Structural stress responses and degradation of dictyosomes in algae analysed by TEM and FIB‐SEM tomography

Abstract

Stress-induced physiological deficiencies in cells are reflected in structural, morphological and functional reactions of organelles. Although numerous investigations have focused on chloroplasts and mitochondria as main targets of different stressors in plant cells, there is insufficient information on the plant Golgi apparatus as stress sensor. By using the advantages of field emission scanning electron microscopy tomography in combination with classical ultrathin sectioning and transmission electron microscopic analyses, we provide structural evidence for common stress responses of the large and highly stable dictyosomes in the algal model system Micrasterias. Stress is induced by different metals such as manganese and lead, by starvation in 9 weeks of darkness or by inhibiting photosynthesis or glycolysis and by disturbing ionic homeostasis via KCl. For the first time a stress-induced degradation pathway of dictyosomes is described that does not follow "classical" autophagy but occurs by disintegration of cisternae into single membrane balls that seem to be finally absorbed by the endoplasmic reticulum (ER). Comparison of the morphological features that accompany dictyosomal degradation in Micrasterias to similar reactions observed during the same stress application in Nitella indicates an ubiquitous degradation process at least in algae. As the algae investigated belong to the closest relatives of higher land plants these results may also be relevant for understanding dictyosomal stress and degradation responses in the latter phylogenetic group. In addition, this study shows that two-dimensional transmission electron microscopy is insufficient for elucidating complex processes such as organelle degradation, and that information from three-dimensional reconstructions as provided by field emission scanning electron microscopy tomography is absolutely required for a comprehensive understanding of the phenomenon.

Micrasterias as a Model System in Plant Cell Biology

“…Dictyosomes of Micrasterias are unique organelles measuring 2–3 μm in diameter and revealing a constant number of 11 cisternae independent from the stage of the cell cycle. They are thus many times larger than in higher plant cells or in other algae (see also Lütz-Meindl et al, 2015 ). In growing cells they are located in great number around the nucleus and along the chloroplast membrane.…”

“…When KCl treatment was continued for up to 24 h, degrading dictyosomes consisting of only two or three coiled cisternae were frequently found in contact with large ER compartments. 3-D analyses by FIB-SEM tomography provided evidence for the dictyosomal degradation process that occurred as consequence of KCl stress ( Figure 8 ; Lütz-Meindl et al, 2015 ). Dictyosomal degradation starts with detachment of single cisternae from a stack.…”

“…Mucilage vesicles in white, multivesicular bodies in yellow (scale bar is 1 μm). Reprinted with permission from Lütz-Meindl et al (2015) , (A) Copyright © 2015 Royal Microscopical Society and from Wanner et al (2013) (B) , Copyright © 2013 Elsevier Inc.…”

Abiotic Stress Tolerance of Charophyte Green Algae: New Challenges for Omics Techniques

“…Nevertheless, these stresses are complex and components other than osmotic dehydration play a role in their effects (e.g., ionic component of the salt stress, Darehshouri and Lütz-Meindl, 2010 ). Recently osmotic stress induced ultrastructural changes leading to degradation of dictyosomes were investigated by TEM and FIB-SEM tomography in the charophyte green algae Micrasterias and Nitella ( Lütz-Meindl et al, 2015 ). During the osmotic acclimation in response to long term hyperosmotic conditions, algae synthesize and accumulate various substances that increase the cellular osmotic value, leading to a negative osmotic potential ( Bisson and Kirst, 1995 ).…”

Charophytes: Evolutionary Giants and Emerging Model Organisms

“…Furthermore, cell morphogenesis is highly sensitive to external stress (e.g., oxidative stress, salinity, heavy metals) that leads to major changes in cell differentiation ( Darehshouri et al, 2008 ; Affenzeller et al, 2009 ; Andosch et al, 2012 ). Micrasterias is an outstanding organism for analyzing these subcellular features as it is easy to maintain and manipulate in the laboratory and it lends itself especially well for acquisition of high resolution imaging using light and electron microscopy including immunocytochemistry, Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) and Electron Energy Loss Spectroscopy (EELS) imaging ( Lutz-Meindl, 2007 ; Eder and Lutz-Meindl, 2008 , 2010 ; Wanner et al, 2013 ; Lutz-Meindl et al, 2015 ). Furthermore, initial molecular analyses including the production of transformed cell lines have further enhanced the use of Micrasterias especially in the molecular dynamics of cell wall processing ( Vannerum et al, 2010 , 2011 , 2012 ).…”