The Journal of General Physiology
Cell MicroControls
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Published online 11 April 2005 doi:10.1085/jgp.200409211
The Rockefeller University Press, 0022-1295 $8.00
JGP, Volume 125, Number 5, 483-492
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AKT2/3 Subunits Render Guard Cell K+ Channels Ca2+ Sensitive

Natalya Ivashikina, Rosalia Deeken, Susanne Fischer, Peter Ache, and Rainer Hedrich

Julius-von-Sachs Institute of Biosciences, Biocenter, Department of Molecular Plant Physiology and Biophysics, University of Würzburg, 97082 Würzburg, Germany

Correspondence to Rainer Hedrich: hedrich{at}botanik.uni-wuerzburg.de

Inward-rectifying K+ channels serve as a major pathway for Ca2+-sensitive K+ influx into guard cells. Arabidopsis thaliana guard cell inward-rectifying K+ channels are assembled from multiple K+ channel subunits. Following the recent isolation and characterization of an akt2/3-1 knockout mutant, we examined whether the AKT2/3 subunit carries the Ca2+ sensitivity of the guard cell inward rectifier. Quantification of RT-PCR products showed that despite the absence of AKT2 transcripts in guard cells of the knockout plant, expression levels of the other K+ channel subunits (KAT1, KAT2, AKT1, and AtKC1) remained largely unaffected. Patch-clamp experiments with guard cell protoplasts from wild type and akt2/3-1 mutant, however, revealed pronounced differences in Ca2+ sensitivity of the K+ inward rectifier. Wild-type channels were blocked by extracellular Ca2+ in a concentration- and voltage-dependent manner. Akt2/3-1 mutants lacked the voltage-dependent Ca2+ block, characteristic for the K+ inward rectifier. To confirm the akt2/3-1 phenotype, two independent knockout mutants, akt2-1 and akt2::En-1 were tested, demonstrating that the loss of AKT2/3 indeed affects the Ca2+ dependence of guard cell inward rectifier. In contrast to AKT2 knockout plants, AKT1, AtKC1, and KAT1 loss-of-function mutants retained Ca2+ block of the guard cell inward rectifier. When expressed in HEK293 cells, AKT2 channel displayed a pronounced susceptibility toward extracellular Ca2+, while the dominant guard cell K+ channel KAT2 was Ca2+ insensitive. Thus, we conclude that the AKT2/3 subunit constitutes the Ca2+ sensitivity of the guard cell K+ uptake channel.

Key Words: Arabidopsis • guard cells • potassium channel • calcium sensitivity • AKT2/3

1 AKT2 (Cao et al., 1995) and AKT3 (Ketchum and Slayman, 1996) represent the same gene. The AKT2 gene contains two start codons 15 amino acids apart. Both AKT2 and its truncated version AKT3 share the same functional properties when expressed in animal cells (c.f. Lacombe et al., 2000; Geiger et al., 2002). Since it has not yet been shown whether AKT2, AKT3, or both channel proteins exist in planta, we name this channel protein AKT2/3.


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