PLANT PHYSIOLOGY , Vol 105, Issue 3 999-1006, Copyright © 1994 by American Society of Plant Biologists
A Cationic Channel in the Guard Cell Tonoplast of Allium cepa
G. Amodeo, A. Escobar and E. Zeiger
Department of Biology (G.A., E.Z.), and Department of Physiology (A.E.), University of California, Los Angeles, California 90024
Stomatal movements depend on an osmoregulation process in which swelling or
shrinking of the guard cells opens or closes the stomatal pore. Ions and
water fluxes are an essential aspect of guard cell osmoregulation. Thus
far, studies of these fluxes have focused on the guard cell plasma
membrane. Guard cells, however, are a multi-compartment system that
includes a prominent vacuole, which has a primary role in turgor
regulation. This study reports on a detailed characterization of an ion
channel at the guard cell tonoplast of Allium cepa (onion). We used
patch-clamp methodology with isolated tonoplast patches to study conduction
and gating at the single channel level. A voltage-dependent
outward-rectifying cationic channel (210 picosiemens) was the dominant
conductance. In symmetrical solutions the channel displayed an ohmic
behavior in its current-voltage relationship. It also showed a very large
rectification in the open probability. The channel was predominantly
cationic and its sequence of ionic selectivity was weak (Na+ > K+ >
Rb+ > Cs+). The channel conductance was not affected by intravacuolar
pH. Analysis of membrane patches with multiple channels showed that the
probability of a channel to open was independent of the opening of the
other channels present in the patch and that there was a conservation of
the open probability for different channels. Ensemble records generated
using a pulse protocol showed slow activation and deactivation kinetics. A
first-latency analysis of single-channel records in response to protocols
with different prepulse duration indicated that this channel has more than
one closed state.
