Lipid droplets in plants: More than a simple fat storage

Lipid droplets (LDs)are found in all lineages of living organisms, whether prokaryotes or eukaryotes. Their structure is rather simple with a core of hydrophobic molecules, mostly Triacylglycerols and Sterol Esters, surrounded by a monolayer of polar lipids with associated proteins. They are constitutively present in storage organs, such as seeds in plants, or adipose tissue in mammals, while they can form in response to various abiotic or biotic stresses in other plant organs and in microalgae. Their biogenesis has been mostly studied in yeast and animals, yet remains poorly understood in plants. In particular, the central role of the photosynthetic plastid in lipid metabolism raises questions about its role in LD biogenesis. Over the years, many functions of LDs have emerged in addition to the obvious role in carbon and energy storage. While some functions appear broadly distributed among organisms and organs, other appear specific to certain tissues and/or lineages. The present review aims at highlighting specific aspects of LD biogenesis and functions in all photosynthetic organisms, from the historic model plant Arabidopsis thaliana to emerging secondary endosymbiont models such as the diatom Phaeodactylum tricornutum.