Host-adapted enzymatic deconstruction of acetylated xylan enables mutualistic colonization of monocot roots

Intracellular accommodation of mutualistic fungi in plant roots depends on selective remodeling of host cell walls while minimizing activation of plant immune responses. In this study, we identify a host-adapted enzymatic module in the root endophyte Serendipita indica that targets acetylated xylan, a major structural component of monocot cell walls. The glycoside hydrolase Si GH11 cleaves the xylan backbone and releases O -acetylated oligosaccharides, which are subsequently deacetylated by the XynE-like esterase Si AXE. These enzymes are co-expressed within a monocot-specific transcriptional program that is enriched in carbohydrate-active enzymes and sugar transporters. Their combined activity enhances enzymatic degradation and facilitates downstream hydrolysis by exo-xylanases, which reduces the production of apoplastic reactive oxygen species triggered by damage-associated molecular patterns. Functional analysis shows that overexpression of Si AXE promotes early root colonization, while deletion of the gene compromises fungal proliferation during later stages. These findings define a coordinated and immune-compatible strategy for host cell wall deconstruction that enables fungal adaptation and endophytic colonization of monocot roots.