Involved in the biosynthesis of beta-1,4-galactan. Can transfer galactose residues from UDP-galactose to beta-1,4-galactopentaose in vitro. Forms specifically beta-1,4-galactosyl linkages and can add successive beta-1,4-galactosyl residues to the acceptor. Beta-1,4-galactans are abundant polysaccharides in plant cell walls and are found as side-chain of rhamnogalacturonan I, which is a major component of pectin. Expressed in root vasculature, mature leaves, trichomes, flowers, siliques and seeds. No visible phenotype under normal growth conditions, but mutant plants have reduced content of beta-1,4-galactan in leaf cell wall. Plants over-expressing GALS1 have a strong increase in beta-1,4-galactan content in leaf cell wall. Belongs to the glycosyltransferase 92 family.