Electrogenic Transport Properties of Growing Arabidopsis Root Hairs

Abstract

Ion transport, measured using double-barreled micropipettes to obtain current-voltage relations, was examined in Arabidopsis thaliana root hairs that continued tip growth and cytoplasmic streaming after impalement with the micropipette. To do this required in situ measurements with no handling of the seedlings to avoid wounding responses, and conditions allowing good resolution microscopy in tandem with the electrophysiological measurements. Two ion transport processes were demonstrated. One was a tetraethylammonium-sensitive potassium ion current, inward at hyperpolarized potentials and outward at depolarized potentials. The addition of tetraethylammonium (a potassium channel blocker) caused the potential to hyperpolarize, indicating the presence of a net inward potassium current through the ion channels at the resting potential. The potassium influx was sufficient to "drive" cellular expansion based upon growth rates. Indeed, tetraethylammonium caused transient inhibition of tip growth. The other electrogenic process was the plasma membrane proton pump, measured by indirect inhibition with cyanide or direct inhibition by vanadate. The proton pump was the dominant contribution to the resting potential, with a very high current density of about 250 microamperes per square centimeter (seen only in young growing root hairs). The membrane potential generated by the proton pump presumably drives the potassium influx required for cellular expansion. The pump appears to be a constant current source over the voltage range -200 to 0 millivolts. With this system, it is now possible to study the physiology of a higher plant cell in dynamic living state using a broad range of cell biological and electrophysiological techniques.

Pressure Regulation of the Electrical Properties of Growing Arabidopsis thaliana L. Root Hairs

“…Growth rates were about 1 pm min-l. Previous experiments demonstrated that growth usually continues after impalement with the microelectrode (Lew, 1991).…”

“…Lew, 1991) and were impaled when they had attained a length of 30 to 100 pm. Growth rates were about 1 pm min-l.…”

“…Current-voltage measurements using a voltage clamp (Smith et al, 1980) have been described previously (Lew, 1991). Voltage clamping for current-voltage measurements was computer-driven through a data acquisition board and operational amplifier configured for voltage clamping.…”

“…A variety of ion transport processes are known to affect Arabidopsis root hair growth. For example, inhibition of the Proton PumP 01 potassium channels transiently inhibits growth (Lew, 1991), and calcium currents inward at the hair tip are associated with actively growing root hairs (Schiefelbein et al, 1992). I find that the electrical properties of the cells are Water relations are a key determinant of growth and development in plants (Milthorpe and Moorby, 1974;Kramer, 1983).…”

“…In this paper, we examine the effect of turgor, directly modulated with a pressure probe Or indirectlY by lular osmotica, on the electrical parameters of actively growing higher celk ActivelY growing root hairs Of Arabidopsis tkaliana were used for the measurements (Lew, 1991) because growth implies that ion transport activity is dYnamicallY maintaining turgor to "drive" c e h l a r exPansion (assuming that turgor would otherwise become a limiting condition as the cell expands). A variety of ion transport processes are known to affect Arabidopsis root hair growth.…”