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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.