Involved in the synthesis of diacylglycerol galactolipids that are specifically found in thylakoid membranes. Specific for alpha-glycosidic linkages. a 1,2-diacyl-3-O-(beta-D-galactosyl)-sn-glycerol + UDP-alpha-D-galactose = 1,2-diacyl-3-O-[alpha-D-galactosyl-(1->6)-beta-D-galactosyl]-sn-glycerol + H(+) + UDP High expression in nodules infected cells, but low in nodule inner cortex and root central cylinder. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.