LIPID-SYNTHESIS AND METABOLISM IN THE PLASTID ENVELOPE

Plastid envelope membranes play a major role in the biosynthesis of gl ycerolipids. In addition, plastids are characterized by the occurrence of plastid-specific membrane glycolipids (galactolipids, a sulfolipid ). Plant lipid metabolism therefore has unique features, when compared to that of other eukaryotic organisms, such as animals and yeast. How ever, the glycerolipid biosynthetic pathway in chloroplasts is almost identical to that found in cyanobacteria, and reflects the prokaryotic origin of the chloroplast. Fatty acids generated in the plastid strom a are substrates for a whole set of enzymes involved in the synthesis of polar lipids of plastid membranes such as galactolipids, the sulfol ipid, the phosphatidylglycerol. In addition, fatty acids are exported outside the plastid where they are used for extraplastidial polar lipi d synthesis (phosphatidylcholine, phosphatidylethanolamine, etc.). Var ious desaturation steps leading to the formation of polyunsaturated fa tty acids occur in various cell compartments, especially in chloroplas ts, using fatty acids esterified to polar lipids as substrates. Furthe rmore, plant glycerolipids can be metabolized by a series of very acti ve envelope enzymes, such as the galactolipid:galactolipid galactosylt ransferase and the acyl-galactolipid forming enzyme. The physiological significance of these enzymes is however largely unknown. One of the most active pathways involved in lipid metabolism and present in envel ope membranes is the oxylipin pathway: polyunsaturated fatty acids tha t are released from polar lipids under various conditions (injury, pat hogen attack) are converted to oxylipin. Thus, the plastid envelope me mbranes are also involved in the formation of signalling molecules.