Fungal xylanase inhibitor. Possesses competitive inhibiting activity against fungal endo-1,4-beta-D-xylanases belonging to glycoside hydrolase family 11 (GH11). May function in plant defense against secreted fungal pathogen xylanases. Is similar to class III chitinases, but does not exhibit chitinase activity. Binds to fungal GH11 xylanases. Constitutively expressed in shoots. Belongs to the glycosyl hydrolase 18 family. Xylanase inhibitor subfamily.