Understanding the metabolic pathways underlying oil production and the precise intracellular localization of lipid droplets is crucial for successfully engineering microalgae for biofuel production. The microalga Chlamydomonas reinhardtii accumulates considerable amounts of starch and triacylglycerol (TAG) under N deprivation or high-light stress conditions. Starch undoubtedly accumulates within the chloroplast, but controversy exists over the localization of the lipid droplets, which are typically cytosolic organelles in other plants and organisms Contrary to previous reports, Moriyama et al. (10.1104/pp.17.01512) present microscopic results showing no evidence for the presence of lipid droplets within the chloroplast stroma. Some lipid droplets, however, existed in close association with the chloroplast or were largely engulfed by the chloroplasts. In the view of the authors, lipid droplets in Chlamydomonas, as in other organisms, are cytosolic structures, distinct from the plastoglobules present in the chloroplast stroma. Since the presumptive “chloroplast lipid droplets” are, in fact, lipid droplets embedded within chloroplast invaginations in association with the outer envelope of the chloroplast without intervention of the endoplasmic reticulum, the possibility exists of direct metabolic flow between the chloroplast and lipid droplets.
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