OAC-39 is an O-acyltransferase required for the synthesis of maradolipids in the dauer larva of C. elegans

Upon overcrowding or low food availability, the nematode C. elegans enters a specialized diapause stage for survival, called the dauer larva. The growth-arrested, non-feeding dauer larva undergoes a profound metabolic and physiologic switch underlying its extraordinary stress resistance and longevity. One of the metabolic signatures of dauer larvae is the accumulation of the disaccharide trehalose, which lowers the sensitivity of worms to desiccation and hyperosmotic shock. Previously, we have found that trehalose is incorporated as a headgroup into dauer-specific 6,6′-di-O-acyltrehalose lipids, named maradolipids. Despite comprising a bulk fraction of the polar lipids in dauer larvae, little is known about the physiological function of maradolipds because the enzyme(s) involved in their synthesis has not yet been identified. Here, we report that the dauer-upregulated O-acyltransferase homolog OAC-39 is essential for the synthesis of maradolipids. This enzyme is enriched at the apical region of the intestinal cells of dauer larvae, where it might participate in the structuring of the gut lumen. As OAC-39 is most probably responsible for the last step of maradolipid synthesis, its identification will pave the way for the elucidation of the function of this obscure class of lipids.

Hyperosmotic Shock Transiently Accelerates Constriction Rate in Escherichia coli

Bacterial cells in their natural environments encounter rapid and large changes in external osmolality. For instance, enteric bacteria such as Escherichia coli experience a rapid decrease when they exit from host intestines. Changes in osmolality alter the mechanical load on the cell envelope, and previous studies have shown that large osmotic shocks can slow down bacterial growth and impact cytoplasmic diffusion. However, it remains unclear how cells maintain envelope integrity and regulate envelope synthesis in response to osmotic shocks. In this study, we developed an agarose pad-based protocol to assay envelope stiffness by measuring population-averaged cell length before and after a hyperosmotic shock. Pad-based measurements exhibited an apparently larger length change compared with single-cell dynamics in a microfluidic device, which we found was quantitatively explained by a transient increase in division rate after the shock. Inhibiting cell division led to consistent measurements between agarose pad-based and microfluidic measurements. Directly after hyperosmotic shock, FtsZ concentration and Z-ring intensity increased, and the rate of septum constriction increased. These findings establish an agarose pad-based protocol for quantifying cell envelope stiffness, and demonstrate that mechanical perturbations can have profound effects on bacterial physiology.

Visualization of Chromatin in the Yeast Nucleus and Nucleolus Using Hyperosmotic Shock

Unlike in most eukaryotic cells, the genetic information of budding yeast in the exponential growth phase is only present in the form of decondensed chromatin, a configuration that does not allow its visualization in cell nuclei conventionally prepared for transmission electron microscopy. In this work, we studied the distribution of chromatin and its relationships to the nucleolus using different cytochemical and immunocytological approaches applied to yeast cells subjected to hyperosmotic shock. Our results show that osmotic shock induces the formation of heterochromatin patches in the nucleoplasm and intranucleolar regions of the yeast nucleus. In the nucleolus, we further revealed the presence of osmotic shock-resistant DNA in the fibrillar cords which, in places, take on a pinnate appearance reminiscent of ribosomal genes in active transcription as observed after molecular spreading (“Christmas trees”). We also identified chromatin-associated granules whose size, composition and behaviour after osmotic shock are reminiscent of that of mammalian perichromatin granules. Altogether, these data reveal that it is possible to visualize heterochromatin in yeast and suggest that the yeast nucleus displays a less-effective compartmentalized organization than that of mammals.

Physical properties of the cytoplasm modulate the rates of microtubule growth and shrinkage

The cytoplasm represents a crowded environment whose properties may change according to physiological or developmental states. Although the effects of crowding and viscosity on in vitro reactions have been well studied, if and how the biophysical properties of the cytoplasm impact cellular functions in vivo remain poorly understood. Here, we probed the effects of cytoplasmic concentration on microtubule (MT) dynamics by studying the effects of osmotic shifts in the fission yeast Schizosaccharomyces pombe. Increasing cytoplasmic concentration by hyperosmotic shock led to proportionate reductions in the rates of interphase MT growth and shrinkage. Conversely, dilution of the cytoplasm in hypoosmotic shifts led to proportionately faster rates. Numerous lines of evidence indicate that these effects were due to biophysical properties of the cytoplasm. These effects were recapitulated in in vitro reconstituted MT assays by modulating viscosity, not by crowding. Our findings suggest that even at normal conditions, the viscous properties of cytoplasm modulate the dynamic reactions of MT polymerization and depolymerization.

Early endocytosis as a key to understanding mechanisms of plasmalemma tension regulation in filamentous fungi

ABSTRACTTwo main systems regulate the plasmalemma tension and provide a close connection of the protoplast with the cell wall in fungi: turgor pressure and actin cytoskeleton. These systems work together with the plasmalemma focal adhesion to the cell wall and their contribution to fungal cell organization has been partially studied, but remains controversial in model filamentous ascomycetes and oomycetes, and even less investigated in filamentous basidiomycetes. Early endocytosis, in which F-actin is actively involved, can be used to research of mechanisms regulating the plasmalemma tension, since the latter influences on the primary endocytic vesicles formation. This study examined the effects of actin polymerization inhibitors and hyperosmotic shock on early endocytosis and cell morphology in two filamentous basidiomycetes. The main obtained results: (i) depolymerization of F-actin leads to the fast formation of primary endocytic vesicles but to inhibition of their scission; (ii) moderate hyperosmotic shock does not affect the dynamics of early endocytosis. These and a number of other results allowed offering a curtain model of regulation the plasmalemma tension in basidiomycetes. According to this model, the plasmalemma tension in many nonapical cells of hyphae is more often regulated not by turgor pressure, but by a system of actin driver cables that are associated with the proteins of focal adhesion sites. The change in the plasmalemma tension occurs similar to the movement of the curtain along the curtain rod using the curtain drivers. This model addresses the fundamental properties of the fungal structure and physiology and requires confirmation, including through the yet technically unavailable high quality labeling of the actin cytoskeleton of basidiomycetes.

Influence of salinity on vegetative growth and photosynthetic pigments of bitter almond rootstock

Bitter almond rootstock is considered one of the most vital rootstocks for stone fruit species but it is classified as a plant sensitive to salinity. This experiment was carried out to study the effect of salt stress on vegetative growth and photosynthetic pigments of bitter almond rootstock as an attempt to sustain growth and increase its tolerance to high salt concentrations. However, the seeds were soaked in salt solution of NaCl as 1, 3, and 5 dsm-1 for 48 hours before stratification. After that, nuts were sown in perlite and treated with different saline solutions subsequently stratified at 6 ℃ for eight weeks. Sprouted seeds were cultivated in pots with a mixture of peat and perlite and treated only with the highest salt concentration 5 dsm-1. The treatments were arranged in a complete randomized block design with three replications. Vegetative traits and photosynthetic pigments content were estimated. The results revealed that soaking and pre-treating seed of bitter almond rootstock by means of high salt concentration 5dsm-1 during the germination period and subsequently after planting produced stronger transplants that had hardening, adaptation and could avoid the hyperosmotic shock of salt stress after planting. It is obvious throughout; increment of stem diameter, plant height, total number of leaves\plant, fresh and dry weight of leaves, photosynthetic pigments and total carbohydrate content of such transplants. While other coming seedlings from low salt concentrations were exposed to hyperosmotic shock and salt injury therefore inhibit growth rate of such plants, increased falling of leaves and finally reduced photosynthetic pigments content in the resulting seedlings.