A. V. Anisimov

Effect of temperature on water transport through aquaporins.

The mean effective water self-diffusion coefficient in maize root segments under the effect of aquaporin blocker (mercuric chloride, 0.1 mM) was measured using the spin-echo NMR method with pulsed magnetic field gradient within the temperature range from 10 to 35 °C. HgCl2 caused the reduction in water diffusion by 30 % as compared to the control samples. Temperature dependences of water self-diffusion coefficients showed two linear regions with different values of Q10 and activation energy, Ea. As the temperature reduced from 20 to 10 °C, Ea values calculated from the Arrhenius plots were close to those of bulk water (20 ± 3 kJ mol−1) and slightly changed for the sample pretreated HgCl2. Within the temperature range from 25 to 35 °C the slope of temperature dependences became steeper and Ea values were 31 ± 3 kJ mol−1 for the control and 40 ± 4 kJ mol−1 for the treated sample. In the vicinity of 20 °C, the temperature dependence of water diffusion via the mercury-sensitive water channels showed extreme value. In the region, the specific area of the mercury-sensitive aquaporins was 0.004 % of the total cell surface area. The data indicate that water transfer via aquaporins is sensitive to temperature, and the contributions of the transmembrane pathways (aquaporins, lipid bilayer) differ in different temperature ranges.

Plant fiber intrusive growth characterized by NMR method

Intrusively growing plant cells insert themselves between surrounding cells, thus increasing the number of membranes on the tissue cross-section. This parameter can be assessed by spin echo NMR method with a magnetic field pulse gradient. Diffusion echo decay was measured for stem regions of long-fiber flax (Linum usitatissimum L.) differing in the stages of primary fiber development, which elongate thousand-fold during intrusive growth. Additionally, the number of fibers on stem cross-sections was counted under microscope. An increase in the slow component of the echo diffusion decay was correlated with an increase in the number of fibers on the stem cross-section in the zone of intrusive growth, while other stem-structure characteristics remained unchanged. Thus, NMR method can be used for characterization of intrusive fiber growth in situ.

Plasmodesmata as a Modulator of Osmotic Water Fluxes in Plants

Solutions to some key problems in the relationships between the structure and functions of plasmodesmata, a component of the plant intercellular communication system, are proposed on the basis of the theory of osmotic flows through porous membranes. The theory accounts for structural characteristics of plasmodesmata, such as their dimension, shape, and length. It considers the steric and adsorption potentials of the solution–cell wall interaction and estimates water and solute (e.g., sucrose) flows under the sustained difference of osmotic pressures at the ends of plasmodesmata. The theory predicts that the water flow through plasmodesmata increases with the widening of the neck constriction and reaches its peak when its size is equal to the diameter of the solute molecule. The water-flow direction was found to depend on the opening of the annulus in neck constrictions at negative adsorption potentials of the plasmodesmata channel walls. Taking into account the presence of sphincters in the neck constrictions, our data suggest the role of plasmodesmata as a modulator of osmotic water fluxes in plants.

Radial diffusion transport of water in various zones of maize root and its sensitivity to mercury chloride

NMR-spin echo method was used for comparative study of radial water diffusion in various zones of maize (Zea mays L., cv. Donskaya 1) primary root. Coefficients of water diffusion varied strongly along the root length; the pattern of variations depended on the period during which the diffusion of water molecules was traced. Water diffusion transport in various root zones was unevently sensitive to mercury chloride, an aquaporin inhibitor. The discovered variations in the mobility of water molecules were assigned to morphological and functional features of cells and tissues in the root zones examined; they were interpreted in terms of variable contribution and redistribution of water flows along several transport pathways. The decrease in diffusional water flows could be caused by cell wall modifications (deposition of suberin) that emerge in the endoderm regions distant from the root apex and diminish the contribution of apoplastic transport.

Effect of water stress and mercuric chloride on the translational diffusion of water in maize seedling roots

Water diffusion in maize roots (Zea mays L., cv. Donskaya 1) was investigated with a pulsed gradient NMR using mercuric chloride as an inhibitor of water channels in cell membranes. A novel operation program was applied that allowed selective evaluation of fractional amounts of water transported through various pathways—the apoplastic, symplasmic, and transmembrane routes. The blockage of water channels with HgCl2 reduced the rates of water diffusion by a factor of 1.5–2. This effect was reversible and was removed by the addition of β-mercaptoethanol. The coefficient of water diffusion changed with time elapsed after the HgCl2 treatment. The effect of water stress on the rates of water diffusion was similar to that of HgCl2. Remarkably, the water-stressed roots of maize seedlings were insensitive to the inhibitor of water channels. The results are interpreted in terms of redistribution of water flows among various routes in plant tissues. Water stress and mercuric chloride treatments decelerate the transmembrane water transport and promote water flow along the apoplastic pathway. These responses might arise from the reversible regulation of water movement along various transport pathways.

Contribution of the actomyosin motor to the temperature-dependent translational diffusion of water by cytoplasmic streaming in Elodea canadensis cells

Summary.The extent to which the actomyosin motor responsible for cytoplasmic streaming contributes to the translational diffusion of water in Elodea canadensis cells was studied by a nuclear magnetic resonance (NMR) spin-echo technique. The relative contribution of the actomyosin motor was determined from the corresponding apparent diffusion coefficient by the Einstein–Smolukhovsky relation. It is equal to the difference between the diffusional displacements of the cytoplasmic and the bulk water (ΔX). The NMR data show that the temperature dependence of ΔX is humpshaped, which is characteristic of enzyme reactions. At the same time, the apparent diffusion coefficient of cytoplasmic water increases with an increase in temperature. The most significant contribution of the actomyosin motor to ΔX is observed at temperatures below 20 °C. Within the temperature range of 20 to 33 °C, ΔX changes only slightly, and a further increase in temperature reduces ΔX to zero.

Symplasmic Transport of Water along the Root Depends on Pressure

The gradient NMR method was applied to study intercellular water flows in root segments of maize (Zea mays L.) under disturbance of root hydrodynamic system by the increase in external pressure up to 4 MPa. The rate of intercellular water flows along the root symplast was found to depend on the magnitude and dynamics of pressure changes. Based on the previously predicted cupola-shaped dependency of water flow on the aperture of plasmodesmal neck constrictions, we assume that the external pressure stimulates (via cytosolic calcium) the activity of contractile structures localized in the neck regions of plasmodesmata. Cells of Chlorella vulgaris were taken for comparison since their water relations and cell structure differ strongly from the root cells of maize. The results show that the diffusional water flow in Chlorella is independent of external pressure both in intact cells and in algae, whose plasma membrane was artificially permeabilized.

Differential sensitivities of water diffusion in maize root cortex and stele to HgCl2-induced blockade of aquaporins

Spin-echo NMR comparative study of water diffusion in the cortex and stele of maize (Zea mays L.) roots was made with the aim to determine predominant pathways of radial water movement in the root. The root parts examined differed in terms of water diffusion coefficients and sensitivity to HgCl2, the aquaporin blocker. These differences are discussed from the viewpoint of unequal contributions of separate transport pathways (apoplastic, symplastic, and transmembrane) to the overall water flow. Characteristics of water diffusion in roots with the endodermis damaged suggest an inconsiderable contribution of the endodermis into resistance to water movement.

Water permeability ofElodeacells and cytoplasm motion

Summary The reduction of the rotational chloroplast motion velocity (CMV) during the exposure of Elodea cells to hypertonic solutions of PEG-4000 is satisfactorily described by an exponent. The dynamics of reduction correlates with the dynamic equilibration time (T eq ) between osmotic potentials of cell cytoplasm and external solution. The acceleration of the chloroplast motion, for example by gadolinium ions, results in the reduction of T eq , and this is supposed to be due to the increase of the con tribution of the near-membrane Nernst layers to transmembrane transfer resistance resulting from the «blowing-off» of a part of the layer by moving cytoplasm.

Hydrodynamics of cytoplasmic streaming and the cytoskeletal tubulin component in cells of Elodea densa internodes.

A spin-echo NMR method was applied to study hydrodynamic parameters of cytoplasmic streaming and to examine translational diffusion of cytoplasmic water in stems of elodea (Elodea densa (Planch.) Casp.) treated with antimitotic drugs. Within the first few hours after colchicine treatment, the dynamics of translational displacement followed the curve with a transient maximum. The action of vinblastine was manifested in a monotonic decrease of the translational diffusion. After 24-h treatments with colchicine and vinblastine, the translational diffusion was slowed down. All these changes induced by antimitotic drugs were observed on the background of unaltered rate of cytoplasmic streaming.