Functional study of Phaeodactylum tricornutum Seipin homolog highlights unique features of lipid droplets biogenesis in diatoms

Diatoms are a major phylum of microalgae, playing crucial ecological roles. They derive from secondary endosymbiosis of a red alga by an unknown heterotrophic eukaryote, leading to a complex intracellular organization. In response to unfavorable conditions (stress), diatoms store oil in lipid droplets (LD), raising interest for applications, in particular biofuels. In spite of numerous investigations aiming to increase their oil content, LD biogenesis mechanisms in these organisms remain poorly understood. In this study, we functionally characterized the homolog of Seipin, a major actor of LD biogenesis, in the diatom Phaeodactylum tricornutum. PtSeipin shares conserved structural features with other Seipins, yet presents unique characteristics, that appear common to diatoms and more broadly Stramenopiles. We provide evidence that Stramenopiles Seipins were inherited from the host during secondary endosymbiosis. The localization of PtSeipin highlights that LD biogenesis can arise simultaneously from the endoplasmic reticulum (ER) and the plastid’s most external membrane. Finally, the knock-out of PtSeipin leads to a strong increase of TAG accumulation, a feature that is not observed in other organisms and is greatly enhanced following high light exposure. Our results suggest a redirection of lipid fluxes towards TAG synthesis, reduced TAG recycling or a combination of both.