CLONING AND ELECTROPHYSIOLOGICAL ANALYSIS OF KST1, AN INWARD RECTIFYING K-CELLS( CHANNEL EXPRESSED IN POTATO GUARD)

Potassium uptake by guard cells represents part of the osmotic motor w hich drives stomatal opening, Patch-clamp measurements have identified inward rectifying K+ channels capable of mediating K+ uptake in guard cells and various other plant cell types, Here we report the molecula r cloning and characterization of a voltage-dependent K+ channel (KST1 ) from potato (Solanum tuberosum L.) guard cells. In situ hybridizatio n shows expression of kst1 in guard cells. Two electrode voltage-clamp arid patch-clamp studies of the gene product after cRNA injection int o Xenopus oocytes identified KST1 as a slowly activating, voltage-depe ndent, inward rectifying K+ channel. The single channel current voltag e curve was linear in the range -160 to +20 mV, with a deduced single channel conductance of 7 pS in symmetrical 100 mM K+. This channel typ e, modulated by pH changes within the physiological range, required AT P for activation. In line with the properties of a K+-selective channe l, KST1 was permeable to K+, Rb+ and NH4+ and excluded Na+ and Li+. Cs + at submillimolar concentrations blocked the channel in a voltage-dep endent manner. Related studies on potato guard cell protoplasts confir med the biophysical characteristics of the kst1 gene product (KST1) in the heterologous expression system. Therefore, KST1 represents a majo r K+ uptake channel in potato guard cells.