Signaling role of action potential in higher plants

The signaling role of action potential (AP) in higher plants is considered. The principles underlying realization of this role and the significance of AP-induced short-term effector response are discussed. The notion is put forward that the effect of propagating AP on plant cells is similar to nonspecific component of the cell functional response to external stimuli.     

Myocardial segment-specific model generation for simulating the electrical action of the heart

Background  

Computer models of the electrical and mechanical actions of the heart, solved on geometrically realistic domains, are becoming an increasingly useful scientific tool. Construction of these models requires detailed measurement of the microstructural features which impact on the function of the heart. Currently a few generic cardiac models are in use for a wide range of simulation problems, and contributions to publicly accessible databases of cardiac structures, on which models can be solved, remain rare. This paper presents to-date the largest database of porcine left ventricular segment microstructural architecture, for use in both electrical and mechanical simulation.     

Mathematical model of the neonatal mouse ventricular action potential

Therapies for heart disease are based largely on our understanding of the adult myocardium. The dramatic differences in action potential (AP) shape between neonatal and adult cardiac myocytes, however, indicate that a different set of molecular interactions in neonatal myocytes necessitates different treatment for newborns. Computational modeling is useful for synthesizing data to determine how interactions between components lead to systems-level behavior, but this technique has not been used extensively to study neonatal heart cell function. We created a mathematical model of the neonatal (day 1) mouse myocyte by modifying, on the basis of experimental data, the densities and/or formulations of ion transport mechanisms in an adult cell model. The new model reproduces the characteristic AP shape of neonatal cells, with a brief plateau phase and longer duration than the adult (action potential duration at 80% repolarization = 60.1 vs. 12.6 ms). The simulation results are consistent with experimental data, including 1) decreased density and altered inactivation of transient outward K+ currents, 2) increased delayed rectifier K+ currents, 3) Ca2+ entry through T-type as well as L-type Ca2+ channels, 4) increased Ca2+ influx through Na+/Ca2+ exchange, and 5) Ca2+ transients resulting from transmembrane Ca2+ entry rather than release from the sarcoplasmic reticulum (SR). Simulations performed with the model generated novel predictions, including increased SR Ca2+ leak and elevated intracellular Na+ concentration in neonatal compared with adult myocytes. This new model can therefore be used for testing hypotheses and obtaining a better quantitative understanding of differences between neonatal and adult physiology.     

高等植物花色苷在液泡中的存在状态及其着色效应

综述了花色苷被摄入液泡的原因、花色苷在液泡中的存在状态及其对植物细胞的着色效应。花色苷在植物细胞质中合成后转运到液泡里是为了解除其对蛋白质和DNA等细胞功能分子的毒性。花色苷的液泡区隔化是花色苷在植物细胞中发挥正常功能的前提。在大多数植物中,花色苷在绝大多数情况下完全溶解在液泡里。但是,花色苷也能在液泡里形成颗粒,这些颗粒可以划分为花色苷体和花色苷液泡包涵体两类。花色苷体由膜包裹,其形成是液泡中小的有色囊泡逐渐合并的结果,发育完全的花色苷体为典型的球状、具比液泡更深的红色;液泡里的花色苷体具高密度,呈现为含高浓度花色苷的不溶性小球;花色苷体的存在可导致液泡的强烈色彩。花色苷液泡包涵体可能具备蛋白质基质,既无膜包裹又无内部结构,其形成是转运进液泡的花色苷与蛋白质基质结合的结果;液泡里的花色苷液泡包涵体形状不规则,象果冻;在花色苷液泡包涵体中,花色苷可能通过氢键连接于蛋白质基质的一个有限空间位点;花色苷液泡包涵体被认为是液泡中花色苷的"陷阱",优先摄取花色素3,5-二糖苷或酰化的花色苷;花色苷液泡包涵体的存在可增加液泡色彩的强度并导致"蓝化"。     

Sulfur metabolism in higher plants: potential for phytoremediation

Sulfur is a major nutrient for all organisms. Plant species have a high biodiversity in uptake, metabolization and accumulation of sulfur so that there are potentials to use plants for phytoremediation of sulfur-enriched sites. A survey of soils enriched with sulfur either naturally or by human activities shows that a surplus of sulfur is mostly accompanied with a surplus of other chemical elements which may limit phytoremediation because these co-occurring elements are more toxic to plants than sulfur. In addition, the accumulation of the other elements makes the plant material (phyto-extraction) less suitable for the use as fodder and for human consumption.     

Detection of sequences with Z-DNA forming potential in higher plants

Sequences of alternating purine-pyrimidine residues with Z-DNA forming potential have been detected in the nuclear DNA of two higher plant species: wheat and radish. Poly (dG-dT) and poly (dG-dC) stretches have been detected by hybridization of the corresponding nick-translated probes to Southern blots. These stretches are scattered throughout the genome and some of them belong to moderately repeated sequence families interspersed with other DNA sequences.     

Reversible changes of extracellular pH during action potential generation in a higher plant Cucurbita pepo

A pH-sensitive electrode was applied to measure activity of H⁺ ions in the medium surrounding excitable cells of pumpkin (Cucurbita pepo L.) seedlings during cooling-induced generation of action potential (AP). Reversible alkalization shifts were found to occur synchronously with AP, which could be due to the influx of H⁺ ions from external medium into excitable cells. Ethacrynic acid (an anion channel blocker) reduced the AP amplitude but had no effect on the transient alkalization of the medium. An inhibitor of plasma membrane H⁺-ATPase, N,N’-dicyclohexylcarbodiimide suppressed both the AP amplitude and the extent of alkalization. In experiments with plasma membrane vesicles, the hydrolytic H⁺-ATPase activity was subjected to inhibition by Ca²⁺ concentrations in the range characteristic of cytosolic changes during AP generation. The addition of a calcium channel blocker verapamil and a chelating agent EGTA to inhibit Ca²⁺ influx from the medium eliminated the AP spike and diminished reversible alkalization of the external solution. An inhibitor of protein kinase, H-7 alleviated the inhibitory effect of Ca²⁺ on hydrolytic H⁺-ATPase activity in plasma membrane vesicles and suppressed the reversible alkalization of the medium during AP generation. The results provide evidence that the depolarization phase of AP is associated not only with activation of chloride channels and Cl^? efflux but also with temporary suppression of plasma membrane H⁺-ATPase manifested as H⁺ influx. The Ca²⁺-induced inhibition of the plasma membrane H⁺-ATPase is supposedly mediated by protein kinases.     

A model for starch breakdown in higher plants

An in vitro system for the breakdown of starch granules by mixtures of α- and β-amylase is developed and discussed with reference to information concerning the degradation of starch in vivo. β-Amylase has no action on starch granules and has very little effect on the rate of starch granule digestion by α-amylase. It does, however, affect the product distribution in an α-amylase digest and is considered to attack dextrin intermediates produced by the action of α-amylase on the starch granules.     

Extension of the neuronal membrane model to account for suppression of the action potential by a constant magnetic field

A biophysical explanation of the reduced excitability in neurons exposed to a constant magnetic field is based on an extended neuronal membrane model. In the presence of a constant magnetic field, reduced excitability is manifested as an increase in the excitation threshold and a decrease in the frequency of action potentials. The proposed explanation for the reduced excitability rests on the well-known Hall effect. The separation of charges resulting from the Lorentz force exerted on moving intracellular ions leads to the formation of a Hall electric field in a direction perpendicular to that of action-potential transmission. Consequently, the ion current for discharging the membrane capacitance is reduced in the presence of a magnetic field, thereby limiting initiation of the action potential. The validity of the proposed biophysical explanation is justified analytically and verified by simulations based on the Hodgkin and Huxley model for the electrical excitability of a neuron. Based on derivation of the current segregation ratio α characterizing the reduction in the stimulating current from first principles, the equivalent circuit model of the neuronal membrane is extended to account for the reduced excitability of neurons exposed to a constant magnetic field.     

A scheme to account quantitatively for the action of phytochrome in etiolated and light-grown plants

Abstract A scheme to account for the phytochrome responses of dark grown and green plants is presented. In contrast to some other models (e.g. Smith, 1970; Schäfer, 1975) it is proposed that both Pfr and the photoconversions of phytochrome are effectors of phytochrome action and interact to promote the observed responses. Under conditions where Ptot is not strongly wavelength dependent (e.g. with short pulses of light and in plants grown for prolonged periods in the light), responses will be predominantly related to Pfr concentration. Where Ptot is the major wavelength dependent variable (e.g. in the ‘high irradiance reaction’ of etiolated seedlings) it is proposed that the photoconversion process itself predominates in controlling the ultimate response. Some responses predicted by the scheme are compared with relevant published data.     

Evidence for a tonoplast-associated form of sucrose synthase and its potential involvement in sucrose mobilization from the vacuole

The following work presents new evidence for a tonoplast localization of sucrose synthase and its functional role during the mobilization of vacuolar sucrose. Highly purified tonoplast vesicles were associated with significant SuSy levels as determined by immuno-recognition, enzymatic activity, and by-product measurements. Total tonoplast-bound SuSy was estimated to be approximately 7% of the total tissue activity. SuSy affinity to the tonoplast was confirmed by the lack of SuSy displacement by ionic washes and also by the tonoplast ability to bind to exogenously added SuSy as compared to the cytosolic marker alcohol dehydrogenase. UL-[(14)C]sucrose-loaded vesicles incubated with ATP and UDP produced [(14)C]UDP-Glc as determined by UDP-Glc dehydrogenase and by the ability of the product to bind to DEAE-cellulose and to co-migrate with authentic UDP-Glc on TLC. ATP alone induced sucrose efflux but not the production of [(14)C]ADP-Glc. Kinetic analysis of [(14)C]UDPG formation under conditions of low sucrose availability suggests sucrose channelling between the ATP-dependent sucrose transporter and SuSy, thus corroborating the association of SuSy with the tonoplast and its involvement in sucrose mobilization from the vacuole.     

Next generation synthetic vectors for transformation of the plastid genome of higher plants

Plastid transformation vectors are E. coli plasmids carrying a plastid marker gene for selection, adjacent cloning sites and flanking plastid DNA to target insertions in the plastid genome by homologous recombination. We report here on a family of next generation plastid vectors carrying synthetic DNA vector arms targeting insertions in the rbcL-accD intergenic region of the tobacco (Nicotiana tabacum) plastid genome. The pSS22 plasmid carries only synthetic vector arms from which the undesirable restriction sites have been removed by point mutations. The pSS24 vector carries a c-Myc tagged spectinomycin resistance (aadA) marker gene whereas in vector pSS30 aadA is flanked with loxP sequences for post-transformation marker excision. The synthetic vectors will enable direct manipulation of passenger genes in the transformation vector targeting insertions in the rbcL-accD intergenic region that contains many commonly used restriction sites. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The relative electrical resistances of the plasmalemma and tonoplast in higher plants

Summary Measurements on 3–6 days old root hair cells of cucumber, oats and maize, in a medium of 1.0 mN KCl plus 1.0 mN CaCl₂, gave an average d.c. surface resistance value of 3,000 ohm·cm² for the plasmalemma and of 3,500 or 3.600 ohm·cm² for the plasmalemma and tonoplast in series. The average value for the combined surface resistance of the two membranes was about 1.2 times that of the plasmalemma alone, showing that in the above cells the latter membrane has a resistance appreciably more than that of the tonoplast.It is concluded that the plasmalemma offers a major barrier to the passive uptake of ions. This conclusion is supported by a consideration of transverse impedance measurements of young and old tissues.     

Sensitivity to phytohormones determined by outer-inner polarity of higher plants: An overall model for phytohormone action

Abstract A generalized model of the higher plant body is proposed in order to assemble the discrete knowledge of the actions, and sites of biosynthesis, of phytohormones. In this model, we attempt to explain the differential sensitivities of different tissues. With this model most effects of plant hormones appear to be reasonable, and even expected. The model is based on a new anatomical and physiological classification of plant tissue. In higher plants the integration of an outer-inner polarity and an upper-lower polarity plays a major role in phytohormone behaviour. Plant tissues and organs which are derived from the cortex of paleophytes (the bud, the mesophyll of the leaf, the cortex of the stem, and the root cap) are classified as the outer pole of the plant. On the other hand, tissues and organs which are derived from the stele of paleophytes (the root, the stele of the shoot, and the vein of the leaf), are classified as the inner pole. It is suggested that tissue sensitivities to phytohormones are mainly determined by the outer-inner polarity. Phytohormones which are synthesized from one pole act on the other, whereas they exert either much less or no effect, or an inverse effect on their own pole. This is shown for both promoters and inhibitors of the phytohormones for both cortical and stelar vegetative tissues of plants.     

Parallel pathways for intracellular Ca2+ release from the vacuole of higher plants

A novel pathway for intracellular Ca²⁺ release via a voltage-gated channel has been diered in the tonoplast of Beta vulgaris (sugar beet) tap roots. The channel is characterized electrophysiologically in isolated vacuoles and radiometrically in tonoplast-enriched microsomes. Single channel properties were studied in excised membrane patches. With 5-20 mM Ca²⁺ as a charge carrier on the vacuolar side, the unitary current saturates to a maximal value of 0.59 ± 0.05 pA as membrane voltage approaches +30 to +50 mV. The maximal slope conductance at non-saturating voltages is 12.45 ± 1.06 pS. Open-state probability increases markedly with positive-going voltage changes in the physiological range. Channel activity is also increased by vacuolar, but not by cytosolic Ca²⁺. The lanthanide Gd³⁺ is alone among a large range of Ca²⁺ channel antagonists in acting as an effective inhibitor of the channel, primarily as a result of a dramatic effect on the open-state probability. We conclude that this voltage-gated channel could constitute an alternative, inositol (1,4,5)-trisphosphate-independent pathway for vacuolar Ca²⁺ release during signal transduction in plants.     

Comporative characteristics of action potentials in animals and higher plants

The current comparative characteristics of action potentials in animals and higher plants are given. The main differences in parameters, mechanisms of generation, and functional significance of propagated electrical signals are examined.