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M H Goldsmith

Publications and source records attributed to M H Goldsmith.

15 recordsLinked to original sources
PubMed1992

K Channels Are Responsible for an Inwardly Rectifying Current in the Plasma Membrane of Mesophyll Protoplasts of Avena sativa.

In whole-cell recording, the conductance of the plasma membrane of protoplasts isolated from mesophyll cells of leaves of oat (Avena sativa) was greater for inward than outward current. The inward current in both the whole-cell mode and with isolated patches was dependent on [K(+)](o). When the membrane voltage was more positive than -50 millivolts, the membrane conductance in the whole-cell mode was low, and K(+) channels in cell-attached or outside-out patches had a low probability of being open. At a membrane voltage more negative than -50 millivolts, the membrane conductance increased by sevenfold in the whole-cell mode, and the probability of the channels being open increased. The inward current was highly selective for K(+) compared with Cs(+), Na(+), choline or Cl(-). Low concentrations of [Cs(+)](o) or [Na(+)](o) blocked the inward current in a strongly voltage-dependent fashion. Comparison of single-channel with the macroscopic current yields an estimate of about 200 inwardly rectifying K(+) channels per cell at a density of 0.035 per square micrometer. At physiological membrane voltages and [K(+)](o) about 10 millimolar, the influx through these channels is sufficient to increase the internal [K(+)] by 2 millimolar per minute. These K(+) channels are activated by membrane voltages in the normal physiological range and could contribute to K(+) uptake whenever the membrane is more negative than the K(+) equilibrium potential.

Journal Article↗
PubMed1992

Ion channels in Arabidopsis plasma membrane : transport characteristics and involvement in light-induced voltage changes.

White light (25 watts per square meter) induced an increase in plasma membrane K(+)-channel activity and a 30- to 70-millivolt transient membrane depolarization (completed in 2-3 minutes) in Arabidopsis thaliana leaf mesophyll cells. Transport characteristics of three types of ion channels in the plasma membrane were determined using inside-out patches. With 220 millimolar K(+) on the cytoplasmic side of the patch and 50 millimolar K(+) in the pipette, (220/50 K), the open-channel current-voltage curves of these channels were sigmoidal and consistent with an enzyme kinetic model. Two channel types were selective for K(+) over Na(+) and Cl(-). One (named PKC1) had a maximum conductance (G(max)) of 44 picosiemens at a membrane voltage (V(m)) of -65 mV in (220/50 K) and is stimulated by light. The other (PKC2) had G(max) = 66 picosiemens at V(m) = 60 millivolts in (220/50 K). The third channel type (PCC1) transported K(+) and Na(+) about equally well but not Cl(-). It had G(max) = 109 picosiemens at V(m) = 55 millivolts in (250/50 K) with 10 millimolar Ca(2+) on the cytoplasmic side. Reducing Ca(2+) to 0.1 millimolar increased PCC1 open-channel currents by approximately 50% in a voltage-independent manner. Averaged over time, PKC2 and PCC1 currents strongly outward rectified and PKC1 currents did so weakly. Reductants (1 millimolar dithiothreitol or 10 millimolar beta-mercaptoethanol) added to the cytoplasmic side of an excised patch increased the open probability of all three channel types.

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PubMed1990

On polar auxin transport in plant cells.

We present here explicit mathematical formulas for calculating the concentration, mass, and velocity of movement of the center of mass of the plant growth regulator auxin during its polar movement through a linear file of cells. The results of numerical computations for two cases, (a) the conservative, in which the mass in the system remains constant and (b) the non-conservative, in which the system acquires mass at one end and loses it at the other, are graphically presented. Our approach differs from that of Mitchison's (Mitchison 1980) in considering both initial effects of loading and end effects of substance leaving the file of cells. We find the velocity varies greatly as mass is entering or leaving the file of cells but remains constant as long as most of the mass is within the cells. This is also the time for which Mitchison's formula for the velocity, which neglects end effects, reflects the true velocity of auxin movement. Finally, the predictions of the model are compared with two sets of experimental data. Movement of a pulse of auxin through corn coleoptiles is well described by the theory. Movement of auxin through zucchini shoots, however, shows the need to take into account immobilization of auxin by this tissue during the course of transport.

Biological Transport↗
PubMed1988

Regulation of electrogenic proton pumping by auxin and fusicoccin as related to the growth of Avena coleoptiles.

The temporal relations between early responses to indoleacetic acid (IAA), proton secretion, hyperpolarization of the membrane potential, and growth change during the incubation of segments of oat (Avena sativa L.) coleoptiles in a low salt medium. When IAA is added after pretreatment of several hours, proton secretion increases after a latency of 7 minutes and reaches its maximum 10 to 15 minutes later. This timing coincides with both the increase in growth of the segments and the hyperpolarization of the membrane potential of parenchyma cells, consistent with the hypothesis that the change in membrane voltage reflects the activity of an electrogenic proton pump. The extent of IAA-induced hyperpolarization is substantially reduced by elevating [KCl](0), most likely because this increases the passive conductance of the membrane. Neither growth nor proton secretion is affected by high [KCl](0) (30 millimolar), indicating that neither process is limited by the magnitude of the membrane potential. These results are consistent with the acid growth hypothesis. Following short incubation times, however, IAA-induced hyperpolarization and growth are detected within 10 minutes, while acidification of the medium is delayed for more than 40 minutes. This result is seemingly in conflict with the acid growth hypothesis, but in freshly cut tissue, the pH of the external medium may not reflect the pH of the epidermal cell walls. The temporal coincidence of auxin-induced growth and hyperpolarization suggests that in freshly isolated segments the hyperpolarization is a more sensitive indication of proton secretion than is acidification of the external aqueous environment.

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PubMed1981

Mathematical analysis of the chemosmotic polar diffusion of auxin through plant tissues.

Equations have been developed to describe the diffusional movement of a weak acid such as the auxin indoleacetic acid through a long file of vacuolated cells, where cellular accumulation is driven by the pH gradients across the cell membranes. If the permeability to the auxin anion is greater at one end of the cell than at the other, diffusional movement takes the form of polar transport, which exhibits: a nearly constant velocity either for the front or for a pulse of radioactive auxin, the capacity to move auxin against an external gradient of concentration, and a polar ratio that increases exponentially with the length of the section. The determinants of velocity include both diffusion through the vacuole and permeation steps at the cell membranes. Except for the permeabilities of the membranes to the anion, values are now available for all of the physical parameters in the equations. With reasonable estimates of permeability coefficients for the anion, the equations predict a velocity of transport of about 1 cm hr(-1), which agrees well with measured values. The analysis indicates, however, that the underlying cellular polarity may be greater than has been heretofore assumed. We thus demonstrate that the hypothesis of chemosmotic polar diffusion is capable of accounting quantitatively for the major features of auxin transport and provides a theoretical framework whose elements can be tested in future experiments.

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PubMed1980

Light-inducible Cytochrome Reduction in Membrane Preparations from Corn Coleoptiles: I. STABILIZATION AND SPECTRAL CHARACTERIZATION OF THE REACTION.

Conditions for obtaining reproducible light-induced reduction of a b-type cytochrome in membrane fractions from coleoptiles of dark-grown Zea mays L. include a glucose-glucose oxidase system that lowers O(2) tension and generates H(2)O(2), substrate amounts of ethylenediaminetetraacetic acid which, in some manner, facilitates photoreduction by both added flavin and the endogenous photoreceptor and a sample temperature below 10 C. Cytochrome reduction could be obtained by photoexcitation of either a tightly bound endogenous receptor, which is probably a flavin, or added riboflavin, flavin mononucleotide, or flavin adenine dinucleotide. The latter flavin was the least effective. The endogenous photoreceptor appears to be rather firmly bound to the membranes, suggesting that this association may also exist in vivo. When any of the above four photoreceptors or methylene blue were used to sensitize the reaction, a cytochrome with a reduced alpha-band near 560 nanometers and a Soret difference peak near 429 nanometers was the electron acceptor. This cytochrome could be clearly distinguished spectrally from other cytochromes that predominated in the membrane preparations.

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PubMed1967

Movement of pulses of labeled auxin in corn coleoptiles.

The transit of indole-3-acetic acid through 20-mm sections of corn coleoptiles can be separated from processes involved in the uptake of auxin by the section and the exit of auxin from the section. Aerobic sections are supplied with an exogenous source of (14)C IAA for a limited time, and after the source is removed, a pulse of (14)C IAA moves down at 12 to 15 mm/hour. After transfer to nitrogen, movement of the pulse at the aerobic rate persists for about 10 minutes; thereafter drops to only 1 to 2 mm/hour and remains at this level during the next 4 hours. Within 2 hours, 70% of the total (14)C in aerobic sections has moved 10 mm or more down the section from the position of the initial peak, whereas after the same time in nitrogen less than 10% of the total (14)C has moved as far. During the migration down the coleoptile, the peak of radioactivity becomes broader and less distinct. This dispersion is more rapid in aerobic than anaerobic sections, but appears to be nonpolar and to occur along the existing concentration gradients. Diffusion probably contributes to this dispersion. In both inhibited and uninhibited sections, the movement of the peak, in contrast to its dispersion, is A) polar (downward) and B) independent of existing concentration gradients. Thus transit within the section possesses the fundamental properties of the overall transport system. The reduced amount of transport in inhibited sections is more likely maintained by glycolysis than by a low level of aerobic respiration dependent on the residual oxygen in the tissue.

Biological Transport↗
PubMed1967

Effect of Inversion on Growth and Movement of Indole-3-acetic Acid in Coleoptiles.

The effect of a 180 degrees displacement from the normal vertical orientation on longitudinal growth and on the acropetal and basipetal movement of (14)C-IAA was investigated in Avena sativa L. and Zea mays L. coleoptile sections. Inversion inhibits growth in intact sections (apex not removed) and in decapitated sections supplied apically with donor blocks containing auxin. Under aerobic conditions, inversion inhibits basipetal auxin movement and promotes acropetal auxin movement, whereas under anaerobic conditions, it does not influence the movement of auxin in either direction. Inversion retards the basipetal movement of the peak of a 30-minute pulse of auxin in corn.The inversion-induced inhibition of basipetal auxin movement is not explained by an effect of gravity on production, uptake, destruction, exit from sections, retention in tissue, or purely physical movement of auxin. It is concluded that inversion (a) inhibits basipetal transport, the component of auxin movement that is metabolically dependent, and as a result (b) inhibits growth and (c) promotes acropetal auxin movement.

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PubMed1966

Movement of indoleacetic acid in coleoptiles of Avena sativa L. II. Suspension of polarity by total inhibition of the basipetal transport.

Acropetal and basipetal movement of indole-3-acetic acid through coleoptiles of Avena sativa L. was studied. Sections 10-mm long were supplied with either apical or basal sources containing C(14) carboxyl-labeled indoleacetic acid (10(-5)m). Anaerobic conditions inhibit metabolically dependent movement (transport) thus reducing basipetal but not acropetal movement. Total inhibition of basipetal transport abolishes the polarity of auxin uptake and movement. The nonpolar movement that remains in anaerobic sections is free diffusion with an average diffusion coefficient of approximately 1 x 10(-4) mm(2) per second. During an 8-hour diffusion, at least the first millimeter of the section comes to equilibrium at approximately the same concentration as the donor.Acropetal movement is probably by diffusion and is accompanied by an aerobic immobilization of indoleacetic acid that increases more than proportionally to concentration. Anaerobic conditions totally prevent this immobilization and reduce acropetal uptake but not the amount of indoleacetic acid moving into the upper parts of the section; there is, therefore, no evidence for acropetal transport. Polarity of auxin movement in aerobic coleoptile sections is achieved by strict basipetal transport of auxin. The basipetal transport may intensify the polarity by recycling auxin that is moving acropetally.

Biological Transport↗
PubMed1966

Maintenance of polarity of auxin movement by basipetal transport.

The polar, basipetal transport of indoleacetic acid helps to maintain polarity of auxin movement in coleoptiles of Avena sativa L. by opposing acropetal diffusion. This conclusion is supported by 3 different kinds of experiments. In all 3 experiments, sections took up (14)C carboxyl-labeled indole-3-acetic acid anaerobically, and the distribution of auxin within all sections was similar at the end of uptake.[LIST: see text].

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