Beschreibung:
<jats:title>Summary</jats:title><jats:p>In search of a K<jats:sup>+</jats:sup> channel involved in phloem transport we screened a <jats:italic>Vicia faba</jats:italic> cotyledon cDNA library taking advantage of a set of degenerated primers, flanking regions conserved among K<jats:sup>+</jats:sup> uptake channels. We cloned VFK1 (for <jats:italic>Vicia faba</jats:italic> K<jats:sup>+</jats:sup> channel 1) characterised by a structure known from the <jats:italic>Shaker</jats:italic> family of plant K<jats:sup>+</jats:sup> channels. When co‐expressed with a KAT1 mutant in <jats:italic>Xenopus</jats:italic> oocytes, heteromers revealed the biophysical properties of a K<jats:sup>+</jats:sup> selective, proton‐blocked channel. Northern blot analyses showed high levels of expression in cotyledons, flowers, stem and leaves. Using <jats:italic>in situ</jats:italic> PCR techniques we could localise the K<jats:sup>+</jats:sup> channel mRNA in the phloem. In the stem <jats:italic>VFK1</jats:italic> expression levels were higher in the lower internodes. There channel transcripts increased in the light and thus under conditions of increased photosynthate allocation. <jats:italic>VFK1</jats:italic> transcripts are elevated in sink leaves, and rise in source leaves during the experimental transition into sinks. Fructose‐ rather than sucrose‐ or glucose‐feeding via the petiole induced <jats:italic>VFK1</jats:italic> gene activity. We therefore monitored the fructose sensitivity of the sieve tube potential through cut aphid stylets. In response to an 1 h fructose treatment the sieve tube potential shift increased from 19 mV to 53 mV per 10‐fold change in K<jats:sup>+</jats:sup> concentration. Under these conditions K<jats:sup>+</jats:sup> channels dominated the electrical properties of the plasma membrane. Based on the phloem localisation and expression patterns of VFK1 we conclude that this K<jats:sup>+</jats:sup> channel is involved in sugar unloading and K<jats:sup>+</jats:sup> retrieval.</jats:p>