Summary
Plastid membranes (thylakoids as well as the two envelope membranes) contain specific polar lipids (galactolipids, sulfolipid, phospholipids), pigments (chlorophylls, carotenoids) and prenylquinones (plastoquinone, tocopherols). In this chapter, we describe our present understanding of the structure of these lipids and their distribution within chloroplasts, of the biosynthetic pathway for glycerolipids, pigments and prenylquinones and of the biochemical properties of the enzymes involved. The biosynthesis of plastid glycerolipids takes place in the inner envelope membrane, which is the site of assembly of fatty acids, glycerol and polar head groups (galactose, for galactolipids; sulfoquinovose, for sulfolipid and glycerol for phosphatidylglycerol). The inner envelope membrane contains all the enzymatic equipment for the biosynthesis of glycerolipids containing almost exclusively a C18/C16 diacylgycerol backbone. In contrast, the origin of plastid glycerolipids with a C18/C18 diacylglycerol backbone is still poorly understood. Then, fatty acids of the newly synthesized molecules are desaturated to form the polyunsaturated molecular species that are characteristic of plastid glycerolipids. Although the sequences of reactions involved in the biosynthesis of plastid prenyllipids (pigments and prenylquinones) have been thoroughly studied, little is known about the precise localization and properties of the different enzymes involved. There is a tight cooperation within chloroplasts between the stroma, the envelope membranes and the thylakoids to form the various pigments (chlorophyll and carotenoids) and prenylquinones (plastoquinone-9 and α-tocopherol), and many of the enzymes involved are in fact located in the inner envelope membrane. Because thylakoids represent the main proportion of the plastid membranes, they contain the largest amount of the plastid lipid constituents, therefore massive transport of lipid molecules from their site of synthesis (envelope membranes) to their site of accumulation (thylakoids) should take place during plastid development. The possible mechanisms that could be involved, i.e. vesicular transport, transfer of lipid monomers through the stroma either by facilitated transport or by spontaneous diffusion of free monomers, lateral diffusion of lipids between membranes at regions of direct intermembrane contact, will be discussed.