Influence of phosphatidylcholine/cholesterol molar ratios in liposomes on cholesterol reactivity with cholesterol:oxygen oxidoreductase]

The reactivity of sonicated phosphatidylcholine-cholesterol liposomes with cholesterol : oxygene oxydoreductase, an enzyme which catalyses the oxidation of the 3 beta hydroxyl group of cholesterol to a ketone group, is compared with that of ternary system phosphatidylcholine-cholesterol-Thesit. Regardless to the phosphatidylcholines nature and the phosphatidylcholine/cholesterol molar ratio (R), the enzymatic oxidation rate of liposomal cholesterol is slower than when the reaction is developed in the present of Thesit, a surfactif agent which destroyes the lamellar particles. This is true whether Thesit is added during preparation of dispersions or during incubation with cholesterol oxydase. The enzymatic oxydation rate of cholesterol of ternary systems phosphatidylcholine-cholesterol-Thesit is independent of the (R) value and the phosphatidylcholine fatty acid unsaturation, whereas that of phosphatidylcholine-cholesterol dispersions depends on these two parameters. The reaction rate increases in the order: dipalmitoylphosphatidylcholine to yolk egg phosphatidylcholines, and dioleylphosphatidylcholine. The optimal conditions for cholesterol oxidation were found to be R = 0.5. This result is not affected by the phosphatidylcholines nature. In order to explain these data, various hypotheses are considered. In particular, the weak liposomal cholesterol reactivity with cholesterol oxidase could result from an inhibitory effect on the enzyme-substrate combination due to the polar phosphorylcholine groups.     

Operation of the glycolate pathway in isolated bundle sheath strands of maize and Panicum maximum

Operation of the glycolate pathway in isolated bundle sheath (BS) strands of two C₄ species was demonstrated from ¹⁴C incorporation into two intermediates, glycine and serine, under conditions favourable for photorespiratory activity. Isolated BS strands fixing ¹⁴CO₂ under light at physiological rates incorporate respectively 3% (Zea mays L., cv. INRA 258) and 7% (Panicum maximum Jacq.) of total ¹⁴C fixed into glycine + serine, at low bicarbonate levels (less than the Km for CO₂ fixation, 0.8 mM). Higher bicarbonate concentrations depressed the percentage of incorporation into the two amino acids. No labelling was observed in the absence of added glutamate. Oxygen was required for glycine + serine labelling, since ¹⁴C incorporation into glycine was largely depressed by argon flushing, and labelling of the two amino acids was nearly suppressed by the addition of the strong reductant, dithionite, especially in maize. Two inhibitors of the glycolate pathway were tested. With α-hydroxypyridine-methanesulfonic acid, an inhibitor of glycolate oxidase, labelling of glycine and serine remained minimal whereas glycolate was accumulated. Isoniazid, an inhibitor of the transformation of glycine to serine induced a 50% increased labelling of glycine in maize BS, and a large decrease in serine labelling. In Panicum, the increase in [¹⁴C]-glycine was 90%. These results suggest that the pathway glycolate → glycine → serine operates in these plants. However, leakage of metabolites occurs in BS cells, especially in maize and a large part of newly formed glycolate, glycine and serine is exported out of the cells. Operation of ribulose-1,5-bisphosphate oxygenase activity in competition with ribulose-1,5-bisphosphate carboxylase is demonstrated by the lowering of total ¹⁴CO₂ fixation when O₂ is increased at low bicarbonate concentration. An interesting feature observed in maize BS, at low bicarbonate concentration, was an increase in ribulose-1,5-bisphosphate labelling when the O₂ level was decreased. This was accompanied by an increase in CO₂ fixation. This could indicate an increased rate in synthesis of ribulose-1,5-bisphosphate (which accumulated) due to a stimulation of ATP synthesis by cyclic photophosphorylation under anaerobic conditions.     

Prevalence of Localized Rearrangements Vs. Transpositions among Events Induced by Drosophila P Element Transposase on a P Transgene

We have studied P transposase-induced events on a P[w] transgene, P[w(d1)], harboring the whole white gene with a 3.44-kb direct duplication of its 5' regulatory sequences (containing the ZESTE-binding region, ZBR). We have recovered mutations leading to an increase or a decrease of zeste(1) repression, generally as the consequence of modifications of the number of ZBR in close physical proximity and/or jumps to other sites. We describe mutants displaying deletions of the original duplicated sequence or increases in the number of repeats from two to three or four. Internal deletions are more frequent than amplifications. Both require the integrity of P-element ends. We have also observed a high frequency of double P elements localized at the original P[w(d1)] insertion site. These double P elements are arranged in nonrandom configurations. We discuss the frequencies and the possible mechanisms leading to the various types of derivatives, in light of the current models for P excision and transposition. We propose that the P transposase induces mainly localized events. Some of these could result from frequent changes of template during gap-repair DNA synthesis, and/or from abortive transposition.     

Evolutionary divergence of anaphase spindle mechanics in nematode embryos constrained by antagonistic pulling and viscous forces

Cellular functions such as cell division are remarkably conserved across phyla. However, the evolutionary principles of cellular organization that drive them are less well explored. Thus, an essential question remains: to what extent do cellular parameters evolve without altering the basic functions they sustain? Here we have observed six different nematode species for which the mitotic spindle is positioned asymmetrically during the first embryonic division. Whereas the C. elegans spindle undergoes oscillations during its displacement, the spindle elongates without oscillations in other species. We asked which evolutionary changes in biophysical parameters could explain differences in spindle motion while maintaining a constant output. Using laser microsurgery of the spindle, we revealed that all species are subjected to cortical pulling forces of varying magnitudes. Using a viscoelastic model to fit the recoil trajectories and with an independent measurement of cytoplasmic viscosity, we extracted the values of cytoplasmic drag, cortical pulling forces, and spindle elasticity for all species. We found large variations in cytoplasmic viscosity, whereas cortical pulling forces and elasticity were often more constrained. In agreement with previous simulations, we found that increased viscosity correlates with decreased oscillation speeds across species. However, the absence of oscillations in some species despite low viscosity can only be explained by smaller pulling forces. Consequently, we find that spindle mobility across the species analyzed here is characterized by a tradeoff between cytoplasmic viscosity and pulling forces normalized by the size of the embryo. Our work provides a framework for understanding mechanical constraints on evolutionary diversification of spindle mobility.     

Knockdown of a laccase in Populus deltoides confers altered cell wall chemistry and increased sugar release

Plant laccases are thought to function in the oxidation of monolignols which leads to higher order lignin formation. Only a hand‐full of laccases in plants have been functionally evaluated, and as such little is known about the breadth of their impact on cell wall chemistry or structure. Here, we describe a previously uncharacterized laccase from Populus, encoded by locus Potri.008G064000, whose reduced expression resulted in transgenic Populus trees with changes in syringyl/guaiacyl ratios as well as altered sugar release phenotypes. These phenotypes are consistent with plant biomass exhibiting reduced recalcitrance. Interestingly, the transgene effect on recalcitrance is dependent on a mild pretreatment prior to chemical extraction of sugars. Metabolite profiling suggests the transgene modulates phenolics that are associated with the cell wall structure. We propose that this particular laccase has a range of functions related to oxidation of phenolics and conjugation of flavonoids that interact with lignin in the cell wall.     

Tolerance of young allis shad Alosa alosa (Clupeidae) to oxy-thermic stress

The ecology of the young stages of allis shad Alosa alosa is poorly documented, although they can be exposed to many pressures during their freshwater phase and their downstream migration. When passing through systems such as the Gironde-Garonne-Dordogne watershed (GGD, SW France), they can be subjected to high temperatures and low levels of oxygen (hypoxia). The aim of this work is to assess the tolerance of young Alosa alosa at four ages (c. 10, 30, 60 and 85 days old) by challenging them to different temperatures (18, 22, 26 and 28°C) together with decreasing oxygen saturation levels (from 100% to 30%). Survival of the 10-day-old individuals was not influenced by oxy-thermic conditions, but high stress levels were detected and perhaps this age class was too fragile regarding the constraint of the experimental design. Survival at 30 and at 60 days old was negatively influenced by the highest temperatures tested alone (from 26°C and from 28°C, respectively) but no effect was detected at 85 days old up to 28°C. A combined effect of temperature and oxygen level was highlighted, with heat accelerating survival decrease when associated with oxygen level depletion: essentially, survival was critical (<50%) at 30 days old at temperature ≥22°C together with 30% O₂; at 60 days old, at temperature = 28°C with 30% O₂; at 85 days old, at temperature ≥26°C with ≤40% O₂. Tolerance to oxy-thermic pressures appeared to be greater among the migratory ages (60 and 85 days old) than among the 30-day-old group. Based on environmental data recorded in the GGD system and on our experimental results, an exploratory analysis allowed a discussion of the possible impact of past oxy-thermic conditions on the local population dynamics between 2005 and 2018. The oxy-thermic conditions that may affect Alosa alosa at ages when they migrate downstream (60 and 85 days old) were not frequently recorded in this period, except in cases of extreme episodes of heat together with hypoxia that occurred in some years, in summertime in the turbidity maximum zone of the Gironde estuary (particularly in the year 2006). Interestingly, oxy-thermic conditions that are likely to threaten the 30-day-old individuals occurred more frequently in the lower freshwater parts of the GGD system between the years 2005 and 2018. In the context of climate change, a general increase in temperature is predicted, as well as more frequent and severe hypoxic events, therefore we suggest that local Alosa alosa population recruitment could encounter critical oxy-thermic conditions more frequently in the future if no adaptive management of water resources occurs.     

Does the presence of grassy strips and landscape grain affect the spatial distribution of aphids and their carabid predators?

• 1 We investigated, over the course of 2 years, the spatial distribution and abundance of two species of aphid, Metopolophium dirhodum and Sitobion avenae, and predatory species of carabid. This was undertaken in 24 wheat fields in ‘coarse‐grain’ and ‘fine‐grain’ landscapes in western France. A greater percentage of the latter landscape was covered by hedgerows and grassland and the total area covered by fields and the average size of the fields were smaller.
• 2 The effects on aphid abundance of the distance from field margins, the presence of grassy strips and carabid abundance were determined in both landscapes.
• 3 Both aphid species were more abundant in the ‘fine‐grain’ landscape, which may have been a result of the higher density of semi‐natural elements. In both types of landscape, the total numbers of aphids were negatively correlated with the distance from the field margin. This may have been because aphids were dispersing from overwintering sites in field margins. The abundance of M. dirhodum was strongly negatively correlated with the presence of grassy strips in the ‘coarse‐grain’ landscape, although there were no such significant correlations for either of the aphid species in the ‘fine‐grain’ landscape.
• 4 Aphid and carabid abundances were negatively correlated in the ‘fine‐grain’ and positively in ‘coarse‐grain’ landscape.
• 5 The results obtained in the present study emphasize the importance of semi‐natural areas in agricultural landscapes in shaping the spatial distribution of aphids and carabid beetles, their natural enemies, at different spatial scales.