pH control of the plant outwardly-rectifying potassium channel SKOR.

SKOR, an Arabidopsis depolarisation-activated K+-selective channel, was expressed in Xenopus oocytes, and external and internal pH effects were analysed. Internal pH was manipulated by injections of alkaline or acidic solutions or by acid load from acetate-containing medium. An internal pH decrease from 7.4 to 7.2 induced a strong (ca. 80%) voltage-independent decrease of the macroscopic SKOR current, the macroscopic gating parameters and the single channel conductance remained unchanged. An external acidification from 7.4 to 6.4 had similar effects. It is proposed that pH changes regulate the number of channels available for activation. Sensitivity of SKOR activity to pH in the physiological range suggests that internal and external pH play a role in the regulation of K+ secretion into the xylem sap.

A shaker-like K(+) channel with weak rectification is expressed in both source and sink phloem tissues of Arabidopsis.

RNA gel blot and reverse transcription-polymerase chain reaction experiments were used to identify a single K(+) channel gene in Arabidopsis as expressed throughout the plant. Use of the beta-glucuronidase reporter gene revealed expression of this gene, AKT2/AKT3, in both source and sink phloem tissues. The AKT2/AKT3 gene corresponds to two previously identified cDNAs, AKT2 (reconstructed at its 5' end) and AKT3, the open reading frame of the latter being shorter at its 5' end than that of the former. Rapid amplification of cDNA ends with polymerase chain reaction and site-directed mutagenesis was performed to identify the initiation codon for AKT2 translation. All of the data are consistent with the hypothesis that the encoded polypeptide corresponds to the longest open reading frame previously identified (AKT2). Electrophysiological characterization (macroscopic and single-channel currents) of AKT2 in both Xenopus oocytes and COS cells revealed a unique gating mode and sensitivity to pH (weak inward rectification, inhibition, and increased rectification upon internal or external acidification), suggesting that AKT2 has enough functional plasticity to perform different functions in phloem tissue of source and sink organs. The plant stress hormone abscisic acid was shown to increase the amount of AKT2 transcript, suggesting a role for the AKT2 in the plant response to drought.

Guard cell inward K+ channel activity in arabidopsis involves expression of the twin channel subunits KAT1 and KAT2.

Stomatal opening, which controls gas exchanges between plants and the atmosphere, results from an increase in turgor of the two guard cells that surround the pore of the stoma. KAT1 was the only inward K(+) channel shown to be expressed in Arabidopsis guard cells, where it was proposed to mediate a K(+) influx that enables stomatal opening. We report that another Arabidopsis K(+) channel, KAT2, is expressed in guard cells. More than KAT1, KAT2 displays functional features resembling those of native inward K(+) channels in guard cells. Coexpression in Xenopus oocytes and two-hybrid experiments indicated that KAT1 and KAT2 can form heteromultimeric channels. The data indicate that KAT2 plays a crucial role in the stomatal opening machinery.

Identification and disruption of a plant shaker-like outward channel involved in K+ release into the xylem sap.

SKOR, a K+ channel identified in Arabidopsis, displays the typical hydrophobic core of the Shaker channel superfamily, a cyclic nucleotide-binding domain, and an ankyrin domain. Expression in Xenopus oocytes identified SKOR as the first member of the Shaker family in plants to be endowed with outwardly rectifying properties. SKOR expression is localized in root stelar tissues. A knockout mutant shows both lower shoot K+ content and lower xylem sap K+ concentration, indicating that SKOR is involved in K+ release into the xylem sap toward the shoots. SKOR expression is strongly inhibited by the stress phytohormone abscisic acid, supporting the hypothesis that control of K+ translocation toward the shoots is part of the plant response to water stress.