Biogenesis of photosynthetic membranes depends on galactolipid synthesis, which relies on several cell compartments, notably the endoplasmic reticulum (ER) and the chloroplast envelope. Galactolipid synthesis involves lipid trafficking between both membrane compartments. In Arabidopsis, ALA10, a phospholipid flippase of the P4 type-ATPase family, counteracts the limitation of monogalactosyldiacylglycerol (MGDG) production and has a positive effect on leaf development. ALA10 locates in distinct domains of the ER depending on the ALIS (ALA interacting subunit) subunit it interacts with: close to the plasma membrane with ALIS1, or next to chloroplasts with ALIS5. It interacts with FAD2 (Fatty acid desaturase 2) and prevents accumulation of linolenic (18:3) containing phosphatidylcholine (PC) stimulating an increase of MGDG synthesis. Here we report that ALA10 interacts with PUB11 (plant U-box type 11), an E3 protein ubiquitin ligase, in vitro and in vivo. ALA10 is however ubiquitinated and degraded by the 26S proteasome in a PUB11-independent process. In pub11 null mutant, the proteasome-dependent degradation of ALA10 is retained and ALA10 is still subject to ubiquitination although its ubiquitination profile appears different. In the absence of PUB11, ALA10 is constrained to the ER close to chloroplasts, which is the usual location when ALA10 is overexpressed. Additionally, in this condition, the decrease of 18:3 containing PC is no longer observed. Taken together these results suggest, that ALA10 contributes in chloroplast-distal ER interacting domains, to reduce the 18:3 desaturation of PC and that PUB11 is involved in reconditioning of ALA10 from chloroplast-proximal to chloroplast-distal ER interacting domains.

In plant cells, phosphatidylcholine (PC) is a major glycerolipid of most membranes but practically lacking from the plastid internal membranes. In chloroplasts, PC is absent from the thylakoids and the inner envelope membrane. It is however the main component of the outer envelope membrane, where it exclusively distributes in the outer monolayer. This unique distribution is likely related with operational compartmentalization of plant lipid metabolism. In this review, we summarize the different mechanisms involved in homeostasis of PC in plant cells. The specific origin of chloroplast PC is examined and the involvement of the P4-ATPase family of phospholipid flippases (ALA) is considered with a special attention to the recently reported effect of the endoplasmic reticulum-localized ALA10 on modification of chloroplast PC desaturation. The different possible roles of chloroplast PC are then discussed and analyzed in consideration of plant physiology.

Photosynthetic membranes, or thylakoids, are the most extensive membrane system found in the biosphere. They form flattened membrane cisternae in the cytosol of cyanobacteria and in the stroma of chloroplasts. The efficiency of light energy capture and conversion, critical for primary production in ecosystems, relies on the rapid expansion of thylakoids and their versatile reorganization in response to light changes. Thylakoid biogenesis results from the assembly of a lipid matrix combined with the incorporation of protein components. Four lipid classes are conserved from cyanobacteria to chloroplasts: mono- and digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidyldiacylglycerol. This review focuses on the production and biophysical properties of galactolipids, making them determinant factors for the nonvesicular/nonlamellar biogenesis and for the three-dimensional architecture of nascent thylakoids. The regulation of MGD1, the committing enzyme of galactolipid biosynthesis in Arabidopsis, via feedback regulatory loops and control of protein binding to membranes, is also detailed.

The biogenesis of photosynthetic membranes relies on galactoglycerolipids, which are synthesized via pathways that are dispatched over several cell compartments. This membrane biogenesis requires both trafficking of lipid intermediates and a tight homeostatic regulation. In this work, we address the role of ALA10 (for aminophospholipid ATPase), a P4-type ATPase, in a process counteracting the monogalactosyldiacylglycerol (MGDG) shortage in Arabidopsis (Arabidopsis thaliana) leaves. ALA10 can interact with protein partners, ALIS1 (for ALA-interacting subunit1) or ALIS5, leading to differential endomembrane localizations of the interacting proteins, close to the plasma membrane with ALIS1 or to chloroplasts with ALIS5. ALA10 interacts also with FATTY ACID DESATURASE2 (FAD2), and modification of ALA10 expression affects phosphatidylcholine (PC) fatty acyl desaturation by disturbing the balance between FAD2 and FAD3 activities. Modulation of ALA10 expression downstream impacts the fatty acyl composition of chloroplast PC. ALA10 expression also enhances leaf growth and improves the MGDG-PC ratio, possibly through MGDG SYNTHASE1 (MGD1) activation by phosphatidic acid. The positive effect of ALA10 on leaf development is significant in conditions such as upon treatment of plants with Galvestine-1, an inhibitor of MGDG synthases, or when plants are grown at chilling temperature.