Thèse Tip-Sense l'Assemblage de Pectine dans la Croissance Apical H/F - 75

Établissement : Université Paris-Saclay GS Biosphera - Biologie, Société, Ecologie & Environnement, Ressources, Agriculture & Alimentation
École doctorale : Sciences du Végétal : du gène à l'écosystème
Laboratoire de recherche : IJPB - Institut Jean-Pierre Bourgin-Sciences du Végétal
Direction de la thèse : Kalina HAAS
Début de la thèse : 2026-10-01
Date limite de candidature : 2026-03-22T23:59:59

Dans le cadre de l'objectif global d'étudier les mécanismes cellulaires et biophysiques sous-jacents à l'expansion des cellules végétales, ce projet examinera le rôle de la formation d'un complexe pectine-protéine (pectine/RALF/LRX) dans l'assemblage de la paroi cellulaire et sa régulation par un complexe capteur de paroi cellulaire membranaire (LLG/RALF/CrRLKL). Le/la candidat(e) étudiera d'abord comment la dégradation enzymatique de la pectine et la formation du complexe pectine/RALF/LRX influencent le devenir de la pectine dans la paroi cellulaire. Il/elle étudiera ensuite l'organisation d'ordre supérieur du complexe capteur de paroi cellulaire in vitro et in vivo.

Plant cells are surrounded by a dynamic, carbohydrate-rich cell wall that must be continuously remodelled to allow growth, morphogenesis and adaptation to environmental cues. Cell wall assembly, is a complex process involving a large panel of polymers: cellulose microfibrils, hemicelluloses, pectins and proteins. This requires a precise coordination through feedback signalling, to accommodate the large stresses exerted by the turgor pressure on the thin expanding cell wall (Haas et al., 2021).
Pectins play a key role in this process, in particular as a structural component in the primary wall through formation of cross-links such as Rhamnogalacturonan-II-borate diester bonds or homogalacturonate (HG)-Ca2+ or HG-extensin crosslinks. The latter include the Leucin Rich Repeat extensin (LRX)-Rapid Alkalinisation Factor (RALF) complexes studied in this project (Moussu et al., 2023; Schoenaers et al., 2024). A part of the pectin is not incorporated, and appears to play a role as a scaffold that is recycled after wall assembly. The proportion of the respective pectin fractions has not been quantified. Knowing this is of importance, since HG is secreted in a highly methylesterified form, which is demethylesterified during the cel wall assembly process thus generating methanol and the greenhouse gas (GHG) methane. The estimation of pectin turnover is therefore important to evaluate the impact of plant biomass production on GHG emissions (Jardine et al., 2024).
Feedback control of wall assembly and growth occurs in part through plasma membrane malectin receptor kinases CrRLK1Ls. CrRLK1Ls have been reported to be part of two types of complexes. A first complex comprising the CrRLK1L FERONIA (FER), another plasma membrane kinase CRK1; the NADPH-oxidase RboH; the H2O2 channel PIP and phytochrome (PHY)B, which regulates ROS production upon high light stress (Mohanty et al., 2025). A second complex, consisting of the GPI-anchored protein LLG- linked through a RALF peptide with CrRLK1Ls, which binds MARIS, a cytoplasmic kinase and MLO, a multi-spanning membrane protein required for Ca2+ influx (Gao et al., 2023). This complex controls cytoplasmic Ca2+ levels, apoplastic alkalinisation and presumably the secretion of cell wall material during cell expansion. It is not known whether CrRLKL's partition between the two complexes or whether they are part of a single large protein or protein-polysaccharide complex.

Provide answers to the following questions:
1.What is the connection of pectin metabolism to cell expansion. Which pectin fraction is incorporated into the wall and which fraction is turned over ? How is this regulated ? What is the role of enzymatic pectin degradation and of Pectin-LRX-RALF complex formation in this process? Which pectin epitopes are recognized by the LRX-RALF complex in vitro and in vivo?
2.Which membrane complex(es) include CrRLK1L cell wall sensors. Is it a single higher order complex or do CrRLK1Ls partition between distinct membrane complexes with specialized functions ?
To address these questions, the project will focus on two tip-growing cell-types studied in the respective host laboratories: root hairs and pollen tubes.

Methode
1.Transient vs permanent role of pectins in cell wall assembly
-Quantify the fate of pectin in root hairs of WT and single and multiple mutants for root hair-specific RALF22 and pectate lyases PLL4/22 using confocal microscopy techniques.
2.Identify and characterize novel components of functional CrRLK1L receptor complexes involved in polarized growth,
-Use proximity labeling approaches (PUP-IT) combined with phosphoproteomics and biochemical characterization of higher-order membrane complexes, and investigate how their assembly regulates downstream signaling pathways.
-Define the molecular and spatial connections between pectin-LRX-RALF and LLG-RALF-CrRLK1L modules, by determining how these complexes interact with specific cell wall polymers in vivo. This will be achieved using epitope- and fluorescently tagged lines together with super-resolution microscopy to resolve their localization, organization and dynamics during pollen tube and root hair growth, complemented by in vitro interaction and stoichiometry analyses.