Involved in the synthesis of monogalactosyldiacylglycerol, the major structural component of photosynthetic membranes and in the chloroplast envelope biogenesis. Can use both prokaryotic (18:1/16:0) or eukaryotic (18:2/18:2) 1,2-diacylglycerol species, but operates with some preference for the eukaryotic one. Plays a minor role in galactolipid synthesis in chloroplasts (PubMed:11553816). Is essential for membrane lipid remodeling in phosphate-starved roots (PubMed:18808455, PubMed:31201686). Acts as the major factor involved in digalactosyldiacylglycerol (DGDG) biosynthesis in phosphate-starved roots (PubMed:18808455). Does not seem to be required for plant growth under nutrient-sufficient conditions (PubMed:18808455). Required for membrane lipid remodeling in plants grown in acidic conditions (PubMed:31201686). a 1,2-diacyl-sn-glycerol + UDP-alpha-D-galactose = a 1,2-diacyl-3-O-(beta-D-galactosyl)-sn-glycerol + H(+) + UDP 1,2-di-(9Z,12Z-octadecadienoyl)-sn-glycerol + UDP-alpha-D-galactose = 1,2-di-(9Z,12Z-octadecadienoyl)-3-beta-D-galactosyl-sn-glycerol + H(+) + UDP 1-(9Z-octadecenoyl)-2-hexadecanoyl-sn-glycerol + UDP-alpha-D-galactose = 1-(9Z-octadecenoyl)-2-hexadecanoyl-3-beta-D-galactosyl-sn-glycerol + H(+) + UDP 1,2-di-(9Z-octadecenoyl)-sn-glycerol + UDP-alpha-D-galactose = 1,2-di-(9Z-octadecenoyl)-3-beta-D-galactosyl-sn-glycerol + H(+) + UDP Inhibited by galvestine-1. Optimum pH is 7.0. Expressed mainly in roots. Detected in flowers, leaves, stems, siliques and pollen tubes. Mostly expressed in early stages of development. Induced by phosphate deprivation (PubMed:11553816, PubMed:14730084, PubMed:16762032). Induced in rosette leaves when grown in acidic soil (PubMed:31201686). Auxin activates expression during Pi starvation, whereas cytokinin represses it. Belongs to the glycosyltransferase 28 family.