Brands_2026_Mol.Plant__

Intracellular accommodation of beneficial fungi requires host cell wall remodeling that avoids excessive immune activation. The root endophyte Serendipita indica, a generalist mutualist capable of colonizing both monocot and dicot plants, employs a monocot-specific enzymatic module to deconstruct acetyl-xylan, the dominant hemicellulose of grasses. Central to this module are the glycoside hydrolase SiGH11, which releases acetylated xylooligosaccharides, and SiAXE, a previously uncharacterized SGNH-like acetyl-xylan esterase that sequentially removes acetyl groups from soluble XOS. Both enzymes are co-expressed within a monocot-enriched transcriptional program that also includes sugar transporters and metabolic regulators. Their coordinated activity, together with co-expressed exo-enzymes, promotes efficient xylan hydrolysis while limiting the prolonged accumulation of immunogenic damage-associated molecular patterns (DAMPs). Functional genetics demonstrated that SiAXE is required for sustained intracellular growth in monocot roots: its deletion impaired colonization, whereas overexpression transiently accelerated entry but provoked immune responses, underscoring the importance of temporal regulation and enzyme coordination for immune-compatible colonization. These findings provide mechanistic insight into an immune-compatible fungal strategy for host cell wall remodeling and reveal how a broadly colonizing mutualist has repurposed ancestral saprotrophic enzymes into specialized host-adapted modules that balance nutrient acquisition with immune modulation.