Modulation and block of the plasma membrane anion channel of guard cells by stilbene derivatives

An anion channel in the plasma membrane of guard cells (GCAC1) provides a regulatory element for the voltage-dependent release of anions during stomatal closure (Keller et al. 1989) as well as excitability (Hedrich et al. 1990). Recognition sites for plant growth hormones on the extracellular surface of GCAC1 further indicate that this channel may also serve as a transduction element in hormone signaling (Marten et al. 1991 a). Stilbene derivatives were used to study the inhibitor-structure channel-function relationship of GCAC1. We have analyzed the activity, voltage-gate and kinetics of this channel as affected by stilbenes. The stilbene derivatives SITS and DNDS caused a shift in activation potential and a decrease in the peak current amplitude. Channel block through the action of DIDS, on the other hand, was not accompanied by a shift in voltage-dependence. Differences in the dose-dependence of the two effects give clues to the presence of channel sites responsible for gate-shifting and block. The ability to inhibit anion currents (K_d) increased in the sequence: SITS (4 µM) < DNDS (0.5 µM) < DIDS (0.2 µM). All inhibitors reversibly blocked the anion channel from the extracellular side. Channel block on the level of single anion-channels is characterized by a reduction of long open-transitions into flickering bursts and a decrease in channel amplitude.Abbreviations DIDS 4,4-Diisothiocyanostilbene-2,2-disulfonic acid - SITS 4-Acetamido-4-isothiocyanostilbene-2,2-disulfonic acid - DNDS 4,4-dinitrostilbene-2,2-disulfonic acid - NPPB 5-Nitro-2-(3-phenylpropylamio)benzoic acid - IAA-94 (6,7-Dichloro2-cyclopentyl-2,3-dihydro-2-methyl-1-oxo-1H-inden-5y1)oxy] acetic acid - A-9-C Anthracene-9-carboxylic acid - TEA Tetraethylammonium