Involved in starch synthesis in endosperm amyloplasts (PubMed:17297616, PubMed:17586688, PubMed:21417378, PubMed:21595523, PubMed:21730357). Plays an important role in the elongation of amylopectin B chains (PubMed:17297616, PubMed:17586688, PubMed:21417378, PubMed:21595523, PubMed:21730357). [(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. Expressed in the endosperm. Expressed in developing caryopsis from 5 to 15 days after flowering. White-core floury endosperm (PubMed:17297616, PubMed:27791174). Increased content of resistant starch (RS) (PubMed:27791174). Endosperm containing increased levels of amylose (PubMed:17297616, PubMed:21417378). Reduced content of long amylopectin chains with a degree of polymerization (DP) of 30 or greater (PubMed:17297616, PubMed:17586688). Reduced content of amylopectin chains with a DP of 6 to 8 and a DP of 16 to 20, but increased content of chains with a DP of 9 to 15 and a DP of 22 to 29 (PubMed:17297616, PubMed:17586688). Decrease in average length of amylopectin chains (PubMed:21417378). Selection of varieties with reduced SSIIIA activity may be a tool to increase resistant starch (RS) content of rice grain. Belongs to the glycosyltransferase 1 family. Bacterial/plant glycogen synthase subfamily.