title: A transient outward-rectifying K+ channel current down-regulated by cytosolic Ca2+ in Arabidopsis thaliana guard cells.
abstract: Sustained (noninactivating) outward-rectifying K+ channel currents have been identified in a variety of plant cell types and species. Here, in Arabidopsis thaliana guard cells, in addition to these sustained K+ currents, an inactivating outward-rectifying K+ current was characterized (plant A-type current: IAP). IAP activated rapidly with a time constant of 165 ms and inactivated slowly with a time constant of 7.2 sec at +40 mV. IAP was enhanced by increasing the duration (from 0 to 20 sec) and degree (from +20 to -100 mV) of prepulse hyperpolarization. Ionic substitution and relaxation (tail) current recordings showed that outward IAP was mainly carried by K+ ions. In contrast to the sustained outward-rectifying K+ currents, cytosolic alkaline pH was found to inhibit IAP and extracellular K+ was required for IAP activity. Furthermore, increasing cytosolic free Ca2+ in the physiological range strongly inhibited IAP activity with a half inhibitory concentration of approximately 94 nM. We present a detailed characterization of an inactivating K+ current in a higher plant cell. Regulation of IAP by diverse factors including membrane potential, cytosolic Ca2+ and pH, and extracellular K+ and Ca2+ implies that the inactivating IAP described here may have important functions during transient depolarizations found in guard cells, and in integrated signal transduction processes during stomatal movements.
authors.affiliation: Department of Biology and Center for Molecular Genetics, University of California, San Diego, La Jolla, CA 92093-0116, USA.
authors.forename: Z M
authors.initials: ZM
authors.lastname: Pei
authors.forename: V M
authors.initials: VM
authors.lastname: Baizabal-Aguirre
authors.forename: G J
authors.initials: GJ
authors.lastname: Allen
authors.forename: J I
authors.initials: JI
authors.lastname: Schroeder