Functions in plant cell wall hydrolysis during mobilization of the endosperm in germinating grain or during the growth of vegetative tissues. Hydrolysis of (1->4)-beta-D-glucosidic linkages in beta-D-glucans containing (1->3)- and (1->4)-bonds. Glycan metabolism; beta-D-glucan degradation. Belongs to the glycosyl hydrolase 17 family.