Involved in sporophytic and gametophytic development. Required for normal leaf development. During pollen formation, required for the formation of the callose wall separating the tetraspores of the tetrad (interstitial wall), but not for the callose wall surrounding the pollen mother cells (peripheral wall). Functionally redudant to CALS11 (GSL1). May play a role later in pollen grain maturation. Required for callose formation induced by wounding and pathogen attack. May interfere with salicylic acid-induced signaling pathway during defense response. 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 Highly expressed in flowers. Expressed at low levels in roots, leaves, stems, cauline leaves and siliques. By salicylic acid (SA). Plants develop smaller leaves, and collapsed and inviable pollen grains. They are resistant to the biotrophic pathogens Erysiphe cichoracearum (powdery mildew), E.orontii and Hyaloperonospora parasitica. Belongs to the glycosyltransferase 48 family.