The characteristics and potential applications of structural lipid droplet proteins in plants

• Plant lipid droplets are structurally robust organelles comprising a protein-rich outer membrane and a hydrophobic core of stored lipids.
• Lipid droplets in seeds and other germinative organs store triacylglycerols as energy sources, and are rich in oleosin-family proteins.
• In other organs, lipid droplets are associated with non-oleosin proteins and perform diverse functions related to environmental interactions.
• Natural and reconstituted artificial lipid droplets have many potential applications in the food, energy, pharmaceutical and cosmetics industries.

Plant cytosolic lipid droplets are storage organelles that accumulate hydrophobic molecules. They are found in many tissues and their general structure includes an outer lipid monolayer with integral and associated proteins surrounding a hydrophobic core. Two distinct types can be distinguished, which we define here as oleosin-based lipid droplets (OLDs) and non-oleosin-based lipid droplets (NOLDs). OLDs are the best characterized lipid droplets in plants. They are primarily restricted to seeds and other germinative tissues, their surface is covered with oleosin-family proteins to maintain stability, they store triacylglycerols (TAGs) and they are used as a source of energy (and possibly signaling molecules) during the germination of seeds and pollen. Less is known about NOLDs. They are more abundant than OLDs and are distributed in many tissues, they accumulate not only TAGs but also other hydrophobic molecules such as natural rubber, and the structural proteins that stabilize them are unrelated to oleosins. In many species these proteins are members of the rubber elongation factor superfamily. NOLDs are not typically used for energy storage but instead accumulate hydrophobic compounds required for environmental interactions such as pathogen defense. There are many potential applications of NOLDs including the engineering of lipid production in plants and the generation of artificial oil bodies.