Developmental and light-dependent regulation of a phloem-localised K+ channel of Arabidopsis thaliana

K+ channels in plants can currently be classified into six families with in dividual members being involved in nutrient uptake, loading of the xylem an d the physiology of stomatal movement. In this study we have focused on akt 2/3. This K+ channel, as shown by GUS-expression analysis, is expressed in the phloem and xylem of the aerial parts of Arabidopsis thaliana. Northern blot analyses revealed the highest akt2/3-concentrations in the flower stal k, followed by the leaf, flower and stem. During the light period (8 am to 4 pm), transcripts reached a peak around noon (11 am), decayed to almost 50 % in the afternoon and reached a low background level in the following dark period. In continuous darkness, however, the K+ channel mRNA content had a lready decreased beyond the background level by noon. In leaves and flower stalk, the light-induced transcription of akt2/3 was suppressed by CO2-free air, indicating that gene activity is under the control of photosynthates. Additionally, when rosette leaves were illuminated and flower stalks shade d, akt2/3-mRNA transcription was still inhibited in the shaded region. This indicates that channel gene activation is sensitive to photosynthesis-deri ved factors from neighboring cells rather than factors mobile in the phloem . We propose that the coupling between sugar production and allocation invo lves the photosynthate- and light-dependent phloem K+ channel AKT2/3.