Required the formation of the callose wall separating the tetraspores (interstitial wall), but not for the callose wall surrounding the pollen mother cells (peripheral wall). Functionally redudant to CALS12 (GSL5). 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 Ubiquitous. Plants develop collapsed and inviable pollen grains. Belongs to the glycosyltransferase 48 family.