Role of the envelope membranes in chloroplast glycerolipid biosynthesis

All plastid membranes are characterized by the presence of large amounts of galactolipids. The galactolipid content of thylakoids is especially high since galactolipids represent up to 80% of the membrane glycerolipids, out of which monogalactosyldiacylglycerol (MGDG) constitutes the main part (50%). In its first part, this chapter describes the structure of the main glycerolipids present in plastids, particularly of galactolipids and details their subcellular localization. Functional analysis of mutants deleted of specific lipids and structural data obtained from Photosystem I and Photosystem II crystallization are presented to characterize the specific role of each glycerolipid. In the second part, this chapter summarizes our current understanding of the role of the chloroplast envelope in glycerolipid biogenesis and points out the emerging lipid trafficking mechanisms that envelope membranes take part in. The building up of eukaryotic and prokaryotic lipid structures proceeds from at least two distinct pathways requiring different trafficking of lipids. Whereas synthesis of prokaryotic glycerolipids is entirely realized in the envelope, synthesis of eukaryotic glycerolipids relies on the transfer of precursors from the endoplasmic reticulum to the envelope. The final assembly of glycerolipids is made in the envelope membranes prior to transfer to thylakoids membranes. A special focus is given to current results concerning MGDG and digalactosyldiacylglycerol (DGDG) synthesis in plants grown in phosphate-deficient conditions.