The K+ channel KZM1 mediates potassium uptake into the phloem and guard cells of the C4 grass Zea mays

In search of K+ channel genes expressed in the leaf of the C-4 plant Zea mays, we isolated the cDNA of KZM1 (for K+ channel Zea mays 1). KZM1 showed highest similarity to the Arabidopsis K+ channels KAT1 and KAT2, which are localized in guard cells and phloem. When expressed in Xenopus oocytes, KZM1 exhibited the characteristic features of an inward-rectifying, potassium-selective channel. In contrast to KAT1- and KAT2-type K+ channels, however, KZM1 currents were insensitive to external pH changes. Northern blot analyses identified the leaf, nodes, and silks as sites of KZM1 expression. Following the separation of maize leaves into epidermal, mesophyll, and vascular fractions, quantitative real-time reverse transcriptase-PCR allowed us to localize KZM1 transcripts predominantly in vascular strands and the epidermis. Cell tissue separation and KZM1 localization were followed with marker genes such as the bundle sheath-specific ribulose-1,5-bisphosphate carboxylase, the phloem K+ channel ZMK2, and the putative sucrose transporter ZmSUT1. When expressed in Xenopus oocytes, ZmSUT1 mediated proton-coupled sucrose symport. Coexpression of ZmSUT1 with the phloem K+ channels KZM1 and ZMK2 revealed that ZMK2 is able to stabilize the membrane potential during phloem loading/unloading processes and KZM1 to mediate K+ uptake. During leaf development, sink-source transitions, and diurnal changes, KZM1 is constitutively expressed, pointing to a housekeeping function of this channel in K+ homeostasis of the maize leaf. Therefore, the voltage-dependent K+-uptake channel KZM1 seems to mediate K+ retrieval and K+ loading into the phloem as well as K+-dependent stomatal opening.