Catalyzes the terminal and only committed step in triacylglycerol (TAG) synthesis by using diacylglycerol (DAG) and fatty acyl-CoA as substrates. Required for storage lipid synthesis (PubMed:12963726, PubMed:26990381). Major DAG esterifying enzyme in stationary phase when TAG production is particularly active (PubMed:12963726). Involved in lipid particle synthesis from the endoplasmic reticulum, promoting localized TAG production at discrete ER subdomains (PubMed:26990381, PubMed:28011631). a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA a 2-acylglycerol + an acyl-CoA = a 1,2-diacyl-sn-glycerol + CoA Glycerolipid metabolism; triacylglycerol biosynthesis. Localizes to sites of lipid droplet biogenesis in the endoplasmic reticulum. Has reduced whole-cell and lipid droplet (LD) TAG levels. Cells lacking both TAG synthase genes (plh1 and dga1) have no LDs and exhibit defects in spore germination and in spore wall integrity. Belongs to the diacylglycerol acyltransferase family.