Spatial determinants in morphogenesis: recovery from plasmolysis in the diatom Ditylum

Uncovering hidden structures: previously undescribed pseudopodia and ectoplasmic structures in planktonic foraminifera

The trophic strategies of cold-water planktonic foraminifera are not well understood due to the challenge of culturing them in polar conditions. Here, we identify previously unknown ectoplasmic and cytoplasmic projections in three species of planktonic foraminifera thriving in polar and subpolar marine environments: Globigerina bulloides, Neogloboquadrina incompta and Neogloboquadrina pachyderma. These structures were observed during routine monitoring of cultured specimens sampled from the Norwegian coast, Greenland Sea and Baffin Bay. Two types of projections were discovered, including permanent and non-permanent structures such as ectoplasmic roots, twigs and twig-like projections, similar to those observed in benthic taxa Cibicides and Cibicidoides. Additionally, a previously undescribed filopodia-like projection was observed in N. pachyderma. We discuss the function, the ecological significance and the potential impact on pelagic processes of the presence of these structures in foraminifera species that occupy diverse niches in the water column. Our findings suggest that these structures may play an important role in the trophic strategies of cold-water planktonic foraminifera, and further research and observations are necessary to fully comprehend their significance in the carbon cycle.

Hypothesis: NDL proteins function in stress responses by regulating microtubule organization

N-MYC DOWNREGULATED-LIKE proteins (NDL), members of the alpha/beta hydrolase superfamily were recently rediscovered as interactors of G-protein signaling in Arabidopsis thaliana. Although the precise molecular function of NDL proteins is still elusive, in animals these proteins play protective role in hypoxia and expression is induced by hypoxia and nickel, indicating role in stress. Homology of NDL1 with animal counterpart N-MYC DOWNREGULATED GENE (NDRG) suggests similar functions in animals and plants. It is well established that stress responses leads to the microtubule depolymerization and reorganization which is crucial for stress tolerance. NDRG is a microtubule-associated protein which mediates the microtubule organization in animals by causing acetylation and increases the stability of α-tubulin. As NDL1 is highly homologous to NDRG, involvement of NDL1 in the microtubule organization during plant stress can also be expected. Discovery of interaction of NDL with protein kinesin light chain- related 1, enodomembrane family protein 70, syntaxin-23, tubulin alpha-2 chain, as a part of G protein interactome initiative encourages us to postulate microtubule stabilizing functions for NDL family in plants. Our search for NDL interactors in G protein interactome also predicts the role of NDL proteins in abiotic stress tolerance management. Based on published report in animals and predicted interacting partners for NDL in G protein interactome lead us to hypothesize involvement of NDL in the microtubule organization during abiotic stress management in plants.

Extensive and intimate association of the cytoskeleton with forming silica in diatoms: control over patterning on the meso- and micro-scale

Background

The diatom cell wall, called the frustule, is predominantly made out of silica, in many cases with highly ordered nano- and micro-scale features. Frustules are built intracellularly inside a special compartment, the silica deposition vesicle, or SDV. Molecules such as proteins (silaffins and silacidins) and long chain polyamines have been isolated from the silica and shown to be involved in the control of the silica polymerization. However, we are still unable to explain or reproduce in vitro the complexity of structures formed by diatoms.

Methods/Principal Finding

In this study, using fluorescence microscopy, scanning electron microscopy, and atomic force microscopy, we were able to compare and correlate microtubules and microfilaments with silica structure formed in diversely structured diatom species. The high degree of correlation between silica structure and actin indicates that actin is a major element in the control of the silica morphogenesis at the meso and microscale. Microtubules appear to be involved in the spatial positioning on the mesoscale and strengthening of the SDV.

Conclusions/Significance

These results reveal the importance of top down control over positioning of and within the SDV during diatom wall formation and open a new perspective for the study of the mechanism of frustule patterning as well as for the understanding of the control of membrane dynamics by the cytoskeleton.

Formation of filopodia in earthworm (Lumbricus terrestris) coelomocytes in response to osmotic stress

Coelomocytes, the immunodefense cells of the earthworm Lumbricus terrestris, are exposed to changing osmotic pressures as the worm's coelomic fluid responds to fluctuating wet-dry conditions of the surrounding soil. Using light and fluorescence microscopy combined with actin and tubulin disrupting drugs, we determined the effects of changing osmotic pressure on coelomocyte morphology. The coelomocytes from L. terrestris respond to an increase in environmental osmotic pressure from isotonic conditions (170 mOsm) to hypertonic conditions (715 mOsm) by changing from a round/petalloid morphology to a filopodial morphology. Cytoskeletal fluorescent staining studies indicate that for filopodia to form, the actin cortical ring, present in most coelomocytes in isotonic conditions, must be disrupted. Breakdown of the actin ring by exposure to a hypertonic environment or actin disrupting drugs allows the formation of actin or tubulin-based filopodia. The filopodia, or podial-like extensions formed by earthworm coelomocytes, may enable the cells to better explore their environment.

Domain-specific mechanosensory transmission of osmotic and enzymatic cell wall disturbances to the actin cytoskeleton

Plant protoplasts are embedded within surrounding cell walls and the cell wall-plasma membrane-cytoskeleton (WMC) structural continuum seems to be crucial for the proper functioning of plant cells. We have utilised the protoplast preparation methodology to study the organisation and the putative components of the WMC continuum. Application of an osmotic agent evoked plasmolysis of the Zea mays root apex cells which appeared to be cell type- and growth stage-specific. Simultaneous use of wall polysaccharide-digesting enzymes selectively severed linkages between the components of the WMC continuum which changed the plasmolytic patterns in various cell types. This was followed by a reorganisation of filamentous actin aimed to reinforce protoplast boundaries and maintain the functioning of intercellular contact sites, especially at the cross walls. Particularly strong effects were evoked by pectin-degrading enzymes. Such treatments demonstrated directly the differentiated composition of various wall domains surrounding individual cells with the pectin-enriched cross walls (synapses), and the cellulose-hemicellulose network dominating the side walls. The same wall-degrading enzymes were used for in vitro digestion of isolated Lupinus albus cell walls followed by the extraction of wall proteins. Selective release of proteins suggested the importance of wall polysaccharide-protein interactions in the maintenance of the functioning and mechanical stability of root cell walls.