VFK1, a Vicia faba K+ channel involved in phloem unloading

In search of a K+ channel involved in phloem transport we screened a Vicia faba cotyledon cDNA library taking advantage of a set of degenerated primer s, flanking regions conserved among K+ uptake channels. We cloned VFK1 (for Vicia faba K+ channel 1) characterised by a structure known from the Shake r family of plant K+ channels. When co-expressed with a KAT1 mutant in Xeno pus oocytes, heteromers revealed the biophysical properties of a K+ selecti ve, proton-blocked channel. Northern blot analyses showed high levels of ex pression in cotyledons, flowers, stem and leaves. Using in situ PCR techniq ues we could localise the K+ channel mRNA in the phloem. In the stem VFK1 e xpression levels were higher in the lower internodes. There channel transcr ipts increased in the light and thus under conditions of increased photosyn thate allocation. VFK1 transcripts are elevated in sink leaves, and rise in source leaves during the experimental transition into sinks. Fructose- rat her than sucrose- or glucose-feeding via the petiole induced VFK1 gene acti vity. We therefore monitored the fructose sensitivity of the sieve tube pot ential 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 ch ange in K+ concentration. Under these conditions K+ channels dominated the electrical properties of the plasma membrane. Based on the phloem localisat ion and expression patterns of VFK1 we conclude that this K+ channel is inv olved in sugar unloading and K+ retrieval.