Boron tolerance in barley is mediated by efflux of boron from the roots

Many plants are known to reduce the toxic effects of high soil boron (B) by reducing uptake of B, but no mechanism for limiting uptake has previously been identified. The B-tolerant cultivar of barley (Hordeum vulgare L.), Sahara, was shown to be able to maintain root B concentrations up to 50% lower than in the B-sensitive cultivar, Schooner. This translated into xylem concentrations that were approximately 64% lower and leaf concentrations 73% lower in the tolerant cultivar. In both cultivars, B accumulation was rapid and reached a steady-state concentration in roots within 3 h. In Schooner, this concentration was similar to the external medium, whereas in Sahara, the root concentration was maintained at a lower concentration. For this to occur, B must be actively extruded from the root in Sahara, and this is presumed to be the basis for B tolerance in barley. The extrusion mechanism was inhibited by sodium azide but not by treatment at low temperature. Several anion channel inhibitors were also effective in limiting extrusion, but it was not clear whether they acted directly or via metabolic inhibition. The ability of Sahara to maintain lower root B concentrations was constitutive and occurred across a wide range of B concentrations. This ability was lost at high pH, and both Schooner and Sahara then had similar root B concentrations. A predictive model that is consistent with the empirical results and explains the tolerance mechanism based on the presence of a borate anion efflux transporter in Sahara is presented.     

A new psittacosaur from Inner Mongolia and the parrot-like structure and function of the psittacosaur skull

We describe a new species of psittacosaur, Psittacosaurus gobiensis, from the Lower Cretaceous of Inner Mongolia and outline a hypothesis of chewing function in psittacosaurs that in many respects parallels that in psittaciform birds. Cranial features that accommodate increased bite force in psittacosaurs include an akinetic skull (both cranium and lower jaws) and differentiation of adductor muscle attachments comparable to that in psittaciform birds. These and other features, along with the presence of numerous large gastroliths, suggest that psittacosaurs may have had a high-fibre, nucivorous (nut-eating) diet.Psittacosaurs, alone among ornithischians, generate oblique wear facets from tooth-to-tooth occlusion without kinesis in either the upper or lower jaws. This is accomplished with a novel isognathous jaw mechanism that combines aspects of arcilineal (vertical) and propalinal (horizontal) jaw movement. Here termed clinolineal (inclined) jaw movement, the mechanism uses posteriorly divergent tooth rows, rather than kinesis, to gain the added width for oblique occlusion. As the lower tooth rows are drawn posterodorsally into occlusion, the increasing width between the upper tooth rows accommodates oblique shear. With this jaw mechanism, psittacosaurs were able to maintain oblique shearing occlusion in an akinetic skull designed to resist high bite forces.     

A kinetic study of human protein arginine N-methyltransferase 6 reveals a distributive mechanism

Human protein arginine N-methyltransferase 6 (PRMT6) transfers methyl groups from the co-substrate S-adenosyl-L-methionine to arginine residues within proteins, forming S-adenosyl-L-homocysteine as well as omega-N(G)-monomethylarginine (MMA) and asymmetric dimethylarginine (aDMA) residues in the process. We have characterized the kinetic mechanism of recombinant His-tagged PRMT6 using a mass spectrometry method for monitoring the methylation of a series of peptides bearing a single arginine, MMA, or aDMA residue. We find that PRMT6 follows an ordered sequential mechanism in which S-adenosyl-L-methionine binds to the enzyme first and the methylated product is the first to dissociate. Furthermore, we find that the enzyme displays a preference for the monomethylated peptide substrate, exhibiting both lower K(m) and higher V(max) values than what are observed for the unmethylated peptide. This difference in substrate K(m) and V(max), as well as the lack of detectable aDMA-containing product from the unmethylated substrate, suggest a distributive rather than processive mechanism for multiple methylations of a single arginine residue. In addition, we speculate that the increased catalytic efficiency of PRMT6 for methylated substrates combined with lower K(m) values for native protein methyl acceptors may obscure this distributive mechanism to produce an apparently processive mechanism.     

Effects and Mechanism Analysis of Combined Infusion by Levosimendan and Vasopressin on Acute Lung Injury in Rats Septic Shock

This research is aimed to discover the influence and underling mechanism of combined infusion of arginine vasopressin with levosimendan on acute lung injury in rat septic shock with norepinephrine supplemented. The traditional fecal peritonitis-induced septic shock model was undergone in rats for study. It is observed that the combined infusion supplemented with norepinephrine brought about a lower mean pulmonary artery pressure; lower high-mobility group box 1 levels, pulmonary levels of interleukin-6, and arterial total nitrate/nitrite; lower apoptotic cells scores and total histological scores; but higher pulmonary gas exchange when compared with the separate infusion group and norepinephrine group. This therapy shows potential clinical beneficial assistance in sepsis-induced acute lung injury. The results suggest the mechanism of such effect is through abating pulmonary artery pressure, and more importantly suppressing inflammatory responses in lung when compared with norepinephrine infusion group and the separate infusion of levosimendan or vasopressin alone.     

农作物Cd耐性的种内和种间差异Ⅱ．种内差

对２８个小麦、１４个大豆品种（或品系）的幼苗培养试验表明,作物对Ｃｄ的耐性具有明显的种内差异;耐性品种与其吸收Ｃｄ相对较少或向地上部运移比例较低有关;部分品种虽吸收Ｃｄ量较高,但仍能正常生长,推断其具有使Ｃｄ转化为活性较低形态,从而减轻毒害的机制．     

Rapid, bilateral changes in growth rate and curvature during gravitropism of cucumber hypocotyls: implications for mechanism of growth control

Abstract. The growth response of etiolated cucumber ( Cucumis sativus L.) hypocotyls to gravitropic stimulation was examined by means of time-lapse photography and high-resolution analysis of surface expansion and curvature. In comparison with video analysis, the technique described here has five- to 20-fold better resolution; moreover, the mathematical fitting method (cubic splines) allows direct estimation of local and integrated curvature. After switching seedlings from a vertical to horizontal position, both upper and lower surfaces of the stem reacted after a lag of about 11 min with a two- to three-fold increase in surface expansion rate on the lower side and a cessation of expansion, or slight compression, on the upper surface. This growth asymmetry was initiated simultancously along the length of the hypocotyl, on both upper and lower surfaces, and did not migrate basipetally from the apex. Later stages in the gravitropic response involved a complex reversal of the growth asymmetry, with the net result being a basipetal migration of the curved region. This secondary growth reversal may reflect oscillatory and or self-regulatory behaviour of growing cells. With some qualifications, the kinetics and pattern of growth response are consistent with a mechanism involving hormone redistribution, although they do not prove such a mechanism. The growth kinetics require a growth mechanism which can be stimulated by two-to three-fold or completely inhibited within a few minutes.     

Influence of joint constraints on lower limb kinematics estimation from skin markers using global optimization

In order to obtain the lower limb kinematics from skin-based markers, the soft tissue artefact (STA) has to be compensated. Global optimization (GO) methods rely on a predefined kinematic model and attempt to limit STA by minimizing the differences between model predicted and skin-based marker positions. Thus, the reliability of GO methods depends directly on the chosen model, whose influence is not well known yet.This study develops a GO method that allows to easily implement different sets of joint constraints in order to assess their influence on the lower limb kinematics during gait. The segment definition was based on generalized coordinates giving only linear or quadratic joint constraints. Seven sets of joint constraints were assessed, corresponding to different kinematic models at the ankle, knee and hip: SSS, USS, PSS, SHS, SPS, UHS and PPS (where S, U and H stand for spherical, universal and hinge joints and P for parallel mechanism). GO was applied to gait data from five healthy males.Results showed that the lower limb kinematics, except hip kinematics, knee and ankle flexion–extension, significantly depend on the chosen ankle and knee constraints. The knee parallel mechanism generated some typical knee rotation patterns previously observed in lower limb kinematic studies. Furthermore, only the parallel mechanisms produced joint displacements.Thus, GO using parallel mechanism seems promising. It also offers some perspectives of subject-specific joint constraints.     

A procedure for estimating the relevant forces in the human knee using a four-bar mechanism

Knee injuries, especially those that affect the cruciate and lateral ligaments, are one of the most serious and frequent pathologies that affect the lower human extremity. Hence, the aim of this study is to develop a dynamic model for the lower extremity capable of estimating forces, forces in the cruciate and collateral ligaments and those normal to the articular cartilage, generated in the knee. The proposed model considers a four-bar mechanism in the knee, a spherical joint in the pelvis and a revolute one in the ankle. The four-bar mechanism is obtained by a synthesis process. The dynamic model includes the inertial properties of the femur, tibia, patella and the foot, the ground reaction force and the most important muscles in the knee. Muscle forces are estimated using an optimisation technique. Results from the application of the model on a real human task are presented.     

Emotion enhances learning via norepinephrine regulation of AMPA-receptor trafficking

Emotion enhances our ability to form vivid memories of even trivial events. Norepinephrine (NE), a neuromodulator released during emotional arousal, plays a central role in the emotional regulation of memory. However, the underlying molecular mechanism remains elusive. Toward this aim, we have examined the role of NE in contextual memory formation and in the synaptic delivery of GluR1-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)-type glutamate receptors during long-term potentiation (LTP), a candidate synaptic mechanism for learning. We found that NE, as well as emotional stress, induces phosphorylation of GluR1 at sites critical for its synaptic delivery. Phosphorylation at these sites is necessary and sufficient to lower the threshold for GluR1 synaptic incorporation during LTP. In behavioral experiments, NE can lower the threshold for memory formation in wild-type mice but not in mice carrying mutations in the GluR1 phosphorylation sites. Our results indicate that NE-driven phosphorylation of GluR1 facilitates the synaptic delivery of GluR1-containing AMPARs, lowering the threshold for LTP, thereby providing a molecular mechanism for how emotion enhances learning and memory.     

The rhythm of yeast

Although yeast are unicellular and comparatively simple organisms, they have a sense of time which is not related to reproduction cycles. The glycolytic pathway exhibits oscillatory behaviour, i.e. the metabolite concentrations oscillate around phosphofructokinase. The frequency of these oscillations is about 1 min when using intact cells. Also a yeast cell extract can oscillate, though with a lower frequency. With intact cells the macroscopic oscillations can only be observed when most of the cells oscillate in concert. Transient oscillations can be observed upon simultaneous induction; sustained oscillations require an active synchronisation mechanism. Such an active synchronisation mechanism, which involves acetaldehyde as a signalling compound, operates under certain conditions. How common these oscillations are in the absence of a synchronisation mechanism is an open question. Under aerobic conditions an oscillatory metabolism can also be observed, but with a much lower frequency than the glycolytic oscillations. The frequency is between one and several hours. These oscillations are partly related to the reproductive cycle, i.e. the budding index also oscillates; however, under some conditions they are unrelated to the reproductive cycle, i.e. the budding index is constant. These oscillations also have an active synchronisation mechanism, which involves hydrogen sulfide as a synchronising agent. Oscillations with a frequency of days can be observed with yeast colonies on plates. Here the oscillations have a synchronisation mechanism which uses ammonia as a synchronising agent.     

京白梨花药开裂异常的解剖学研究

比较研究了京白梨(Pyrus ussuriensis Maxim.)、鸭梨、雪花梨花药开裂特性、环境条件对花药开裂的影响以及花药开裂异常的解剖学结构,结果表明:(1)京白梨花药开裂率明显低于鸭梨和雪花梨,使其产生花粉的量较少,从而导致传粉能力较弱.(2)京白梨、鸭梨、雪花梨在环境温度为25℃,相对湿度在 60%～80%时最适宜花药开裂,但京白梨花药开裂率始终低于鸭梨和雪花梨.(3)在京白梨花药发育后期不能形成开裂腔是引起花药不开裂的主要原因,而导致开裂腔不能形成的原因可能与单核期的绒毡层细胞不退化和药室内壁细胞不进行'U'字型加厚有关.     

New insight into the formation mechanism of imidazolium-based halide salts

By performing density functional theory calculations, the Menshutkin reaction between the N-methyl imidazole with chloroethane is reexamined to rationalize the experimental discovery. The calculated results show that the reaction proceeds via a S_N2 mechanism with a barrier of 119.1 kJ mol^-1 which is much lower than that reported in previous literature according to a five-membered transition state mechanism. Moreover, it is found that the barrier is further reduced to 98.1 kJ mol^-1 in toluene solution. The present result validates the experimental finding that the Menshutkin reaction for synthesizing N-alkyl imidazolium halide salts proceed smoothly at lower heating temperature.     

Intragenomic bet-hedging

The notion of intragenomic bet-hedging is introduced by modeling a system where one locus is seen as setting the “environment” for selection in a two-locus genetic system. Using a spatially structured simulation model I show that bet-hedging alleles with a lower mean fitness and lower variance of fitness across genotypes at a different locus can go to fixation, potentially providing a mechanism for the reduction of severe heterozygote advantage.     

Mechanism of AMPA receptor activation by partial agonists: disulfide trapping of closed lobe conformations

The mechanism by which agonist binding to an ionotropic glutamate receptor leads to channel opening is a central issue in molecular neurobiology. Partial agonists are useful tools for studying the activation mechanism because they produce full channel activation with lower probability than full agonists. Structural transitions that determine the efficacy of partial agonists can provide information on the trigger that begins the channel-opening process. The ligand-binding domain of AMPA receptors is a bilobed structure, and the closure of the lobes is associated with channel activation. One possibility is that partial agonists sterically block full lobe closure but that partial degrees of closure trigger the channel with a lower probability. Alternatively, full lobe closure may be required for activation, and the stability of the fully closed state could determine efficacy with the fully closed state having a lower stability when bound to partial relative to full agonists. Disulfide-trapping experiments demonstrated that even extremely low efficacy ligands such as 6-cyano-7-nitroquinoxaline-2,3-dione can produce a full lobe closure, presumably with low probability. The results are consistent the hypothesis that the efficacy is determined at least in part by the stability of the state in which the lobes are fully closed.     

Density-dependent reproduction in the European rabbit: a consequence of individual response and age-dependent reproductive performance

Density dependence of reproduction has generally been proposed to be caused by habitat heterogeneity and by the individual response of reproductive output. However, a further mechanism might generate density dependence of average reproductive rates. High density situations might be associated with a high proportion of first-season breeders which often show a principally lower reproductive performance. We tested for the existence of the latter mechanism as well as for density-dependent individual changes of reproductive effort in a population of European rabbits living in a homogeneous grassland habitat. The study was conducted over a period of eleven years. Overall, a strong relationship between mean reproductive rates and the breeding density of females was apparent. All necessary conditions for the presence of a density-dependent effect caused by age-dependent reproduction were fulfilled: Fluctuations of breeding density were paralleled by variations in the proportion of one-year-old females. These one-year-old, first-season breeders showed a consistently lower reproductive performance than older females, which might be caused by their lower body mass and their lower social rank. However, we also found strong evidence for density-dependent response of individual reproductive effort: Individual changes in fecundity over successive years were explained by changes in the breeding density of females. The results suggest that density dependence of reproduction in European rabbits is due to an interaction of age-dependent reproductive performance together with short-term fluctuations in breeding density, and a density-dependent, individual based response of reproductive rates. We further conclude that the lower reproductive performance of first-season breeders in age-structured animal populations may contribute substantially to interannual, and under particular circumstances to density-dependent variations of mean reproductive rates.     

Molecular origin of polyglutamine aggregation in neurodegenerative diseases

Expansion of polyglutamine (polyQ) tracts in proteins results in protein aggregation and is associated with cell death in at least nine neurodegenerative diseases. Disease age of onset is correlated with the polyQ insert length above a critical value of 35-40 glutamines. The aggregation kinetics of isolated polyQ peptides in vitro also shows a similar critical-length dependence. While recent experimental work has provided considerable insights into polyQ aggregation, the molecular mechanism of aggregation is not well understood. Here, using computer simulations of isolated polyQ peptides, we show that a mechanism of aggregation is the conformational transition in a single polyQ peptide chain from random coil to a parallel beta-helix. This transition occurs selectively in peptides longer than 37 glutamines. In the beta-helices observed in simulations, all residues adopt beta-strand backbone dihedral angles, and the polypeptide chain coils around a central helical axis with 18.5 +/- 2 residues per turn. We also find that mutant polyQ peptides with proline-glycine inserts show formation of antiparallel beta-hairpins in their ground state, in agreement with experiments. The lower stability of mutant beta-helices explains their lower aggregation rates compared to wild type. Our results provide a molecular mechanism for polyQ-mediated aggregation.     

The two mechanisms of intercostal muscle action on the lung.

The mechanisms of respiratory action of the intercostal muscles were studied by measuring the effect of external forces (F) applied to the ribs and by modeling the effect of F exerted by the intercostal muscles. In five dogs, with the airway occluded, cranial F were applied to individual rib pairs, from the 2nd to the 11th rib pair, and the change in airway opening pressure (Pao) was measured. The ratio Pao/F increases with increasing rib number in the upper ribs (2nd to 5th) and decreases in the lower ribs (5th to 11th). These data were incorporated into a model for the geometry of the ribs and intercostal muscles, and Pao/F was calculated from the model. For interspaces 2-8, the calculated values agree reasonably well with previously measured values. From the modeling, two mechanisms of intercostal muscle action are identified. One is the well-known Hamberger mechanism, modified to account for the three-dimensional geometry of the rib cage. This mechanism depends on the slant of an intercostal muscle relative to the ribs and on the resulting difference between the moments applied to the upper and lower ribs that bound each interspace. The second is a new mechanism that depends on the difference between the values of Pao/F for the upper and lower ribs.     

Field theoretic study of bilayer membrane fusion: II. Mechanism of a stalk-hole complex

We use self-consistent field theory to determine structural and energetic properties of intermediates and transition states involved in bilayer membrane fusion. In particular, we extend our original calculations from those of the standard hemifusion mechanism, which was studied in detail in the first article of this series, to consider a possible alternative to it. This mechanism involves non-axial stalk expansion, in contrast to the axially symmetric evolution postulated in the classical mechanism. Elongation of the initial stalk facilitates the nucleation of holes and leads to destabilization of the fusing membranes via the formation of a stalk-hole complex. We study properties of this complex in detail, and show how transient leakage during fusion, previously predicted and recently observed in experiment, should vary with lipid architecture and tension. We also show that the barrier to fusion in the alternative mechanism is lower than that of the standard mechanism by a few k(B)T over most of the relevant region of system parameters, so that this alternative mechanism is a viable alternative to the standard pathway. We emphasize that any mechanism, such as this alternative one, which affects, even modestly, the line tension of a hole in a membrane, affects greatly the ability of that membrane to undergo fusion.     

Signal transformation and pattern recognition in visual pathways

A model of the visual pathways it has been developed consisting of two neural systems working together: a feed-forward mechanism given by convolutions through different layers and a feed-back one consiting of the comparison of the results of the first one. The first mechanism would be predominant at lower centers, and it would be able, for example, to explain the increase in contrast sensitivity at different stations of visual pathways. The second mechanism seems apter to explain the behaviour of complex and hypercomplex cells whose activity is to a great extent independent of the position of the stimulus in the receptive field. The ideas developed in this paper could have some implications for building a machine able to recognize patterns independently of their position.     

SOS chromotest and mutagenicity in Salmonella: evidence for mechanistic differences

The kinetics of micronucleated polychromatic erythrocytes (MN-PCE) induction by methylnitrosourea (MNU) was determined in mice with the purpose of discerning whether or not the kinetics reflects the mechanism of chromosome break induction. A very long latency period (LP) was observed which is not compatible with an agent that does not require metabolic activation or incorporation to DNA for acting, but this is consistent with the mechanism demonstrated earlier that MNU causes chromosome breaks throughout the repair of mismatches induced by the alkylation of bases in a previous division. This is also supported by the presence of two rates of MN-PCE induction with respect to dose, which suggests that MN-PCE are induced by two mechanisms, an efficient one induced with the lower dose, and another less efficient one induced with higher doses. A similar behavior was observed in the curve of LP vs. dose, the lower dose causes 8 h of LP and higher doses increase LP but not proportionally to dose. The lower dose did not cause a reduction in the proportion of polychromatic erythrocytes, suggesting that this dose did not produce an important cytotoxic effect that could explain the long LP.