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Differential expression and regulation of K+ channels in the maize coleoptile: molecular and biophysical analysis of cells isolated from cortex and vasculature

Bauer, C.S., Hoth, S., Haga, K., Philippar, K., Aoki, N., Hedrich, R.
The plant journal 2000 v.24 no.2 pp. 139-145
coleoptiles, potassium, Zea mays, ion transport, cortex, genes, Xenopus laevis, oocytes, acidification, hydrogen ions, plant proteins, cesium, barium, plant vascular system, inorganic ions, calcium, plasma membrane
Recently, two K+ channel genes, ZMK1 and ZMK2, were isolated from maize coleoptiles. They are expressed in the cortex and vasculature, respectively. Expression in Xenopus oocytes characterized ZMK1 as an inwardly rectifying K+ channel activated by external acidification, while ZMK2 mediates voltage-independent and proton-inhibited K+ currents. In search of the related gene products in planta, we applied the patch-clamp technique to protoplasts isolated from the cortex and vasculature of Zea mays coleoptiles and mesocotyls. In the cortex, a 6-8 pS K+ channel gave rise to inwardly rectifying K+ currents. Like ZMK1, this channel was activated by apoplastic acidification. In contrast, protoplasts from vascular tissue expressing the sucrose transporter ZmSUT1 were dominated by largely voltage-independent K+ currents with a single-channel conductance of 22pS. The pronounced sensitivity to the extracellular protons Ca2+, Cs+ and Ba2+ is reminiscent of ZMK2 properties in oocytes. Thus, the dominant K+ channels in cortex and vasculature most likely represent the gene products of ZMK1 and ZMK2. Our studies on the ZMK2-like channels represent the first in planta analysis of a K+ channel that shares properties with the AKT3 K+ channel family.