MULTIPLE GENES, TISSUE-SPECIFICITY, AND EXPRESSION-DEPENDENT MODULATION CONTRIBUTE TO THE FUNCTIONAL DIVERSITY OF POTASSIUM CHANNELS IN ARABIDOPSIS-THALIANA

K+ channels play diverse roles in mediating K+ transport and in modula ting the membrane potential in higher plant cells during growth and de velopment. Some of the diversity in K+ channel functions may arise fro m the regulated expression of multiple genes encoding different K+ cha nnel polypeptides. Here we report the isolation of a novel Arabidopsis thaliana cDNA (AKT2) that is highly homologous to the two previously identified K+ channel genes, KAT1 and AKT1. This cDNA mapped to the ce nter of chromosome 4 by restriction fragment length polymorphism analy sis and was highly expressed in leaves, whereas AKT1 was mainly expres sed in roots. In addition, we show that diversity in K+ channel functi on may be attributable to differences in expression levels. increasing KAT1 expression in Xenopus oocytes by polyadenylation of the KAT1 mRN A increased the current amplitude and led to higher levels of KAT1 pro tein, as assayed in western blots. The increase in KAT1 expression in oocytes produced shifts in the threshold potential for activation to m ore positive membrane potentials and decreased half-activation times. These results suggest that different levels of expression and tissue-s pecific expression of different K+ channel isoforms can contribute to the functional diversity of plant K+ channels. The identification of a highly expressed, leaf-specific K+ channel homolog in plants should a llow further molecular characterization of K+ channel functions for ph ysiological K+ transport processes in leaves.