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

Abstract

Based on 3D sections through cells of Botryococcus braunii, the structure of three domains of endoplasmic reticulum, and their spatial and functional relationships to other organelles are clarified. Oil production by photosynthetic microalgae has attracted attention since these oils can be converted into renewable, carbon-neutral 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 ribosome-free 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.

Stay in Touch—The Cortical ER of Moss Protonemata in Osmotic Stress Situations

“…The cortical ER of plant cells consists of a network of tubules and sheets [11,[23][24][25]. Single tubules can form polygons of varying diameters and are connected by three-way junctions [26].…”

Chlamydomonas reinhardtii cellular compartments and their contribution to intracellular calcium signalling

“…The ER is the major Ca 2+ store capable of its release in most eukaryotes, and is the largest and the major rapidly exchanging Ca 2+ store in animal cells ( Raffaello et al , 2016 ). ER network 3D reconstruction in the green alga Botryococcus braunii has shown that most of the ER is present as sheets or fenestrated sheets, differing from the pattern in land plant cells, with a definite larger contact area with organelles ( Suzuki et al , 2018 ). Besides being in continuity with the double nuclear membrane, the ER has been shown to be in strict contact with chloroplasts, oil bodies, or plasma membrane with specific domains ( Suzuki et al , 2018 ).…”

“…ER network 3D reconstruction in the green alga Botryococcus braunii has shown that most of the ER is present as sheets or fenestrated sheets, differing from the pattern in land plant cells, with a definite larger contact area with organelles ( Suzuki et al , 2018 ). Besides being in continuity with the double nuclear membrane, the ER has been shown to be in strict contact with chloroplasts, oil bodies, or plasma membrane with specific domains ( Suzuki et al , 2018 ). As proposed in land plants, such a unique architecture may facilitate the establishment of a communication network between cell compartments where, through specific contact sites, organelles could exchange metabolites and ions, such as Ca 2+ (recently reviewed in plants by Wang and Dehesh, 2018 ).…”

Untersuchung zur Sekretion und Abtrennung von Lipiden aus Mikroalgen

“…Lycopapentaen (3 %) macht Lycopadien mit 95 % den größten Anteil der gebildeten Kohlenwasserstoffe aus (Thapa et al 2016). Ausgehend von FPP wird wie bei den Stämmen der B-Race zunächst Geranylgeranylpyrophosphat (GGPP) gebildet (Molnár et al 2012) Race-spezifisch zu unterscheiden (Hirose et al 2013;Suzuki et al 2013;Suzuki et al 2018).…”

“…Mit Hilfe von fluoreszenz-und elektronenmikroskopischen Untersuchungen konnte bei den Stämmen der Races A und B ein direkter Zusammenhang zwischen Zellzyklus und Kohlenwasserstoffsekretion ermittelt werden (Hirose et al 2013;Suzuki et al 2013;Suzuki et al 2018) Chloroplastensignale festgestellt werden (Suzuki et al 2013). Das Fehlen der Chloroplastensignale bei den Schlüsselenzymen SSL-1, -2 und -3, welche die finalen Schritte von Farnesylpyrophosphat zu C 30-Botryococcene (SSL-1 und SSL-3) bzw.…”