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. In endosperm it forms both homotetramers and heterotetramers with SS2, all three possible heterotetramers are formed. Highly expressed in developing endosperm and in roots and, at lower levels, in coleoptiles and aleurone. In 3 day old roots it is detected in cap cells and along the vascular strand, starting just after the meristemic region. In 9 day old leaves it is found in the phloem. In seeds it is distributed throughout the endosperm and also found in the assimilate-unloading tissues, the nucellar projection, the vascular area and at a high concentration in the chalazal region. Activity increases as seeds develop. Belongs to the glycosyltransferase 1 family. Plant sucrose synthase subfamily.