Arabidopsis galactolipid biosynthesis and lipid trafficking mediated by DGD1. The photosynthetic apparatus in plant cells is associated with membranes of the thylakoids within the chloroplast and is embedded into a highly specialized lipid matrix. Diacylglycerol galactolipids are common in thylakoid membranes but are excluded from all others. Isolation of the gene DGD1, encoding a galactosyltransferase-like protein, now provides insights into assembly of the thylakoid lipid matrix and subcellular lipid trafficking in Arabidopsis thaliana.