Figure 7.

K⁺ (⁸⁶Rb⁺) uptake in plants overexpressing AtCHX13. Seven-day-old seedlings grown hydroponically in B5 medium were washed and transferred to 0.5× Murashige and Skoog liquid medium without K⁺ for 2 d. After this K⁺ depletion, the seedling roots were used for K⁺ uptake assays using an ⁸⁶Rb⁺ tracer. Data points represent means ± sd. At least three independent experiments were performed with duplicates for each treatment. A and B, Time course of K⁺ (⁸⁶Rb⁺) uptake at 0.02 (A) and at 20 (B) mm external K⁺. C, Uptake of K⁺ (⁸⁶Rb⁺) as a function of external K⁺ concentration. D, AtCHX13-mediated high-affinity K⁺ uptake. The AtCHX13-dependent K⁺ (⁸⁶Rb⁺) uptake was obtained by subtracting the uptake in vector control from that in the 35S∷AtCHX13 seedlings. Inset (D), Lineweaver-Burk plot of AtCHX13-dependent K⁺ uptake. [S], K⁺ concentration (μm); V, K⁺ uptake rate (nmol g⁻¹ h⁻¹). E, Effects of pH and CCCP on AtCHX13-mediated K⁺ (⁸⁶Rb⁺) uptake at 20 μm external [K⁺]. Different pH values were obtained by adjusting MES-Tris combinations. Effect of CCCP on AtCHX13-mediated K⁺ uptake was tested at pH 4.3. Protonophore CCCP was added to the uptake medium 5 min prior to K⁺ and Rb⁸⁶ tracer addition. The AtCHX13-dependent K⁺ uptake is compared to the uptake in vector control from that in the 35S∷AtCHX13 seedlings. Data show net uptake at 10 min.