Organogenesis at the shoot apex: An attempt at modelization

A geometrical model of the emergence of a primordium at the shoot apex in dicotyledons is proposed. It is based on recent fundamental results on plant morphogenesis, i.e.:

1. the emergence is preceded by the reorganization of the microtubules of the cortical cytoskeleton, leading to a new orientation of the synthesis of the cell wall microfibrils;
2. the resulting global stress is related to the general orientation of the cell growth.

So the model sums up the continuous interactions linking the microtubules, the microfibrils and the cell growth axis. This paper tries to answer three questions which are essential from a botanical point of view:

1. Why does the principal stem shift its growth direction after each lateral emergence?
2. Why do the three axes involved in any ramification (namely the old and the new principal stems and the lateral emergence) exhibit a plane configuration whereas this is an essentially three dimensional phenomenon?
3. Does phyllotaxis exclusively depend upon the local emergence of a primordium?

A come and go between the botanical knowledge and the mathematical model leads to an integrated view of the compatibility mechanisms linking the different microtubules and microfibrils networks, without forgetting the apical dome restoration. A geometrical formalism allows a modern redefinition of both the “generating centre” and the “organizing centre” and their field effects.     

Seasonal change in photoperiodic response of the larvae of the lacewing Nineta pallida

Larvae of the lacewing Nineta pallida (Schneider), collected in the field during two seasons, from September to July, were reared in the laboratory under short- or long-day light conditions at 21°C. In autumn and winter, artificial short days delayed the first ecdysis. The influence on the duration of the first instar was maximal (3.4 times longer) when the short days began at hatching time, and later regularly diminished. In spring, the second and third instars showed a reversed response so that the long days now increased the duration of development, although development took no more than 1.4 time as long as in short days. A similar effect appeared in field-collected third instars on and after mid June, reaching its maximum (1.8 time until the cocoon spinning) in July. This sort of photoperiodic effect on the larval development is new to the seasonal adaptation of the life cycle in insects.

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Résumé Des formes préimaginales (oeufs, puis larves) de N. pallida sont récoltées sur des conifères de montagne (Pyrénées), chaque mois depuis septembre jusqu'en juillet en deux saisons (1983/84 et 1985/86). Elles sont ensuite élevées au laboratoire à 21°C, soit en jours longs (JL=L16:D8), soit en jours courts (JC=L8:D16).Le développement embryonnaire est légèrement plus long s'il se fait en JC. Pour les larves de premier stade récoltées en automne et en hiver, les JC retardent considérablement la première mue et prolongent aussi le deuxième stade qui en provient. L'influence retardatrice est maximale (3,4 fois) lorsque les JC agissent dès l'éclosion. Elle diminue ensuite progressivement et devient insignificante pour les larves récoltées à artir de février.Au printemps, les larves récoltées au deuxième stade ainsi que les troisièmes stades qui en découlent présentent une réaction inverse: ce sont alors les JL qui augmentent la durée du dévelopement, toutefois, pas plus de 1,4 fois par rapport aux JC. Un effet de même ordre se manifeste sur les larves de troisième stade récoltées à partir de juin, atteignant son maximum (1,8 fois) dans le lot de larves de juillet, c'est-à-dire peu avant la fin de la croissance pondérale larvaire et le coconnage.Un tel retardement du développement larvaire hivernal, prolongé au printemps et au début de l'été par une inversion de la réponse à la photopériode, est nouveau comme élément d'adaptation saisonnière du cycle naturel chez les insectes.

     

Regulation of carbon flow in Selenomonas ruminantium grown in glucose-limited continuous culture.

We have applied a model that permits the estimation of the sensitivity of flux through branch point enzymes (D. C. LaPorte, K. Walsh, and D. E. Koshland, J. Biol. Chem. 259:14068-14075, 1984) in order to analyze the control of flux through the lactate-acetate branch point of Selenomonas ruminantium grown in glucose-limited continuous culture. At this branch point, pyruvate is the substrate of both the NAD-dependent L-(+)-lactate dehydrogenase (LDH) and the pyruvate:ferredoxin oxidoreductase (PFOR). The LDH was purified, and it exhibited positive cooperativity for the binding of pyruvate. The LDH had an [S].5 for pyruvate of 0.43 mM, a Hill coefficient of 2.4, and a K' equal to 0.13 mM. The PFOR, assayed in cell extracts, exhibited Michaelis-Menten kinetics for pyruvate, with a Km of 0.49 mM. Carbon flux through the LDH and the PFOR increased 80-fold and 3-fold, respectively, as the dilution rate was increased from 0.07 to 0.52 h-1 in glucose-limited continuous culture. There was nearly a twofold increase, from 6.5 to 11.2 mumol min-1 mg of protein-1 in the specific activity (i.e., maximum velocity) of the LDH at dilution rates of 0.11 and 0.52 h-1, respectively. A flux equation was used to calculate the intracellular concentration of pyruvate; a fourfold increase in pyruvate, from 0.023 to 0.093 mM, was thereby predicted as the dilution rate was increased from 0.07 to 0.52 h-1. When these calculated values of intracellular pyruvate concentration were inserted into the flux equation, the predicted values of flux through the LDH and the PFOR were found to match closely the flux actually measured in the chemostat-grown cells. Thus, the 80-fold increase in flux through the LDH was due to a twofold increase in the maximum velocity of the LDH and a fourfold increase in the intracellular pyruvate concentration. In addition, the flux through the LDH exhibited ultrasensitivity to changes in both the maximum velocity of the LDH and the intracellular concentration of pyruvate. The flux through the PFOR exhibited ultrasensitivity to changes in the maximum velocity of the LDH and hyperbolic sensitivity to changes in the intracellular concentration of pyruvate.     

Cellular Localization of the Molecular Forms of Acetylcholinesterase in Primary Cultures of Rat Sympathetic Neurons and Analysis of the Secreted Enzyme

The secretion and cellular localization of the molecular forms of acetylcholinesterase (AChE) were studied in primary cultures of rat sympathetic neurons. When cultured under conditions favoring a noradrenergic phenotype, these neurons synthesized and secreted large quantities of the tetrameric G4, and the dodecameric A12 forms, and minor amounts of the G1 and G2 forms. When these neurons adopted the cholinergic phenotype, i.e., in the presence of muscle-conditioned medium, the development of the cellular A12 form was completely inhibited. These neurons secreted only globular, mainly G4, AChE. Both cellular and secreted A12 AChE in adrenergic cultures aggregated at an ionic strength similar to that of the culture medium, raising the hypothesis that this form was associated with a polyanionic component of basal lamina. In noradrenergic neurons, 60-80% of the catalytic sites were exposed at the cell surface. In particular, 80% of G4 form, but only 60% of the A12 form, was external, demonstrating for the A12 form a sizeable intracellular pool. The hydrophobic character of the molecular forms was studied in relation to their cellular localization. As in muscle cells, most of the G4 form was membrane-bound. Whereas 76% of the cell surface A12 form was solubilized in the aqueous phase by high salt concentrations, only 50% of the intracellular A12 form was solubilized under these conditions. The rest of intracellular A12 could be solubilized by detergents and was thus either membrane-bound or entrapped in vesicles originating from, e.g., the Golgi apparatus.     

Investigation of Dopamine Content, Synthesis, and Release in the Rabbit Retina In Vitro: I. Effects of Dopamine Precursors, Reserpine, Amphetamine, and l-DOPA Decarboxylase and Monoamine Oxidase Inhibitors

The basal catecholamine content of rabbit retina was determined by liquid chromatography with electrochemical detection (LC-EC) and 3,4-dihydroxyphenylethylamine (dopamine, DA) found to be the major catecholamine. The immediate DA precursor, 3,4-dihydroxyphenylalanine (L-DOPA), and the metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were also detected at about 2.8% and 17% of DA levels, respectively. When added exogenously, L-tyrosine did not increase the rate of DA synthesis over the basal level. In contrast, exogenous L-DOPA led to a 3.5-fold increase in DA, and to a 20-fold increase in DOPAC content. The monoamine oxidase inhibitors pargyline and (-)-deprenyl differentially affected the degradation of DA, since 100 microM pargyline was apparently more effective than 100 microM (-)-deprenyl. Reserpine and (+/-)-amphetamine each induced a Ca2+-independent decrease of DA stores. The separate actions of reserpine and (+/-)-amphetamine in lowering tissue DA levels were additive, suggesting two separate pools of DA available for release from presynaptic stores. The present study demonstrates that the LC-EC technique may be used to investigate the modulation of the synthesis and release of retinal DA in vitro, without the prior uptake of radiolabelled transmitter.     

The Photocycle of Bacteriorhodopsin in the Two-Dimensional Orthorombic Crystal Form of Purple Membrane

Laser flash photolysis and low-temperature absorption studies of the photocycle of orthorhombic purple membrane (o-PM) reveal the existence of the same K, L, and M intermediates as found in the native hexagonal purple membrane (h-PM). However, the 0 intermediate is missing in the o-PM. The absorption spectrum of the K intermediate of o-PM is blueshifted by ~15 nm relative to the K intermediate found in the hexagonal purple membrane. The decay relaxation time constants of M in the o-PM are higher by more than an order of magnitude than the corresponding relaxation time constants in the h-PM. Similarly to the h-PM, the decay of M depends on the pulse width of excitation. The time-independent anisotropy factor obtained in photoselection studies of the M intermediate demonstrates the complete immobility of bacteriorhodopsin (bR) within the o-PM matrix. The same anisotropy factor of 0.3 obtained for o-PM and for h-PM suggests that in both crystalline lattices the transition moment of the retinal chromophore has similar angles with the plane of the membrane. The dependence of the decay kinetics of M on its occupancy may suggest the existence of kinetic coupling between neighboring bR molecules.     

Identification of Glycolytic Enzyme Polypeptides on the Two-Dimensional Protein Map of Saccharomyces cerevisiae and Application to the Study of Some Wine Yeasts

Using a modification of the basic two-dimensional polyacrylamide gel electrophoresis technique, the polypeptides of the protein map of Saccharomyces cerevisiae involved in glycolysis were investigated. This study resulted in a reassignment of two of the seven glycolytic enzyme polypeptides previously identified (Ludwig et al., Mol. Cell. Biol. 2:117-126, 1982), those corresponding to phosphoglycerate kinase and to alcohol dehydrogenase. It also resulted in the identification of two additional glycolytic polypeptides, the enolase B monomer and the glyceraldehyde phosphate dehydrogenase B monomer. The glycolytic enzymes polypeptides so identified were investigated in 5 laboratory strains (all S. cerevisiae) and in 11 commerical strains used for wine making (S. cerevisiae and Saccharomyces bayanus). It appeared highly significant that a particular electrophoretic variant of the glyceraldehyde phosphate dehydrogenase B monomer was found only in the wine yeasts. Furthermore, it was strongly suggested that S. cerevisiae and S. bayanus strains are distinguishible on the basis of a different electrophoretic migration of the enolase B monomer.