Abstract
Knowledge of ion fluxes in the dumbell-shaped guard cells of grass species has been limited by the difficulty of obtaining isolated epidermes or guard-cell protoplasts for use in radioactive-tracer or electrophysiological studies. We describe here a method for isolating guard-cell protoplasts from Zea mays L. Whole-cell patch clamp has been used to measure K+-channel current across the plasma membrane surrounding these protoplasts. Two populations of K+-permeable channels have been identified. Hyperpolarization of the membrane to potentials (Vm) more negative than -100 mV results in inward K+ current through one population of channels. Inward current activation is faster than in the dicotyledon, Vicia faba L. (mean activation half-time 26 ms (Z. mays) versus 123 ms (V. faba) at Vm=-180 mV). Steady-state current density is less than in V. faba (-22 μA · cm−2 (Z. mays) versus -40 μA · cm−2 (V. faba) at Vm=- 180 mV in 12 mM external K+). Depolarization of the membrane to potentials more positive than -20 mV results in outward K+ current through a second population of channels; these channels activate and (upon repolarization of the membrane) deactivate more slowly than in V. faba (mean activation half-time 375 ms (Z. mays) versus 187 ms (V. faba) at Vm=+ 80 mV) but result in a similar steady-state current density (23.8 μA · cm−2 (Z. mays) versus 28.7 μA · cm−2 (V. faba) at Vm= + 80 mV with 105 mM internal K+). Omission of K+ eliminates the current. The K+ current is sensitive to both internal and external Ca2+ concentration: increasing internal Ca2+ from 2 nM to 0.2 μM or increasing external Ca2+ from 1 mM to 8.5 mM reduces the magnitude of both inward and outward current.
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Abbreviations
- Ex :
-
electrochemical equilibrium potential for ion x
- Px :
-
permeability of ion x
- Vm :
-
membrane potential
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The research described in this paper was supported by grants to S.M.A. from the McKnight Foundation and from the National Science Foundation (DCB-89-04041). K.F.-G. is a Charles Gilbert Heydon Travelling Fellow. The authors are grateful to Ms. J. Sherwood for maintenance of Z. mays seedlings, to Drs. W. Gilbert and K.C. Bansal for assistance with photography and to Dr. V. Ling for comments on the manuscript.
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Fairley-Grenot, K.A., Assmann, S.M. Whole-cell K+ current across the plasma membrane of guard cells from a grass: Zea mays . Planta 186, 282–293 (1992). https://doi.org/10.1007/BF00196258
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DOI: https://doi.org/10.1007/BF00196258