A role for lipid trafficking in chloroplast biogenesis

Chloroplasts are the defining plant organelle carrying out photosynthesis. Photosynthetic complexes are embedded into the thylakoid membrane which forms an intricate system of membrane lamellae and cisternae. The chloroplast boundary consists of two envelope membranes controlling the exchange of metabolites between the plastid and the extraplastidic compartments of the cell. The plastid internal matrix (stroma) is the primary location for fatty acid biosynthesis in plants. Fatty acids can be assembled into glycerolipids at the envelope membranes of plastids or they can be exported and assembled into lipids at the endoplasmic reticulum (ER) to provide building blocks for extraplastidic membranes. Some of these glycerolipids, assembled at the ER, return to the plastid where they are remodeled into the plastid typical glycerolipids. As a result of this cooperation of different subcellular membrane systems, a rich complement of lipid trafficking phenomena contributes to the biogenesis of chloroplasts. Considerable progress has been made in recent years towards a better mechanistic understanding of lipid transport across plastid envelopes. Lipid transporters of bacteria and plants have been discovered and their study begins to provide detailed mechanistic insights into lipid trafficking phenomena relevant to chloroplast biogenesis.