Abstract text: Hydathodes are specialized leaf-margin glands that exude xylem sap as droplets when its influx exceeds the evapotranspiration rate, a phenomenon known as guttation. Hydathodes consist of tracheids supplying a specialized tissue of small, thin-walled cells—the epithem—connected to permanently open water pores, forming a direct interface between the vascular system and the environment. Because guttation fluid provides favourable conditions for microbial growth, hydathodes are privileged entry points for vascular pathogens causing systemic diseases, such as Xanthomonas (Xcc). In response, plants have evolved defence mechanisms to limit pathogen colonisation and invasion through hydathodes, although the underlying immune processes remain poorly understood. We hypothesise that continuous cell wall remodelling in the epithem generates cell wall–derived elicitors (oligosaccharides, OS) that may shape the hydathode microbiome by promoting beneficial microorganisms and restricting pathogen development. We are analysing the OS composition of guttation fluid using oligoglycomic LC–MS approaches to investigate their potential roles in triggering defence responses or affecting microbial growth. We will also characterise associated signalling pathways involving receptors and pectin methylesterases (PMEs) localised in Arabidopsis thaliana hydathodes. This should allow us to determine whether PME-mediated pectin demethylesterification promotes pectin degradation in the epithem, generating oligogalacturonides that could keep the hydathodes defenses continuously activated.