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 experiment s were used to identify a single K+ channel gene in Arabidopsis as expresse d throughout the plant. Use of the beta-glucuronidase reporter gene reveale d expression of this gene, AKT2/AKT3, in both source and sink phloem tissue s. The AKT2/AKT3 gene corresponds to two previously identified cDNAs, AKT2 (reconstructed at its 5' end) and AKT3, the open reading frame of the latte r being shorter at its 5' end than that of the former. Rapid amplification of cDNA ends with polymerase chain reaction and site-directed mutagenesis w as performed to identify the initiation codon for AKT2 translation. All of the data are consistent with the hypothesis that the encoded polypeptide co rresponds to the longest open reading frame previously identified (AKT2). E lectrophysiological characterization (macroscopic and single-channel curren ts) of AKT2 in both Xenopus oocytes and COS cells revealed a unique gating mode and sensitivity to pH (weak inward rectification, inhibition, and incr eased rectification upon internal or external acidification), suggesting th at 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.