Lipid Trafficking in Plant Photosynthetic Cells

Abstract

Each of the various membranes in plant cells has a specific glycerolipid composition, which is kept relatively stable in different cells and different plants. Lipid homeostasis effectors, particularly lipid transporters, remain largely uncharacterized. Recent progresses in the field rely on the analysis of chloroplast lipid homeostasis as a model of choice. Galactolipids are the main lipids of chloroplast membranes. Galactolipid synthesis occurs in the chloroplast envelope membranes and depends on the fine exchange of lipid intermediates between the envelope membranes. This synthesis is also highly dependent on supply of lipid precursors synthesized in the endoplasmic reticulum membranes. Phosphatidic acid is an important lipid intermediate that is generated in the envelope but also in various extraplastidic membranes belonging to the endosomal network. It was recently shown that extraplastidic phosphatidic acid is one of the galactolipid precursors. As it is also a signalling molecule in plant cells, it could be a regulator of the lipid synthesis pathway. Trafficking of phosphatidic acid in the envelope is, therefore, a key step for synthesis of chloroplast lipids. After their completion, galactolipids are transferred to the thylakoids possibly through vesicles. The stability of the membrane lipid composition indicates a tight regulation at the subcellular level. This control is, however, modified when the plant is deprived of phosphate resulting in enrichment of digalactosyldiacylglycerol in mitochondrial membranes, the tonoplast and the plasma membrane. The transfers of specific lipids into and out of the chloroplast envelope, particularly the transfer of digalactosyldiacylg-lycerol to the mitochondria, are enhanced. The molecular mechanisms potentially involved in transport of these glycerolipids are surveyed.