P-239 - Cooperative enzymatic deacetylation of xylan facilitates Serendipita indica colonization of monocot roots

The beneficial endophytic fungus Serendipita indica colonizes roots through an initial biotrophic phase, followed by a late-stage transition characterized by localized, non-necrotrophic host cell death and the secretion of hydrolytic enzymes. The biological significance of this biphasic lifestyle and the specific roles of the secreted enzymes involved, remains unclear. Here, we identified a set of late stage expressed, monocot specific, co-regulated genes encoding proteins involved in carbohydrate uptake and metabolism, including secreted glycoside hydrolases (GHs) and carbohydrate esterases (CEs). Using recombinant enzyme assays and oligosaccharide mass spectrometry profiling, we demonstrated that S. indica GHs and CEs act cooperatively to hydrolyze acetyl-xylan. Genetic manipulation of extracellular acetyl-xylan esterase activity in transgenic S. indica led to altered colonization patterns, underscoring its functional importance. Furthermore, enzymatic treatment of barley cell walls modified the immunogenic properties of the resulting xylooligosaccharides, suggesting a role in host immune modulation. Our findings reveal that S. indica employs a coordinated enzymatic strategy to modify plant cell wall components at the late stage of colonization. This process likely supports fungal proliferation by optimizing nutrient acquisition and reshaping host immune responses, shedding new light on the functional role of hydrolytic enzymes in plant-fungal interactions.