Les gibbérellines (GA) sont une classe de phytohormones modulant différents aspects du développement des plantes. La biosynthèse des GA est catalysée par l’activité de différentes classes d’enzymes permettant la formation des formes bioactives. Si les mutants de biosynthèse sont nains, un excès de l’hormone provoque croissance excessive et stérilité. Ainsi, les plantes ont développé des mécanismes efficaces leur permettant de maintenir une concentration optimale de GA bioactives. Un niveau supplémentaire de régulation peut être constitué par une séparation spatiale de la biosynthèse dans différents types cellulaires et organes. A l’aide d’approches variées, nous démontrons qu’une forme intermédiaire est mobile sur de longues distances. Ce transport s’effectue à travers les vaisseaux vasculaires de la plante, et pourrait impliquer des transporteurs. Ensembles, nos résultats révèlent la nature et les propriétés biologiques du transport de GA sur de longues distances chez Arabidopsis. / Gibberellins (GA) are a class of diterpenoid hormones regulating major aspects of plant growth. GA biosynthesis from GGDP is catalyzed by the activity of different classes of enzymes leading to the formation of the active forms of GA. Thus GA biosynthesis mutants are dwarfs and late flowering, while GA overdose causes excessive growth and sterility. Therefore plants have evolved efficient mechanisms to maintain optimal levels of bioactive GA. However, an additional level of regulation may reside in the separation of the GA biosynthetic pathway into distinct cell types and organs. Through micro-grafting, genetic and biochemical approaches, we demonstrate that a GA intermediate is mobile over long distances in Arabidopsis. Moreover, this transport occurs through vascular tissues of the plant, and may involve specific transporters. Altogether, our results reveal the nature and the biological properties of GA long distances transport in Arabidopsis.

Dans les cellules végétales, les canaux et les transporteurs anioniques sont essentiels pour les fonctions clés telles que la nutrition, l'homéostasie ionique et la tolérance aux stress biotiques ou abiotiques. Chez Arabidopsis thaliana, les membres de la famille CLC (pour ChLoride Channel), situés sur le tonoplaste, sont requis pour l'homéostasie du nitrate (AtCLCa et AtCLCb) ou impliqués dans la tolérance au sel (AtCLCc).Dans mon travail de thèse, j’ai identifié et caractérisé un canal chlorure, AtCLCg, chez A. thaliana. L'étude de la protéine fusion AtCLCg::GFP a révélé que cette protéine est localisée sur le tonoplaste. Deux lignés mutants indépendants d’insertion ADN-T, atclcg ont été sélectionnés. Les études physiologiques sur ces deux lignés ont démontré qu’AtCLCg joue un rôle dans le passage de chlorure mais pas dans l'homéostasie du nitrate au travers du tonoplaste. En effet, aucune différence de contenu en nitrate (NO3-) racinaire et foliaire n’a été observée entre le sauvage et les mutants dans nos conditions. Par contre, les plantes mutantes présentent un phénotype par rapport au sauvage lorsqu'elles se développent sur milieu de croissance contenant 75 mM NaCl: (i) une diminution de 20% de la masse fraîche ; (ii) une diminution de 16% de la longueur de racines primaires et une réduction de 19% du nombre de racines secondaires ; (iii) une sur-accumulation de 21% et 26% de chlorure et sulfate foliaire, respectivement. Ces phénotypes sont abolis chez les lignés complétées avec 35S::AtCLCg. De plus, les mutants atclcg présentent un phénotype similaire à la présence de 75 mM KCl, mais aucune différence n'est détectée en réponse à 140 mM mannitol. Ce résultat suggère que le phénotype d'hypersensibilité des mutants atclcg dépend du chlorure et non du l'effet osmotique du stress salin.Sachant qu’AtCLCg et AtCLCc partagent un haut degré d'homologie, environ 75% d'identité au niveau des protéines, et que les deux sont impliquées dans la réponse au stress salin de la plante, nous avons généré le double mutant atclcc/atclcg. L’analyse phénotypique a montré que le double mutant ne présente pas un phénotype additif sur milieu de stress 75 mM NaCl. En parallèle, l'analyse de l'expression des gènes a montré qu’AtCLCg est réprimé dans le fond mutant atclcc, et inversement. Par ailleurs, l'analyse de l'expression de gène rapporteur démontre que PAtCLCg::GUS est fortement exprimé dans les cellules du mésophylle alors qu’une forte expression de PAtCLCc::GUS dans les cellules de garde et le pollen est observé. Ainsi, l’ensemble de ces résultats montrent que ces deux protéines AtCLCc et AtCLCg sont impliquées dans la réponse au stress salin de la plante, mais elles n’ont pas de fonction redondante. / In plant cells, anion channels and transporters are essential for key functions such as nutrition, ion homeostasis and, resistance to biotic or abiotic stresses. In Arabidopsis thaliana, members of the ChLoride Channel (CLC) family located on the tonoplast have been shown to be required for nitrate homeostasis (AtCLCa, AtCLCb) or involved in salt tolerance (AtCLCc). In this study, we identified and characterized the chloride channel AtCLCg in A. thaliana. Use of an AtCLCg:GFP fusion revealed the localization of this protein on the tonoplast. Studies on the disruption of the AtCLCg gene by a T-DNA insertion in two independent lines demonstrated that AtCLCg is involved in response to salt stress and not in nitrate homeostasis in our conditions. Although no difference in shoot and root NO3- content is observed, mutant plants show a phenotype compared to wild-type when they are grown on 75 mM NaCl: (i) a decrease by 20% of total plant fresh weight; (ii) a diminution by 16% of primary root length and a reduction by 19% of secondary root number; (iii) an over-accumulation of chloride and sulfate in shoots by 21% and 26% respectively. These phenotypes are abolished in complemented lines with 35S::AtCLCg. atclcg mutants show a similar phenotype in the presence of 75 mM KCl, but no difference is detected in response to 140 mM mannitol. This result suggests that the hypersensitivity phenotype of atclcg mutant depends on the ionic component and not on osmotic effect of salt stress.Knowing that AtCLCg and AtCLCc share a high degree of homology, approximately 75% of identity at protein level, and both are involved in response to salt stress, we generated a clcc/clcg double mutant. Phenotypic analysis showed that the two KO mutations do not have additive effect under salt stress of 75 mM NaCl. In parallel, gene expression analysis showed that AtCLCg is repressed in the clcc mutant background, and conversely. Expression analysis of reporter gene displayed a different pattern for PAtCLCg::GUS, strongly expressed in mesophyll cells, compared with a strong expression of PAtCLCc::GUS in guard cells and pollen. Altogether these results demonstrate that both AtCLCc and AtCLCg are involved in response to salt stress but they are not functionally redundant.

Artemisia annua L. is a common type of wormwood that grows throughout the world. Artemisinin, a terpene compound in A. annua, has recently been recognized as the most promising antimalaria drug. Artemisinin and other types of terpenoids are synthesized and accumulated in glandualr trichomes that appear on the surface of leaf, stem and flower bud. To identify new genes involved in artemisinin biosynthesis and trichome function in A. annua, a normalized glandular trichome cDNA collection was sequenced by Roche GS FLX pyrosequencing system. Two sequencing runs generated totally 85M nucleotides which were further assembled into 190,377 unigenes (42,678 contigs and 147,699 sigletons). Putative functions were assigned to the unigenes based on Blast search against GeneBank database. Many terpene biosynthesis pathway genes were identified from the pyrosequencing ESTs. Together with other identified A. annua terpene pathway genes, a global view of terpene biosynthesis in glandular trichomes of A. annua were re-established. Meanwhile, a WD repeat protein, AaWD40, which show high amino acid sequence similarity with its Arabidopsis ortholog, AtTTG1 (AT5G24520) was identified. To investigate the functional relevance of AaWD40 to its Arabidopsis counterpart, genetic complementation test using Arabidopsis mutants was conducted. When AaWD40 was transformed into Arabidopsis transparent testa glabrous1 (ttg1-1) mutant, the anthocyanins and proanthocyanidin (PAs) production in seeds were restored, and the trichomeless phenotype of ttg1-1 mutant was rescued. In addition, over-expression of AaWD40 and AtTTG1 modulated the expression of WUS and CLVs genes which are required to maintain the stem-cell niche of Arabidopsis shoot apex. Transcriptomic profiling of transgenic Arabidopsis over-expressing AaWD40, TTG1, or ttg1-1 mutant revealed lists of genes modulated by these two WD40 genes homologue and gene ontology (GO) analysis suggested that the top-ranked categories are defense, stress response and developmental programme. We hypothesize that WD40 repeat protein act as a crucial regulatory factor in a wide variety of cellular functions in A. thaliana. / Wang, Wei. / Advisers: Guo Dianjing; Jiang Liwen. / Source: Dissertation Abstracts International, Volume: 73-02, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 82-105). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

La méthylation des histones constitue un niveau important de contrôle épigénétique chez les eucaryotes. Mes études portent sur la caractérisation des facteurs potentiellement intervenant dans la mise en place et la lecture de la méthylation pour mieux apprécier son rôle et des mécanismes sous-jacents dans la régulation de la transcription et du développement des plantes chez l’Arabidopsis thaliana. Ainsi, la première partie de mes travaux de thèse a contribué à l’étude d’une protéine à domaine SET (SET DOMAIN GROUP7, SDG7) et à montrer que SDG7 est nécessaire au bon déroulement de l'induction de VIN3 et du processus de vernalisation pour la floraison. Nos résultats suggèrent que SDG7 pourrait méthyler une protéine non-histone encore inconnue dans la régulation de la transcription et le contrôle de la durée de vernalisation. La deuxième partie de ma thèse porte sur l’étude de SDG8 et les H2B-UBIQUITIN-ligases HUB1/HUB2 pour examiner un cross-talk éventuel entre la triméthylation de H3K36 (H3K36me3) et la monoubiquitination d’H2B (H2Bub1). Nous avons montré que H3K36me3 et H2Bub1 sont déposés largement indépendamment, qui diffère d’une dépendance hiérarchique de déposition préalablement observée chez la levure. La dernière partie de ma thèse a permis l’identification des protéines HUA2/HULK2 à domaine PWWP comme lecteurs éventuels de H3K36me3 dans la régulation de la floraison et du développement des plantes. / Histone methylation is one of the keys epigenetic marks evolutionarily conserved in eukaryotes. My study focuses on the characterization of factors potentially involved in the deposition and reading of lysine (K) methylation to appreciate its role and underlying mechanisms in the regulation of transcription and plant development, using Arabidopsis thaliana as a model organism. In the first part of my thesis, I report on our study of SET DOMAIN GROUP7 (SDG7), a protein containing the evolutionarily conserved SET domain, which is generally recognized as a signature of K-methyltransferases. We found that SDG7 plays an important role in the regulation of VIN3 induction associated with cold duration measure during vernalization treatment. Intriguingly, levels of several different histone methylations were found unchanged in the sdg7 mutant plants and the recombinant SDG7 protein failed to show a histone-methyltransferase activity in vitro. We thus conclude that SDG7 might methylate a yet unknown non-histone protein to regulate transcription and proper measurement of the duration of cold exposure in the vernalization process. In the second part, I studied interaction between SDG8 and HISTONE MONOUBIQUITINATION1 (HUB1) and HUB2. My results unravel that H3K36me3 and H2Bub1 are deposited largely independently in Arabidopsis, which is in contrast to the dependent crosstalk of these two different epigenetic marks previously reported in yeast. In the last part of my thesis, I report on the identification of the PWWP-domain proteins HUA2/HULK2 as readers of H3K36me3 and demonstrate that sdg8 and hua2 genetically interacts in the regulation of flowering time.

Yu, Pui Man. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 68-78). / Abstracts in English and Chinese. / Acknowledgements --- p.iv / Abstract --- p.v / Table of Contents --- p.ix / List of Figures --- p.xii / List of Tables --- p.xiv / List of Abbreviations --- p.xv / Chapter Chapter 1. --- General Introduction --- p.1 / Chapter Chapter 2. --- Literature Review --- p.5 / Chapter 2.1 --- The importance of terpenoids --- p.5 / Chapter 2.2 --- The difficulties in synthesizing terpenoids --- p.8 / Chapter 2.3 --- Structure and classification of terpenoids --- p.9 / Chapter 2.4 --- MVA and MEP pathways of terpenoid biosynthesis in higher plants --- p.12 / Chapter 2.4.1 --- The MVA pathway --- p.16 / Chapter 2.4.2 --- The MEP pathway --- p.18 / Chapter 2.5 --- The crosstalk between MVA and MEP routes --- p.20 / Chapter 2.6 --- The farnesyl diphosphate is a key enzyme in terpenoid biosynthetic pathway --- p.20 / Chapter 2.7 --- The glutaredoxin system --- p.22 / Chapter Chapter 3. --- Materials and Methods --- p.25 / Chapter 3.1 --- Plant materials and growth condition --- p.25 / Chapter 3.2 --- DNA extraction and screening of fps mutants --- p.25 / Chapter 3.3 --- Validation of the fps mutant by semi-quantitative RT-PCR --- p.26 / Chapter 3.4 --- Semi-quantitative RT-PCR analysis of the fps mutants --- p.28 / Chapter 3.5 --- Genechip analysis of fps mutants --- p.29 / Chapter 3.6 --- Enzyme assays --- p.29 / Chapter 3.7 --- Triterpene and sterol analysis of fps mutants --- p.30 / Chapter 3.8 --- Preparation of carotenoid standards for carotenoid analysis --- p.31 / Chapter 3.9 --- Carotenoids analysis of fps mutants by HPLC --- p.31 / Chapter 3.10 --- Subcellular localization of FPS 1 and FPS2 by transient expression --- p.33 / Chapter Chapter 4. --- Results --- p.36 / Chapter 4.1 --- Screening of fpsl and fps2 homozygous mutants --- p.36 / Chapter 4.2 --- Validation of fps mutants by RT-PCR and enzyme activity assay --- p.36 / Chapter 4.3 --- Genechip analysis of two fps mutants --- p.40 / Chapter 4.3.1 --- Quality control and normalization of microarray sample --- p.40 / Chapter 4.3.2 --- Normalization and identification of differentially expressed genes --- p.42 / Chapter 4.3.3 --- GO annotation of differentially expressed genes in fps mutants --- p.43 / Chapter 4.3.4 --- Genes participate in stress and defense response were differentially expressed in both fpsl and fps2 mutants --- p.48 / Chapter 4.3.5 --- Genes in the plastidial pathway were down-regulated --- p.51 / Chapter 4.4 --- Effects of FPS mutations on pathway enzymes --- p.53 / Chapter 4.5 --- Effects of fps mutants on terpenoids and sterol metabolism --- p.55 / Chapter 4.6 --- Comparison on carotenoids and chlorophyll contents --- p.55 / Chapter 4.7 --- Subcellular localization of FPS 1 and FPS2 --- p.61 / Chapter Chapter 5. --- Discussion --- p.62 / Chapter Chapter 6. --- Conclusion --- p.67 / Reference --- p.68 / Appendices --- p.79 / Appendix A. Primers designed for homozygous mutant screening for fps mutants --- p.79 / "Appendix B. Primer pairs designed for fps ORF, common sequence and fpsl specific region" --- p.80 / Appendix C. Primer pairs designed for studying expression level of the downstream genes of FPS --- p.81 / Appendix D. Annotations of differentially expressed genes in fpsl mutant --- p.82 / Appendix E. Annotations of differentially expressed genes in fps2 mutant --- p.84 / Appendix F. Log fold changes of terpenoid pathway genes involved in FPS mutants --- p.94

San, Wan Yan. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 75-78). / Abstracts in English and Chinese. / Thesis/Assessment Committee --- p.i / Statement --- p.ii / Abstract --- p.iii / 摘要 --- p.v / Acknowledgements --- p.vi / Table of Contents --- p.viii / List of Tables --- p.x / List of Figures --- p.xi / List of Abbreviations --- p.xii / Chapter Chapter 1 --- General Introduction --- p.1 / Chapter 1.1 --- The Plant Secretory Pathway --- p.1 / Chapter 1.2 --- AtEMP70 As a Potential Candidate in PVC Proteomics Analysis --- p.4 / Chapter 1.3 --- EMP70 Protein Family --- p.6 / Chapter 1.3.1 --- Arabidopsis EMP70 Protein Family --- p.6 / Chapter 1.3.2 --- EMP70 Homologs Among Different Species --- p.9 / Chapter 1.4 --- Aims of This Study --- p.10 / Chapter Chapter 2 --- Materials and Methods --- p.12 / Chapter 2.1 --- Generation of Arabidopsis cDNA --- p.12 / Chapter 2.2 --- Plasmid Construction --- p.13 / Chapter 2.3 --- Transformation of Tobacco BY-2 Cells --- p.14 / Chapter 2.4 --- Confocal Immunofluorescence Studies --- p.15 / Chapter 2.5 --- Drug Treatments --- p.16 / Chapter 2.6 --- Transient Expression in Protoplasts --- p.16 / Chapter 2.7 --- Generation of Antibodies --- p.18 / Chapter 2.8 --- SDS-PAGE and Western Blot Analysis --- p.19 / Chapter 2.9 --- Microsomal Protein Extraction --- p.21 / Chapter 2.10 --- Subcellular Fractionation --- p.21 / Chapter 2.11 --- Membrane Strip-off --- p.23 / Chapter Chapter 3 --- Results --- p.24 / Chapter 3.1 --- Subcellular Localization Study of GFP-tagged AtEMP2 Fusions via Transient Expression --- p.24 / Chapter 3.1.1 --- AtEMP2-GFP Localized to TGN in BY-2 Protoplasts --- p.24 / Chapter 3.1.2 --- AtEMP2-GFP Localized to TGN in Arabidopsis Protoplasts --- p.30 / Chapter 3.1.3 --- N-terminal GFP-tagged AtEMP2 Fusions Localized to the Golgi Apparatus in Arabidopsis Protoplasts --- p.33 / Chapter 3.2 --- Generation and Characterization of Transgenic Tobacco BY-2 Cells and Arabidopsis PSB-L Cells Expressing AtEMP2-GFP Fusion --- p.36 / Chapter 3.2.1 --- Subcellular Localization of AtEMP2-GFP Fusion in Transgenic BY-2 Cell Lines --- p.36 / Chapter 3.2.2 --- Subcellular Localization of AtEMP2-GFP Fusion in Transgenic Arabidopsis PSB-D Cell Lines --- p.39 / Chapter 3.3 --- Immunofluorescent Labeling Study --- p.41 / Chapter 3.3.1 --- ManI Antibodies Did Not Label the Punctate Organelles --- p.41 / Chapter 3.3.2 --- AtEMP2 Antibodies Labeled the Golgi Apparatus --- p.43 / Chapter 3.4 --- Generation of AtEMP70 Antibodies --- p.46 / Chapter 3.5 --- Western Blot Analysis --- p.50 / Chapter 3.5.1 --- Heat Treatment Caused Aggregation of AtEMP2-GFP Fusion Proteins --- p.51 / Chapter 3.5.2 --- Size Change of AtEMP2-GFP Fusion Proteins in Response to Heat Treatment --- p.52 / Chapter 3.5.3 --- Aggregation Formation of AtEMP2-T7 Fusion Proteins in 95°C --- p.56 / Chapter 3.5.4 --- Distribution of Endogenous AtEMP70 in Arabidopsis Wild Type Cells --- p.58 / Chapter 3.6 --- Subcellular Fractionation --- p.61 / Chapter 3.6.1 --- C-terminal GFP- or T7-tagged Fusion Affected the Subcellular Localization of AtEMP2 --- p.61 / Chapter 3.6.2 --- Endogenous AtEMP70 Localized to the Golgi Apparatus --- p.64 / Chapter Chapter 4 --- Discussion and Future Perspectives --- p.67 / Chapter 4.1 --- Discussion --- p.67 / Chapter 4.1.1 --- ER Export Signal in the Cytosolic Tail of AtEMP70 --- p.71 / Chapter 4.1.2 --- Potential Golgi Retention Signal in the Cytosolic Tail of AtEMP70 --- p.73 / Chapter 4.2 --- Future Perspectives --- p.74 / References --- p.75

Dans les cellules eucaryotes, différents types de ribosomes coexistent. Les ribosomes mitochondriaux synthétisent les quelques protéines codées par l’ADN mitochondrial, qui sont essentielles au fonctionnement de l’organisme. Ces ribosomes sont particulièrement divergents des ribosomes procaryotes, mais sont également très différents entre les eucaryotes. Mon travail de thèse s'est concentré sur la caractérisation de la structure et de la composition en protéines du ribosome mitochondrial de la plante modèle Arabidopsis thaliana. Des approches biochimiques complémentaires ont permis d’identifier 19 protéines uniquement trouvées dans le mitoribosome de plante, parmi lesquelles 10 sont des protéines PPR, des protéines particulièrement abondantes chez les plantes. Les mutations des gènes codant pour ces PPR ribosomales (rPPR) mènent à l’apparition de phénotypes macroscopiques distincts, notamment une létalité ou des retards de croissance importants. L'analyse moléculaire du mutant rppr1 par profilage des ribosomes, ainsi que l'analyse du taux de protéines mitochondriales, révèlent que la protéine rPPR1 est un facteur de traduction générique, ce qui constitue une nouvelle fonction des protéines PPR. De plus, la cryo-électron microscopie a été utilisée pour déterminer l’architecture tridimensionnelle de ce mitoribosome. Cette approche a révélé la structure unique du mitoribosome de plante, caractérisée par une très grande petite sous-unité ribosomale ayant un domaine additionnel jamais décrit jusqu’à présent. Globalement, mes résultats ont montré que le mitoribosome d’Arabidopsis est complètement différent des ribosomes bactériens et des autres mitoribosomes eucaryotes, à la fois en terme de structure mais aussi de composition, permettant ainsi de mieux comprendre l’évolution de ce composant central de l’expression génétique. / Ribosomes are the molecular machines translating the genetic information carried by mRNA into protein. Different translation machineries co-exist in eukaryote cells. While cytosolic translation is comparatively well characterized, it remains the most elusive step of gene expression in mitochondria. In plants, while numerous pentatricopeptide repeat (PPR) proteins are involved in all steps of gene expression, their function in translation remains unclear. My work focused on the biochemical characterisation of Arabidopsis mitochondrial ribosomes and the identification of its protein composition. Complementary biochemical approaches identified 19 plant specific mitoribosome proteins, among which 10 are PPR proteins. The knock out mutations of ribosomal PPR (rPPR) genes result in distinct macroscopic phenotypes including lethality or severe growth delays. The molecular analysis of rPPR1 mutants, using ribosome profiling, as well as the analysis of mitochondrial protein levels, revealed that rPPR1 is a generic translation factor, which is a novel function for PPR proteins. Finally, single particle cryo-electron microscopy was used and revealed the unique structural architecture of Arabidopsis mitoribosomes, characterised by a very large small ribosomal subunit, larger than the large subunit, with a novel head domain. Overall, my results showed that Arabidopsis mitoribosomes are completely distinct from bacterial and other eukaryote mitoribosomes, both in terms of structure and of protein content.

L'ARN interférence est un mécanisme moléculaire conservé chez les Eucaryotes dont les principaux acteurs sont les protéines ARGONAUTE (AGO). Chez les plantes, AGO1 est une protéine essentielle à la croissance et la défense antivirale. Elle utilise des petits ARNs comme sondes pour reconnaître et réguler des ARN messagers. Les virus ont développé des suppresseurs de l'ARN interférence pour surmonter cette défense. L'un d'entre eux, P0 du virus de la mosaïque jaune du navet, est comme une protéine F-box qui détourne le complexe SCF, une ubiquitine ligase E3, et conduit AGO1 vers la protéolyse ubiquitine-dépendante. Cette dégradation utilise la vacuole au lieu du protéasome 26S, généralement associé à la dégradation ubiquitine-dépendante. Ce mécanisme de protéolyse n'est pas compris et est aussi apparent quand AGO1 est déstabilisé de manière endogène, suggérant que P0 utilise une voie déjà existante. Une protéine F-box d'Arabidopsis, FBW2, a été décrite comme impactant l'homéostasie d'AGO1 indépendamment du protéasome. Mon projet de thèse visait à caractériser l'activité F-box de FBW2 et à comprendre la relation entre AGO1 et FBW2 ainsi que ses conséquences sur l'ARN interférence. Les résultats obtenus dans ce manuscrit montrent que le complexe SCFFBW2 interagit avec AGO1 et déclenche sa dégradation via un processus indépendant de l'autophagie ou du protéasome, tout en n'affectant que faiblement l'ARN interférence. FBW2 ciblerait en fait un sous-ensemble de protéines AGO1 qui semble ne pas contenir de petits ARNs. Cette régulation jouerait un rôle de surveillance pour prévenir une activité délétère d'AGO1 en absence de petits ARNs. / RNA silencing is a conserved molecular mechanism in eukaryotes, of which the main effectors are the ARGONAUTE (AGO) proteins. In plants, AGO1 is a protein that is essential for growth and antiviral defence. It uses small RNAs as probe to recognize and regulate messenger RNAs. Viruses have developed suppressors of RNA silencing to overcome this defence. One of these, P0 from the Turnip Yellows Virus, acts as an F-box protein to hijack the SCF complex, an E3 ubiquitin ligase, and guide AGO1 to the ubiquitin-dependent proteolysis. This degradation uses the vacuole instead of the 26S proteasome, generally associated with ubiquitin-dependant proteolysis. This proteolysis mechanism is not understood and is also apparent when AGO1 is endogenously destabilized, suggesting that P0 uses an already existing pathway. An Arabidopsis F-box protein, FBW2, has been shown to impact AGO1 homeostasis independently from the proteasome. My PhD project aimed at characterizing FBW2 F-box activity and understanding the relationship between AGO1 and FBW2, as well as its consequences on the RNA silencing. The results obtained in this manuscript show that the SCFFBW2 interacts with AGO1 and triggers its degradation through an autophagy- and proteasome- independent process, while only weakly affecting the RNA silencing. FBW2 would actually target a subset of AGO1 proteins, which appears not to contain small RNAs. This regulation would play a surveillance role in order to prevent a deleterious activity of AGO1 in absence of small RNAs.

No description available.

Abstract not available