CHARACTERIZATION OF THE GLYCEROLIPID COMPOSITION AND BIOSYNTHETIC CAPACITY OF PEA ROOT PLASTIDS

The glycerolipid composition of pea (Pisum sativum L.) root plastids a nd their capacity to synthesize glycerolipids from [UL-C-14]glycerol-3 -phosphate were determined. Pea root plastids primarily consist of mon ogalactosyldiacylglycerol, triacylglycerol, phosphatidylcholine, digal actosyldiacylglycerol, and diacylglycerol. Maximum rates of total glyc erolipid 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 pr opane (pH 7.5), and 0.31 M sorbitol. Glycerolipid biosynthesis was com pletely dependent on exogenously supplied ATP, coenzyme A, and a dival ent cation, whereas the remaining cofactors improved their activity fr om 1.3- to 2.4-fold. Radioactivity from glycerol-3-phosphate was recov ered predominantly in phosphatidic acid, phosphatidylglycerol, diacylg lycerol, and triacylglycerol with lesser amounts in phosphatidylcholin e and monoacylglycerol. The proportions of the various radiolabeled li pids that accumulated were dependent on the pH and the concentration o f ATP and glycerol-3-phosphate. The data presented indicate that pea r oot plastids can synthesize almost all of their component glycerolipid s and that glycerolipid biosynthesis is tightly coupled to de novo fat ty acid biosynthesis, pH and the availability of ATP may have importan t roles in the regulation of lipid biosynthesis at the levels of phosp hatidic acid phosphatase and in the reactions that are involved in pho sphatidylglycerol and triacylglycerol biosynthesis.