We have characterized transcripts for three potassium channel homologs in the AKT/KAT subfamily (Shaker type) from the common ice plant ( Mesembryanthemum crystallinum), with a focus on their expression during salt stress (up to 500 m m NaCl). Mkt1 and 2, Arabidopsis AKT homologs, and Kmt1, a KAT homolog, are members of small gene families with two to three isoforms each. Mkt1 is root specific; Mkt2 is found in leaves, flowers, and seed capsules; and Kmt1 is expressed in leaves and seed capsules. Mkt1 is present in all cells of the root, and in leaves a highly conserved isoform is detected present in all cells with highest abundance in the vasculature. MKT1 for which antibodies were made is localized to the plasma membrane. Following salt stress, MKT1 (transcripts and protein) is drastically down-regulated, Mkt2 transcripts do not change significantly, and Kmt1 is strongly and transiently (maximum at 6 h) up-regulated in leaves and stems. The detection and stress-dependent behavior of abundant transcripts representing subfamilies of potassium channels provides information about tissue specificity and the complex regulation of genes encoding potassium uptake systems in a halophytic plant.
My flash recovery serial key. Potassium, the most abundant cation in plant cells, plays essential roles in maintaining the membrane potential, ion homeostasis, in enzyme activation, signal transduction, and many other physiological processes. The molecular mechanism of potassium uptake by plant roots, loading, and transport within plants has been a focus of study during the last decade. Following the isolation of plant potassium channels by yeast ( Saccharomyces cerevisiae) complementation (; ), several proteins capable of transporting potassium have been reported in Arabidopsis, potato ( Solanum tuberosum), barley ( Hordeum vulgare), wheat ( Triticum aestivum), and a few other species.
These membrane proteins include inwardly rectifying channels (IRC) and two types of carriers: those in the HKT and HAK/KUP families, respectively (;;; ). Here, we report the characterization of transcripts encoding potassium channel homologs in the Shaker-type subfamily from a halophytic plant, the common ice plant ( Mesembryanthemum crystallinum). We focus on transcript behavior, comparing plants grown under control conditions with plants stressed by high sodium chloride. As in animal systems, three families of plant potassium channels are known ().
They contain a characteristic pore-forming (P) domain conferring ion selectivity, but they differ in the number of transmembrane (TM) and P domains. A nomenclature has become established by which two subfamilies of plant Shaker-type channels became known as members of either the AKT or KAT family (). All AKT- and KAT-type channels consist of six TM regions with one P region, but their electrophysiological features vary, as well as the regulation of channel activities (;; for review, see ). In contrast with the outward-rectifying animal Shaker channels (ORC), the functionally characterized KAT1 and AKT1 channels in Arabidopsis and their homologs in other species are IRC (; ). AKT differ from the KAT type by the presence of carboxy-terminal ankyrin repeat domains, possibly for anchoring to the cytoskeleton. So far, all potassium channels show high specificity for K + over other alkali cations, making unlikely candidates for significant inadvertent sodium intrusion even at high Na + to K + ratios (; ).