Regulation of guard cell ion channels

Stomatal guard cells regulate the stomatal aperture to allow gas exchange whilst minimising water loss from the plant. Stomatal movements are driven by changes in the ion transport across the various membrane systems through ion channels and pumps. Complex signaling events are involved in the regulation of ion channels during stomatal movements. Whilst considerable progress has been made in the understanding of ion transport processes, some questions still remain. Regulation of guard cell ion channels by the gaseous free radical nitric oxide, (NO), was investigated using electrophysiological and Ca2+ imaging techniques. Treatment of intact guard cells with NO donor and scavenger compounds revealed that NO modulates the Ca2+-sensitive inward rectifier K+ channel (IK,in) and anion channel (Ici) by enhancing Ca2+ release from internal stores. The NO-enhanced Ca2+ release is via a cGMP / cADPR pathway, thus showing similarities with animal NO signalling. Furthermore, abscisic acid (ABA) regulation of Ik,in and Ici is abolished by the nitric oxide scavenger compounds, placing NO as a key component of ABA-mediated ion channel regulation. NO regulation of IK,in and Ici was abolished by the broad range protein kinase inhibitor staurosporine, which blocked the NO enhancement of Ca2+ release from internal stores. Stomatal movements also involve changes in volume and surface area, which must be accommodated by delivery and retrieval of membrane material. Fusion and fission of membrane material requires protein machinery called SNARE proteins, which include syntaxins. The discovery of a syntaxin, NtSyr1, which is involved in hormonal control of ion channel gating, along with patch clamp measurements of cell surface area, raised the possibility of interaction between membrane traffic and ion channel regulation. To explore this possibility further, experiments were carried out using pharmacological tools and transgenic plants expressing the cytosolic portion (SP2) of NtSyr1 under the control of a dexamethasone inducible promoter (dexSP2-14 plants). Experiments with the membrane traffic inhibitor brefeldin A (BFA) and the actin antagonist latrunculin B (LATB), showed coupling of lK,in channel activity with membrane traffic and both IK,in and the outward rectifier K+ channel (IK,out) with actin filaments. Furthermore, analysis of voltage evoked Ca2+ signals in dexSP2-14 plants showed that voltage evoked Ca2+ signals were altered by SP2 protein expression. These data, along with stomatal aperture measurements, suggest a coupling of membrane trafficking and cell volume regulation processes with ion channel control