Differential regulation of K+ channels in Arabidopsis epidermal and stelar root cells

Abstract

The patch clamp technique was applied to protoplasts isolated from the epidermis and pericycle of Arabidopsis roots and their plasma membrane currents investigated. In the whole cell configuration, all protoplasts from the epidermis exhibited depolarization-activated time-dependent outwardly rectifying (OR) currents whereas OR currents were present in only 50% of cells from the pericycle. The properties of the OR currents in the epidermis and pericycle were compared with respect to their selectivity, pharmacology and gating. The time-dependent activation kinetics, selectivity and sensitivity to extracellular tetraethyl ammonium of the OR current in each cell type were not significantly different. The reversal potential (Erev) of the OR currents indicated that they were primarily due to the movement of K+. However, the gating properties of the OR currents from the epidermis differed markedly from those exhibited in the pericycle. Although both cell types displayed OR currents with voltage-dependent gating modulated in a potassium-dependent fashion [i.e. the activation threshold (V0.5) was displaced to more positive voltages as extracellular K+ increased], the OR currents in the epidermis also displayed voltage-independent gating by extracellular K + which dramatically regulated current density. In the present study, reducing extracellular K+ activity from 40 to 0.87 mM reduced the OR current density in epidermal cells by approximately 80%. The chord conductance of the OR current saturated as a function of extracellular K+ and could be fitted with a Michaelis-Menten function to yield a binding constant (Km) of 10.5 mM. The ability of other monovalent cations to substitute for K+-gating of the OR currents was also investigated and shown to exhibit a relative sequence of K+ ≥ Rb+ > Cs+ > Na+ ≥ Li+ (Eisenmann sequence IV) with respect to efficacy of gating. Furthermore, single channel recordings demonstrated that channel activity rather than the single channel conductance was modulated by extracellular K+. In contrast, OR current density in the pericycle was largely independent of extracellular K+. It is suggested that the contrasting gating properties of the K+ channels in the epidermis and pericycle reflect their different physiological roles, particularly with respect to their role in K+ (nutrient) transport from the soil solution to the shoot.