Potassium Uptake Supporting Plant Growth in the Absence of AKT1 Channel Activity Inhibition by Ammonium and Stimulation by Sodium

A transferred-DNA insertion mutant of Arabidopsis that lacks AKT1 inward-rectifying K 1 channel activity in root cells was obtained previously by a reverse-genetic strategy, enabling a dissection of the K 1 -uptake apparatus of the root into AKT1 and non-AKT1 components. Membrane potential measurements in root cells demonstrated that the AKT1 component of the wild-type K 1 permeability was between 55 and 63% when external [K 1 ] was between 10 and 1,000 m M, and NH 4 1 was absent. NH 4 1 specifically inhibited the non-AKT1 component, apparently by competing for K 1 binding sites on the transporter(s). This inhibition by NH 4 1 had significant consequences for akt1 plants: K 1 permeability, 86 Rb 1 fluxes into roots, seed germination, and seedling growth rate of the mutant were each similarly inhibited by NH 4 1 . Wild-type plants were much more resistant to NH 4 1 . Thus, AKT1 channels conduct the K 1 influx necessary for the growth of Arabidopsis embryos and seedlings in conditions that block the non-AKT1 mechanism. In contrast to the effects of NH 4 1 , Na 1 and H 1 significantly stimulated the non-AKT1 portion of the K 1 permeability. Stimulation of akt1 growth rate by Na 1 , a predicted consequence of the previous result, was observed when external [K 1 ] was 10 m M. Collectively, these results indicate that the AKT1 channel is an important component of the K 1 uptake apparatus supporting growth, even in the “high-affinity” range of K 1 concentrations. In the absence of AKT1 channel activity, an NH 4 1 -sensitive, Na 1 /H 1 -stimulated mechanism can suffice. key words: Arabidopsis • plant nutrition • root • transferred-DNA insertion mutant i n t r o d u c t i o n It has been known since the work of Knop and Sachs over 130 yr ago that plants cannot grow in the absence of potassium (Pfeffer, 1900). It is their most abundant inorganic constituent, contributing importantly to the osmotic potential and electrolytic character of cytoplasm. The plasma membrane is typically more permeable to K 1 than to other ions, so the difference in its concentration across the membrane has a large influence on the membrane potential and, hence, cell physiology. Another reason for its essentiality is that some enzymes require K 1 as a cofactor. The mechanism by which cells concentrate K 1 from dilute extracellular sources such as soil has received considerable attention because plant growth depends directly on it. Early kinetic studies by Epstein et al. (1963) gave evidence of two distinct uptake mechanisms: a high affinity system operating over micromolar concentration ranges and a low affinity system that predominates when [K 1 ] ext is in the millimolar range. Recent measurements of K 1 electrochemical potential gradients were incorporated into this classical model to create the widely held view that active transport is necessary when [K 1 ] ext is less than z 300 m M, but that a passive mechanism suffices at higher values of [K 1 ] ext (Maathuis and Sanders, 1993, 1994, 1997; Walker et al., 1996a). This important thermodynamic information was readily integrated with ground-breaking molecular advances occurring at about the same time. Genes encoding passive K 1 channels and active K 1 cotransporters were cloned by complementation of yeast mutants, functionally characterized after heterologous or ectopic expression, and demonstrated to be expressed in roots (reviewed in Smart et al., 1996; de Boer, 1999). These advances collectively gave rise to the dominant view that transporters such as HKT1 (Schachtman and Schroeder, 1994; Rubio et al., 1995; Gassmann et al., 1996; Wang et al., 1998) and the KUP family (Quintero and Blatt, 1997; Santa-Maria et al., 1997; Fu and Luan, 1998; Kim et al., 1998) are responsible for “high affinity” K 1 uptake and that inward-rectifying K 1 channels such as AKT1 (Sentenac et al., 1992; Basset et al., 1995; Rebecca E. Hirsch’s present address is Department of Zoology, University of Wisconsin, Madison, WI 53706. Bryan D. Lewis’ present address is Department of Biology, Clarke College, Dubuque, IA 52001. Address correspondence to Edgar P. Spalding, Department of Botany, University of Wisconsin, 430 Lincoln Drive, Madison, WI 53706. Fax: 608-262-7509; E-mail: [email protected] on Jne 6, 2017 D ow nladed fom Published June 1, 1999