentielles de PtAIL1. Mes résultats montrent le rôle de PtAIL1 dans la formation des racines adventives / My thesis was part of the european project ENERGYPOPLAR aiming to improve biomass yield of poplar hybrids in a intensive forestry system. Clonal propagation, by use of cuttings, is a very powerful tool in the fast production of poplars. A key step of this process is the adventitious root formation. Therefore, I investigated the rooting behavior of several selected poplar genotypes. Differences in rooting capacity could be shown suggesting the existence of easy-to-root and difficult-to-root genotypes. The main goal of my thesis was to identify key genes involved in the adventitious rooting process in poplar, with particular interest in the role of transcription factors (TFs). An initial genome-wide transcript profiling of the different stages of adventitious root formation in P. trichocarpa was carried-out. Auxin signaling seems to be involved in poplar adventitious root development, since the transcript level of several gene families like PIN, GH3, AUX/IAA, AUX/LAX and ARF were regulated. Further, the AP2/ERF transcription factors family and in particular AIL1 were identified as important factors involved in adventitious rooting. The phenotypic characterization of poplar transgenics lines (35s::AIL1 and RNAi lines) showed differences in the chronology of adventitious root formation and in the number of roots formed. In order to investigate the pathway(s) associated with PtAIL1 expression, transcriptomic analyses of transgenic lines and wild type were performed. Agamous-like and myb transcription factors were identified as possible candidate genes.PtAIL1 is one of the key regulators involved in adventitious root formation in poplar