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. May preferentially transfer short chains during branching. Responsible for the synthesis of about 75% of the amylopectin found in the starch granules of mature embryos. 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. Expressed in roots, leaves, stipules, pods and flowers. Highly expressed during early stages of embryo development. Decreasing expression during the embryo maturation. Wrinkled seeds. The wrinkled seeds phenotype (rr) studied by Mendel in 1866 is probably caused by an 800 bp insertion in the SBEI gene, leading to the loss of the last 61 amino acids of the protein and a complete absence of activity. Belongs to the glycosyl hydrolase 13 family. GlgB subfamily. On the garden pea - Issue 159 of April 2014