Thesis defense: Luka Lelas

Role of The Plant Cell Wall and Cell Wall-derived Oligosaccharides in Plant-Pathogen Interactions

The plant cell wall (PCW) is a carbohydrate-based matrix that not only confers mechanical strength to cells but is also involved in numerous physiological processes that shape the plant and allow it to adapt to its environment. Among these is plant immunity, particularly the production of oligosaccharides (OS). These molecules are damage-associated molecular patterns generated by the breakdown of cell wall polysaccharides. They result from action of the enzymatic arsenal deployed by pathogens during invasion of plant tissues. OS thus represent both emerging defense signals and promising tools for crop stimulation and plant protection. The growing interest in this field over recent decades has led to significant advances in understanding the structure, chemistry, and bioactivity of these molecules. However, due to the complexity of PCW chemistry and the diversity of enzymes involved in its degradation during plant–pathogen interactions, the range of OS studied remains relatively limited. This thesis aimed to broaden this aspect of OS research and to explore their effects and the role of PCW in three relevant pathosystems, tomato / Botrytis cinerea, Arabidopsis thaliana / B. cinerea and wheat / Zymoseptoria tritici. This was achieved through an interdisciplinary approach that combined chemistry, biochemistry, molecular biology and phytopathology, and, most importantly, the application of liquid chromatography coupled to mass spectrometry, which allowed detailed exploration of OS. First, this led to the discovery of the accumulation of inositol phosphate glycans, originating from plant plasma membrane glycosylinositol phosphorylceramides (GIPCs), during the A. thaliana–B. cinerea interaction. Furthermore, their production was shown to occur through the action of a putative fungal sphingomyelinase secreted by the invading pathogen, and their presence was found to correlate with lesion development in A. thaliana. Next, the exploration of the tomato / B. cinerea pathosystem revealed differences in tomato susceptibility, determined by varietal and age-related factors that can be explained by differences in pectin composition. Profiling of OSs released during the infection process qualitatively resembled previous observations in A. thaliana, and assays in which tomato leaves were coated with selected OSs and infected with multiple B. cinerea loss-of-function mutants for key enzymatic virulence factors revealed that their promoting or inhibiting effects on infection depend on their chemical structure. Selected OS groups were also tested in assays designed to reveal their effects at the molecular level, again demonstrating that their bioactivity depends on their chemical structure. Finally, the study of wheat cell wall changes triggered by infection with Z. tritici, the causal agent of Septoria tritici blotch, focused on two key elements that define the outcome of the pathogenic interaction: the Z. tritici avirulence factor AvrStb6 and the wheat Stb6 WAKL receptor that is involved in the recognition of AvrStb6. Using near isogenic wheat lines differing only in the presence of the resistance gene Stb6, and fungal strains differing in the presence or absence of AvrStb6, global compositional changes during infection were evaluated, and it was established that fine-scale PCW changes can also be monitored. In conclusion, from cell wall remodeling during pathogen infection and the diversity of oligosaccharides produced in this context to the link between their chemical structure and bioactivity, this thesis expands our understanding of how plasma membrane- and cell wall-derived oligosaccharides and PCW remodeling shape plant–pathogen interactions, uncovering mechanisms that refine our knowledge of plant cell wall biology, glycobiology and phytopathology.

Director:  Samantha Vernhettes, "Glycanes et Signalisation" GAS team, IJPB, Versailles

Co-supervision:  Aline Voxeur, "Glycanes et Signalisation" GAS team, IJPB, Versailles
Jury members
> Anissa Lounes-Hadj Sahraoui (Rapportrice) - UCEIV, Université du Littoral Côte d’Opale, Dunkerque
> Olivier van Wuytswinkel (Rapporteur) - BIOPI, Université Picardie Jules Verne, Amiens
> Jean-Félix Dallery (Examinateur) - INRAE, BIOGER, ECCP, Palaiseau
> Marie-Laure Follet-Gueye (Examinatrice) - GlycoMEV, Université de Rouen Normandie, Mont Saint-Aignan 
> > Marianne Delarue (Examinatrice) - Université Paris-Saclay, IPS2, Gif-sur-Yvette


Research developed at the Institute Jean-Pierre Bourgin for Plant Sciences.