3D reconstruction of endoplasmic reticulum in a hydrocarbon-secreting green alga, Botryococcus braunii (Race B)

• Published in: Planta 2018-03, Vol.247 (3), p.663-677
• Main Authors: Suzuki, Reiko, Nishii, Ichiro, Okada, Shigeru, Noguchi, Tetsuko
• Format: Article
• Language: English
• Subjects: Agriculture; Algae; Aquatic plants; Biomedical and Life Sciences; Biosynthesis; Botryococcus braunii; Chlorophyta - metabolism; Chlorophyta - ultrastructure; Chloroplasts; Cytoplasm; Ecology; Endoplasmic reticulum; Endoplasmic Reticulum - physiology; Endoplasmic Reticulum - ultrastructure; Forestry; Free surfaces; Freezing; Hydrocarbons; Hydrocarbons - metabolism; Image processing; Image reconstruction; Image transmission; Imaging, Three-Dimensional - methods; Life Sciences; Mammalian cells; Microscopy, Electron; Microscopy, Fluorescence; Oils & fats; Organelles; Organelles - physiology; Organelles - ultrastructure; ORIGINAL ARTICLE; Petroleum production; Photosynthesis; Plant cells; Plant Sciences; Ribosomes; Secretion; Transport
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/s00425-017-2811-8

Oil production by photosynthetic microalgae has attracted attention since these oils can be converted into renewable, carbonneutral fuels. The green alga B. braunii accumulates large amounts of hydrocarbons, 30–50% of cell dry weight, in extracellular spaces rather than its cytoplasm. To advance the knowledge of hydrocarbon biosynthesis and transport pathways in this alga, we utilized transmission EM combined with rapid freezing and image reconstruction. We constructed detailed 3D maps distinguishing three ER domains: rdER with ribosomes on both sides, rsER with ribosomes on one side, and sER without ribosomes. The rsER and sER domains were especially prominent during the oil body formation and oil secretion stages. The ER contacted the chloroplasts, oil bodies, or plasma membrane via the rsER domains, oriented with the ribosomefree surface facing the organelles. We discuss the following transport pathway for hydrocarbons and their precursors in the cytoplasm: chloroplast → endoplasmic reticulum (ER) → oil bodies → ER → plasma membrane → secretion. This study represents the first 3D study of the three-domain classification (rdER, rsER and sER) of the ER network among eukaryotic cells. Finally, we propose the novel features of the ERs in plant cells that are distinct from the latest proposed model for the ERs in mammalian cells.