The plant hormones gibberellins (GAs) and auxin display overlapping activities in the regulation of multiple developmental processes, including the differential growth that mediates the response to tropic stimuli and the formation of the apical hook. Several mechanisms have been proposed that explain the interaction between these two hormones, such as the regulation of auxin transport by GAs, and the regulation of GA biosynthesis by auxin. GAs are known to exert their action at the transcriptional level by promoting the degradation of DELLA proteins, which in turn interact with numerous transcription factors and modulate their activity. We have identified INDOLE-3-ACETIC ACID METHYLTRANSFERASE 1 (IAMT1) as one of the earliest target genes upregulated after conditional expression of the DELLA protein GAI in Arabidopsis thaliana. In this Thesis, we have addressed two main issues: (1) the contribution of IAMT1 to auxin homeostasis and its biological relevance; and (2) the molecular mechanism by which DELLAs are able to induce the expression of IAMT1. Using combinations of iamt1 loss-of-function mutants and reporter lines for auxin accumulation and activity, we have found that IAMT1 activity is essential for proper generation and maintenance of the auxin gradients that underlie differential growth. According to our results, the role of IAMT1 would be to restrict polar auxin transport especially during the response to tropic stimuli, preventing excessive auxin accumulation in the responding tissues, and IAMT1 exerts this function, at least in part, by inhibiting the expression of the PIN genes, encoding auxin efflux carriers. Regarding the regulation of IAMT1 expression by DELLAs, dissection of the promoter, in silico analysis of putative DELLA partners, and molecular genetic analysis of reporter lines has allowed us to identify two mechanisms with different relevance depending on the environmental conditions, and through different cis elements. In etiolated seedlings, DELLA proteins are recruited by DORNRÖSCHEN (DRN) to the IAMT1 promoter to induce IAMT1 expression. In the light and in a temperature-dependent manner, DELLA proteins inhibit the DNA-binding activity of PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and BRI1 EMS-SUPPRESSOR1(BES1), which act as repressors of IAMT1 expression. The work presented here highlights how GAs may affect local accumulation of auxin, being particularly relevant in processes that involve differential growth. / Abbas, M. (2014). The control of auxin homeostasis through the regulation of IAMT1 by DELLA proteins [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/39348 / TESIS

Chez les plantes, la croissance est essentielle à la régulation de la forme de tout organe en développement. A l'échelle de la cellule, la croissance résulte de la compétition entre la pression interne de la cellule et la rigidité de sa paroi. Des hétérogénéités de ces grandeurs à l'échelle du tissu mènent alors à une croissance différentielle puis à des mouvements à l'échelle de l'organe entier. Ces mouvements macroscopiques peuvent être interprétés comme une manifestation des processus de croissance microscopiques. Nous proposons de mettre à profit ce lien entre croissance et mouvements afin d'approcher la croissance d'une nouvelle manière : de l'organe à la cellule. Nos travaux portent sur les feuilles composées d'Averrhoa carambola qui montrent deux mouvements typiques des feuilles en croissance,le déroulement et la nutation. Nous montrons dans un premier temps que la forme de la feuille en croissance est régulée de manière active. Nous étudions ensuite la cinématique du déroulement et de la nutation. Ce faisant, nous mettons en évidence une relation particulière entre croissance et croissance différentielle à partir de laquelle nous construisons un modèle cinématique de nutation. Conformément à de précédents résultats, ce modèle suggère que la croissance peut s'accompagner de contractions locales. Enfin, à l'échelle du tissu, la mécanique des parois cellulaires et leur composition sont étudiées. Nos résultats révèlent des hétérogénéités spatiales de ces deux paramètres au sein de la feuille, potentiellement cohérents avec la direction du mouvement de nutation. / In plants, growth is essential to the shape regulation of developing organs. It is also the key for every plant to adapt to its environment. At the cell level, growth relies on a competition between the inner pressure of the cell and the rigidity of its cell wall. Heterogeneities of these quantities across the tissue can result in differential growth and lead to motions of the whole organ. These macroscopic motions can thus be read as an outward signal of the cellular mechanisms underlying shape regulation and growth itself. Here, we propose to take benefit from this link between growth and motions to gain a new insight on growth through a multiscale approach, from the organ to the cell wall. We focus on the compound leaves of Averrhoa carambola which display two marked motions, unfurling and nutation, widespread among developing leaves. First, we show that the shape of the leaf results from an active regulation of its macroscopic mechanical properties. The kinematics of the two latter motions are then compared to the associated growth. Doing so, we put forward the specific patterns of growth and differential growth underlying the development of the leaf. Based on these results, we build a kinematic model for nutation which, accordingly with previous findings, suggests the occurrence of local contractions during growth. At the cell wall level, our results reveal spatial heterogeneities of both its rigidity and biochemical status within the leaf. These heterogeneities might be consistent with the direction of the nutation.

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