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 col eoptiles. They are expressed in the cortex and vasculature, respectively. E xpression 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 is olated from the cortex and vasculature of Zea mays coleoptiles and mesocoty ls. In the cortex, a 6-8 pS K+ channel gave rise to inwardly rectifying Kcurrents. Like ZMK1, this channel was activated by apoplastic acidification . In contrast, protoplasts from vascular tissue expressing the sucrose tran sporter ZmSUT1 were dominated by largely voltage-independent K+ currents wi th a single-channel conductance of 22 pS. 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 ZMK 2-like channels represent the first in planta analysis of a K+ channel that shares properties with the AKT3 K+ channel family.