Involved in the synthesis of the major structural component of photosynthetic membranes. Required for proper thylakoid membrane biogenesis. Does not discriminate between prokaryotic (18:1/16:0) or eukaryotic (18:2/18:2) 1,2-diacylglycerol species, but operates with some preference for the prokaryotic one. Is responsible for most galactolipid synthesis in chloroplasts (PubMed:10869420, PubMed:11171188, PubMed:11553816, PubMed:17940034). Required for the formation of thylakoid membranes and functional photosynthetic electron transport during cotyledons greening in young seedlings (PubMed:25253888). May link galactolipid synthesis with the coordinated transcriptional regulation of chloroplasts and other organelles during cotyledon greening (PubMed:25253888). 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 Activated by phosphatidate (PA) and phosphatidylglycerol (PG) (PubMed:20023301). Inhibited by galvestine-1 (PubMed:21946275). Optimum pH is 7.5. Homodimer. Expressed in roots, stems, leaves, flowers, siliques and seeds. Highly expressed throughout whole developmental stages. Transient increase in embryos at the globular stage. Induced by illumination and cytokinin treatment in etiolated seedlings. Not induced by phosphate deprivation. Plants show defects in chloroplast biogenesis and a dwarf and albino phenotype. The amount of monogalactosyldiacylglycerol was reduced by 98% in the mutant leaves, indicating that MGD2 or MGD3 cannot compensate for loss of MGD1 function. Belongs to the glycosyltransferase 28 family.