Molecular Machinery of Lipid Droplet Degradation and Turnover in Plants

Abstract

Lipid droplets (LDs) are important organelles conserved across eukaryotes with a fascinating biogenesis and consumption cycle. Recent intensive research has focused on uncovering the cellular biology of LDs, with emphasis on their degradation. Briefly, two major pathways for LD degradation have been recognized: (1) lipolysis, in which lipid degradation is catalyzed by lipases on the LD surface, and (2) lipophagy, in which LDs are degraded by autophagy. Both of these pathways require the collective actions of several lipolytic and proteolytic enzymes, some of which have been purified and analyzed for their in vitro activities. Furthermore, several genes encoding these proteins have been cloned and characterized. In seed plants, seed germination is initiated by the hydrolysis of stored lipids in LDs to provide energy and carbon equivalents for the germinating seedling. However, little is known about the mechanism regulating the LD mobilization. In this review, we focus on recent progress toward understanding how lipids are degraded and the specific pathways that coordinate LD mobilization in plants, aiming to provide an accurate and detailed outline of the process. This will set the stage for future studies of LD dynamics and help to utilize LDs to their full potential.

Keywords: lipases; lipid droplet; lipolysis; lipophagy; triacylglycerol hydrolysis.

Figure 1

Model of lipid droplet degradation during seed germination in plants. (A) Lipolysis. Prior to TAG degradation, the structural proteins and LD phospholipid monolayer are partially degraded mainly via the ubiquitin–proteasome-dependent system and phospholipase activities, respectively. Subsequent TAG mobilization is catalyzed by lipid-mobilizing enzymes, such as SDP1 and lipoxygenase (LOX). The released FAs are transported into peroxisomes for β-oxidation and converted to acetyl-CoA. Next, acetyl-CoA enters the glyoxylate cycle, through which succinate is synthesized. Then, succinate is transported into the mitochondrion and converted to malate through a part of the Krebs cycle. Finally, soluble sugars are synthesized via gluconeogenesis. (B) Lipophagy. The process may occur by vacuolar membrane invagination, which increases, and LD is engulfed. Inside the vacuole, a single bilayer membrane autophagic body containing LD is released. Finally, the autophagic body is degraded and lipids in LDs are hydrolyzed into FAs and glycerol.