Fungi hijack a plant apoplastic endoglucanase to release a ROS scavenging β-glucan decasaccharide to subvert immune responses

Plant pathogenic and beneficial fungi have evolved several strategies to evade immunity and cope with host-derived hydrolytic enzymes and oxidative stress in the apoplast, the extracellular space of plant tissues. Fungal hyphae are surrounded by an inner, insoluble cell wall (CW) layer and an outer, soluble extracellular polysaccharide (EPS) matrix. Here we show by proteomics and glycomics that these two layers have distinct protein and carbohydrate signatures, implicating different biological functions. The barley (Hordeum vulgare) {beta}-1,3-endoglucanase HvBGLUII, which belongs to the widely distributed apoplastic glycoside hydrolase 17 family (GH17), releases a conserved {beta}-1,3;1,6-glucan decasaccharide ({beta}-GD) from the EPS matrices of fungi with different lifestyles and taxonomic positions. This low molecular weight {beta}-GD does not activate plant immunity, is resilient to further enzymatic hydrolysis by {beta}-1,3-endoglucanases due to the presence of three {beta}-1,6-linked glucose branches and can scavenge reactive oxygen species. Additionally, exogenous application of {beta}-GD leads to enhanced fungal colonization in barley. Our data highlights the hitherto undescribed capacity of this often overseen fungal EPS layer to act as an outer protective barrier important for fungal accommodation within the hostile environment at the apoplastic plant-microbe interface. SignificanceHere we identify and characterize a conserved {beta}-1,3;1,6-glucan decasaccharide with antioxidant activity released from the fungal extracellular polysaccharide (EPS) matrix by the activity of a plant apoplastic endoglucanase. In addition, we provide a quantitative proteomic analysis of the fungal EPS and cell wall (CW) layers. HIGHLIGHTSO_LIThe fungal extracellular polysaccharide (EPS) matrix and the cell wall (CW) are specific layers with distinct protein and carbohydrate signatures C_LIO_LIA conserved {beta}-1,3;1,6-glucan decasaccharide ({beta}-GD) is released from the EPS matrices of different fungi by the activity of the barley {beta}-1,3-endoglucanase BGLUII, a member of the widely distributed apoplastic GH17 family C_LIO_LIThe {beta}-GD efficiently scavenges reactive oxygen species (ROS) and enhances fungal colonization C_LIO_LIThe immunomodulatory potential as microbe-associated molecular pattern (MAMP) as well as the biochemical activity as ROS scavenger of soluble low molecular weight {beta}-glucans are defined by the presence of {beta}-1,6-glucose branches C_LI