Converts glucose 1-phosphate to UDP-glucose, which is the major glycosyl donor for polysaccharides. Acts redundantly with UGP2 and is essential for the synthesis of sucrose, starch and cell wall, and callose deposition (PubMed:19366709, PubMed:20435647). Involved in the regulation of the programmed cell death (PCD) induced by the fungal toxin fumonisin B1 (FB1) (PubMed:23438466). alpha-D-glucose 1-phosphate + H(+) + UTP = diphosphate + UDP-alpha-D-glucose A number of isoforms are produced. According to EST sequences. Expressed in roots, rosette leaves, cauline leaves, stems, 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.