Galactolipid export from chloroplasts towards mitochondria – ChloroMitoLipid

Plant cell membranes are mainly constituted of glycerolipids. Membrane glycerolipids can be classified in two groups: phospholipids containing phosphate, mainly synthesised in the endoplasmic reticulum (ER), and glycolipids – galactolipids and sulfolipids - without phosphate, synthesised in chloroplasts, and being the major constituents of photosynthetic membranes. When plants are deprived of phosphate, a frequent natural situation that limits plant growth, they adapt to the environment by increasing their phosphate absorption, decreasing their phosphate consumption and mobilising phosphate cell reserves. Phospholipids contain up to one third of intracellular phosphate and for that reason represent a non negligible phosphate source. In phosphate deprived plants, phospholipid amount decreases. Phospholipid hydrolysis occurs in the cytosol, outside chloroplasts, and is concomitant to an increase of non-phosphorus galactolipid synthesis within plastid membranes. I previously showed that theses two phenomenon corresponded to a conversion of phospholipids into galactolipids. Phospholipid diacylglycerol (DAG) moiety exhibits a characteristic fatty acid signature that can be followed from the ER to the chloroplast, indicating the existence of a transfer of this DAG moiety. This ER-to-plastid transfer feeds galactolipid synthesis, more specifically digalactosyldiacylglycerol (DGDG) synthesis. DGDG is then transferred to non plastidial membranes such as mitochondria, tonoplast or plasma membranes, replacing phospholipids. During my PhD, I showed that DGDG transfer between chloroplast and mitochondria occurred in membrane contact zone formed by apposition of chloroplast and mitochondria membranes. The hosting team “Biogenesis, dynamics and homeostasis of membrane lipids” of the Laboratoire de Physiologie Cellulaire et Végétale in Grenoble has an international expertise in the study of plastid galactolipid synthesis. My project concerns a new aspect in the team, unaddressed so far, but complementary. I will study, in phosphate deprivation context, lipid trafficking between membranes and more precisely DGDG transfers from chloroplasts towards mitochondria. The aim of the project is to identify proteins involved in DGDG traffic and to analyse molecular mechanisms involved in this process. My objective is to identify and to characterise new components acting at the level of DGDG export from chloroplast to mitochondria, by focusing on the three partners of this transport: (1) the chloroplast envelope with galactolipid synthesis enzymes providing DGDG to be exported; (2) the transport machinery per se, that operates the transfer of DGDG; (3) the mitochondria envelope receiving DGDG, with a highlight on ALA10, a flippase that is overexpressed during phosphate deprivation – flippase meaning an enzyme that transfer a lipid inside a membrane from one bilayer to another one. The identification of the DGDG transfer actors will give us new data on intermembrane lipid trafficking that is up to now poorly understood, and will help us to comprehend, at the level of the membrane, plant adaptation mechanisms to environment changes such as phosphate deprivation.