Biosynthesis and Function of Chloroplast Lipids

Abstract

Chloroplast membranes are composed of four unique lipids, including monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidyl-glycerol (PG). These lipids are crucial for maintaining the function of chloroplasts not simply because they account for a large fraction of the photosynthetic membranes, but because they are assembled into the photosynthetic machinery and are, therefore, directly involved in photosynthetic processes. Indeed, Ara-bidopsis mutants of these lipids possess some photosynthetic defects. Diacylglycerol (DAG), a common precursor of the glycolipids, is produced by both prokaryotic and eukaryotic pathways but the detailed mechanism of DAG supply to chloroplasts remains ambiguous. Because most of the genes encoding the lipid-synthesizing enzymes have been identified in this decade, significant progress delineating the physiological functions and regulatory mechanisms of lipid biosynthesis in chloroplasts has been achieved. In Arabidopsis, two types of MGDG synthases, Type A (AtMGD1) and Type B (AtMGD2, AtMGD3), were identified and their distinct functions in chloroplasts have been unveiled. Type A MGDG synthase is involved in the bulk of MGDG synthesis whereas Type B MGDG synthase is induced under phosphate (Pi)-limited conditions. Two genes, DGD1 and DGD2 , for DGDG synthases, which are involved in DGDG synthesis, were identified. DGD1 is the predominant DGDG synthase whereas DGD2 is induced under Pi-limited growth conditions. SQDG synthesis is mediated by two enzymes, SQD1 and SQD2. The key enzyme for PG synthesis is PG phosphate synthase, which is encoded by two genes, PGP1 and PGP2 . Plants have homeostatic mechanisms to balance the amount of these lipids by regulating their biosyntheses under various environmental conditions, such as limiting Pi, which stimulates replacement of phospholipids with glycolipids through regulation of enzymes involved in lipid biosynthesis.