A transient outward-rectifying K+ channel current down-regulated by cytosolic Ca2+ in Arabidopsis thaliana guard cells

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 ≈ 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.

Duke Scholars

Author Zhen-Ming Pei Biology