FIGURE 4.

Different hypotheses regarding lipid droplet (LD) biogenesis in Phaeodactylum tricornutum. EpM, epiplastidial membrane; PpM, Periplastidial membrane; P-oEM, plastid outer envelope membrane; P-iEM, plastid inner envelope membrane; ER, endoplasmic reticulum; VN, vesicular network; SQDG, sulfoquinovosyldiacylglycerol; PC, phosphatidylcholine; DGTA, diacylglycerylhydroxymethyl-N,N,N-trimethyl-β-alanine; TAG, triacylglycerol). (A) Lipid droplets emerge from the EpM. The EpM membrane contains PC and specific DGTA species, but also SQDG. The abundance of C16:2 on the betaine lipid DGTA found in the lipid droplet monolayer (Lupette et al., 2019) supports a plastid emerging hypothesis as this fatty acid is major in the plastid but very minor in endomembranes (Abida et al., 2015). The origin of SQDG in the EpM is unknown and may be explained either by an export from the plastid or a synthetic pathway located in the EpM. (B) Lipid droplets form at an interface between the ER and the EpM. PC and DGTA can originate from both membranes, while SQDG comes from the EpM as described above. The peculiar DGTA composition (Lupette et al., 2019) suggests that only certain species are present at LD biogenesis sites although the sorting mechanism is unknown.