Catalyzes the formation of the alpha-1,6-glucosidic linkages in starch by scission of a 1,4-alpha-linked oligosaccharide from growing alpha-1,4-glucan chains and the subsequent attachment of the oligosaccharide to the alpha-1,6 position. Essential during embryogenesis. Transfers a segment of a (1->4)-alpha-D-glucan chain to a primary hydroxy group in a similar glucan chain. Glycan biosynthesis; starch biosynthesis. Monomer. Mostly expressed in flowers and inflorescence, and, to a lower extent, in seedlings, roots, stems, leaves, siliques and seeds. Embryolethal. Embryo developmental arrests at the heart stage associated with reduced cell divisions and abnormal cell differentiation, thereby leading to defects in setting up the shoot apical meristem, embryonic vascular tissues and cotyledons. Modified starch composition. This phenotype is enhanced when associated with SBE2.1 and SBE2.2 disruptions. Belongs to the glycosyl hydrolase 13 family. GlgB subfamily.