Vanadate inhibition of stomatal opening in epidermal peels of Commelina communis : Cl(-) interferes with vanadate uptake.

An H(+) ATPase at the plasma-membrane of guard cells is thought to establish an electrochemical gradient that drives K(+) and Cl(-) uptake, resulting in osmotic swelling of the guard cells and stomatal opening. There are, however, conflicting results regarding the effectiveness of the plasma-membrane H(+)-ATPase inhibitor, vanadate, in inhibiting both H(+) extrusion from guard cells and stomatal opening. We found that 1 mM vanadate inhibited light-stimulated stomatal opening in epidermal peels of Commelina communis L. only at KCl concentrations lower than 50 mM. When impermeant n-methylglucamine and HCl (pH 7.2) were substituted for KCl, vanadate inhibition was still not observed at total salt concentrations≥50 mM. In contrast, in the absence of Cl(-), when V2O5 was used to buffer KOH, vanadate inhibition of stomatal opening occurred at K(+) concentrations as high as 70 mM. Partial vanadate inhibition was observed in the presence of the impermeant anion, iminodiacetic acid (100 mM KHN(CH2CO2H)2). These results indicate that high concentrations of permeant anions prevent vanadate uptake and consequently prevent its inhibitory effect. In support of this hypothesis, an inhibitor of anion uptake, anthracene-9-carboxylic acid, partially prevented vanadate inhibition of stomatal opening. Other anion-uptake inhibitors (1 mM 4,4-diisothiocyanatostilbene-2,2'-disulfonic acid, 1 mM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, 200 μM Zn(2+)) were not effective. Decreased vanadate inhibition at high Cl(-)/vanadate ratios may result from competition between vanadate and Cl(-) for uptake. Unlike metabolic inhibitors, vanadate did not affect the extent of stomatal closure stimulated by darkness, further indicating that the observed action of vanadate represents a specific inhibition of the guard-cell H(+) ATPase.