Possesses endo-beta-mannanase activity in vitro. May be involved in seed germination by weakening the endosperm cap prior to radicle emergence. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Expressed specifically in the endosperm cap before radicle emergence. By gibberellin. Belongs to the glycosyl hydrolase 5 (cellulase A) family.