Involved in lipid metabolism and lipid droplet (LD) morphology, number, and size. Facilitates the formation of large-sized LDs and modulates triacylglycerol accumulation. Induces probably a reorganization of the endoplasmic reticulum into LD-forming domains. Localized to the lipid droplet-forming sites. Expressed in seeds and young seedlings. Not detected in leaves. The N-terminal domain (1-98) determines the lipid droplet size. Knockdown mutants have a reduced accumulation of oil per seed and a reduced overall seed size. Belongs to the seipin family.