LIPID METABOLISM IN LEAVES OF AN 18/4-PLANT, ECHIUM PLANTAGINEUM - A MODEL OF GALACTOLIPID BIOSYNTHESIS IN 18/3- AND 18/4-PLANTS

摘要

All of the major leaf glycerolipids of Echium plantagineum, a member of the Boraginaceae, contain two unusual fatty acids, gamma-linolenic acid (gamma 18:3) and octadecatetraenoic acid (18:4). gamma 18:3 is synthesized by an omega 12-desaturation of linoleic acid (18:2) in cytosolic phosphatidylcholine (PC); 18:4 is synthesized by an further desaturation of alpha-linolenic acid (alpha 18:3) and/or gamma 18:3. The positional distribution of gamma 18:3 and 18:4, and a comparison of the fatty acid molecular species composition of the major leaf glycerolipids were used to show that each of the chloroplast glycerolipids is synthesized from different diacylglycerol (DAG) precursors. Our results show that (a) the bulk of the chloroplastic monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are synthesized by separate pathways from different cytosolic pools of precursor DAG, and (b) the DAG backbone of sulfoquinovosyldiacylglycerol (SQDG) is derived from cytosolic precursor(s) in a pathway similar to the bulk MGDG. A comparison of lipid metabolism in 18:4-plants with 16:3- and 18:3-plants has led us to propose a new model for the biosynthesis of MGDG, DGDG and SQDG in 18:3- and 18:4-plants. In this model there are three pathways of biosynthesis in each type of plant; two pathways, in association with the inner envelope membrane, synthesize the majority of MGDG and SQDG, while a separate pathway in the outer envelope synthesizes predominantly DGDG. [References: 31]