Marie-Christine Ralet, Directrice de recherche, Rapporteur
Sylvia Colliec-Jouault, Chercheur Ifremer, Rapporteur
Azeddine Driouich, Professeur des universités, Examinateur
François-yves Bouget, Directeur de recherche, Président du jury
Cécile Hervé, Chargée de recherche CRNS, Co-directrice de thèse
Bernard Kloareg, Professeur des universités, Directeur de thèse
The cell wall of brown algae is a major cell compartment involved in many physiological responses including cell growth, development, or in adaptation to the physico-chemical changes of the environment. Like other photosynthetic organisms (plants, red and green algae), brown algae have a cell wall mainly composed of polysaccharides, but taking into account phylogenetic distances, the compounds are distinct, with notably alginates and sulfated fucans. Most knowledge on cell wall compositions comes from chemical extractions carried out on whole algal plants, with the induced lost of most cellular information. Today, monoclonal antibodies specifically directed towards alginates and sulfated fucans have been developed and characterized. These tools can be used to precisely localize at a cellular and tissue level their particular polysaccharide fractions. In addition to the information on the structure and composition of the cell wall, these antibodies allow to study the biological roles of the cell wall in many functional responses, including during early development. In this study, two model organisms of brown algae, the zygote of Fucus and the sporophytes of Ectocarpus, were used. In term of cell wall composition, glycoproteins known as arabinogalactan proteins (AGPs) have been identified as minor components in the cell walls of brown algae. They were shown to have a functional role in the Fucus embryogenesis. The use of our specific monoclonal antibodies allowed to locate alginates and fucans in cell walls during the development of our models. Notably, I have shown the involvement of sulfated fucans in normal apical growth and the crucial role of sulfate in this process.
Keywords: Cell wall, Alginates, Sulfated fucans, AGPs, Fucus zygote, Ectocarpus