Cellular Organization and Regulation of Plant Glycerolipid Metabolism

Abstract

Great strides have been made in understanding how membranes and lipid droplets are formed and maintained in land plants, yet much more is to be learned given the complexity of plant lipid metabolism. A complicating factor is the multi-organellar presence of biosynthetic enzymes and unique compositional requirements of different membrane systems. This necessitates a rich network of transporters and transport mechanisms that supply fatty acids, membrane lipids and storage lipids to their final cellular destination. Though we know a large number of the biosynthetic enzymes involved in lipid biosynthesis and a few transport proteins, the regulatory mechanisms, in particular, coordinating expression and/or activity of the majority remain yet to be described. Plants undergoing stress alter their membranes' compositions, and lipids such as phosphatidic acid have been implicated in stress signaling. Additionally, lipid metabolism in chloroplasts supplies precursors for jasmonic acid (JA) biosynthesis, and perturbations in lipid homeostasis has consequences on JA signaling. In this review, several aspects of plant lipid metabolism are discussed that are currently under investigation: cellular transport of lipids, regulation of lipid biosynthesis, roles of lipids in stress signaling, and lastly the structural and oligomeric states of lipid enzymes.

Keywords: Lipid transport; Lipids; Membrane.

Fig. 1

Scheme of PA synthesis in the plastid and the ER. Denoted with letters are outstanding transport mechanisms proposed and yet to be characterized: the unknown exporter of acyl groups across the outer envelope membrane (OEM) of the plastid (A) and the transport mechanism by which plastid-assembled PA is moved through the IEM of the plastid (B). FAS, fatty acid synthesis; FAX1, fatty acid exporter 1; GPAT, glycerol-3-P acyltransferase; LPAT, lyso-phosphatidate acyltransferase; TGD 1, 2, 3, 4, 5, trigalactosyldiacylglycerol 1, 2, 3, 4, 5.

Fig. 2

Proposed movement of galactolipids after synthesis from diacylglycerol (DAG). Denoted with letters are the unknown mechanisms of transport of MGDG and DGDG (A and B, respectively) to the THY, after synthesis by MGDG synthase 1 (MGD1) at the IEM and DGDG synthase 1 (DGD1) at the outer envelope membrane (OEM). PAP, phosphatidic acid phosphatase.