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. 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 roots, stems, seeds, inflorescences, flowers and leaves, and, to a lower extent, in seedlings. Induced by light, preferentially when associated with glucose, fructose or sucrose treatment, but repressed by darkness. Modified starch composition. This phenotype is enhanced when associated with SBE2.2 and SBE3 disruptions. Belongs to the glycosyl hydrolase 13 family. GlgB subfamily.