May aid fertilization by loosening the cell wall of the stigma and style, thereby facilitating penetration of the pollen tube. Acts selectively on grass cell walls, which are relatively poor in pectins and xyloglucans and rich in glucuronoarabinoxylans and (1-3),(1-4)-beta-D-glucans, when compared with cell walls of other angiosperms, including other monocots (By similarity). Causes an allergic reaction in human. Binds to IgE. Belongs to the expansin family. Expansin B subfamily.