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. Keywords: K(+) channel, voltage-independent, proton block, maize coleoptile.