Protein kinase and phosphatase regulation during abscisic acid signaling and ion channel regulation in guard cells

The regulation of gas exchange between leaves and the atmosphere is controlled by stomatal pore apertures. Water vapor diffusion through stomatal pores is responsible for transpirational water loss of plants and therefore is detrimental to plants during drought stress (Mansfield et al., 1990). Plants adapt to changes in environmental conditions by adjusting the aperture of stomatal pores in the leaf epidermis. Guard cells, which in pairs surround stomatal pores, determine the stomatal pore aperture. Stomatal closing is mediated by potassium and anion efflux from guard cells and parallel malate metabolism (Raschke, 1979; MacRobbie, 1981). Environmental factors such as CO2 concentrations, humidity, temperature, light quality and intensity and the plant hormone abscisic acid affect stomatal movements. Abscisic acid (ABA), is synthesized in response to drought, and triggers the signaling cascade in guard cells leading to stomatal closing. Guard cells provide a well-suited system for characterizing the physiological functions of ion channels in higher plants and for unraveling very early signal transduction events. Several major classes of ion channels were identified by patch clamp studies of the plasma membrane of Vicia faba guard cells including outward-and inward-conducting K+ channels and anion channels (for reviews; Schroeder and Hedrich, 1989; Hetherington and Quatrano, 1991; Assmann, 1993; Ward et al., 1995). In addition, non-selective ABA-activated Ca2+-permeable ion channels (Schroeder, 1990) and several differing types of single stretch-activated channels (Cosgrove and Hedrich, 1991) were found. In a proposed model for roles of guard cell ion channels, stomatal opening and stomatal closing are accompanied by regulation of several plasma membrane ion channels and proton pumps in parallel (Schroeder and Hedrich, 1989). Stomatal opening requires K+ uptake (Raschke, 1979). Based on biophysical, pharmacological and cell physiological studies inward-rectifying K+ channels may provide a major pathway for enabling K+ uptake during stomatal opening (Schroeder et al., 1987). The driving force for channel-mediated K+ uptake can be generated by the plasma membrane proton pump, which is activated by light signals and inactivated by ABA (Assmann et al., 1985; Shimazaki et al., 1986; Goh et al., 1996). Calcium is known to reduce stomatal opening (DeSilva et al., 1985). In correlation to this physiological response, elevation in the cytosolic Ca2+ concentration inhibits both plasma membrane proton pump activities (Kinoshita et al., 1995) and inward-rectifying K+ channels (Schroeder and Hagiwara, 1989). This review will focus mainly on new findings pertaining to ion channel regulation during ABA-induced stomatal closing and to the proposed underlying signal transduction mechanisms.