Characterization of the Glycerolipid Composition and Biosynthetic Capacity of Pea Root Plastids

The glycerolipid composition of pea (Pisum sativum L.) root plastids and their capacity to synthesize glycerolipids from [UL-14C]glycerol-3-phosphate were determined. Pea root plastids primarily consist of monogalactosyldiacylglycerol, triacylglycerol, phosphatidylcholine, digalactosyldiacylglycerol, and diacylglycerol. Maximum rates of total glycerolipid biosynthesis were obtained in the presence of 2.4 mM glycerol-3-phosphate, 15 mM KHCO3, 0.2 mM sodium-acetate, 0.5 mM each of NADH and NADPH, 0.05 mM coenzyme A, 2 mM MgCl2, 1 mM ATP, 0.1 M Bis-Tris propane (pH 7.5), and 0.31 M sorbitol. Glycerolipid biosynthesis was completely dependent on exogenously supplied ATP, coenzyme A, and a divalent cation, whereas the remaining cofactors improved their activity from 1.3- to 2.4-fold. Radioactivity from glycerol-3-phosphate was recovered predominantly in phosphatidic acid, phosphatidylglycerol, diacylglycerol, and triacylglycerol with lesser amounts in phosphatidylcholine and monoacylglycerol. The proportions of the various radiolabeled lipids that accumulated were dependent on the pH and the concentration of ATP and glycerol-3-phosphate. The data presented indicate that pea root plastids can synthesize almost all of their component glycerolipids and that glycerolipid biosynthesis is tightly coupled to de novo fatty acid biosynthesis. pH and the availability of ATP may have important roles in the regulation of lipid biosynthesis at the levels of phosphatidic acid phosphatase and in the reactions that are involved in phosphatidylglycerol and triacylglycerol biosynthesis.