Differential expression and regulation of K(+) channels in the maize coleoptile: molecular and biophysical analysis of cells isolated from cortex and vasculature.
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 22 pS. The pronounced sensitivity to the extracellular protons Ca(2+), Cs(+) and Ba(2+) 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.


BACKGROUND
K(+) channel, voltage-independent, proton block, maize coleoptile.
Bauer
C S
CS
Julius-von-Sachs Institut für Biowissenschaften, Molekulare Pflanzenphysiologie und Biophysik, Universität Würzburg, Julius-von-Sachs-Platz 2, 97082 Würzburg, Germany.
Hoth
S
S
Haga
K
K
Philippar
K
K
Aoki
N
N
Hedrich
R
R
D018548:Cotyledon / Q000166:cytology / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D010940:Plant Proteins; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000378:metabolism*; D011523:Protoplasts / Q000378:metabolism; D003313:Zea mays
D010940:Plant Proteins; D015221:Potassium Channels; C417562:ZMK1 protein, Zea mays; C417563:ZMK2 protein, Zea mays; D011188:Potassium