SNAP exposure dramatically reduced the amplitude of IK,in. the control of guard cell movements. does not display a wilty phenotype (14). Therefore, although NO seems to play a role in water-stress signaling, its scenario within ABA-related signaling pathways and its relationship MF1 to ion transport that drives stomatal movement has remained unclear. ABA closes stomata by regulating guard cell membrane transport to promote osmotic solute loss. Among its actions, ABA increases cytosolic-free [Ca2+] ([Ca2+]i) and cytosolic pH (pHi); these signals inactivate inward-rectifying K+ channels (IK,in) to prevent K+ uptake and activate outward-rectifying K+ channels (IK,out) and Cl- (anion) channels (ICl) in the plasma membrane to facilitate solute efflux (9, 10, 17). To explore NO function in guard cells and its association with ABA transmission transduction, we recorded guard cell membrane current under voltage clamp and [Ca2+]i using fura 2 fluorescence percentage imaging. Our results demonstrate that NO promotes intracellular Ca2+ launch and therefore regulates guard cell ion channels via a subset of signaling pathways enlisted by ABA. Materials and Methods Flower Material and Electrophysiology. Protoplasts and epidermal pieces were prepared from L., and procedures were carried out Regadenoson on a Zeiss Axiovert microscope with 63 very long working range differential interference contrast microscopy optics (18, 19). Patch pipettes were pulled having a Narashige (Tokyo) PP-83 puller, and currents were recorded and analyzed as explained (18, 20). Voltage-clamp recordings and fura 2 injections of intact guard cells were carried out by impalement with two- and three-barrelled microelectrodes (19, 20). [Ca2+]i Measurements. [Ca2+]i was determined by fura 2 fluorescence percentage imaging having a GenIV-intensified Pentamax-512 charge-coupled device camera (Princeton Devices, Trenton, NJ) (20). Measurements were corrected for background before loading and analyzed with Common Imaging software (Press, PA). Fura 2 fluorescence was calibrated and after permeabilization (19). Estimations of loading indicated final fura 2 concentrations 10 M (19). Numerical Analysis. Currents from intact cells were recorded and analyzed with HENRY II software (Y-Science, Glasgow, U.K., Channel amplitudes were determined from point-amplitude histograms of openings 5 ms in duration beyond closed levels, and channel number, openings, and probabilities were determined as explained (18, 20). Results are reported as means SE. Chemicals and Solutions. Intact cells were bathed in 5 mM Ca-Mes, pH 6.1 [Mes titrated to its pKa with Ca(OH)2] with 10 mM KCl or 15 mM CsCl/15 mM tetraethylammonium-Cl to verify Cl- currents (21). Protoplasts were bathed in Ba2+-Hepes, pH 7.5 [Hepes buffer titrated to its pKa with Ba(OH)2] modified to 300 milliosmolar with sorbitol, and pipettes were filled with similar solutions. For cell-attached recording, pipette and bath contained 30 mM Ba2+; for whole-cell recording, pipettes contained 1 mM Ba2+ and (Mg2+)2ATP, and the bath contained 30 mM Ba2+; and for excised, inside-out patches, pipettes contained 30 mM Ba2+, and the bath contained 1 mM Ba2+ and (Mg2+)2ATP. guard cells under voltage clamp. Fig. 1 shows current traces and steady-state Regadenoson currentCvoltage curves from one guard cell recorded before and after a 60-s exposure to 10 M SNAP, yielding 10 nM NO per min. Voltage methods positive of -50 mV were designated by an outward current, standard of IK,out, that relaxed to a new steady state with half-times near 300 ms; methods Regadenoson bad of -120 mV offered an inward current.