Magnesium-dependent phosphatidate phosphatase which catalyzes the dephosphorylation of phosphatidate to yield diacylglycerol. Acts redundantly with PAH1 to repress phospholipid biosynthesis at the endoplasmic reticulum (ER). May function indirectly as repressor of multiple enzymes involved in phospholipid biosynthesis. Is involved in the pathway of galactolipid synthesis in the ER, which is required for the membrane lipid remodeling, an essential adaptation mechanism to cope with phosphate starvation. a 1,2-diacyl-sn-glycero-3-phosphate + H2O = a 1,2-diacyl-sn-glycerol + phosphate Optimum pH is 6.5. A number of isoforms are produced. According to EST sequences. Expressed in roots, leaves, stems, flowers, siliques, embryos and mature seeds. Contains 1 Asp-Xaa-Asp-Xaa-Thr (DXDXT) motif, a catalytic motif essential for phosphatidate phosphatase activity. No visible phenotype under normal growth conditions, but the double mutants pah1 and pah2-1 show reduced growth. Belongs to the lipin family.