Converts glucose 1-phosphate to UDP-glucose, which is the major glycosyl donor for polysaccharides. Acts redundantly with UGP1 and is essential for the synthesis of sucrose, starch and cell wall, and callose deposition. alpha-D-glucose 1-phosphate + H(+) + UTP = diphosphate + UDP-alpha-D-glucose Expressed in cauline leaves, flowers and siliques. By sucrose. Reduced number of seeds (PubMed:19366709). The double mutants upg1 and ugp2 display severe growth defects and male sterility due to the absence of callose deposition around microspores (PubMed:20435647). Belongs to the UDPGP type 1 family.