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 required for membrane lipid remodeling in phosphate-starved roots (PubMed:18808455, PubMed:31201686). Acts as the minor 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 8.5. Expressed mainly in floral buds. Detected in roots, leaves, stems, siliques and pollen tubes. Low and continuous expression throughout whole developmental stages. 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. Truncated N-terminus.