Local Ionic Environment of Plant Membranes: Effects on Membrane Functions

Molecular components and biochemistry of electron transport in plant plasma membranes (review)

It is worthwhile emphasizing the importance of electron transport across lipid membranes. Mitochondrial and electron transport in chloroplasts were elucidated in great detail many years ago. Plasma membrane-bound electron transfer may be involved in several processes such as membrane energization, signalling, regulation of transport and/or growth, and generation or scavenging of free radicals. We here give an overview of plasma membrane-bound electron transfer, of possible compounds of the electron transporting systems isolated from plasma membranes, and of their biochemical characteristics. Both the progress made in purification of redox enzymes and compounds and data from biochemical characterization of the activities found, support the discussion concerning models of the molecular structure of the electron transport systems of plant plasma membranes.

Cation exchange in isolated xylem cell walls of tomato. I. Cd2+ and Rb+ exchange in adsorption experiments

Purified xylem cell walls were prepared from isolated xylem bundles of tomato (an inbred line of Lycopersicon esculentum Mill, cv. Tiny Tim). Adsorption and exchange experiments were carried out with ¹¹⁵ Cd²⁺ , ⁸² Rb⁺ and ⁸² Br^- . The application of γ-ray spectroscopy permitted the simultaneous measurement of several ions applied together. The cell-wall water volume was shown to be independent of the external pH and solution ionic strength, possibly due to the presence of lignin. The Donnan Free Space (DFS) volume could be determined as a constant 0.15 dm³ per kg cell-wall dry weight. Consequently, the total cell-wall cation exchange capacity (CEC) could be estimated based on the DFS volume, and amounted to approximately 1000 mol m^-3 negative charges. The results of Cd²⁺ -Rb⁺ exchange experiments indicated an apparent CEC value of about 350-450 mol m^-3 DFS, at external pH ∼ 4. These data are in agreement with earlier reports on xylem wall CEC, and indicate the weak acid characteristics of the charge groups. The rational selectivity coefficient R^Cd_Rb , of the cell wall was shown to depend on external ion fractions and ionic strength, with a maximum R^Cd_Rb of 450 at ionic Cd²⁺ fraction near 0.3, based at the smallest experimental ionic strength of the external solution. The adsorption of Cd²⁺ , applied at relatively high concentrations, was shown to be stimulated by simultaneous application of high Rb⁺ concentrations.

Cation stimulation of the proton-translocating redox activity at the plasmalemma of Catharanthus roseus cells

Cultured Catharanthus roseus cells exhibit transmembrane ferricyanide reduction through a plasma membrane redox system which may be associated with proton translocation. Evidence shows that endogenous pyridine nucleotides serve as hydrogen donors for the reaction. The proton translocating function of the redox system is confirmed, in intact cells and isolated protoplasts, by the ability of Ca(2+) and other cations to increase both the redox activity and the efflux of protons. The role of the cations is seen to be not a simple general charge screening phenomenon as already described. By using ionic surfactants (CP(+), SDS(-)) it was shown that the net surface charge of the membrane can interact in the activation process via a cation attraction effect. It is proposed that specific binding of cations to the plasma membrane could alter the conformation of the redox system facilitating its interaction with NADH.

Electrostatic surface properties of plasmalemma vesicles from oat and wheat roots. Ion binding and screening investigated by 9-aminoacridine fluorescence

Right-side-out and sealed plasmalemma vesicles were isolated from roots of spring wheat (Triticum aestivum L. cv. Drabant) and oat (Avena sativa L. cv. Brighton) by two-phase partition in a medium containing sucrose (0.25 mol l(-1)). Oat root plasmalemma vesicles were discovered to contain a strongly fluorescent compound with an emission maximum at 418 nm. The surface potential of the membranes was monitored by 9-aminoacridine fluorescence and the effect of protein concentration, mannitol versus sucrose, absence of osmoticum, concentrations of salt, and titrations with chelators investigated. It is concluded that i) protein concentrations of less than 50 μg ml(-1) for oat and 100 μg ml(-1) for wheat plasmalemma vesicles should be used to avoid serious problems with non-linearity of response of 9-aminoacridine fluorescence, ii) mannitol can be used instead of sucrose as the osmoticum, iii) the vesicles were ruptured in the absence of osmoticum allowing us to monitor both sides of the membranes, iv) plasmalemma vesicles from oat roots are more negative than vesicles from wheat roots, and v) oat and wheat root plasmalemma vesicles are isolated with about the same amounts of bound Ca(2+) and Mg(2+). These bound divalent cations may not, however, reflect the in-vivo conditions since the tissues were homogenised in the presence of ethylenediaminetetraacetic acid.