Multiple inward channels provide flexibility in Na+/K+ discrimination at the plasma membrane of barley suspension culture cells

Abstract

Ion transport across the plasma membrane of suspension-culture cells derived from immature barley embryos has been studied in low (15 mM KCl) and high (additional 150 mM NaCl) salt conditions to understand how plants discriminate between K(+) and Na(+) during ion uptake. In both media about 50% of the cells exhibited resting potentials more negative than any of the passive diffusion potentials. In whole-cell patch clamp experiments membrane hyperpolarization activated large inward currents. Whilst the instantaneous current components did not discriminate between K(+) and Na(+), the time-dependent current, I(in), was selective for K(+) over Na(+). Further analysis of I(in) revealed the following properties: double exponential current activation (time-constants 0.03 s and 0.3 s, half activation potential - 171 mV); no inactivation; complete block by Ba(2+) (30 mM in 100 mM KCl) and part block by TEA(+) (maximum 50% with 20 mM); dependence on millimolar concentrations of cytoplasmic ATP; no block by external or cytoplasmic Na(+). The selectivity sequences K(+) ≫ Rb(+) > NH(+)(4) > Na(+) ≫ Cl(-) and K(+) ≫ NH(+)(4) > Na(+) > Rb(+) were determined from measurements of reversal potentials and relative steady-state currents respectively. P(Na):P(K) was 0.07 ± 0.02 (from reversal potentials) and I(Na):I(K) was 0.17 + 0.05 (from relative currents). A high variance among the observed permeability ratios suggested that several channels with different ion-selectivities contributed to the time-dependent whole-cell currents. In single channel experiments, several inward channels with distinct properties were found. The major channels were (i) a voltage-gated, K(+)-selective channel (12 pS), (ii) an ATP-activated non-selective cation channel (7 pS) and (iii) an inward-rectifying anion-channel (150 pS, all unitary conductances given for 100 mM KCI). No significant differences were found in whole-cell currents or single-channel characteristics between cells that had been adapted to a high-salt growth-medium (150 mM NaCl) and non-adapted cells. The idea that differential regulation of plasma membrane ion channels gives rise to a physiological flexibility, allowing the cells to control Na(+) uptake under varying external conditions, is discussed.

The role of root apoplastic transport barriers in salt tolerance of rice (Oryza sativa L.)

“…At the cellular level, the positive role played by calcium is very clear from studies on membrane integrity under salt stress (Cramer et al 1985Lynch et al 1987). Calcium is also reported to inhibit the nonselective cation channels and outward rectifying K + channels which are candidates for transporting Na + into cells (Amtmann et al 1997;Tyerman and Skerrett 1999;White 1999;Davenport and Tester 2000).…”

Selectivity of various types of salt-resistant plants for K+ over Na+

“…Potassium, another Group 1 cation, may be equally toxic to Na þ when at high concentrations and yet plants have evolved to use K þ as an important component of osmotic adjustment and an essential macronutrient. Consequently, the selectivity for K þ over Na þ is of major importance to plants in saline conditions (Pitman, 1984) and such selectivity has been the subject of considerable recent research (Amtmann et al, 1997;Amtmann & Sanders, 1999;Maathuis & Amtmann, 1999;Schachtman & Liu, 1999;Box & Schachtman, 2000;Liu et al, 2000a,b;Rubio et al, 2000). The physiology of salt resistance at whole-plant level has been studied in detail ( Jeschke & Stelter, 1983;Flowers, 1985;Munns & Termaat, 1986;Pitman, 1988;Jeschke & Pate, 1991;Noble & Rogers, 1992;Yeo, 1994;Garcia et al, 1997), but the physiological criteria in identifying different types of salt-resistant plants have, to our knowledge, not been reported.…”

Enhancement of Na+ Uptake Currents, Time-Dependent Inward-Rectifying K+ Channel Currents, and K+Channel Transcripts by K+ Starvation in Wheat Root Cells

“…A Na ϩ -permeable channel was characterized in protoplasts of maize root cortical cells (Roberts and Tester, 1997). In barley suspension-cultured cells, instantaneously activating inward-rectifying currents that were permeable to Na ϩ and K ϩ were identified (Amtmann et al, 1997). From guard cells of two Aster species, non-rectifying cation channels that possess similar characteristics as the Na ϩ -dependent inward currents from roots have been reported (Véry et al, 1998).…”