Organelle interactions and possible degradation pathways visualized in high‐pressure frozen algal cells

Abstract

SummaryOrganelle interactions, although essential for both anabolic and catabolic pathways in plant cells have not been examined in detail so far. In the present study the structure of different organelle-organelle, organelle-vesicle and organelle-membrane interactions were investigated in growing and nongrowing cells of the green alga Micrasterias denticulata by use of high pressure freeze fixation and energy filtering transmission electron microscopy. It became clear that contacts between mitochondria always occur by formation of a cone-shaped protuberance of one of the mitochondria which penetrates into its fusion partner. In the same way, structural interactions between mitochondria and mucilage vesicles and between microbodies and mucilage vesicles are achieved. Lytic compartments contact mitochondria or mucilage vesicles again by forming protuberances and by extending their contents into the respective compartment. Detached portions of mitochondria are found inside lytic compartments as a consequence of such interactions. Mitochondria found in contact with the plasma membrane reveal structural disintegration. Our study shows that interactions of organelles and vesicles are frequent events in Micrasterias cells of different ages. The interactive contacts between lytic compartments and organelles or vesicles suggest a degradation pathway different from autophagy processes described in the literature. Both the interactions between vesicles and organelles and the degradation pathways occur independently from cytoskeleton function as demonstrated by use of cytochalasin D and the microtubule inhibitor amiprophos-methyl.

Changes in Phenolic Compounds and Cellular Ultrastructure of Arctic and Antarctic Strains of Zygnema (Zygnematophyceae, Streptophyta) after Exposure to Experimentally Enhanced UV to PAR Ratio

“…Control and 4-day UV-exposed samples were high-pressure frozen and freeze-substituted according to the methods of Aichinger and Lütz-Meindl [2] with modifications. Prior to freezing, samples were transferred to 150 mM sucrose for 1 min.…”

Salt stress-induced cell death in the unicellular green alga Micrasterias denticulata

“…Cells 48 h after mitosis were treated with 200 mM KCl for 3, 20, and 24 h, 300 mM KCl for 24 h, 200 mM NaCl for 3 h and 24 h, and 339 mM sobitol for 3 h, respectively. Cells were fixed by a high pressure freeze-fixation method as described previously ( Meindl et al , 1992 ; Aichinger and Lütz-Meindl, 2005 ) and were embedded in Agar low viscosity resin (LV Resin, VH1 and VH2 Hardener, and LV Accelerator; Agar Scientific, Essex, Great Britain). Ultrathin sections were placed on Formvar coated copper grids for TEM analysis and viewed in a LEO 912 transmission electron microscope (Zeiss, Oberkochen, Germany) with an in-column energy filter.…”

“…Second, salt stress may become an increasing threat for Micrasterias due to fertilization of the natural habitat by farming or due to salinization by road salt. Many physiological and ultrastructural studies have already been performed in Micrasterias which has increased our knowledge about cell differentiation, growth, and physiology in this unicellular green alga ( Kiermayer, 1981 ; Meindl, 1993 ; Lütz et al , 1997 ; Weiss et al , 1999 ; Oertel et al , 2004 ; Aichinger and Lütz-Meindl, 2005 ; Darehshouri et al , 2008 ) and represent the basis for the present investigation.…”

Plasmolysis effects and osmotic potential of two phylogenetically distinct alpine strains of Klebsormidium (Streptophyta)

“…Samples were dehydrated in increasing ethanol concentrations. Alternatively, control samples and samples exposed to 2,000 mM sorbitol for 3 h were fixed by high-pressure freeze fixation followed by freeze substitution with 1% OsO 4 and 0.05% uranyl acetate in acetone (according to the methods of Aichinger and Lütz-Meindl 2005).…”