Activation Kinetics of the K + Outward Rectifying Conductance (KORC) in Xylem Parenchyma Cells from Barley Roots L.H. Wegner*, A.H. De Boer Faculty of Biology, Department of Molecular Genetics, Section of Plant Physiology, Vrije Universiteit Amsterdam, De Boelelaan 1087, 1081HV Amsterdam, The Netherlands Received: 20 November 1998/Revised: 1 April 1999 Abstract. The activation kinetics of outward currents in protoplasts from barley root xylem parenchyma was in- vestigated using the patch-clamp technique. The K + out- ward rectifying conductance (KORC), providing the main pathway for K + transport to the xylem, could be described in terms of a Hodgkin-Huxley model with four independent gates. Gating of KORC depended on volt- age and the external K + concentration. An increase in the external K + concentration resulted in a shift in the voltage dependence of gating. This could be explained by a K + dependence of the rate constant b for channel closure, indicating binding of K + to a regulatory site exposed to the bath. Occasionally, KORC was observed to inactivate; this inactivation occurred and vanished spontaneously. In some of the whole cell and excised patch recordings, a stepwise increase in outward current was observed upon a depolarizing voltage pulse, indicat- ing that several populations of ‘sleepy’ channels existed in the plasma membrane that activated with a certain lag time. It is discussed whether this observation can be ex- plained by a putative subunit, which retards channel ac- tivation, or by a scheme of cooperative gating. A quan- titative description of outward rectifying K + channels in xylem parenchyma cells is a major step forward towards a mathematical model of salt transport into the xylem. Key words: Plant ion channel — Hodgkin-Huxley for- malism — Cooperative gating — K + dependent gating Introduction Release of K + by xylem parenchyma cells in the root occurs via outwardly rectifying K + channels (KORCs) in the plasma membrane of these cells (Wegner & Raschke, 1994; Roberts & Tester, 1995, 1997; Wegner & De Boer, 1997). The central role of these channels for the trans- location of potassium from the root to the shoot was first demonstrated by Wegner and De Boer (1997). By per- fusing the stele of young barley seedlings with the K + channel blocker TEA, accumulation of K + in the shoot was completely inhibited. Further evidence for K + chan- nels serving as major pathways for K + release to the xylem sap came from the recent report by Gaymard et al. (1998) on the cloning of a K + outward rectifier exclu- sively expressed in root stelar cells of Arabidopsis (SKOR, stelar K + outward rectifier). A highly homolo- gous sequence with a similar expression pattern has also been found in maize, (C. Alcon and H. Sentenac, per- sonal communication), suggesting that SKOR may be common to dicots and monocots. In knockout mutants of Arabidopsis lacking SKOR, shoot K + content and xy- lem sap concentration of K + were reduced significantly. The electrophysiological properties of SKOR were stud- ied by heterologous expression in Xenopus oocytes. Several lines of evidence indicate that the properties of SKOR are identical with those of KORC (Wegner & De Boer, 1997; Gaymard et al., 1998): (i) The activation potential of the channel shifts with a change in the ex- ternal K + concentration, (ii) whole cell currents activate with sigmoidal kinetics upon a stepwise depolarization of the membrane, (iii) besides K + , the channel translo- cates Ca 2+ , (iv) the expression of SKOR is down- regulated by the phytohormone ABA, leading to a de- crease of the outward K + currents in stelar cells when plants were pretreated with ABA (Roberts, 1998). *Present address: Am Lehrstuhl fu ¨ r Biotechnologie, Universita ¨t Wu ¨rz- burg, Biozentrum, Am Hubland, D-97074 Wu ¨rzburg, Germany Correspondence to: L.H. Wegner J. Membrane Biol. 170, 103–119 (1999) The Journal of Membrane Biology © Springer-Verlag New York Inc. 1999