Involved in starch synthesis in endosperm amyloplasts (PubMed:16443699, PubMed:21417378, PubMed:21730357). Plays a role in the elongation of amylopectin chains (PubMed:16443699, PubMed:21417378, PubMed:21730357). Synthesizes preferentially amylopectin chains with a degree of polymerization (DP) of 7 to 11 by elongating chains with a DP of 4 to 7 (PubMed:16443699). Generates distincly chains with a DP of 8 to 12 chains from short chains with a DP of 6 to 7 (PubMed:16443699). [(1->4)-alpha-D-glucosyl](n) + ADP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + ADP + H(+) Glycan biosynthesis; starch biosynthesis. Amyloplast or chloroplast, soluble. Leaves and immature seeds. Decrease in average length of amylopectin chains (PubMed:21417378). Reduced content of amylopectin chains with a degree of polymerization (DP) of 8 to 12, but increased content of chains with a DP of 6 to 7 and a DP of 16 to 19 (PubMed:16443699). Three forms of soluble starch synthase were purified: RSS1, RSS2 and RSS3. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.