We are using the yeast two-hybrid (Y2H) system to identify novel protein-protein interactions (PPI) relevant to wood formation. Bait proteins for Y2H binary assays and screening against a xylem cDNA prey library were selected from approximately 400 Populus trichocarpa genes that are at least 8-fold more highly expressed in differentiating secondary xylem versus phloem-cambium, and designated here as poplar biomass (PB) genes. Here we report some of the interactions involving selected PB proteins and efforts to characterize their functions in Populus and Arabidopsis. Members of the ROP GTPase family, PB15 in poplar and ROP11 in Arabidopsis, interact with the domain of unknown function (DUF) 620 (DUF620) proteins (e.g., PB129 in poplar). Ectopic co-expression of PB15 and PB129 in Arabidopsis caused outgrowths at the base of flower pedicels and altered leaf morphology. Interestingly, the co-expression phenotype could not be observed in transgenic plants that are only expressing either one of the interacting partners separately. Transgenics altered in expression of PB15 and/or PB129 were prepared in Populus and characterization of transgenic trees will be performed in greenhouse and field. In addition to DUF620 family proteins, ROP11 also interacts with the COP9 subunit CSN5A in Arabidopsis. We confirmed the interaction of ROP11 and CSN5A in Y2H and employed available mutants for ROP11 and CSN5A in Arabidopsis to genetically characterize this interaction. Surprisingly, loss of ROP11 was found to rescue the csn5a-2 pleiotropic phenotype. Ectopic expression of a ROP11 dominant negative mutant in the csn5a-2 background also complemented the stunted growth phenotype. Transcript analysis and gel blot assays showed that CSN5A transcript levels remained unchanged in all rescue lines, whereas CSN5A protein levels increased relative to WT. Taken together, we concluded that ROP11 negatively regulate CSN5A protein level in plant by some as yet unknown mechanism. / Ph. D.

Le rétromère est un complexe multiprotéique qui régule le trafic intracellulaire et qui est conservé chez les eucaryotes. Chez la levure et les animaux, le complexe rétromère est impliqué dans le recyclage des régulateurs de la polarité cellulaire. Chez Arabidopsis thaliana, les travaux de notre équipe ont démontré que ce complexe est requis pour la localisation polaire des transporteurs d’auxine de la famille PIN. Au cours de mon travail de thèse, j’ai approfondi la caractérisation de la fonction du composant AtVPS29 du rétromère et démontré son rôle dans la régulation de la croissance cellulaire polarisée et dans la polarité planaire chez la plante modèle Arabidopsis thaliana. Les mutants perte de fonction vps29 présentent des défauts de croissance polaire des tubes polliniques et des poils absorbants, et également des défauts de morphogenèse des cellules épidermiques des cotylédons. Les petites GTPases ROP (Rho-Of-Plant) sont les régulateurs majeurs de la polarité cellulaire chez les plantes. ROP1 est requis pour la croissance du tube pollinique, et ROP2 pour la morphogenèse des poils absorbants et des cellules épidermiques des cotylédons. Le travail décrit ici est destiné à comprendre les liens fonctionnels et moléculaires entre AtVPS29 et les protéines ROP. Mes résultats suggèrent que l’activité de ROP2 est dépendante de VPS29. J’ai également établi que VPS29 était nécessaire pour assurer la localisation subcellulaire de ROP1 et de ROP2. Chez les mutants vps29, l’analyse de la voie de signalisation ROP2 suggère que l’activité de ROP2 est fortement dérégulée. Ainsi, nos données montrent que le complexe rétromère de plante régule le trafic membranaire des protéines ROP, assurant la localisation de leur activité dans les bons territoires cellulaires, et in fine, la croissance polaire des cellules. Ces données suggèrent que le complexe rétromère de plante pourrait réguler la formation et/ou le maintien de la polarité chez différents types de cellules végétales. Ils soulignent également l’importance du trafic endocytique dans la régulation de la polarité cellulaire chez les plantes. / The retromer complex is a coat pentameric complex, strongly conserved among eukaryotes and involved in intracellular trafficking. In yeast and animals, the retromer complex participates in the polar targeting of regulators of cell polarity. Our group has previously shown that the retromer is required for the polar localization of auxin carrier proteins of the PIN-FORMED (PIN) family in Arabidopsis thaliana. Here, I demonstrate that the plant retromer component AtVPS29 regulates cell polarity, with a specific focus on polar cell growth and planar cell polarity in the model plant A. thaliana. Null vps29 mutants display defects in both pollen tube and root hair tip growth, as well as in the diffuse polar growth of epidermal pavement cells. Rho-of-plant (ROP) small GTPases are master regulators of cell polarity in plants. ROP1 is required for pollen tube growth, while ROP2 coordinates root hair and pavement cell morphogenesis. The aim of my work was focused on the functional and the molecular links between the AtVPS29 and ROP proteins. My results indicate that ROP2 signaling activity is VPS29-dependent in root hairs and in pavement cells. They also suggest that VPS29 is required for the proper localization of ROP1 and ROP2. The analysis of ROP2 signaling pathway in the vps29 mutant, suggest that ROP2 activity is completely deregulated in the absence of VPS29. Altogether, my data indicate that the plant retromer mediates ROP membrane trafficking to allow the proper localization of its activity, a prerequisite for cell polar growth. These findings suggest that the plant retromer might regulate maintenance and/or establishment of cell polarity in various cell types and further emphasize the importance of endocytic recycling in the regulation of polarity in plants.