The ontogeny of lipid bodies (spherosomes) in plant cells

Maturing embryos of 16 oil plants, anise suspension culture cells, and Neurospora crassa cells were prepared for electron microscopy at different stages during massive lipid accumulation. Lipid-rich structures of certain species were best preserved by dehydration of fixed tissues in ethanol without propylene oxide, embedding in Spurr's Medium, and polymerization at room temperature. In all cells examined, spherical lipid bodies (spherosomes) showed a moderately osmiophilic, amorphous matrix and displayed a delimiting half-unit membrane when sectioned medially. Associations with the endoplasmic reticulum (ER) were viewed at any stage during lipid body development but with different frequency in the different plant species. Plastids of fat-storing cells exhibited conspicuously undulate outer and inner envelope membranes that formed multiple contact sites with each other and protuberances into both cytoplasm and stroma. Some species, e.g., Linum, have plastids with tubular structures that connect the inner membrane to the thylakoid system; in addition, in the stroma vesicles fusing with or apparently passing through the envelope were observed. The outer envelope membrane may be associated with ER-like cytoplasmic membrane structures. In addition, lipid bodies of various sizes were found in contact with the plastid envelope. The ultrastructural observations are interpreted to match the published biochemical evidence, indicating that both plastids and ER may be involved in the synthesis of storage lipids and lipid body production.