Probably involved in callose synthesis, but not required for callose formation after wounding or pathogen attack. During plant growth and development, callose is found as a transitory component of the cell plate in dividing cells, is a major component of pollen mother cell walls and pollen tubes, and is found as a structural component of plasmodesmatal canals. [(1->3)-beta-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->3)-beta-D-glucosyl](n+1) + H(+) + UDP Belongs to the glycosyltransferase 48 family.