Plastoglobules: A hub of lipid metabolism in the chloroplast

Chloroplasts in plant and algae are the home of photosynthesis and many essential metabolic pathways. The photosynthetic light reactions occur at an extensive membrane system, the thylakoids. The main constituents of thylakoid membranes are the galactolipids, in particular monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). Apart from these, thylakoid membranes contain a number of other lipid molecules that function as electron transporters (plastoquinone) or as lipid antioxidants (tocopherols) and that are free to diffuse in the membrane system. Others such as phylloquinone, carotenoids and chlorophylls are mostly associated with protein components of the photosystems that reside in the thylakoids. The thylakoids are extremely dynamic and rapidly respond to environmental stresses. Plastoglobules (PG), lipoprotein particles or lipid droplets associated with thylakoids, playa central role in these responses. PG are thought to allocate stored electron carriers as well as lipid antioxidants to the membranes. PG also contribute to the enzymatic capacity for antioxidant and electron carrier synthesis and metabolism. During senescence and under nitrogen deprivation PG actively participate in membrane catabolism. PG become supersized while absorbing triacylglycerols and fatty acid phytol esters resulting from galactolipid and chlorophyll degradation. This chapter aims at giving a broad overview of the role of PG in chloroplast lipid metabolism.