Electrical Potentials of Plant Cell Walls in Response to the Ionic Environment

Abstract

Electrical potentials in cell walls ( Wall ) and at plasma membrane surfaces ( PM ) are determinants of ion activities in these phases. The PM plays a demonstrated role in ion uptake and intoxication, but a comprehensive electrostatic theory of plant-ion interactions will require further understanding of Wall . Wall from potato (Solanum tuberosum) tubers and wheat (Triticum aestivum) roots was monitored in response to ionic changes by placing glass microelectrodes against cell surfaces. Cations reduced the negativity of Wall with effectiveness in the order AlThis order resembles substantially the order of plant-root intoxicating effectiveness and indicates a role for both ion charge and size. Our measurements were combined with the few published measurements of Wall , and all were considered in terms of a model composed of Donnan theory and ion binding. Measured and model-computed values for Wall were in close agreement, usually, and we consider Wall to be at least proportional to the actual Donnan potentials. Wall and PM display similar trends in their responses to ionic solutes, but ions appear to bind more strongly to plasma membrane sites than to readily accessible cell wall sites. Wall is involved in swelling and extension capabilities of the cell wall lattice and thus may play a role in pectin bonding, texture, and intercellular adhesion.The cell wall (CW) is composed of various crosslinked units (macrofibrils, microfibrils, micelles, cellulose units, and linked agents such as neutral sugars, pectin, proteins, and ions; Cosgrove, 1997; Buchanan et al., 2000). The CW behaves as an ion exchanger where the fixed CW charges interact with exchangeable ions in the surrounding solution (Briggs and Robertson, 1957; Gillet and Lefèbvre, 1981; Sentenac and Grignon, 1981; Irwin et al., 1985; Richter and Dainty, 1989a, 1989b, 1990a, 1990b Grignon and Sentenac, 1991). The net CW charge is negative and results from weakly dissociating acidic groups having pK a values similar to those of polyGalUA, the principal origin of the negative charges (Ritchie and Larkum, 1982; Saftner and Raschke, 1981; Richter and Dainty, 1989a; Buchanan et al., 2000). Some positive charges occur too, mainly associated with CW proteins (Cassab and Varner, 1988; Buchanan et al., 2000).The CW determines cell dimensions (Taiz, 1984) and intracellular volume. The volume of the CW is a consequence of the dimensions of its internal spaces, i.e. the distances between the intra-CW units (Shomer et al., 1984; Shomer and Levy, 1988), which are determined by the repulsive strengths of diffuse double layers (Shomer et al., 1991). The swelling of parenchyma CW is restrained by the increase of valence and concentration of the exchangeable ions and with the decrease of the dielectric constant of the bulk solution (Shomer et al., 1991; Shomer, 1995). Although cell expansion and CW extension have been studied comprehensively (Cosgrove, 1997), the properties governing the volume of the CW, from the point of view of the dimensions of its internal spaces...

Plasma Membrane Surface Potential: Dual Effects upon Ion Uptake and Toxicity

“…E m is composed of three potential differences (E m = E m,surf + c 0 o 2 c 0 i ) and can be measured comparatively easily by the insertion of microelectrodes into cells (Nobel, 1991). The cell wall appears to have comparatively little influence on c 0 o and ion concentrations at the PM surface (Gage et al, 1985;Shomer et al, 2003;Kinraide, 2004). Hence, c 0 o is controlled by the composition of the soil solution and little, or not at all, by soil solid matter lying external to the cell walls, except as that solid matter influences the soil solution.…”

Possible Influence of Cell Walls upon Ion Concentrations at Plasma Membrane Surfaces. Toward a Comprehensive View of Cell-Surface Electrical Effects upon Ion Uptake, Intoxication, and Amelioration

“…In several studies microelectrodes were pushed against cell surfaces and the potentials were recorded as the composition of the bathing solutions was changed (Nagai and Kishimoto, 1964;Saftner and Raschke, 1981;Shomer et al, 2003). These measurements corresponded substantially with expectation in that the depolarizing effectiveness of ions was related to size and charge (Shomer et al, 2003), corresponded to the strength of adsorption by CWs as determined by chemical assays (Franco et al, 2002), was in agreement with z-potential responses of CW fragments (O'Shea et al, 1990), and corresponded roughly to the depolarizing effectiveness against PMs (Shomer et al, 2003).…”

“…In fact, all of the values in the ''Donnan-Plus-Binding for CW'' column in Table I appear to be small, except for the value for K R,H . Chemical assays of isolated CW material indicate much greater values (references in Shomer et al, 2003), but the values presented in Table I were determined in vivo or at least in situ and pertain to sites that were accessible to measurements of electrical potential and available to reversible binding. If the reported values for R Total 5 1 M referred to sites available to reversible binding, then c CW,medium 2200 mV in 0.1 mM NaCl.…”

“…Optimized values for total negative sites (R Total ) were computed for each study, but a single suite of binding constants was evaluated for the pooled PM data and for the pooled CW data (see Table I). The figure is redrawn from Shomer et al (2003). Table II. In solution number 1, all solutes are at a minimum relative to the other solutions so that c CW,medium (250.3 mV) is more negative than elsewhere.…”

“…This is a possibility because glass microelectrodes and z-potential measurements may not access directly the electrical potentials of the CW interior (see discussion in Shomer et al, 2003). The effect of this is illustrated in Figure 5 for La…”