Journal of Experimental Botany

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Journal of Experimental Botany, Vol. 52, No. 363, pp. 1959-1967, October 1, 2001
© 2001 Oxford University Press

Original Papers

The role of ion channels in light-dependent stomatal opening

Petra Dietrich^1,3, Dale Sanders² and Rainer Hedrich¹

¹ Julius-von-Sachs-Institut für Biowissenschaften, Lehrstuhl für Molekulare Pflanzenphysiologie und Biophysik, Julius-von-Sachs-Platz 2, D-97082 Würzburg, Germany
² The Plant Laboratory, Biology Department, University of York, PO Box 373, York YO10 5YW, UK

Received 12 April 2001; Accepted 19 June 2001

	Abstract

Top Abstract Introduction Blue light-dependent proton... Coupling of H+-ATPase and... The K+ channel-intrinsic pH... Stomatal opening in KAT1... Guard cell signal transduction References

 
Stomatal opening represents a major determinant of plant productivity and stress management. Because plants lose water essentially through open stomata, volume control of the pore-forming guard cells represents a key step in the regulation of plant water status. These sensory cells are able to integrate various signals such as light, auxin, abscisic acid, and CO₂. Following signal perception, changes in membrane potential and activity of ion transporters finally lead to the accumulation of potassium salts and turgor pressure formation. This review analyses recent progress in molecular aspects of ion channel regulation and suggests how these developments impact on our understanding of light- and auxin-dependent stomatal action.

Key words: Guard cell, light, auxin, K⁺ channel, H⁺-ATPase, 14-3-3 protein.

	Introduction

Top Abstract Introduction Blue light-dependent proton... Coupling of H+-ATPase and... The K+ channel-intrinsic pH... Stomatal opening in KAT1... Guard cell signal transduction References

 
Stomata optimize the uptake of CO₂ and concomitant loss of water vapour. This dynamic valve is based on a proper control of the turgor pressure in guard cells which, in pairs, surround the stomatal pore. During the past 400 million years of plant evolution the number of stomata dramatically increased. Phenomena, especially the step from microphylla to macrophylla about 360 million years ago, correlated with the drop in atmospheric CO₂-concentration (Beerling et al., 2001). Due to their inability to leave their habitats, plants developed strategies to circumvent the limitations in their surroundings. Under the selection pressure of dry and salty soils, or of low CO₂-concentration (high altitudes) a coevolution of different photosynthetic types and stomatal behaviour took place. C₃, C₄, and CAM plants are distinguished on the basis of their carbon metabolism and water-use efficiency. In addition, there have evolved C₃-C₄ intermediates and C₃ plants which switch to CAM when subjected to limited water supply or increased salt concentration. Today, many crop plants belong to the C₃ type. These plants lose about 600 molecules H₂O per fixed CO₂ molecule (Fig. 1), a ratio determined by infrared gas analysis (Fig. 1A). C₃ crops are cultured in more moderate climates, while C₄ and CAM plants, which are characterized by respective water-use efficiencies of about 100 and 10 molecules H₂O per CO₂ fixed, are found in the hot and dry regions of the earth (Fig. 1B). C₄ plants, which operate a more efficient CO₂ assimilation due to very low photorespiration rates, are able to assimilate similar carbon quantities as C₃ plants with reduced stomatal aperture and water loss.

View larger version (93K): [in this window] [in a new window]   Fig. 1. Gas exchange measurement on a plant leaf. (A) An infrared gas analyser determines changes in CO2 assimilation and H2O loss. A cuvette covers the defined leaf area under investigation. (B) Stoma embedded in the lower epidermis of a Vicia faba leaf. In this C3 species about 600 molecules H2O (blue) evaporate for one assimilated CO2 molecule (yellow).

 
CAM plants, which live in the desert, close their stomata during the day and open them at night when the water vapour gradient is less steep. The control of stomatal movement in these plant types has puzzled scientists for decades. Independently of whether they open in the light or dark, guard cell swelling and stomatal opening is accompanied by an accumulation of potassium salts (Fig. 2). While K⁺ uptake occurs mainly via highly selective K⁺ channels, Cl^- ions enter the cytoplasm probably via a H⁺/Cl^- symport mechanism. In addition, malate^2- is synthesized following starch breakdown. Depending on the growth conditions and time of the day, stomatal movement may also rely on sugar accumulation (Talbott and Zeiger, 1998; Tallman and Zeiger, 1988).

View larger version (171K): [in this window] [in a new window]   Fig. 2. Components of the osmotic motor. Cl- and K+ enter the guard cell cytoplasm and vacuole to drive turgor formation. In addition to Cl- uptake from the medium, malate2- synthesis contributes to anion accumulation. Following the osmotic gradient, water entry results in stomatal opening. AthH2, an Arabidopsis water channel expressed in guard cells, may contribute to water fluxes and is upregulated by blue light (Kaldenhoff et al., 1995). Figures 1 and 2 are reprinted from the scientific movie, ‘From Phenomenon to Molecule’ (R Hedrich and H Busch, 1998, produced by Institut für den Wissenschaftlichen Film IWF gem. GmbH, 1998). Order numbers are: C2014, C7034, C7041 for part I through III.

 
As an interface between the photosynthetic cells and the atmosphere, guard cells are equipped with a CO₂ sensor. When the CO₂ concentration in the substomatal cavity drops, for example, when photosynthetic activity is high, stomata open and enable the influx of CO₂. Likewise, at high CO₂ concentration in the absence of photosynthesis, stomata close and prevent water loss. It is thus tempting to speculate that guard cells sense light via the internal CO₂ concentration. Various studies, however, demonstrated that guard cells respond to the blue and red parts of the visible light spectrum (Serrano et al., 1988; Sharkey and Raschke, 1981). Red light-mediated stomatal opening is dependent on photosynthetic electron transport in the guard cell chloroplast. In contrast to red light, low quantum fluxes of blue light elicit stomatal opening, indicating that red (chlorophyll) and blue light receptors work together. This review focuses on the molecular components of the osmotic motor of guard cells, which in a light-dependent manner generates and accumulates osmotica to drive stomatal opening (Fig. 2; Raschke et al., 1988).

	Blue light-dependent proton pumping

Top Abstract Introduction Blue light-dependent proton... Coupling of H+-ATPase and... The K+ channel-intrinsic pH... Stomatal opening in KAT1... Guard cell signal transduction References

 
A prerequisite for channel-mediated K⁺-uptake during stomatal opening is a hyperpolarization of the membrane negative of the Nernst potential for potassium. This hyperpolarization can result from proton extrusion via the H⁺-ATPase (Assmann et al., 1985; Blatt, 1988; Lohse and Hedrich, 1992; Roelfsema et al., 1998), an electroenzyme residing in the guard cell plasma membrane at high density (Becker et al., 1993).

Only recently were the first recordings published of the membrane potential, together with ion channel activities, in guard cells of intact plants (Fig. 3; Roelfsema et al., 2001). Upon a dark–light transition the plasma membrane hyperpolarizes. When the photosynthetic capacity was saturated using a beam of red light, additional red light proved ineffective in modulating the membrane potential. By contrast, additional blue light could trigger a negative shift of the membrane potential. This blue light-dependent activation of the H⁺-ATPase demonstrates that guard cells in intact plants respond qualitatively and in a similar manner to blue light as isolated guard cell protoplasts (Assmann et al., 1985; Gotow et al., 1985; Schroeder, 1988; Shimazaki et al., 1986).

View larger version (141K): [in this window] [in a new window]   Fig. 3. In planta recordings of the guard cell membrane potential. Cartoon: double-barrelled microelectrodes are introduced into a guard cell of an intact Vicia faba plant. Dark–light transitions elicit changes in the membrane potential (lower trace) and ion channel activity (not shown). (Modified after Roelfsema et al., 2001.)

 
While it can be assumed that in the presence of continuous blue light the responsible photoreceptors desensitize (Iino et al., 1985; Roelfsema et al., 2001), the nature of the blue light receptor of guard cells is under debate (for review, see Assmann and Shimazaki, 1999). Although the Arabidopsis mutant hy4 (cry1) lacks the blue light-inhibition of hypocotyl growth, its blue light-dependent stomatal opening is not affected. CRY1 is therefore unlikely to function as the blue light receptor of guard cells (Eckert and Kaldenhoff, 2000; Frechilla et al., 1999; Lascève et al., 1999). Likewise, loss of function mutants with respect to the flavin- and pterin-containing photoreceptor CRY2 or the NPH1 (non-phototropic hypocotyl 1) protein kinase lacked a blue light-related stomatal phenotype (Lascève et al., 1999). Instead, the carotenoid zeaxanthin has been suggested to mediate blue light-sensitivity in guard cells (Zeiger, 2000). In line with this hypothesis, the absence of zeaxanthin in the Arabidopsis mutant npq1 correlated with the lack of blue light-sensitivity of mutant stomata in epidermal peels (Frechilla et al., 1999). A detailed infrared gas analysis (cf. Fig. 1A) on the light-sensitivity of stomata in various Arabidopsis mutants, however, could demonstrate that the blue light-dependent stomatal opening in npq1 was comparable to the wild type (Eckert and Kaldenhoff, 2000). Very recently, the NPH1 homologue NPL1 (NPH-like 1) has been shown to be required for the light avoidance response of chloroplasts in Arabidopsis leaves (Kagawa et al., 2001). Therefore, this blue light receptor or that responsible for the chloroplast accumulation response in weak light and homologues thereof may underlie blue light-dependent guard cell processes as well.

	Coupling of H+-ATPase and K+ uptake channels

Top Abstract Introduction Blue light-dependent proton... Coupling of H+-ATPase and... The K+ channel-intrinsic pH... Stomatal opening in KAT1... Guard cell signal transduction References

 
Accumulation of potassium during stomatal opening is thought to be mediated by inward-rectifying K⁺ channels which open upon hyperpolarization (Assmann and Haubrick, 1996; Grabov and Blatt, 1998a; Hedrich et al., 1998; MacRobbie, 1998; Thiel and Wolf, 1997; Ward and Schroeder, 1997; Willmer and Fricker, 1996; Zimmermann et al., 1999). Apoplastic K⁺ concentrations during stomatal opening decrease from about 15 mM in the dark to 3 mM in the light (Felle et al., 2000; Szyroki et al., 2001). Together with cytoplasmic K⁺ concentrations rising from about 100 to 400 mM (Raschke, 1979), stomatal opening will result in a negative shift of the Nernst potential for K⁺ (E_K), from about -60 to -130 mV. Irrespective of these changes in E_K, K⁺ uptake channels will be gated open with the same voltage-sensitivity (Blatt, 1992; Brüggemann et al., 1999a). Thus, only at membrane potentials negative of both the activation potential of the K⁺ channel and the E_K accumulation of K⁺ salts will take place.

In the light, average resting potentials of -112 mV have been measured in hyperpolarized guard cells of intact Vicia faba plants (Fig. 3; Roelfsema et al., 2001). In this hyperpolarized state of the guard cell which accompanies stomatal opening, the membrane potential lies only slightly negative of the activation potential of the K⁺ uptake channel, a voltage gradient sufficient for K⁺ accumulation via inward-rectifying K⁺ channels (Blatt, 1991; Roelfsema and Prins, 1998; Thiel et al., 1992). Besides this direct electrical coupling of the H⁺-ATPase and K⁺ uptake channels, regulatory mechanisms through pump-driven apoplastic acidification have been shown to alter the K⁺ uptake capacity. Impalement studies on guard cells within epidermal strips and patch-clamp analysis on protoplasts derived thereof have reported on the acid activation of K⁺ uptake channels (Blatt, 1992; Brüggemann et al., 1999b; Dietrich et al., 1998; Ilan et al., 1996). Therefore, voltage- and proton-dependent coupling of the H⁺-ATPase and K⁺ uptake channels seem to represent part of the mechanism underlying stomatal opening. Apoplastic acidification acts through a change in the voltage-sensitivity and number of K⁺ uptake channels (Blatt, 1992; Dietrich et al., 1998; Hoth et al., 1997a; Ilan et al., 1996). Assuming one titratable pH-sensitive site, a pK_a value of 5.3 was determined for K⁺ uptake channels in Vicia faba (Ilan et al., 1996). This value lies well within the range of light-induced apoplastic pH changes in this plant species (Fig. 4). A detailed comparison of the guard cell inward rectifier from different species revealed maximum pH-sensitivities (pK_a) at pH 6.2 in Solanum tuberosum and Nicotiana tabacum and pH 4.8 in Arabidopsis thaliana (Brüggemann et al., 1999b; Dietrich et al., 1998). K⁺ uptake into guard cells therefore seems to depend on the rate of H⁺ pumping and apoplastic pH buffer capacity of the individual species.

View larger version (118K): [in this window] [in a new window]   Fig. 4. In planta technique non-invasively to monitor changes in ion concentrations around stomata. Ion-selective microelectrodes are introduced through an open stoma. Changes in K+, Cl-, Ca2+, and H+ can be followed (Felle et al., 2000). Upon a light–dark transition, the H+-concentration of the stomatal apoplast decreases (lower trace), and vice versa, starting from pH 5.0 in the light-adapted Vicia faba leaf. Note that the arrow indicates an acidification by 0.5 pH units. pH-recording with kind permission by Hubert Felle, University of Giessen.

 

	The K+ channel-intrinsic pH sensor

Top Abstract Introduction Blue light-dependent proton... Coupling of H+-ATPase and... The K+ channel-intrinsic pH... Stomatal opening in KAT1... Guard cell signal transduction References

 
Insights into the molecular mechanism of the pH-dependence were obtained by a mutational analysis of the K⁺ uptake channel KST1, which is highly expressed in guard cells of Solanum tuberosum (Hoth et al., 1997a, b; Hoth and Hedrich, 1999a, b; Müller-Röber et al., 1995). Together with KAT1, the guard cell homologue cloned from Arabidopsis thaliana (Anderson et al., 1992; Nakamura et al., 1995), KST1 represents a member of a large plant K⁺ channel family (Fig. 5A; for review, see Hedrich and Dietrich, 1996; Hedrich et al., 1998). The common structure is characterized by six transmembrane domains, a pore region between the fifth and sixth membrane spanning segment, and cytoplasmic N- and C-termini (Fig. 5B; Becker et al., 1996; Dreyer et al., 1998; Hoth et al., 1997b; Marten and Hoshi, 1997; Nakamura et al., 1997; Uozumi et al., 1998). Assembly of four identical or even different subunits results in a functional channel protein (Daram et al., 1997; Dreyer et al., 1997; Ehrhardt et al., 1997; Pilot et al., 2001). Since KST1 and KAT1 activities very closely resemble the native K⁺ inward-rectifying currents from guard cells of Solanum tuberosum and Arabidopsis thaliana they were assumed to represent the molecular mechanism of K⁺ uptake into guard cells (Fig. 6; Bei and Luan, 1998; Brüggemann et al., 1999b; Dietrich et al., 1998; Ichida et al., 1997; Müller-Röber et al., 1995; Pei et al., 1997; Roelfsema and Prins, 1997). Besides similarities in voltage-dependence, kinetics and selectivity, the heterologously expressed K⁺ channels retained the pH-sensitivity of the functional K⁺ channels recorded in their natural environment. To identify the functional domain responsible for acid activation in KST1, two histidines, which are exposed to the extracellular face of the membrane, were mutated (Hoth et al., 1997a). When both histidines, one in the linker between S3 and S4 and one in the pore region (Fig. 5B), were replaced by alanines, the double mutant lost its pH-dependence. Further mutations within the fourth transmembrane segment (S4 in Fig. 5B) revealed the interaction of the pH sensor with this voltage-sensing domain (Hoth and Hedrich, 1999a). In line with the more acidic pK_a value for the guard cell K⁺ uptake channel in Arabidopsis, mutations of the pore histidine in KAT1 did not affect the pH sensitivity (Hoth and Hedrich, 1999a). Therefore, a more complex pH-sensing mechanism was postulated to account for acid activation in KAT1.

View larger version (28K): [in this window] [in a new window]   Fig. 5. Plant K+ channel family. (A) Phylogenetic tree of Shaker-like plant K+ channels (modified after Ache et al., 2000). Five subfamilies can be distinguished on the basis of their amino acid sequence homology. (B) Predicted structure of one K+ channel subunit. N- and C-terminus face the cytoplasmic side of the membrane; S, transmembrane segment; P, pore region.

 

View larger version (23K): [in this window] [in a new window]   Fig. 6. Electrophysiological recordings in vivo and after heterologous expression of guard cell channels. Upper graph: Whole-cell recording of a guard cell protoplast (left) revealing the activation of K+ uptake channels upon membrane hyperpolarization (right). Lower graph: Double-electrode voltage-clamp technique applied to a KST1-expressing Xenopus oocyte (left). Inward K+ currents similar to those observed in vivo (upper graph) can be resolved (right). (Modified after Brüggemann et al., 1999b.)

 
In parallel with the external acidification, a rise in intracellular H⁺-concentration was shown to precede auxin-induced stomatal opening (Gehring et al., 1990; Irving et al., 1992). Cytosolic acidification in response to auxin caused the activation of the guard cell K⁺ inward rectifier (Blatt and Thiel, 1994; Grabov and Blatt, 1997; Thiel et al., 1993) as well as KAT1 expressed in oocytes (Hoshi, 1995). The K⁺ uptake channel from guard cells of Arabidopsis thaliana and KAT1 are characterized by the same pH-dependent activation kinetics with a pK_a of about 6. Thus, the internal pH-sensor was also assumed to involve histidine residues in the respective K⁺ channel protein. A detailed mutagenesis approach with respect to all cytoplasmic histidines in KAT1 identified a histidine in the linker between S2 and S3 as part of the acid activation process (Tang et al., 2000).

	Stomatal opening in KAT1 knock out Arabidopsis

Top Abstract Introduction Blue light-dependent proton... Coupling of H+-ATPase and... The K+ channel-intrinsic pH... Stomatal opening in KAT1... Guard cell signal transduction References

 
Using the K⁺ channel inhibitor Ba²⁺ (Schroeder et al., 1987) attempts have been undertaken to determine the impact on the guard cell K⁺ inward rectifier on stomatal movement (Kelly et al., 1995). However, stomatal opening in Vicia faba could not be prevented by these methods. After it was shown that KAT1 is expressed in guard cells of Arabidopsis (Nakamura et al., 1995), a KAT1 channel mutant with enhanced Cs⁺-sensitivity was overexpressed in this species (Ichida et al., 1997). This approch generated a transgenic line with a higher sensitivity of stomatal opening toward this alkali metal. The isolation and phenotypical analysis of a KAT channel knockout mutant revealed, however, that mutant stomata, deficient in this K⁺ uptake channel, open in response to light or low CO₂ concentrations and close in the presence of ABA similarly to wild type (Szyroki et al., 2001). Thus, KAT1 appears not to be essential for stomatal movement. In a search for K⁺ channels which compensate the KAT1 defect, real-time RT-PCR on isolated guard cell protoplasts identified additional K⁺ channel transcripts (Szyroki et al., 2001). In wild type and KAT1 knockout Arabidopsis, the voltage-dependent K⁺ channels KAT2 (Pilot et al., 2001) and AKT1 together with the largely voltage-independent channel AKT2/3 and K⁺ channels of unknown voltage-dependence (AtKC1, AKT5, and AKT6) colocalize in guard cells. Compared to KAT1 in the wild type, the other K⁺ channel genes are expressed at lower levels, leading to a severe decrease of inward K⁺ currents in guard cell protoplasts of KAT1 knockout plants. Therefore, it can be concluded that multiple K⁺ channels provide for a K⁺ channel homeostasis. This guarantees a proper function of these sensory cells at the interface between the plant body and the environment.

	Guard cell signal transduction

Top Abstract Introduction Blue light-dependent proton... Coupling of H+-ATPase and... The K+ channel-intrinsic pH... Stomatal opening in KAT1... Guard cell signal transduction References

 
While reports accumulate on cytosolic Ca²⁺- and pH-changes associated with ABA- and CO₂-induced stomatal closure, the role of these signals for light- and auxin-stimulated stomatal opening is still unclear (Assmann, 1999; Assmann and Shimazaki, 1999; MacRobbie, 1998). When K⁺ currents before and after a light-induced hyperpolarization were compared in guard cells of intact Vicia faba plants, a small decrease in inward K⁺ currents was observed, while currents mediated by depolarization-activated K⁺ channels remained unaltered (Roelfsema et al., 2001). Since the latter channel type is affected by cytoplasmic pH-changes too (Blatt, 1992; Ilan et al., 1994), it might be concluded that light treatment does not significantly act through changes in cytoplasmic pH in planta. Inhibition of inward- but not outward-rectifying K⁺ channels after light treatment is thus in line with an increase in Ca²⁺-concentration during stomatal opening (Blatt et al., 1990; Luan et al., 1993). Likewise, cytoplasmic Ca²⁺-signals have been proposed to precede auxin-induced stomatal opening (Irving et al., 1992).

Calcium
Ca²⁺-permeable channels in the plasma membrane have been shown to be involved in ABA-induced stomatal closure (Grabov and Blatt, 1998b, 1999). However, besides the obvious contribution of vacuoles to guard cell osmotic relationships, vacuoles might impact significantly on signalling processes by virtue of their ability to store Ca²⁺. A surprisingly wide array of Ca²⁺-permeable channels is present in the vacuolar membrane. Voltage-gated Ca²⁺ channels, activated by membrane hyperpolarization are quite highly selective for Ca²⁺ over K⁺, suggesting a role in physiological Ca²⁺ mobilization (Allen and Sanders, 1994b). Channel activity is decreased as a function of luminal pH, and it is therefore noteworthy that modest alkalinization of guard cell vacuoles has been reported during stomatal opening (Bowling and Edwards, 1984).

Cation-selective channels that are activated by cytosolic free Ca²⁺ may potentially catalyse a Ca²⁺-activated pathway for cation release from the vacuole. These slowly-activating vacuolar (SV) channels (Hedrich and Neher, 1987) were first discovered in vacuoles isolated from barley mesophyll protoplasts (Hedrich et al., 1986), but have been shown subsequently to be ubiquitously distributed among plant cell vacuoles (Allen and Sanders, 1997; Hedrich et al., 1988). One attractive model, first proposed by Ward and Schroeder, envisages that Ca²⁺ might activate vacuolar Ca²⁺ release through SV channels, thereby eliciting Ca²⁺-induced Ca²⁺ release (Ward and Schroeder, 1994). This would endow the channel with a role in amplifying Ca²⁺ signals, perhaps as they initiate at the plasma membrane or through ligand-gated vacuolar channels. Nevertheless, a critical question remains concerning whether SV channels are sufficiently Ca²⁺ permeable to contribute to Ca²⁺ mobilization and do have sufficient activity at the negative membrane voltages (Allen et al., 1998; Bewell et al., 1999; Pei et al., 1999; Potossin et al., 1997; Schulz-Lessdorf and Hedrich, 1995). Therefore, the potential role of SV channels in all of these responses remains contentious, even among the authors of this review.

Redox state
Redox agents might also serve to co-ordinate transport activities at the two membranes. Guard cell Ca²⁺ channels in the plasma membrane are effectively activated by H₂O₂ (Pei et al., 2000), while reducing agents activate vacuolar SV channels (Carpaneto et al., 1999). Hydrogen peroxide is also a potent inhibitor of InsP₃- and cADPR-gated Ca²⁺ channels (GD Dickinson and D Sanders, unpublished observations). Again, the physiological significance of these observations has yet to be rationalized. It seems possible that the redox state effectively switches the poise of the cell between states in which extracellular Ca²⁺ is used as a primary supply of Ca²⁺ for signalling (oxidizing conditions) and states in which intracellular Ca²⁺ stores are mobilized (reducing conditions).

InsP₃ and cADP ribose
In addition to these voltage-dependent channels, ligand-gated Ca²⁺ release channels have been identified in the vacuole. In guard cells, inositol 1,4,5-trisphosphate- (InsP₃-) gated channels can mobilize vacuolar Ca²⁺ (Allen et al., 1995; Allen and Sanders, 1994a; Gilroy et al., 1990). Distinct and potent gating of Ca²⁺ channels by cyclic ADP-ribose (cADPR) has suggested a role for ryanodine receptor-like channels in Ca²⁺ release from guard cell vacuoles, and this notion is supported by the findings that antagonists of cADPR production are inhibitors of stomatal closure in response to ABA (Leckie et al., 1998). The relative roles of InsP₃ and cADPR in controlling vacuolar mobilization of Ca²⁺ in guard cells during the stomatal closing response have yet to be explored.

14-3-3 proteins
14-3-3 proteins, which activate tomato-cell outward rectifying K⁺ channels and plasma membrane H⁺ pumps in a variety of tissues (Baunsgaard et al., 1998; Booij et al., 1999) and down-regulate mitochondrial and chloroplast ATP synthases (Bunney et al., 2001), also markedly reduce currents through SV channels in mesophyll cells (van den Wijngaard et al., 2001). For the guard cell H⁺-ATPase from Vicia faba the involvement of 14-3-3 proteins in the blue light-dependent activation of the pump could be demonstrated (Kinoshita and Shimazaki, 1999). Thereby, the binding of 14-3-3 proteins to the phosphorylated autoinhibitory C-terminal domain prevents its interaction with the catalytic site leading to a high-activity state of the H⁺-ATPase. In addition to blue light, hyperactivation of the plasma membrane H⁺-ATPase and stomatal opening are also observed after treatment with the fungal elicitor fusicoccin (Blatt, 1988). In contrast to blue light-stimulation, fusicoccin seems to stabilize the complex between H⁺-ATPase and 14-3-3-protein even in the absence of a phosphorylated residue (Baunsgaard et al., 1998; Fullone et al., 1998). Fusicoccin has been shown to inhibit guard cell outward K⁺ currents (Blatt and Clint, 1989), but a mechanism involving down-regulation via dissociation of 14-3-3 proteins has not yet been analysed. Therefore, details of any co-ordinating role for 14-3-3 proteins have yet to be established. Only recently, the guard cell K⁺ outward rectifier, GORK, has been cloned (Ache et al., 2000) enabling the analysis of how 14-3-3 proteins are involved in guard cell ion channel regulation and stress management.

	Acknowledgments

 
RH gratefully acknowledges funding through the DFG (SFB 251).

	Notes

 
³ To whom correspondence should be addressed. Fax: +49 931 888 6157.

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Top Abstract Introduction Blue light-dependent proton... Coupling of H+-ATPase and... The K+ channel-intrinsic pH... Stomatal opening in KAT1... Guard cell signal transduction References

 
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