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Understanding the early mechanisms of graft union formation in Arabidopsis

Abstract : Grafting consists of the cutting and joining of two plants together to form one unique chimeric organism; it is done to combine biological traits of the two plants such as resistance to soil borne pathogens and high fruit quality in one plant. To survive, grafted plants, where all physical cell communications between aerial and root parts are interrupted by cutting, have to interact together to establish new exchange pathways. Nowadays, considerable efforts have been done to understand the developmental mechanisms underlying graft union formation in terms of hormone signaling, gene expression and histology however ultrastructural data are still lacking.Because of the difficulty of reliably targeting the graft interface under the electron microscope very few ultrastructural data of the graft interface are available; however such precise data is essential to understand the establishment of scion/rootstock communications.In this context, the aim of my thesis was to characterize the ultrastructural developments occurring during graft union formation to provide insights into the cellular mechanisms involved. First, a correlative light and electron microscopy (CLEM) approach was implemented on grafted hypocotyls of Arabidopsis thaliana and in vitro grafts of grapevine. Although quality obtained on the grapevine model still needs to be improved, the fluorescence and ultrastructure preservation obtained for A. thaliana allowed me to reveal four potential pathways of communication between the scion and rootstock: exosomes, plasmodesmata, phloem and xylem. The ultrastructural characterization of de novo plasmodesmata formed at the graft interface, by electron tomography, showed that more than 30 % of the attempts to form symplastic connection failed and resulted in the formation, both at scion or rootstock side of the cell wall, of hemi-plasmodesmata that do no span the cell wall. Moreover, the observations of events of secondary plasmodesmata biogenesis seem to indicate that plasmodesmata can be formed successfully between cells where there is a synchronous thinning of the cell wall. Finally, for the first time cytoplasmic exchanges were shown to happen at the graft interface and confirm that grafting combined with our CLEM approach could be a way to induce and study transplastomic events. They could play a role in wound healing or vascular reconnection. Additionally, biological tools have been developed to follow, in the future, the permeability of plasmodesmata at the graft interface of A. thaliana. The integration of ultrastructural and dynamic studies on different mutants will allow us to decipher the functional establishment of the cell-to-cell connectivity during the graft union formation.
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Clément Chambaud. Understanding the early mechanisms of graft union formation in Arabidopsis. Vegetal Biology. Université de Bordeaux, 2019. English. ⟨NNT : 2019BORD0395⟩. ⟨tel-02926313⟩

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