Xyloglucan structure-dependent cell wall remodelling elicited by the exopolysaccharide of Hymenoscyphus fraxineus, the causal agent of ash dieback.

The plant cell wall is a dynamic matrix integrating mechanical, biochemical and immune signals during plant-microbe interactions. Although fungal exopolysaccharides (EPS) modulate these interactions, their structures and roles in cell wall remodelling remain unclear. Here we characterize the EPS secreted by Hymenoscyphus fraxineus, the causal agent of ash dieback, and demonstrate its biological activity in Fraxinus excelsior and Arabidopsis thaliana. Biochemical analyses revealed that the EPS is enriched in mixed linkage β-glucans (MLG). EPS exposure triggered substantial remodelling of cell wall architecture, including changes in monosaccharide profiles, increased crystalline cellulose deposition and extensive reorganisation of xyloglucan. These responses were altered in xyloglucan-deficient mutants, underscoring the requirement of specific wall components for effective glycan recognition and downstream signalling. EPS treatment also elicited oxidative burst and callose deposition in ash. In summary, we show that H. fraxineus EPS is composed mainly of small glucan molecules of 4-30 kDa in size that trigger cell wall-mediated immune signalling and structural remodelling. Our results highlight the sensory role of plant cell wall in the recognition of fungal glycans and further our understanding of glycan-driven immunity. The strong elicitor activity of EPS suggests potential applications in mitigating ash dieback.