Sucrose-cleaving enzyme that provides UDP-glucose and fructose for various metabolic pathways. an NDP-alpha-D-glucose + D-fructose = a ribonucleoside 5'-diphosphate + H(+) + sucrose Forms homotetramers and heterotetramers with SS1, all three possible heterotetramers are formed. Abundant in developing endosperm, low in aleurone, and undetected in coleoptiles and roots. Also detected in crude extracts of anthers and in immature embryos. Activity increases as seeds develop. Belongs to the glycosyltransferase 1 family. Plant sucrose synthase subfamily.