Involved in triacylglycerol (TAG) synthesis (PubMed:17084870). Catalyzes the acylation of the sn-3 hydroxy group of sn-1,2-diricinolein using ricinoleoyl-CoA (PubMed:17084870). a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA (12R)-hydroxy-(9Z)-octadecenoyl-CoA + 1,2-di-(12R-hydroxy-9Z-octadecenoyl)-sn-glycerol = 1,2,3-tri-(12R-hydroxy-9Z-octadecenoyl)-glycerol + CoA Glycerolipid metabolism; triacylglycerol biosynthesis. Expressed in endosperm of developing seeds from 10 to 30 days after pollination, with a peak at 25 days. Belongs to the diacylglycerol acyltransferase family.