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

Abstract

SummaryRecently, 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 acidi®cation, 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 acidi®cation. In contrast, protoplasts from vascular tissue expressing the sucrose transporter ZmSUT1 were dominated by largely voltageindependent 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 ®rst in planta analysis of a K + channel that shares properties with the AKT3 K + channel family.

Review: Nutrient loading of developing seeds

“…This causes a sigmoid shaped I-V curve with a negative conductance at membrane potentials between -220 and −140 mV (Zhang et al 2004b). These characteristics are in contrast to the KAT1 channels, but they do resemble those of the phloem-related K + inward rectifying channels such as AKT2/3 (Marten et al 1999), ZMK2 (Bauer et al 2000) and VFK1 (Ache et al 2001). Moreover, the cotyledon cation channel differs from the characterised plant KAT1 and ATK2/3 channels in terms of selectivity for univalent cations.…”

Outer Pore Residues Control the H+ and K+ Sensitivity of the Arabidopsis Potassium Channel AKT3

“…Members of the AKT2/3 subfamily (Cao et al, 1995), such as AKT2/3 and ZMK2, exhibit weak rectification properties only, allowing the uptake of potassium ions at membrane potentials negative and potassium release positive to E K (Marten et al, 1999;Philippar et al, 1999;Bauer et al, 2000;Lacombe et al, 2000b;Dreyer et al, 2001). Furthermore, AKT2/3-like channels are inhibited by extracellular protons.…”

Sucrose Transporter ZmSut1 Expression and Localization Uncover New Insights into Sucrose Phloem Loading

“…Because no Suc was detectable in the wild-type guttation fluid, this finding indicated that a high level of Suc was present in the apoplasm (which is contiguous with the xylem transpiration stream) of zmsut1 mutants and supported a role for ZmSut1 in phloem loading. In addressing this potential role, one previous study evaluated ZmSut1 expression in vein-enriched versus nonvein tissues dissected from the coleoptile, the protective sheath enclosing the germinating shoot, and found higher expression in the vein-enriched tissue (Bauer et al, 2000). However, to our knowledge, no previous studies have examined the cellular and subcellular expression of ZmSut1 and the ZmSUT1 protein, respectively, to evaluate this proposed role in the phloem loading of Suc in the leaf blade.…”