A Cationic Channel in the Guard Cell Tonoplast of Allium cepa

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.