Ion channels in guard cells of Arabidopsis thaliana (L.) Heynh.

Abstract

Despite the availability of many mutants for signal transduction, Arabidopsis thaliana guard cells have so far not been used in electrophysiological research. Problems with the isolation of epidermal strips and the small size of A. thaliana guard cells were often prohibiting. In the present study these difficulties were overcome and guard cells were impaled with double-barreled microelectrodes. Membrane-potential recordings were often stable for over half an hour and voltage-clamp measurements could be conducted. The guard cells were found to exhibit two states. The majority of the guard cells had depolarized membrane potentials, which were largely dependent on external K+ concentrations. Other cells displayed spontaneous transitions to a more hyperpolarized state, at which the free-running membrane potential (Em) was not sensitive to the external K+ concentration. Two outward-rectifying conductances were identified in cells in the depolarized state. A slow outward-rectifying channel (s-ORC) had properties resembling the K(+)-selective ORC of Vicia faba guard cells (Blatt, 1988, J Membr Biol 102: 235-246). The activation and inactivation times and the activation potential, all depended on the reversal potential (Erev) of the s-ORC conductance. The s-ORC was blocked by Ba2+ (K1/2 = 0.3-1.3 mM) and verapamil (K1/2 = 15-20 microM). A second rapid outward-rectifying conductance (r-ORC) activated instantaneously upon stepping the voltage to positive values and was stimulated by Ba2+. Inward-rectifying channels (IRC) were only observed in cells in the hyperpolarized state. The activation time and activation potential of this channel were not sensitive to the external K+ concentration. The slow activation of the IRC (t1/2 approximately 0.5 s) and its negative activation potential (Vthreshold = -155 mV) resemble the values found for the KAT1 channel expressed in Saccharomyces cerevisiae (Bertl et al., 1995, Proc Natl Acad Sci USA 92: 2701-2705). The results indicate that A. thaliana guard cells provide an excellent system for the study of signal transduction processes.

A transient outward-rectifying K + channel current down-regulated by cytosolic Ca 2+ in Arabidopsis thaliana guard cells

“…1A). At the beginning of whole-cell recordings (before ''rundown'') we found large noninactivating K ϩ out currents in Arabidopsis guard cells (data not shown) that were analogous to recently reported K ϩ out currents (6). These noninactivating K ϩ out currents in V. faba guard cells have been reported to also show rundown (3).…”

The Arabidopsis outward K + channel GORK is involved in regulation of stomatal movements and plant transpiration

“…In planta (electro)physiological analyses have led to the conclusion that K ϩ release from the guard cell, leading to stomatal closure, involves the activity of K ϩ -selective slowly activating voltage-gated outwardly rectifying channels (6,26). The functional features of these channels characterized in vivo in guard cells of Arabidopsis and of a number of other species are very similar to those displayed by the Arabidopsis SKOR and GORK channels expressed in heterologous systems (10,27).…”

“…simplest hypothesis is that a single gene, GORK, encodes the major voltage-gated outwardly rectifying K ϩ channel characterized in this membrane (6).…”

Increased Functional Diversity of Plant K+ Channels by Preferential Heteromerization of the Shaker-like Subunits AKT2 and KAT2

“…The absence of instantaneous current might be explained by the weakness of the leak current in AKT2/KAT1 heteromers, as shown in Baizabal-Aguirre et al (35). In guard cells, dominant K ϩ currents are generated by the activity of voltage-gated K ϩ channels (65)(66)(67)(68). There is co-existence of KAT1, GORK, KAT2, AKT1, AKT2, and AtKC1 transcripts (41,69).…”