Extracellular Ca²⁺ Ameliorates NaCl-Induced K⁺ Loss from Arabidopsis Root and Leaf Cells by Controlling Plasma Membrane K⁺-Permeable Channels

Specificity and pharmacology of net K⁺ flux responses. A, Transient K⁺ fluxes in response to isotonic mannitol (white symbols) and sodium gluconate (gray symbols). Fluxes were measured from the mature epidermis of Arabidopsis wild-type roots after 1 to 1.5 h incubation in basic measuring solution. Data are mean ± se (n = 8). B, Effect of cation channel blockers (TEACl, 10 mm; verapamil, 20 μm) on the magnitude of the peak K⁺ efflux measured in response to 50 mm NaCl. Data are mean ± se (n = 6–8).

Salinity-induced net Na⁺ flux kinetics measured from Arabidopsis wild-type root (mature epidermis; gray symbols) and leaf tissue (mesophyll; white symbols). Bath [Ca²⁺] was 0.1 mm. Data are mean ± se (n = 6–8).

Effect of salinity (50 mm NaCl) on changes in cytosolic free K⁺ concentration in mature epidermal Arabidopsis (wild-type Columbia) root cells at low (0.1 mm) and high (10 mm) bathing [Ca²⁺]. One (of five) representative example for each treatment is shown.

A and B, Salinity-induced net K⁺ (A) and Na⁺ (B) flux kinetics measured from mature root epidermis of Arabidopsis wild-type (gray symbols) and akt1 (white symbols) plants. Roots were incubated in basic measuring solution for about 1 h before 50 mm NaCl was added. Data are mean ± se (n = 6–9). C, Effect of supplemental Ca²⁺ on net K⁺ fluxes measured from the mature epidermis of Arabidopsis wild-type (gray symbols) and akt1 (white symbols) roots in the presence of 50 mm NaCl. Data are mean ± se (n = 6–8). Roots were exposed to salinity in low Ca²⁺ (0.1 mm) solution for 3 h prior to Ca²⁺ treatment.

Whole-cell plasma membrane currents in protoplasts derived from Arabidopsis mature root epidermis in control (Na⁺-free) conditions (A–E) and in 50 mm extracellular Na⁺ solution (F–I). A, Typical currents (recorded from the same protoplast) in 10 mm (top) and 0.1 mm (bottom) extracellular Ca²⁺ in control solution. B to D, Mean ± se I-V relationships (n = 4–9) for total (B), instantaneous (C), and time-dependent (D) currents measured at 0.1, 1, and 10 mm extracellular Ca²⁺ in Na⁺-free solution. F, Typical currents (recorded from the same protoplast) in 10 mm (top) and 0.1 mm (bottom) extracellular Ca²⁺ with 50 mm Na⁺ present in the bath. G to I, Mean I-V relationships (n = 5–6) for total (G), instantaneous (H), and time-dependent (I) currents measured at 0.1, 1, and 10 mm extracellular Ca²⁺ in the presence of 50 mm Na⁺ in the bath. Time-dependent currents were calculated as described in “Materials and Methods.” In all sections except E, PS contained 50 mm potassium gluconate and 30 mm KCl. E, Typical currents in 0.1 mm extracellular Ca²⁺ when K⁺ salts in PS were replaced by 30 mm TEACl. Extracellular K⁺ was 0.1 mm throughout. All concentrations are given in mm.

Whole-cell plasma membrane currents (protoplasts derived from Arabidopsis mature root epidermis) with 50 mm Na⁺ present in the patch pipette (A–D), and when 50 mm Na⁺ was present both in the bath and PS (E–H). A, Typical currents (recorded from the same protoplast) in 10 mm (top) and 0.1 mm (bottom) extracellular Ca²⁺ for Na⁺ present in PS only. B to D, Mean ± se I-V relationships (n = 4–5) for total (B), instantaneous (C), and time-dependent (D) currents measured at 0.1, 1, and 10 mm extracellular Ca²⁺. E, Typical currents (recorded from the same protoplast) in 10 mm (top) and 0.1 mm (bottom) extracellular Ca²⁺ for symmetrical 50 mm Na⁺ conditions. F to H, Mean ± se I-V relationships (n = 4–5) for total (F), instantaneous (G), and time-dependent (H) currents measured at 0.1, 1, and 10 mm extracellular Ca²⁺. Time-dependent currents were calculated as described in “Materials and Methods.” PS contained 50 mm potassium gluconate, 30 mm KCl, and 50 mm sodium gluconate. Extracellular K⁺ was 0.1 mm. All concentrations are given in mm.