Download Roles of ion channels and transporters in guard cell signal

FEBS Letters 581 (2007) 2325–2336
Minireview
Roles of ion channels and transporters in guard cell signal transduction
Sona Pandey, Wei Zhang, Sarah M. Assmann*
Biology Department, Penn State University, 208 Mueller Laboratory, University Park, PA 16802, United States
Received 27 February 2007; revised 3 April 2007; accepted 3 April 2007
Available online 17 April 2007
Edited by Julian Schroeder and Ulf-Ingo Flügge
Stomata are microscopic pores in the epidermes of aerial
plant parts that allow both CO2 influx for photosynthesis
and water vapor loss. Stomatal apertures are regulated by
the reversible swelling and shrinking of the surrounding pair
of guard cells (GC), which thereby control rates of gas exchange [1]. During stomatal opening, an increase in guard-cell
volume (swelling) is driven by uptake and intracellular generation of solutes, which decreases guard cell water potential
and creates a driving force for water uptake into the guard
cells. Due to the radial reinforcement of the guard cell walls,
the resultant increase in turgor causes the two guard cells of
the stomate to separate, widening the stomatal pore. During
stomatal closure, there is a reduction in guard cell solute content and volume, which results in guard cell deflation and a
narrowing of the stomatal aperture.
Since mature guard cells lack plasmodesmata, all solute uptake and efflux must occur via ion channels and ion transporters situated in the plasma membrane (PM). Ion channels are
proteins that mediate energetically downhill ion fluxes via
ion movement through a regulated proteinaceous pore. Transporters include pumps, which use energy directly, usually from
ATP, to drive transport against a free energy gradient, as well
as carriers, symporters, and antiporters. Rates of solute flux
through transporters are orders of magnitude slower than
fluxes through channels. Ion currents, channels, and transporters discussed in this review are summarized in Table 1.
The suite of transport events that operates during stomatal
opening and closure has been extensively studied. Models
highlighting the ion channels and transporters involved in stomatal opening and closure are given in Fig. 1. During stomatal
opening, H+-ATPase-mediated H+ efflux from the cytosol
hyperpolarizes the membrane potential beyond the equilibrium
potential for K+ and activates voltage-regulated inward K+
channels, leading to K+ influx. Malate2 production from
starch breakdown provides the major anionic species that
accumulates during stomatal opening. Transporter-mediated
uptake of Cl and NO
3 can also contribute to intracellular solute buildup, as can import or synthesis of sugars. During stomatal closure, membrane depolarization occurs due to
inhibition of H+-ATPase activity and activation of anion channels that mediate passive efflux of Cl, malate2, and NO
3.
Membrane depolarization creates a driving force for K+ efflux
via outwardly-rectifying K+ channels that are activated by
depolarization. Elevation of cytosolic free Ca2+ concentration
2+
2+
ðCa2þ
cyt Þ via Ca -permeable channels at the PM as well as Ca release channels situated in endomembranes is frequently
observed to precede or accompany stomatal closure. As large
amounts of cell solutes are stored in the vacuole, coordinated
regulation of solute fluxes at the PM and tonoplast is especially
central to turgor control in guard cells [2].
Our knowledge of the roles of ion channels and transporters
in the control of stomatal aperture came initially from electrophysiological and pharmacological studies. These channels
and transporters have distinctive current/voltage relationships,
some of which are illustrated in Fig. 2. Recent work has also
elucidated the molecular basis of some of these fundamental
processes. This review briefly describes our current understanding of the properties and regulation of guard cell ion
channels and transporters of the plasma membrane and tonoplast, and their roles in known signal transduction pathways.
Models depicting the essential signaling proteins and secondary messengers involved in regulating guard cell ion transport
are shown in Fig. 3.
*
Corresponding author. Fax: +1 814 865 9131.
E-mail address: [email protected] (S.M. Assmann).
2. Plasma membrane ion transporters and channels
Abstract Stomatal complexes consist of pairs of guard cells
and the pore they enclose. Reversible changes in guard cell volume alter the aperture of the pore and provide the major regulatory mechanism for control of gas exchange between the plant
and the environment. Stomatal movement is facilitated by the
activity of ion channels and ion transporters found in the plasma
membrane and vacuolar membrane of guard cells. Progress in recent years has elucidated the molecular identities of many guard
cell transport proteins, and described their modulation by various
cellular signal transduction components during stomatal opening
and closure prompted by environmental and endogenous stimuli.
Ó 2007 Federation of European Biochemical Societies. Published
by Elsevier B.V. All rights reserved.
Keywords: Guard cell; Ion channel; Ion transporter; Signal
transduction; Stomata
1. Introduction
Ca2þ
cyt ,
2+
Abbreviations:
Cytosolic free Ca concentration; pHcyt, cytosolic pH; GC, guard cell; PM, plasma membrane; ROS, reactive
oxygen species; CNGC, cyclic nucleotide gated channel; GLR,
glutamate receptor family protein
2.1. H+-ATPases: properties and regulation
H+-ATPases belong to the family of P-type ATPases and are
functional equivalents of the Na+/K+-ATPases of animals. In
0014-5793/$32.00 Ó 2007 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.febslet.2007.04.008
2326
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
Table 1
Ion currents, ion channels, and ion transporters discussed in this review, listed in the order mentioned in the text
Name of current, channel, or transporter
+
(PM) H -ATPase
Dual affinity NO
3 transporter
(PM) inward K+ channel
(PM) weakly rectifying K+ channel
(PM) outward K+ channel
(PM) Slow (S-type) anion current
(PM) Rapid (R-type) anion current
ABC transporter that modulates S-type currents and
hyperpolarization-activated Ca2+currents
(PM) Hyperpolarization-activated Ca2+ current
Cyclic nucleotide-gated Ca2+-permeable channel
Glutamate receptor family (may conduct Ca2+ and/or activate a
Ca2+ conductance)
(V) V-ATPases
(V) V-PPases
þ
(V) 2NO
3 =H antiporter
Ca2+ ATPases
Ca2+/H+ antiporters
(V) FV cation currents
(V) VK K+ current
(V) SV K+, Ca2+ currents
(V) InsP3-, InsP6, and cADPR-activated Ca2+ release channels
(V) CDPK-activated anion current
Genes encoding the transport function in Arabidopsis
AHA1-AHA11
AtNRT1.1 (CHL1)(also NRT1.2,and a family of NRT2 genes)
KAT1, KAT2, AKT1, AKT5, SPIK, AKT2/3, ATKC1
AKT2/3
GORK, SKOR
NA (Possibilities are >120 ABC transporter genes and seven
AtCLC genes)
NA
AtMRP5
NA
20 CNGC genes
20 GLR genes
26 VHA genes which together encode the 12 different V-ATPase
subunits
AVP1, AVP2, AVPL1
AtCLCa (in total 7 AtCLC genes)
ACA1-ACA11
CAX1-CAX10
NA
KCO/TPK1 (in total 6 KCO/TPK genes)
TPC1
NA
NA
PM = plasma membrane; V = vacuolar membrane (tonoplast). If not enough information is available to definitively localize the entire gene family, or
if different family members are known to localize to multiple membrane types, then no intracellular location is listed. NA = no information available.
See Fig. 1 for information on which of these genes/proteins have been detected in guard cells to date.
Fig. 1. Ion channels and transporters functioning in stomatal movements. The left stomate shows transport proteins active during stomatal opening
and the right stomate shows transport proteins active during stomatal closure.
plants, H+-ATPases are encoded by multi-gene families with
11 and 10 genes present in the fully sequenced genomes of Arabidopsis and rice respectively. All 11 of the Arabidopsis genes
are expressed in guard cells [3]. These proteins may have
redundant functions or they may achieve functional specificity
at the signal transduction level (specific pathways, interaction
partners, etc.).
During stomatal opening, membrane hyperpolarization
facilitates K+ entry into the guard cells. Hyperpolarization is
driven by the activity of H+-ATPase pumps located in the
PM [4,5]. The electrochemical gradient thus generated acts as
a driving force for a number of other ion/solute fluxes. Irreversible activation of the H+-ATPase by the fungal phytotoxin
fusicoccin leads to inability of stomates to close and results in
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
2327
Fig. 2. Idealized current/voltage relationships of guard cell plasma membrane ion channels and the plasma membrane H+-ATPase. (A) Inward
potassium current (cf. [70]). (B) Outward potassium current (cf. [70]). (C) S-type anion current (cf. [67,70]). (D) R-type anion current (cf. [92]). (E)
Calcium current (cf. [117]). (F) H+-ATPase pump current (cf. [157]). Currents are color-coded according to the color scheme of Fig. 1.
Fig. 3. Models of guard cell signal transduction pathways affecting transport. Only regulators that have been demonstrated to participate in
modulation of transport functions are shown, so the models show only a subset of the current information on guard cell signal transduction (cf. [57]
for more information) and are depicted in a simplified, largely linear fashion. (A) Model depicting ion channels and transporters that are active
during stomatal opening, and the regulatory proteins and secondary messengers which regulate them. (B) Model depicting ion channels and
transporters that are active during stomatal closure, and the regulatory proteins and secondary messengers which regulate them. Note that although
dominant negative abi1-1 and abi1-2 mutations render stomata ABA-hyposensitive, recessive mutations in these genes render stomata ABAhypersensitive [158]. For this reason, the ABI1 and ABI2 proteins are depicted as negative regulators of ABA response.
leaf wilting, emphasizing the importance of H+-ATPases in the
regulation of stomatal movement. In guard cells, H+-ATPase
activity is regulated by blue light, auxin, ABA and Ca2+.
2.2. H+-ATPases: roles in guard cell signal transduction
Blue light-activation of PM H+-ATPases is one of the best
studied processes in guard cell function. Studies in Vicia faba
2328
and Arabidopsis show that blue light activates the H+-ATPase
through a phosphorylation/ dephosphorylation-based mechanism [6,7]. When inactive, the C terminus of the H+-ATPase
acts as an autoinhibitory domain. Upon blue light illumination, a type 1 protein phosphatase is activated [8] which, via
an unknown mechanism, activates a protein kinase that phosphorylates the C terminus of the H+-ATPase [9–11]. Consequent H+-ATPase binding with 14-3-3 protein results in
displacement of the auto-inhibitory domain and activation of
the H+-ATPase [11] (Fig. 3).
Identification of the photoreceptors that perceive the blue
light signal upstream of the type 1 protein phosphatase came
when studies with the double blue light-receptor mutant phot1
phot2 demonstrated essentially no blue light-stimulated stomatal opening in this mutant [12]. Compared to wild-type plants
or single phot1 or phot2 mutant plants, guard cell protoplasts
from phot1 phot2 double mutant plants show severely reduced
blue light-induced proton pumping and ATP hydrolysis, and
loss of 14-3-3-protein binding, confirming a sequence of events
starting at perception of blue light by PHOT1 PHOT2 receptors leading to 14-3-3 binding, H+-ATPase activation, H+
extrusion and stomatal opening [3,12]. In addition, the cryptochrome blue light receptors Cry1 and Cry2 have also been proposed to regulate blue light-induced stomatal opening
collectively with PHOT1 PHOT2, however their roles in ion
transport regulation have not been demonstrated [13].
Similar to blue light, auxins also cause swelling of guard
cells, leading to opening of the stomatal pores, and auxins have
been shown to increase ATP-dependent pump currents across
the guard cell PM [14,15]. Conversely, abscisic acid (ABA)
strongly inhibits light-stimulated stomatal opening. ABA also
inhibits blue light-stimulated H+ pumping and ATP hydrolysis, via inhibition of known pathway components such as
H+-ATPase phosphorylation and binding of 14-3-3 protein
[16]. These four effects were antagonized by ascorbate, a scavenger of reactive oxygen species (ROS), suggesting that ROS
production is an early element in this signaling cascade
(Fig. 3). Indeed, ABA stimulates ROS production in GC
[17,18]. Oscillatory increases in Ca2þ
cyt are another consequence
of ABA exposure, and elevated Ca2+ concentrations inhibit
H+ pumping and ATP hydrolysis in guard cell microsomes
[19].
2.3. Anion transporters
It has been known for several decades that guard cells can
take up Cl ions during stomatal opening [20], but the molecular basis of this mechanism is still unknown. The electrochemical gradient for Cl across the membrane dictates that
Cl uptake is an energy-requiring process, and it is hypothesized that influx occurs via H+/anion symporters or OH/anion antiporters.
NO
3 uptake can also contribute to stomatal opening, particularly when Cl is not available [21], as is evident from studies
with Arabidopsis mutants of the nitrate transporter, AtNRT1.1
(CHL1), which mediates both high and low-affinity NO
3 uptake. The chl1 mutant shows reduced stomatal opening and
guard cell NO
3 content when NO3 is provided in the medium
in the absence of Cl , indicating that AtNRT1.1 is the transporter responsible for NO
3 uptake by guard cells. When grown
in NO
3 -containing substrates, chl1 mutant plants show reduced
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
stomatal opening and decreased rates of water loss as compared
with wild-type plants, confirming the importance of guard cell
NO
3 uptake by AtNRT1.1 under these conditions [21].
2.4. K+ channels: properties and regulation
Plant K+ channels expressed at the GC PM play a major role
in the K+ uptake and release that modulates GC turgor and volume [22,23]. Plant K+ channels were first identified in Vicia faba
GC by electrophysiological studies [24,25]. The Arabidopsis K+
channels AKT1 and KAT1 were the first plant ion channels to
be identified at the molecular level [26–28]. Nine K+-channel
genes have now been identified in Arabidopsis: KAT1, KAT2,
AKT1, AKT5, SPIK, AKT2/3, AtKC1, SKOR and GORK [29].
Plant K+ channels exhibit significant sequence and structural
similarity with voltage-dependent Shaker-type ion channels of
animals; however, while Shaker channels are outwardly-rectifying, individual plant K+ channels may be inwardly-rectifying,
outwardly-rectifying, or weakly rectifying (Fig. 2). The inward
K+ channels KAT1, KAT2, AKT1, the outward K+ channel
GORK, the K+ channel AKT2/3, whose rectification properties
are dependent on its phosphorylation status [30], and the subunit AtKC1, which forms a heteromeric channel with KAT1
or AKT1 [31,32] are expressed in guard cells.
Shaker channels in animals function as tetramers, and coassembly of different monomers can affect channel characteristics. Because the individual Arabidopsis K+ channels that are
expressed in guard cells form functional channels when expressed in heterologous systems, it seems that these plant channels can form homomers. However, from analyses in
heterologous systems, interactions have been found between
the inward rectifiers KAT1 and AKT1, KAT1 and AtKC1
[31], AKT2 and KAT1 [33], KAT1 and KAT2 [34], AKT1
and AKT2, AKT1 and AtKC1, AKT2 and AtKC1 [29], and
such interactions can affect the current magnitude of the co-expressed channel [33].
Evidence for the importance of K+ channel heteromerization
in planta is provided by the observation that inward K+ channels from akt2/3 knockout plants are still detected, but these
channels have lost susceptibility to voltage-dependent channel
block by Ca2+ [35]. This result implies that, in wild-type guard
cells, heteromerization of other inward K+ channels with
AKT2/3 is responsible for production of the characteristic inward K+ current that is subject to Ca2+ block. K+ channel beta
subunits such as KAB1 may also modulate K+ current characteristics, possibly via interaction with KAT1, similar to some
of their mammalian homologs [36].
2.5. K+ channels: roles in guard cell signal transduction
2.5.1. Identification of specific K+ channel proteins that
mediate GC K+ currents. K+ channels mediate K+ uptake and
release during stomatal movements. Thus, knockout mutation
of GORK, which encodes the solitary Kþ
out channel expressed in
guard cells, results in elimination of outwardly-rectifying K+
currents and impaired stomatal closure [37]. Overexpression
of a mutant version of KAT1 with reduced susceptibility to
Cs+ block of this inward K+ channel results in decreased Cs+
inhibition of light-induced stomatal opening [38]. Conversely,
dominant negative point mutations of KAT1 reduce Kþ
in currents and impair light-induced stomatal opening [39]. These results suggest that KAT1, either as a homomer or a heteromer,
plays a key role in K+ uptake during stomatal opening. How-
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
ever, kat1 knockout plants show essentially normal inward K+
currents and stomatal opening [40] possibly because in the absence of KAT1, other inward K+ channels are upregulated;
this does not appear to occur at the transcriptional level [40]
but could occur at the functional, i.e. post-transcriptional level. Additional regulation of K+ channels by trafficking and
relocalization may also be an important determinant [41]. Recent elegant studies with transiently expressed GFP-tagged
KAT1 and dominant negative SNARE fragments show that
KAT1 is localized at the PM in punctuate structures and that
syntaxins regulate the trafficking and clustering of these channels [41]. Actin cytoskeleton stability also plays a role in K+
channel regulation: drugs that induce actin cytoskeleton depolymerization (cytochalasin D) increase inward K+ currents in
Vicia faba guard cells, whereas phalloidin, which stabilizes
the actin cytoskeleton, inhibits these currents [42,43], and perhaps these effects reflect alterations in K+ channel trafficking
(Fig. 3).
2.5.2. Regulation of GC K+ currents by Ca2þ
cyt and cytosolic
pH (pHcyt). Inward K+ currents of guard cells are inhibited
2+
by elevation in Ca2þ
entry
cyt . In a separate mechanism, Ca
from the apoplast at hyperpolarized membrane potentials
physically blocks the channel lumen, reducing K+ influx
[35,44] (Figs. 1 and 3). Among Kþ
in channels, expression in heterologous systems demonstrates that KAT1 is activated by
acidification of either cytosol or apoplast [45]; KAT2 is activated by external acidic pH [34], and channels of the AKT2
subfamily are inhibited by acidification as a result of H+ block
of the channel pore [46,47]. In planta, whole cell inward K+
currents of V. faba guard cells are enhanced by external acidification but show little response to changes in internal pH
[48,49].
Outward K+ currents of guard cells are insensitive to Ca2+regulation, but show distinct regulation by pH (Fig. 3). When
expressed in Xenopus oocytes, GORK activity is inhibited by
external acidification [50], and studies of guard cell outward
K+ channels in whole cells and membrane patches similarly
show that availability of the outward K+ channels for activation by depolarization is decreased by external acidification
[48,51]. Conversely, outward K+ currents of guard cells are
promoted by cytosolic alkalinization via a membrane delimited pathway, i.e. pHcyt regulation of outward K+ channel
availability can be observed in isolated guard cell membrane
patches [48,52,53].
2.5.3. Regulation of GC K+ currents by kinases and
phosphatases. Experiments using phosphatase and kinase
inhibitors during whole cell recordings have long suggested
the importance of phosphorylation/dephosphorylation in the
regulation of plant K+ channels, e.g. the guard cell inward
K+ current was reduced in response to protein phosphatase
2B inhibitors [54]. Moreover, protein phosphatase 1 and 2A
inhibitors, calyculin A and okadaic acid, inhibited inward
and outward K+ currents in V. faba guard cells [55].
Biochemical evidence for the role of kinases in K+ channel
regulation was revealed by phosphorylation of KAT1 by a calcium-dependent protein kinase (CDPK) in V. faba guard cells
[56]. The behavior of heterologously expressed AKT2 channels
is modulated by phosphorylation, as activators of PKA type
kinases abolish inward rectification [30].
2.5.4. Regulation of GC K+ currents by ABA-initiated
signaling cascades. ABA-induced stomatal aperture regulation is one of the best characterized signal transduction path-
2329
ways in guard cells [57]. Loss-of-function mutants in two of
the three recently identified ABA-receptors show impairment
of guard cell function [58–60]. ABA inhibition of GC inward
K+ currents is very well elucidated [61], while ABA-promotion
of outward K+ currents is observed in some experimental systems [61,62]. Channel relocalization mediated by syntaxins
might comprise one component of K+ channel regulation,
since addition of a truncated form of Nt-SYR1 syntaxin to tobacco guard cells suppresses the effect of ABA on both inward
and outward K+ currents [63].
ABA response is typically although not invariably followed
by elevation of pHcyt and oscillation of Ca2þ
cyt in the guard cells
[64] (Fig. 3). Increase in pHcyt possibly occurs via increased
vacuolar H+ uptake [65] and results in an increase in the availability of voltage-activated outward K+ channels in the PM,
promoting K+ efflux and ABA-induced stomatal closure. An
2+
increase in Ca2þ
influx from outcyt could either occur from Ca
side the cell and/or Ca2+ release from internal stores, and
inhibits inward K+ currents (Figs. 1, and 3).
Guard-cell K+ currents showed reduced sensitivity to ABA
in transgenic plants overexpressing dominant negative alleles
of the protein phosphatase 2C gene, ABI1 [66,67], showing
involvement of protein phosphatases in K+ channel regulation.
Heterotrimeric G-proteins are also involved in regulation of
inward K+ currents during ABA signaling in guard cells. The
first evidence for a role of G proteins in inhibition of inward
K+ channels in guard cells came from electrophysiological
studies in which GTP and GDP analogues were introduced
to the cytosol of Vicia faba guard cell protoplasts [68,69]. Arabidopsis plants lacking the Ga subunit of G proteins (GPA1)
show reduced inhibition of inward K+ currents, consistent with
reduced guard cell ABA-insensitivity in stomatal opening [70].
Sphingosine-1-phosphate (S1P) [71] and related sphingosine
metabolites appear to function upstream of GPA1 in this signaling cascade: ABA stimulates the activity of sphingosine kinase, which generates S1P, and S1P application inhibits inward
K+ currents in wild-type but not gpa1 guard cells [72] (Fig. 3).
S1P is a known ligand of G-protein-coupled receptors in mammals, although a S1P receptor in plants remains to be identified.
Production of ROS such as hydrogen peroxide (H2O2) and
the reactive nitrogen species nitric oxide (NO) in response to
ABA are recently discovered signaling mechanisms in guard
cells [73–76]. Pharmacological manipulations coupled with
mutant analysis of both ROS and NO pathways indicate that
ABA-induced H2O2 production stimulates NO production and
NO is required for H2O2 induced-stomatal closure. Conversely, NO does not stimulate H2O2 production in guard cells
nor requires H2O2 for its effect on stomatal closure thus NO
possibly acts downstream of ROS production [77]. NO-stimulated intracellular Ca2+-release inhibits inward K+ currents in
a phosphorylation-dependent manner. Broad range protein kinase inhibitors inhibit intracellular Ca2+ release and render inward K+ currents insensitive to pharmacologically produced
NO [74], suggesting that NO-induced Ca2+ release is kinasedependent, and that the NO effect on inward K+ channels is
due to Ca2+-based inhibition [78]. Thus, the overall signaling
cascade appears to be a pathway from ABA to ROS production to NO production to alteration in protein phosphorylation status to endomembrane Ca2+ release to Kþ
in channel
inhibition; however, additional parallel pathways may exist.
In addition to its effect on inward K+ currents, NO at high
2330
concentrations also inhibits outward K+ currents through a
mechanism that is independent of Ca2+ or pH changes and
dependent on an oxidizing environment, which may promote
protein nitrosylation [79] (Fig. 3).
2.6. Anion channels: properties and regulation
Anion channel currents have been reported in the PM of
many plant cell types [80–83], however, in contrast to cation
channels, little is known about the molecular identity of anion
channels. One candidate gene family is the CLC family, because CLC homologs in prokaryotes mediate Cl flux. However to date, none of the seven CLC family members in
Arabidopsis has been localized to the PM or shown to function
as an anion channel in planta [84,85]. Another candidate gene
family is the ATP-binding cassette (ABC) superfamily [86,87].
The ABC transporter known as the ‘‘cystic fibrosis transmembrane conductance regulator’’ (CFTR) can function as a Cl
channel in mammalian cells, and pharmacological inhibitors
of the CFTR also modulate anion (as well as K+) currents in
guard cells [88–90].
Guard cell PM anion channels mediate anion efflux and are
classified electrophysiologically as rapid (R-type), or slow
(S-type) anion channels [80–83,91] (Fig. 2). The R-type channels exhibit rapid activation kinetics over a narrow voltage
range. The S-type channels activate over a much broader range
of voltages and exhibit slow activation and deactivation kinetics. S-type channels are a major component of the membrane
depolarization mechanisms that drive stomatal closure [91]
(Fig. 1). In the presence of extracellular anions, including malate2 and Cl, which would accumulate in the apoplast during
stomatal closure, the voltage-regulation of guard cell R-type
channels shifts to more negative voltages. This promotes anion
efflux through the channels and in this manner R-type channels can also contribute to membrane depolarization [62,92].
Both R- and S-type channels are permeable to a range of anions, including those of physiological relevance to guard cell
function: malate2, Cl, and NO
3 . Both channels can be activated by elevation in Ca2þ
cyt [80,81,93] , although electrophysiological measurements on intact guard cells in situ demonstrate
that these channels are not obligately dependent on Ca2+ elevation for their activity. However, the channels do require
2þ
the physiological resting level of Ca2þ
cyt , as when Cacyt is buffered to non-physiologically low levels (below 100 nM),
ABA-activation of anion channels is interrupted [94]. As described in more detail below, physiological and genetic manipulations implicate protein kinase(s) as important activators of
S-type channels.
2.7. Anion channels: roles in guard cell signal transduction
ABA activates both S-type and R-type anion channels in GC
[67,92]. Regulation of the S-type channels has been studied in
more detail. Recent work has suggested the involvement of a
PM localized, ATP binding cassette (ABC) protein, AtMRP5,
in ABA-activation of anion channels [95,96]. Although
AtMRP5 itself does not result in currents when expressed in insect cells or Xenopus oocytes, insertional mutation of AtMRP5
in Arabidopsis impairs ABA and Ca2þ
cyt activation of slow (Stype) anion channels as well as PM-Ca2+ channels. Thus, the
authors propose that AtMRP5 is a plasma membrane-localized regulator of these channels [95].
Guard cells can elevate Ca2þ
cyt following ABA exposure and
they can also elevate pHcyt (Fig. 3). Based on whole-cell
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
recording of gpa1 knockout vs. wild-type guard cells in the
presence or absence of a pH clamp, it seems that ABA can activate S-type channels either via a GPA1-based signaling cascade or via a pHcyt -based signaling cascade. When both of
these mechanisms are blocked, ABA activation of S-type currents is greatly reduced, as is ABA-induced stomatal closure
[70]. S1P appears to function solely in the G-protein branch
of this pathway, as S1P application activates S-type channels
in wild-type guard cells but not in gpa1 mutant guard cells [72].
Mutants deficient in a farnesyl transferase subunit (era1) or
in a RNA cap-binding protein (abh1) are ABA hypersensitive
and show enhanced ABA-activation of S-type currents [91,97].
In addition, patch clamp recordings of S-type anion currents in
dominant negative mutants of type 2C protein phosphatases
which are ABA hyposensitive (abi1-1 and abi2-1) [67] or direct
measurement of anion currents in intact guard cells of tobacco
plants expressing the dominant negative ABI1 gene [98],
showed a reduction in ABA-activation of S-type channels in
these genotypes, indicating the importance of protein dephosphorylation (Fig. 3).
Following initial pharmacological studies [99], evidence for
involvement of phosphorylation events, both Ca2+-independent and Ca2+-dependent, in ABA regulation of S-type channels has been provided by capitalizing on the tools afforded
by molecular genetics. First came the cloning and characterization of a V. faba Ca2+-independent ABA-activated protein kinase (AAPK) [100–102], with an ortholog in Arabidopsis
named OST1 [103]. Expression of a dominant negative form
of AAPK resulted in insensitivity of ABA-induced closure of
guard cells and impaired ABA-activation of S-type anion currents. More recently, involvement of two calcium-dependent
protein kinases (CDPKs), CPK3 and CPK6 in regulation of
these channels has been demonstrated in Arabidopsis [104].
Guard cells of double cpk3 cpk6 knock-out mutants show impaired Ca2+-induced activation of S-type anion currents (as
would be expected if Ca2+ was acting upstream, or at the level
of, the CDPK), reduced sensitivity to ABA regulation of these
channels, and a reduced sensitivity to ABA- and Ca2+-induced
stomatal closure in mutant plants (Fig. 3). R-type current
behavior was only slightly affected by these mutations [104].
Given that genetic manipulations have implicated RNA processing, protein farnesylation, protein dephosphorylation,
both Ca2+-dependent and Ca2+-independent protein kinases,
sphingosine kinases, G-proteins, and pHcyt in S-type anion
channel regulation, the complexity of the intracellular signaling cascade regulating these channels is very apparent. Modeling approaches such as those developed by Li et al. [57] can
help to organize this information and infer a hierarchy of signaling events.
2.8. Ca2+ channels: properties and regulation
Ca2+ entry activates many physiological processes induced
during ABA-stimulated stomatal closure. At the molecular level, it is predicted that 41 genes in Arabidopsis may potentially
encode non-selective cation channels with permeability to
Ca2+. These genes are divided into three families of which
one, TPC1, has a single family member that appears tonoplast-localized [105] (Fig. 1). The other two families are the
20 member cyclic nucleotide gated channel (CNGC) family
[106], and the 20 member glutamate receptor family (GLR)
[107]. Work on CNGC1 and CNGC2 [108–111], indicates that
CNG channels are permeable to cations, including K+ and
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
Ca2+, and are regulated by cyclic nucleotides and calmodulin.
cAMP can activate a Ca2+ (Ba2+) current in Arabidopsis guard
cell protoplasts, suggesting a possible role of CNG channels in
Ca2+-based signaling in this cell type [112].
In Arabidopsis roots, glutamate induces large and rapid elevation of [Ca2+]cyt and transient membrane depolarization
[113], suggesting that GLRs either conduct Ca2+ or activate
another channel that does so [114]. GLR3.4 is ubiquitously expressed throughout the plant, including guard cells, and so
may play role in Ca2+-based guard cell signaling [115].
Hyperpolarization- and ABA-activated Ca2+ currents are
observed in guard cells [116,117] however, the relationship between these currents and CNGC or GLR proteins remains unknown. In addition, genetic disruption of AtMRP5 impairs
activation of these currents, but there is no evidence to
date that AtMRP5 functions as a Ca2+-permeable channel
[95,96].
2.9. Ca2+ channels: roles in guard cell signal transduction
External Ca2+ application inhibits stomatal opening and
promotes stomatal closure. ABA can elevate Ca2þ
cyt , although
it does not invariably do so [93,118,119]. Upon elevation of
+
Ca2þ
cyt , inward K channels are inhibited and S- and R-type anion channels are activated. A detailed discussion of roles for
cytosolic Ca2+ in initiation and maintenance of stomatal closure is beyond the focus of this review, but is available elsewhere [119,120]. It is certain that that PM Ca2+-channels are
important players in regulation of GC Ca2þ
cyt status, and these
channels are regulated by many of the same proteins and
metabolites involved in GC ABA response. For example, genetic studies with era1 and abh1 ABA hypersensitive mutants
showed enhanced ABA sensitivity of PM Ca2+ currents, while
cpk3 and cpk6 mutants show ABA hyposensitivity of these
currents [104,121,122] (Fig. 3); all these phenotypes are consistent with the aforementioned effects of these mutations on
ABA modulation of S-type anion currents.
Blocking H2O2 production in guard cells inhibits ABA-induced stomatal closure emphasizing the role of ROS during
this process [117]. The PM ABA-activated Ca2+ currents of
guard cells are likewise activated by H2O2 [116,117]. Disruption of two partially redundant Arabidopsis guard cell-expressed NADPH oxidase catalytic subunit genes, AtrbohD
and AtrbohF, causes impairment of: (1) ABA-induced stomatal
closing; (2) ABA promotion of ROS production and; (3) ABAinduced cytosolic Ca2+ increases, and also leads to disruption
of ABA-activation of PM Ca2+-permeable channels in guard
cells, confirming the regulation of these Ca2+ channels by cellular redox status [18] (Fig. 3).
In isolated GC membrane patches, ATP or type 1/2A phosphatase inhibitors prolong Ca2+ channel activity, suggesting
protein phosphorylation as a proximate regulator of these
channels [123]. Dominant negative mutations in PP2C-type
protein phosphatases abi1 and abi2 disrupt ABA-activation
of Ca2+channels [17,124]. ROS production was also disrupted
in the abi1-1 mutant but not in the abi2-1 mutant, leading to
the hypothesis that abi2-1 affects responses downstream of
ROS production [17] and abi1-1 affects responses upstream
of ROS production (Fig. 3). However, there is also evidence
that ROS regulate ABI1 activity (discussed in [57]). Additionally abi1-1, but not abi2-1, inhibits ABA activation of OST1
protein kinase [125], so the relative positions of these components in the signaling cascade remain uncertain.
2331
3. Vacuolar ion transporters and channels
The vacuole makes up most of the volume of mature plant
cells, including guard cells. During stomatal movements, the
volume of guard cells can change by more than 40%, and total
vacuolar volume changes rapidly during these processes [126].
These observations indicate the important roles of ion fluxes
across the tonoplast during stomatal movements [2]. Because
of the relative inaccessibility of the tonoplast in intact cells,
most of the data on tonoplast ion channels have been obtained
either by patch clamping of isolated vacuoles, or by tracer flux
measurements on isolated guard cells [2]. Below we discuss the
identity, regulation and functional roles of vacuolar ion channels and transporters of guard cells. TIP-like aquaporins are a
class of vacuolar water channels and have been discussed elsewhere in detail [127].
3.1. Vacuolar ion transporters: properties and regulation
Vacuoles serve as a H+ sink, and the accumulation of H+
into the vacuole depends on the activity of vacuolar-type
H+-ATPases (V-ATPases) [6] and H+ translocating pyrophosphatases (V-PPases). Regulation of these enzymes can thus affect the secondary messenger, pHcyt [128,129]. Primary H+
uptake across the tonoplast also indirectly mediates the movements of other solutes between vacuoles and cytoplasm, via
H+-coupled antiporters and modulation of the tonoplast membrane potential (Fig. 1).
Evidence for tonoplast H+-ATPases in plants came from
patch clamp recordings on isolated vacuoles of various plant
cell types, including guard cells [130,131]. At the molecular level, plant vacuolar H+-ATPases (V-ATPases), like those of
other eukaryotes, have a multiple subunit composition [132].
Genes for at least 26 different V-ATPase subunits have been
identified in the Arabidopsis genome, with some subunits encoded by small gene families [132]. By contrast, V-PPases are
large homodimeric enzymes which utilize energy from pyrophosphate (PP) hydrolysis to drive H+ uptake into the vacuole
[133,134]. Despite their names, neither V-ATPases nor VPPases show exclusive localization at the tonoplast [134,135].
While molecular genetic elucidation of the role of the VPPases in guard cell function is still lacking, genetic evidence
for the importance of the V-ATPase in guard cells is provided
by a study on the V-ATPase subunit mutant, det3, of Arabidopsis [136]. In wild-type Arabidopsis guard cells, ABA, oxidative stress, cold, and external calcium elicited Ca2+ oscillations
of differing amplitudes and frequencies and induced stomatal
closure. In the det3 mutant guard cells although the cold and
ABA-induced Ca2þ
cyt oscillations and resulting stomatal closure
were normal, external Ca2+ and oxidative stress induced a nonoscillating, prolonged Ca2þ
cyt increase and these stimuli no longer induced stomatal closure [136]. This result indirectly implicates the V-ATPase in control of Ca2+ release, likely via
effects on tonoplast membrane potential.
The importance of the H+ gradient in transport of other solutes across the tonoplast is exemplified by the case of NO
3
storage. As discussed earlier, under some conditions NO
3 is
an important osmoticum for stomatal opening, and is presumþ
ably stored in the vacuole [137]. Recently a 2NO
3 =1H anti
porter for vacuolar NO3 accumulation in Arabidopsis
mesophyll cells was identified as AtCLCa, a protein homologous to the prokaryotic CLC family of Cl channels and
H+/Cl exchange proteins [138,139]. The potential roles of
2332
AtCLCa and the other six members of the AtCLC family in
guard cell physiology will be an exciting topic for future research.
Vacuoles serve as a major Ca2+ store, and Ca2+ transporters
and channels within the tonoplast undoubtedly participate in
2+
Ca2þ
can accumulate to millimocyt signal generation [140]. Ca
lar levels in the vacuole, and this accumulation is promoted by
high affinity P-type Ca2+ ATPases [141] and moderate affinity
Ca2+/H+ antiporters (CAX), each of which are encoded by
multigene families in Arabidopsis [141–143]. The specific roles
of these two types of Ca2+ transporters in guard cell function
have yet to be elucidated.
3.2. Vacuolar cation and anion channels: properties and
regulation
Several cation channel activities have been characterized in
guard cell vacuolar membranes (Fig. 1) including fast vacuolar
(FV), K+-selective vacuolar (VK) cation channels, and slow
vacuolar (SV) channels [2,144]. In isolated patch-clamped vacuoles, all of these channels can mediate K+ uptake or release,
depending on the electrochemical gradient for K+. FV channels activate quickly in response to voltage, are inhibited at elevated (above 100 nM) Ca2+ concentrations, and are activated
by cytosolic alkalinization. VK channels do not exhibit voltage-regulation and are activated as Ca2þ
cyt concentrations increase from resting levels. VK currents may be mediated by
members of the TPK/KCO K+ channel family, encoded in
Arabidopsis by six genes. KCO1/TPK1 was the first K+ channel to be localized to the plant tonoplast [145] and, like VK
currents of guard cells, when heterologously expressed in yeast
KCO1 channels exhibit K+ selectivity, activation by Ca2+, and
voltage independence [146].
At 500 nM Ca2þ
cyt and above, SV channels, which are voltageregulated and activate slowly following a change in membrane
potential, are activated. SV ion channel activity has been
extensively characterized in guard cells [144,147,148]. SV type
vacuolar currents appear to be lost in mesophyll cells of
knockout mutants of TPC1, a two-pore Ca2+-induced Ca2+ release channel, suggesting that SV channels not only mediate
K+ flux but also function as conduits for Ca2+ release [105].
However, whether a voltage-dependent shift of SV channel
activity leads to Ca2+ efflux in guard cells under physiological
conditions is currently under debate [105,149–151]. The release
of Ca2+ from the vacuole additionally involves several kinds of
Ca2+-permeable channels that have been implicated by physiological experiments but not yet characterized at the molecular
level, including InsP3- and InsP6- and cADPR-sensitive Ca2+release channels [120].
With regard to anion channels of the tonoplast, still little is
known. It has been hypothesized that some plant CLC family
members may function as vacuolar anion channels [62]. To
date, Cl and malate2 conductances that are activated by recombinant CDPK have been described at the guard cell tonoplast [152]; the conductances show slightly different kinetics,
and thus whether they are mediated by the same or different
molecular entities is an open question.
3.3. Vacuolar ion transport: roles in guard cell signal
transduction
In the context of guard cell signaling, pharmacological and
biochemical evidence that ABA elevates cytosolic concentra-
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
tions of secondary messengers such as Ca2+, InsP3- and InsP6and cADPR which in turn can activate endomembrane Ca2+release provides a link between ABA and regulation of
tonoplast ion transport. Arabidopsis tpc1 knockout plants
are impaired in Ca2+-induced, but not ABA-induced stomatal
closure, a phenotype they share with the det3 mutants. However, due to the fact that both K+ and Ca2+ can permeate
through the TPC1 channel it is still debated whether the phenotype observed is derived from altered K+ fluxes or from
TPC1-dependent Ca2+ signaling. Given recent evidence for
roles of CDPKs in transducing ABA regulation of anion and
Ca2+ channels at the GC plasma membrane [104], it is anticipated that new connections may also soon emerge between
ABA, CDPKs and regulation of tonoplast transport. A possible link between extracellular Ca2+ concentration and tonoplast transport is indicated by the recent observation that the
2+
generation of Ca2þ
cyt oscillation is regulated by external Ca
concentration via an apoplastic Ca2+ sensor (CAS)-mediated
CAS-InsP3 pathway in Arabidopsis [153].
4. Conclusions and perspectives
Guard cells are an attractive model system for the study of
ion channel and ion transporter regulation. Transport proteins
of GCs integrate the plant’s response to its changing environment into readily observable outputs at the molecular (ionic
currents), physiological (stomatal apertures) and whole plant
(transpiration) levels. Active research in the past few years
has identified a number of physiological, biochemical and
molecular aspects of guard cell function. Molecular identities
of a number of ion channels and transporters are now established, and aspects of the cellular regulatory mechanisms have
been revealed for well known responses such as blue light-induced stomatal opening and ABA-induced stomatal closure.
However, gaping holes remain in our understanding of the
physiology of GCs and the mechanisms underlying their function.
One of the main areas to explore is the identification of new
components governing guard cell physiology, including the yet
elusive anion channel and transporter proteins. In addition,
some channels, transporters, and regulatory mechanisms have
been described in other cell types, but whether the same mechanisms prevail in guard cells remain to be evaluated (cf. Table
1). Many channels and transport proteins have been identified
on the basis of their similarity to their mammalian counterparts however, novel plant-specific proteins or plant-specific
functions of known proteins may exist. For proteins known
to be involved in GC signaling, FRET-based studies could
be helpful in analyzing subcellular co-localization and interaction [154]. Recent development of mass spectrometry-based
proteomic approaches coupled with purification of protein
complexes, as well as study of membrane protein interactions
using the split ubiquitin based system [155,156], should also
prove useful in identification of new protein partners of known
components. Large scale genomic and proteomic analysis of
guard cells could also help to identify novel regulators of ion
channels and transporters. The availability of knockout mutants in most genes of the Arabidopsis genome provides a powerful tool to directly assess the roles of candidate proteins in
guard cell function. With the many tools available to stomatal
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
physiologists, we can look forward to continued rapid progress
in the field of guard cell signaling.
Acknowledgements: Financial sources: Research on guard cell signaling
in our laboratory is supported by NSF Grant 6-2066-01 and USDA
Grant 2006-35100-17254 to S.M.A.
References
[1] Nilson, S.E. and Assmann, S.M. (2007) The control of transpiration. Insights from Arabidopsis. Plant Physiol. 143, 19–27.
[2] MacRobbie, E.A.C. (2006) Control of volume and turgor in
stomatal guard cells. J. Membr. Biol. 210, 131–142.
[3] Ueno, K., Kinoshita, T., Inoue, S., Emi, T. and Shimazaki, K.
(2005) Biochemical characterization of plasma membrane H+ATPase activation in guard cell protoplasts of Arabidopsis
thaliana in response to blue light. Plant Cell Physiol. 46, 955–963.
[4] Assmann, S.M., Simoncini, L. and Schroeder, J.I. (1985) Blue
light activates electrogenic ion pumping in guard cell protoplasts
of Vicia faba L. Nature 318, 285–287.
[5] Shimazaki, K., Iino, M. and Zeiger, E. (1986) Blue lightdependent proton extrusion by guard-cell protoplasts of Vicia
faba. Nature 319, 324–326.
[6] Sze, H., Li, X.H. and Palmgren, M.G. (1999) Energization of
plant cell membranes by H+-pumping ATPases: regulation and
biosynthesis. Plant Cell 11, 677–689.
[7] Morsomme, P. and Boutry, M. (2000) The plant plasma
membrane H+-ATPase: structure, function and regulation.
Biochim. Biophys. Acta 1465, 1–16.
[8] Takemiya, A., Kinoshita, T., Asanuma, M. and Shimazaki, K.
(2006) Protein phosphatase 1 positively regulates stomatal
opening in response to blue light in Vicia faba. Proc. Natl.
Acad. Sci. USA 103, 13549–13554.
[9] Kinoshita, T. and Shimazaki, K. (1999) Blue light activates the
plasma membrane H+-ATPase by phosphorylation of the Cterminus in stomatal guard cells. EMBO J. 18, 5548–5558.
[10] Emi, T., Kinoshita, T. and Shimazaki, K. (2001) Specific binding
of vf14-3-3a isoform to the plasma membrane H+-ATPase in
response to blue light and fusicoccin in guard cells of broad
bean. Plant Physiol. 125, 1115–1125.
[11] Kinoshita, T., Emi, T., Tominaga, M., Sakamoto, K., Shigenaga, A., Doi, M. and Shimazaki, K. (2003) Blue-light- and
phosphorylation-dependent binding of a 14-3-3 protein to
phototropins in stomatal guard cells of broad bean. Plant
Physiol. 133, 1453–1463.
[12] Kinoshita, T., Doi, M., Suetsugu, N., Kagawa, T., Wada, M.
and Shimazaki, K. (2001) phot1 and phot2 mediate blue light
regulation of stomatal opening. Nature 414, 656–660.
[13] Mao, J., Zhang, Y.-C., Sang, Y., Li, Q.-H. and Yang, H.-Q.
(2005) A role for Arabidopsis cryptochromes and COP1 in the
regulation of stomatal opening. Proc. Natl. Acad. Sci. USA 102,
12270–12275.
[14] Lohse, G. and Hedrich, R. (1992) Characterization of the
plasma-membrane H+-ATPase from Vicia faba guard cells.
Planta 188, 206–214.
[15] Dietrich, P., Sanders, D. and Hedrich, R. (2001) The role of ion
channels in light-dependent stomatal opening. J. Exp. Bot. 52,
1959–1967.
[16] Zhang, X., Wang, H., Takemiya, A., Song, C., Kinoshita, T. and
Shimazaki, K. (2004) Inhibition of blue light-dependent H+
pumping by abscisic acid through hydrogen peroxide-induced
dephosphorylation of the plasma membrane H+-ATPase in
guard cell protoplasts. Plant Physiol. 136, 4150–4158.
[17] Murata, Y., Pei, Z.M., Mori, I.C. and Schroeder, J. (2001)
Abscisic acid activation of plasma membrane Ca2+ channels in
guard cells requires cytosolic NAD(P)H and is differentially
disrupted upstream and downstream of reactive oxygen species
production in abi1-1 and abi2-1 protein phosphatase 2C
mutants. Plant Cell 13, 2513–2523.
[18] Kwak, J.M., Mori, I.C., Pei, Z.M., Leonhardt, N., Torres, M.A.,
Dangl, J.L., Bloom, R.E., Bodde, S., Jones, J.D.G. and
Schroeder, J.I. (2003) NADPH oxidase AtrbohD and AtrbohF
2333
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
genes function in ROS-dependent ABA signaling in Arabidopsis.
EMBO J. 22, 2623–2633.
Kinoshita, T., Nishimura, M. and Shimazaki, K. (1995) Cytosolic concentration of Ca2+ regulates the plasma membrane H+ATPase in guard cells of fava bean. Plant Cell 7, 1333–1342.
Van Kirk, C.A. and Raschke, K. (1978) Presence of chloride
reduces malate production in epidermis during stomatal opening. Plant Physiol. 61, 361–364.
Guo, F.Q., Young, J. and Crawford, N.M. (2003) The nitrate
transporter AtNRT1.1 (CHL1) functions in stomatal opening
and contributes to drought susceptibility in Arabidopsis. Plant
Cell 15, 107–117.
Véry, A.-A. and Sentenac, H. (2003) Molecular mechanisms and
regulation of K+ transport in higher plants. Annu. Rev. Plant
Biol. 54, 575–603.
Gambale, F. and Uozumi, N. (2006) Properties of Shaker-type
potassium channels in higher plants. J. Membr. Biol. 210, 1–19.
Schroeder, J.I., Raschke, K. and Neher, E. (1987) Voltage
dependence of K+ channels in guard-cell protoplasts. Proc. Natl.
Acad. Sci. USA 84, 4108–4112.
Blatt, M.R. (1988) Potassium-dependent, bipolar gating of K+
channels in guard cells. J. Membr. Biol. 102, 235–246.
Schachtman, D.P., Schroeder, J.I., Lucas, W.J., Anderson, J.A.
and Gaber, R.F. (1992) Expression of an inward-rectifying
potassium channel by the Arabidopsis KAT1 cDNA. Science
258, 1654–1658.
Sentenac, H., Bonneaud, N., Minet, M., Lacroute, F., Salmon,
J.M., Gaymard, F. and Grignon, C. (1992) Cloning and
expression in yeast of a plant potassium ion transport system.
Science 256, 663–665.
Anderson, J.A., Huprikar, S.S., Kochian, L.V., Lucas, W.J. and
Gaber, R.F. (1992) Functional expression of a probable
Arabidopsis thaliana potassium channel in Saccharomyces cerevisiae. Proc. Natl. Acad. Sci. USA 89, 3736–3740.
Pilot, G., Pratelli, R., Gaymard, F., Meyer, Y. and Sentenac, H.
(2003) Five-group distribution of the Shaker-like K+ channel
family in higher plants. J. Mol. Evol. 56, 418–434.
Michard, E., Lacombe, B., Poree, F., Mueller-Roeber, B.,
Sentenac, H., Thibaud, J.B. and Dreyer, I. (2005) A unique
voltage sensor sensitizes the potassium channel AKT2 to
phosphoregulation. J. Gen. Physiol. 126, 605–617.
Dreyer, I., Antunes, S., Hoshi, T., Muller-Rober, B., Palme, K.,
Pongs, O., Reintanz, B. and Hedrich, R. (1997) Plant K+ channel
a-subunits assemble indiscriminately. Biophys. J. 72, 2143–2150.
Reintanz, B., Szyroki, A., Ivashikina, N., Ache, P., Godde, M.,
Becker, D., Palme, K. and Hedrich, R. (2002) AtKC1, a silent
Arabidopsis potassium channel a-subunit modulates root hair
K+ influx. Proc. Natl. Acad. Sci. USA 99, 4079–4084.
Baizabal-Aguirre, V.M., Clemens, S., Uozumi, N. and Schroeder, J.I. (1999) Suppression of inward-rectifying K+ channels
KAT1 and AKT2 by dominant negative point mutations in the
KAT1 a-subunit. J. Membr. Biol. 167, 119–125.
Pilot, G., Lacombe, B., Gaymard, F., Cherel, I., Boucherez, J.,
Thibaud, J.B. and Sentenac, H. (2001) Guard cell inward K+
channel activity in Arabidopsis involves expression of the twin
channel subunits KAT1 and KAT2. J. Biol. Chem. 276, 3215–
3221.
Ivashikina, N., Deeken, R., Fischer, S., Ache, P. and Hedrich, R.
(2005) AKT2/3 subunits render guard cell K+ channels Ca2+
sensitive. J. Gen. Physiol. 125, 483–492.
Zhang, X., Ma, J. and Berkowitz, G.A. (1999) Evaluation of
functional interaction between K+ channel a- and b-subunits and
putative inactivation gating by co-expression in Xenopus laevis
oocytes. Plant Physiol. 121, 995–1002.
Hosy, E., Vavasseur, A., Mouline, K., Dreyer, I., Gaymard, F.,
Poree, F., Boucherez, J., Lebaudy, A., Bouchez, D., Very, A.-A.,
Simonneau, T., Thibaud, J.B. and Sentenac, H. (2003) The
Arabidopsis outward K+ channel GORK is involved in regulation of stomatal movements and plant transpiration. Proc. Natl.
Acad. Sci. USA 100, 5549–5554.
Ichida, A.M., Pei, Z.M., Baizabal-Aguirre, V.M., Turner, K.J.
and Schroeder, J.I. (1997) Expression of a Cs+-resistant guard
cell K+ channel confers Cs+-resistant, light-induced stomatal
opening in transgenic Arabidopsis. Plant Cell 9, 1843–1857.
2334
[39] Kwak, J.M., Murata, Y., Baizabal-Aguirre, V.M., Merrill, J.,
Wang, M., Kemper, A., Hawke, S.D., Tallman, G. and
Schroeder, J.I. (2001) Dominant negative guard cell K+ channel
mutants reduce inward-rectifying K+ currents and light-induced
stomatal opening in Arabidopsis. Plant Physiol. 127, 473–485.
[40] Szyroki, A., Ivashikina, N., Dietrich, P., Roelfsema, M.R.,
Ache, P., Reintanz, B., Deeken, R., Godde, M., Felle, H.,
Steinmeyer, R., Palme, K. and Hedrich, R. (2001) KAT1 is not
essential for stomatal opening. Proc. Natl. Acad. Sci. USA 98,
2917–2921.
[41] Sutter, J.U., Campanoni, P., Tyrrell, M. and Blatt, M.R. (2006)
Selective mobility and sensitivity to SNAREs is exhibited by the
Arabidopsis KAT1 K+ channel at the plasma membrane. Plant
Cell 18, 935–954.
[42] Hwang, J.U., Suh, S., Yi, H., Kim, J. and Lee, Y. (1997) Actin
filaments modulate both stomatal opening and inward K+channel activities in guard cells of Vicia faba L.. Plant Physiol.
115, 335–342.
[43] Liu, K. and Luan, S. (1998) Voltage-dependent K+ channels as
targets of osmosensing in guard cells. Plant Cell 10, 1957–1970.
[44] Fairley-Grenot, K.A. and Assmann, S.M. (1992) Permeation of
Ca2+ through K+ channels in the plasma membrane of Vicia faba
guard cells. J. Membr. Biol. 128, 103–113.
[45] Hoth, S., Geiger, D., Becker, D. and Hedrich, R. (2001) The
pore of plant K+ channels is involved in voltage and pH sensing:
Domain-swapping between different K+ channel a-subunits.
Plant Cell 13, 943–952.
[46] Langer, K., Ache, P., Geiger, D., Stinzing, A., Arend, M., Wind,
C., Regan, S., Fromm, J. and Hedrich, R. (2002) Poplar
potassium transporters capable of controlling K+ homeostasis
and K+-dependent xylogenesis. Plant J. 32, 997–1009.
[47] Latz, A., Ivashikina, N., Fischer, S., Ache, P., Sano, T., Becker,
D., Deeken, R. and Hedrich, R. (2006) In planta AKT2 subunits
constitute a pH- and Ca2+-sensitive inward rectifying K+
channel. Planta 225, 1179–1191.
[48] Blatt, M.R. (1992) K+ channels of stomatal guard cells.
Characteristics of the inward rectifier and its control by pH. J.
Gen. Physiol. 99, 615–644.
[49] Ilan, N., Schwartz, A. and Moran, N. (1996) External protons
enhance the activity of the hyperpolarization-activated K+
channels in guard cell protoplasts of Vicia faba. J. Membr. Biol.
154, 169–181.
[50] Ache, P., Becker, D., Ivashikina, N., Dietrich, P., Roelfsema,
M.R. and Hedrich, R. (2000) GORK, a delayed outward rectifier
expressed in guard cells of Arabidopsis thaliana, is a K+-selective,
K+-sensing ion channel. FEBS Lett. 486, 93–98.
[51] Ilan, N., Schwartz, A. and Moran, N. (1994) External pH effects
on the depolarization-activated K+ channels in guard cell
protoplasts of Vicia faba. J. Gen. Physiol. 103, 807–831.
[52] Blatt, M.R. and Armstrong, F. (1993) K+ channels of stomatal
guard cells: abscisic-acid-evoked control of the outward rectifier
mediated by cytoplasmic pH. Planta 191, 330–341.
[53] Miedema, H. and Assmann, S.M. (1996) A membrane-delimited
effect of internal pH on the K+ outward rectifier of Vicia faba
guard cells. J. Membr. Biol. 154, 227–237.
[54] Luan, S., Li, W., Rusnak, F., Assmann, S.M. and Schreiber, S.L.
(1993) Immunosuppressants implicate protein phosphatase regulation of K+ channels in guard cells. Proc. Natl. Acad. Sci. USA
90, 2202–2206.
[55] Thiel, G. and Blatt, M.R. (1994) Phosphatase antagonist
okadaic acid inhibits steady-state K+ currents in guard cells of
Vicia faba. Plant J. 5, 727–733.
[56] Li, J., Lee, Y-R.J. and Assmann, S.M. (1998) Guard cells possess
a calcium-dependent protein kinase that phosphorylates the
KAT1 potassium channel. Plant Physiol. 116, 785–795.
[57] Li, S., Assmann, S.M. and Albert, R. (2006) Predicting essential
components of signal transduction networks: a dynamic model
of guard cell abscisic acid signaling. PLoS Biol. 4, 1732–1748.
[58] Razem, F.A., El-Kereamy, A., Abrams, S.R. and Hill, R.D.
(2006) The RNA-binding protein FCA is an abscisic acid
receptor. Nature 439, 290–294.
[59] Shen, Y.-Y., Wang, X.-F., Wu, F.-Q., Du, S.-Y., Cao, Z., Shang,
Y., Wang, X.-L., Peng, C.-C., Yu, X.-C., Zhu, S.-Y., Fan, R.-C.,
Xu, Y.-H. and Zhang, D.-P. (2006) The Mg-chelatase H subunit
is an abscisic acid receptor. Nature 443, 823–826.
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
[60] Liu, X.-G., Yue, Y.-L., Li, B., Nie, Y.-L., Li, W., Wu, W.-H.
and Ma, L.-G. (2007) A G protein-coupled receptor is a plasma
membrane receptor for the plant hormone abscissic acid. Science
315, 1712–1716.
[61] Blatt, M.R. (1990) Potassium channel currents in intact stomatal
guard cells: rapid enhancement by abscisic acid. Planta 180, 445–
455.
[62] Roelfsema, M.R.G. and Hedrich, R. (2005) In the light of
stomatal opening: new insights into ’the Watergate’. New Phytol.
167, 665–691.
[63] Leyman, B., Geelen, D., Quintero, F.J. and Blatt, M.R. (1999) A
tobacco syntaxin with a role in hormonal control of guard cell
ion channels. Science 283, 537–540.
[64] McAinsh, B. (1990) Abscisic acid-induced elevation of guard
cell cytosolic Ca2+ precedes stomatal closure. Nature 343, 186–
188.
[65] Grabov, A. and Blatt, M.R. (1997) Parallel control of the
inward-rectifier K+ channel by cytosolic free Ca2+ and pH in
Vicia guard cells. Planta 201, 84–95.
[66] Armstrong, F., Leung, J., Grabov, A., Brearley, J., Giraudat, J.
and Blatt, M.R. (1995) Sensitivity to abscisic acid of guard-cell
K+ channels is suppressed by abi1-1, a mutant Arabidopsis gene
encoding a putative protein phosphatase. Proc. Natl. Acad. Sci.
USA 92, 9520–9524.
[67] Pei, Z.-M., Kuchitsu, K., Ward, J.M., Schwarz, M. and
Schroeder, J.I. (1997) Differential abscisic acid regulation of
guard cell slow anion channels in Arabidopsis wild-type and abi1
and abi2 mutants. Plant Cell 9, 409–423.
[68] Fairley-Grenot, K. and Assmann, S.M. (1991) Evidence for Gprotein regulation of inward K+ channel current in guard cells of
fava bean. Plant Cell 3, 1037–1044.
[69] Assmann, S.M. (2002) Heterotrimeric and unconventional GTP
binding proteins in plant cell signaling. Plant Cell 14 (Suppl.),
S355–S373.
[70] Wang, X.-Q., Ullah, H., Jones, A.M. and Assmann, S.M. (2001)
G protein regulation of ion channels and abscisic acid signaling
in Arabidopsis guard cells. Science 292, 2070–2072.
[71] Ng, C.K.Y., Carr, K., McAinsh, M.R., Powell, B. and Hetherington, A.M. (2001) Drought-induced guard cell signal transduction involves sphingosine-1-phosphate. Nature 410, 596–599.
[72] Coursol, S., Fan, L.M., Le Stunff, H., Spiegel, S., Gilroy, S. and
Assmann, S.M. (2003) Sphingolipid signalling in Arabidopsis
guard cells involves heterotrimeric G proteins. Nature 423, 651–
654.
[73] Pei, Z.-M., Murata, Y., Benning, G., Thomine, S., Klusener, B.,
Allen, G.J., Grill, E. and Schroeder, J.I. (2000) Calcium channels
activated by hydrogen peroxide mediate abscisic acid signalling
in guard cells. Nature 406, 731–734.
[74] Garcia-Mata, C., Gay, R., Sokolovski, S., Hills, A., Lamattina,
L. and Blatt, M.R. (2003) Nitric oxide regulates K+ and Cl
channels in guard cells through a subset of abscisic acid-evoked
signaling pathways. Proc. Natl. Acad. Sci. USA 100, 11116–
11121.
[75] Lamattina, L., Garcia-Mata, C., Graziano, M. and Pagnussat,
G. (2003) Nitric oxide: the versatility of an extensive signal
molecule. Annu. Rev. Plant Biol. 54, 109–136.
[76] Desikan, R., Cheung, M.K., Bright, J., Henson, D., Hancock,
J.T. and Neill, S.J. (2004) ABA, hydrogen peroxide and nitric
oxide signalling in stomatal guard cells. J. Exp. Bot. 55, 205–212.
[77] Bright, J., Desikan, R., Hancock, J.T., Weir, I.S. and Neill, S.J.
(2006) ABA-induced NO generation and stomatal closure in
Arabidopsis are dependent on H2O2 synthesis. Plant J. 45, 113–
122.
[78] Sokolovski, S., Hills, A., Gay, R., Garcia-Mata, C., Lamattina,
L. and Blatt, M.R. (2005) Protein phosphorylation is a
prerequisite for intracellular Ca2+ release and ion channel
control by nitric oxide and abscissic acid in guard cells. Plant
J. 43, 520–529.
[79] Sokolovski, S. and Blatt, M.R. (2004) Nitric oxide block of
outward-rectifying K+ channels indicates direct control by
protein nitrosylation in guard cells. Plant Physiol. 136, 4275–
4284.
[80] Schroeder, J.I. and Hagiwara, S. (1989) Cytosolic calcium
regulates ion channels in the plasma membrane of Vicia faba
guard cells. Nature 338, 427–430.
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
[81] Hedrich, R., Busch, H. and Raschke, K. (1990) Ca2+ and
nucleotide dependent regulation of voltage dependent anion
channels in the plasma membrane of guard cells. EMBO J. 9,
3889–3892.
[82] Schroeder, J.I. (1995) Anion channels as central mechanisms for
signal transduction in guard cells and putative functions in roots
for plant-soil interactions. Plant Mol. Biol. 28, 353–361.
[83] Roberts, S.K. (2006) Plasma membrane anion channels in higher
plants and their putative functions in roots. New Phytol. 169,
647–666.
[84] Hechenberger, M., Schwappach, B., Fischer, W.N., Frommer,
W.B., Jentsch, T.J. and Steinmeyer, K. (1996) A family of
putative chloride channels from Arabidopsis and functional
complementation of a yeast strain with a CLC gene disruption. J.
Biol. Chem. 271, 33632–33638.
[85] Geelen, D., Lurin, C., Bouchez, D., Frachisse, J.M., Lelievre, F.,
Courtial, B., Barbier-Brygoo, H. and Maurel, C. (2000) Disruption of putative anion channel gene AtCLC-a in Arabidopsis
suggests a role in the regulation of nitrate content. Plant J. 21,
259–267.
[86] Theodoulou, F.L. (2000) Plant ABC transporters. Biochim.
Biophys. Acta 1465, 79–103.
[87] Rea, P.A. (2007) Plant ATP-binding cassette transporters. Annu.
Rev. Plant. Biol. 58, 347–375.
[88] Leonhardt, N., Marin, E., Vavasseur, A. and Forestier, C. (1997)
Evidence for the existence of a sulfonylurea-receptor-like protein
in plants: Modulation of stomatal movements and guard cell
potassium channels by sulfonylureas and potassium channel
openers. Proc. Natl. Acad. Sci. USA 94, 14156–14161.
[89] Leonhardt, N., Vavasseur, A. and Forestier, C. (1999) ATP
binding cassette modulators control abscisic acid regulated slow
anion channels in guard cells. Plant Cell 11, 1141–1152.
[90] Leonhardt, N., Bazin, I., Richaud, P., Marin, E., Vavasseur, A.
and Forestier, C. (2001) Antibodies to the CFTR modulate the
turgor pressure of guard cell protoplasts via slow anion channels.
FEBS Lett. 494, 15–18.
[91] Schroeder, J.I., Allen, G.J., Hugouvieux, V., Kwak, J.M. and
Waner, D. (2001) Guard cell signal transduction. Annu. Rev.
Plant Physiol. Plant Mol. Biol. 52, 627–658.
[92] Roelfsema, M.R.G., Levchenko, V. and Hedrich, R. (2004) ABA
depolarizes guard cells in intact plants, through a transient
activation of R- and S-type anion channels. Plant J. 37,
578–588.
[93] Marten, H., Konrad, K.R., Dietrich, P., Roelfsema, M.R. and
Hedrich, R. (2007) Ca2+-dependent and -independent abscisic
acid activation of plasma membrane anion channels in guard
cells of Nicotiana tabacum. Plant Physiol. 143, 28–37.
[94] Levchenko, V., Konrad, K.R., Dietrich, P., Roelfsema, M.R.G.
and Hedrich, R. (2005) Cytosolic abscisic acid activates guard
cell anion channels without preceding Ca2+ signals. Proc. Natl.
Acad. Sci. USA 102, 4203–4208.
[95] Suh, S.J., Wang, Y.F., Frelet, A., Leonhardt, N., Klein, M.,
Forestier, C., Mueller-Roeber, B., Cho, M., Martinoia, E. and
Schroeder, J. (2006) The ATP binding cassette transporter
AtMRP5 modulates anion and Ca2+ channel activities in
Arabidopsis guard cells. J. Biol. Chem. 282, 1916–1924.
[96] Gaedeke, N., Klein, M., Kolukisaoglu, U., Forestier, C., Muller,
A., Ansorge, M., Becker, D., Mamnun, Y., Kuchler, K., Schulz,
B., Mueller-Roeber, B. and Martinoia, E. (2001) The Arabidopsis
thaliana ABC transporter AtMRP5 controls root development
and stomata movement. EMBO J. 20, 1875–1887.
[97] Hugouvieux, V., Murata, Y., Young, J.J., Kwak, J.M., Mackesy, D.Z. and Schroeder, J.I. (2002) Localization, ion channel
regulation, and genetic interactions during abscisic acid signaling
of the nuclear mRNA cap-binding protein, ABH1. Plant Physiol.
130, 1276–1287.
[98] Grabov, A., Leung, J., Giraudat, J. and Blatt, M.R. (1997)
Alteration of anion channel kinetics in wild-type and abi1-1
transgenic Nicotiana benthamiana guard cells by abscisic acid.
Plant J. 12, 203–213.
[99] Schmidt, C., Schelle, I., Liao, Y.J. and Schroeder, J.I. (1995)
Strong regulation of slow anion channels and abscisic acid
signaling in guard cells by phosphorylation and dephosphorylation events. Proc. Natl. Acad. Sci. USA 92, 9535–9539.
2335
[100] Li, J. and Assmann, S.M. (1996) An abscisic acid-activated and
calcium-independent protein kinase from guard cells of fava
bean (Vicia faba L.). Plant Cell 8, 2359–2368.
[101] Mori, I.C. and Muto, S. (1997) Abscisic acid activates a 48kilodalton protein kinase in guard cell protoplasts. Plant Physiol.
113, 833–839.
[102] Li, J., Wang, X.-Q., Watson, M.B. and Assmann, S.M. (2000)
Regulation of abscissic acid-induced stomatal closure and anion
channels by guard cell AAPK kinase. Science 287, 300–303.
[103] Mustilli, A.C., Merlot, S., Vavasseur, A., Fenzi, F. and
Giraudat, J. (2002) Arabidopsis OST1 protein kinase mediates
the regulation of stomatal aperture by abscisic acid and acts
upstream of reactive oxygen species production. Plant Cell 14,
3089–3099.
[104] Mori, I.C., Murata, Y., Yang, Y., Munemasa, S., Wang, Y.F.,
Andreoli, S., Tiriac, H., Alonso, J.M., Harper, J.F., Ecker, J.R.,
Kwak, J.M. and Schroeder, J.I. (2006) CDPKs CPK6 and CPK3
function in ABA regulation of guard cell S-type anion- and
Ca2+-permeable channels and stomatal closure. PLoS Biol. 4,
1749–1762.
[105] Peiter, E., Maathuis, F.J.M., Mills, L.N., Knight, H., Pelloux, J.,
Hetherington, A.M. and Sanders, D. (2005) The vacuolar Ca2+activated channel TPC1 regulates germination and stomatal
movement. Nature 434, 404–408.
[106] Finn, J.T., Grunwald, M.E. and Yau, K.W. (1996) Cyclic
nucleotide-gated ion channels: an extended family with diverse
functions. Annu. Rev. Physiol. 58, 395–426.
[107] Lacombe, B., Becker, D., Hedrich, R., DeSalle, R., Hollmann,
M., Kwak, J.M., Schroeder, J.I., Le Novère, N., Nam, H.G.,
Spalding, E.P., Tester, M., Turano, F.J., Chiu, J. and Coruzzi,
G. (2001) The identity of plant glutamate receptors. Science 292,
1486–1487.
[108] Kohler, C., Merkle, T. and Neuhaus, G. (1999) Characterisation
of a novel gene family of putative cyclic nucleotide- and
calmodulin-regulated ion channels in Arabidopsis thaliana. Plant
J. 18, 97–104.
[109] Leng, Q., Mercier, R.W., Yao, W. and Berkowitz, G.A. (1999)
Cloning and first functional characterization of a plant cyclic
nucleotide-gated cation channel. Plant Physiol. 121, 753–761.
[110] Leng, Q., Mercier, R.W., Hua, B.G., Fromm, H. and Berkowitz,
G.A. (2002) Electrophysiological analysis of cloned cyclic
nucleotide-gated ion channels. Plant Physiol. 128, 400–410.
[111] Ali, R., Zielinski, R.E. and Berkowitz, G.A. (2006) Expression of
plant cyclic nucleotide-gated cation channels in yeast. J. Exp.
Bot. 57, 125–138.
[112] Lemtiri-Chlieh, F. and Berkowitz, G.A. (2004) Cyclic adenosine
monophosphate regulates calcium channels in the plasma
membrane of Arabidopsis leaf guard and mesophyll cells. J.
Biol. Chem. 279, 35306–35312.
[113] Dennison, K.L. and Spalding, E.P. (2000) Glutamate-gated
calcium fluxes in Arabidopsis. Plant Physiol. 124, 1511–1514.
[114] Qi, Z., Stephens, N.R. and Spalding, E.P. (2006) Calcium entry
mediated by GLR3.3, an Arabidopsis glutamate receptor with a
broad agonist profile. Plant Physiol. 142, 963–971.
[115] Meyerhoff, O., Müller, K., Roelfsema, M., Latz, A., Lacombe,
B., Hedrich, R., Dietrich, P. and Becker, D. (2005) AtGLR3.4, a
glutamate receptor channel-like gene is sensitive to touch and
cold. Planta 222, 418–427.
[116] Hamilton, D.W.A., Hills, A., Kohler, B. and Blatt, M.R. (2000)
Ca2+ channels at the plasma membrane of stomatal guard cells
are activated by hyperpolarization and abscisic acid. Proc. Natl.
Acad. Sci. USA 97, 4967–4972.
[117] Pei, Z.M., Murata, Y., Benning, G., Thomine, S., Klusener, B.,
Allen, G.J., Grill, E. and Schroeder, J.I. (2000) Calcium channels
activated by hydrogen peroxide mediate abscisic acid signalling
in guard cells. Nature 406, 731–734.
[118] Romano, L.A., Jacob, T., Gilroy, S. and Assmann, S.M. (2000)
Increases in cytosolic Ca2+ are not required for abscisic acidinhibition of inward K+ currents in guard cells of Vicia faba L..
Planta 211, 209–217.
[119] Israelsson, M., Siegel, R.S., Young, J., Hashimoto, M., Iba, K.
and Schroeder, J.I. (2006) Guard cell ABA and CO2 signaling
network updates and Ca2+ sensor priming hypothesis. Curr.
Opin. Plant Biol. 9, 654–663.
2336
[120] Hetherington, A.M. and Brownlee, C. (2004) The generation of
Ca2+ signals in plants. Annu. Rev. Plant Biol. 55, 401–427.
[121] Allen, G.J., Murata, Y., Chu, S.P., Nafisi, M. and Schroeder, J.I.
(2002) Hypersensitivity of abscisic acid-induced cytosolic calcium increases in the Arabidopsis farnesyltransferase mutant
era1-2. Plant Cell 14, 1649–1662.
[122] Hugouvieux, V., Kwak, J.M. and Schroeder, J.I. (2001) An
mRNA cap binding protein, ABH1, modulates early abscisic
acid signal transduction in Arabidopsis. Cell 106, 477–487.
[123] Kohler, B. and Blatt, M.R. (2002) Protein phosphorylation
activates the guard cell Ca2+ channel and is a prerequisite for
gating by abscisic acid. Plant J. 32, 185–194.
[124] Allen, G.J., Kuchitsu, K., Chu, S.P., Murata, Y. and Schroeder,
J.I. (1999) Arabidopsis abi1-1 and abi2-1 phosphatase mutations
reduce abscisic acid induced cytoplasmic calcium rises in guard
cells. Plant Cell 11, 1785–1798.
[125] Yoshida, R., Umezawa, T., Mizoguchi, T., Takahashi, S.,
Takahashi, F. and Shinozaki, K. (2006) The regulatory domain
of SRK2E/OST1/SnRK2.6 interacts with ABI1 and integrates
abscisic acid (ABA) and osmotic stress signals controlling
stomatal closure in Arabidopsis. J. Biol. Chem. 281, 5310–5318.
[126] Shope, J.C., DeWald, D.B. and Mott, K.A. (2003) Changes in
surface area of intact guard cells are correlated with membrane
internalization. Plant Physiol. 133, 1314–1321.
[127] Kaldenhoff, R. and Fischer, M. (2006) Aquaporins in plants.
Acta Physiol. 187, 169–176.
[128] Gaxiola, R.A., Fink, G.R. and Hirschi, K.D. (2002) Genetic
manipulation of vacuolar proton pumps and transporters. Plant
Physiol. 129, 967–973.
[129] Blatt, M.R. (2000) Cellular signaling and volume control in
stomatal movements in plants. Annu. Rev. Cell Dev. Biol. 16,
221–241.
[130] Schulz-Lessdorf, B., Dietrich, P., Marten, I., Lohse, G., Busch,
H. and Hedrich, R. (1994) Coordination of plasma membrane
and vacuolar membrane ion channels during stomatal movement. Symp. Soc. Exp. Biol. 48, 99–112.
[131] Allen, G.J. and Sanders, D. (1996) Control of ionic currents in
guard cell vacuoles by cytosolic and luminal calcium. Plant J. 10,
1055–1069.
[132] Padmanaban, S., Lin, X., Perera, I., Kawamura, Y. and Sze, H.
(2004) Differential expression of vacuolar H+-ATPase subunit c
genes in tissues active in membrane trafficking and their roles in
plant growth as revealed by RNAi. Plant Physiol. 134, 1514–
1526.
[133] Drozdowicz, Y.M. and Rea, P.A. (2001) Vacuolar H+ pyrophosphatases: from the evolutionary backwaters into the mainstream. Trends Plant Sci. 6, 206–211.
[134] Schumacher, K. (2006) Endomembrane proton pumps: connecting membrane and vesicle transport. Curr. Opin. Plant Biol. 9,
595–600.
[135] Li, J., Yang, H., Peer, W.A., Richter, G., Blakeslee, J., Bandyopadhyay, A., Titapiwantakun, B., Undurraga, S., Khodakovskaya, M., Richards, E.L., Krizek, B., Murphy, A.S., Gilroy, S.
and Gaxiola, R. (2005) Arabidopsis H+-PPase AVP1 regulates
auxin-mediated organ development. Science 310, 121–125.
[136] Allen, G.J., Chu, S.P., Schumacher, K., Shimazaki, C.T.,
Vafeados, D., Kemper, A., Hawke, S.D., Tallman, G., Tsien,
R.Y., Harper, J.F., Chory, J. and Schroeder, J.I. (2000)
Alteration of stimulus-specific guard cell calcium oscillations
and stomatal closing in Arabidopsis det3 mutant. Science 289,
2338–2342.
[137] Schumaker, K.S. and Sze, H. (1987) Decrease of pH gradients in
+
tonoplast vesicles by NO
3 and Cl : evidence for H -coupled
anion transport. Plant Physiol. 83, 490–496.
[138] de Angeli, A., Monachello, D., Ephritikhine, G., Frachisse,
J.M., Thomine, S., Gambale, F. and Barbier-Brygoo, H. (2006)
The nitrate/proton antiporter AtCLCa mediates nitrate accumulation in plant vacuoles. Nature 442, 939–942.
[139] Schroeder, J.I. (2006) Physiology: nitrate at the ion exchange.
Nature 442, 877–878.
[140] Allen, G.J., Chu, S.P., Harrington, C.L., Schumacher, K.,
Hoffmann, T., Tang, Y.Y., Grill, E. and Schroeder, J.I. (2001) A
S. Pandey et al. / FEBS Letters 581 (2007) 2325–2336
[141]
[142]
[143]
[144]
[145]
[146]
[147]
[148]
[149]
[150]
[151]
[152]
[153]
[154]
[155]
[156]
[157]
[158]
defined range of guard cell calcium oscillation parameters
encodes stomatal movements. Nature 411, 1053–1057.
Geisler, M., Axelsen, B.K., Harper, J.F. and Palmgren, M.G.
(2000) Molecular aspects of higher plant Ca2+-ATPases. Biochim. Biophys. Acta 1465, 52–78.
Cheng, N.H., Pittman, J.K., Shigaki, T., Lachmansingh, J.,
LeClere, S., Lahner, B., Salt, D.E. and Hirschi, K.D. (2005)
Functional association of Arabidopsis CAX1 and CAX3 is
required for normal growth and ion homeostasis. Plant Physiol.
138, 2048–2060.
Cai, X. and Lytton, J. (2004) The cation/Ca2+ exchanger
superfamily: phylogenetic analysis and structural implications.
Mol. Biol. Evol. 21, 1692–1703.
Allen, G.J. and Sanders, D. (1995) Calcineurin, a type 2B protein
phosphatase, modulates the Ca2+-permeable slow vacuolar ion
channel of stomatal guard cells. Plant Cell 7, 1473–1483.
Czempinski, K., Frachisse, J.M., Maurel, C., Barbier-Brygoo, H.
and Mueller-Roeber, B. (2002) Vacuolar membrane localization
of the Arabidopsis ‘two-pore’ K+ channel KCO1. Plant J. 29,
809–820.
Bihler, H., Eing, C., Hebeisen, S., Roller, A., Czempinski, K.
and Bertl, A. (2005) TPK1 is a vacuolar ion channel different
from the slow-vacuolar cation channel. Plant Physiol. 139, 417–
424.
Ward, J.M. and Schroeder, J.I. (1994) Calcium-activated K+
channels and calcium-induced calcium release by slow vacuolar
ion channels in guard cell vacuoles implicated in the control of
stomatal closure. Plant Cell 6, 669–683.
Schulz-Lessdorf, B. and Hedrich, R. (1995) Protons and calcium
modulate SV-type channels in the vacuolar-lysosomal compartment-channel interaction with calmodulin inhibitors. Planta 197,
655–671.
Bewell, M.A., Maathuis, F.J.M., Allen, G.J. and Sanders, D.
(1999) Calcium-induced calcium release mediated by a voltageactivated cation channel in vacuolar vesicles from red beet.
FEBS Lett. 458, 41–44.
Ivashikina, N. and Hedrich, R. (2005) K+ currents through SVtype vacuolar channels are sensitive to elevated luminal sodium
levels. Plant J. 41, 606–614.
Scholz-Starke, J., Carpaneto, A. and Gambale, F. (2006) On the
interaction of neomycin with the slow vacuolar channel of
Arabidopsis thaliana. J. Gen. Physiol. 127, 329–340.
Pei, Z.M., Ward, J.M., Harper, J.F. and Schroeder, J.I. (1996) A
novel chloride channel in Vicia faba guard cell vacuoles activated
by the serine/threonine kinase, CDPK. EMBO J. 15,
6564–6574.
Tang, R.-H., Han, S., Zheng, H., Cook, C.W., Choi, C.S.,
Woerner, T.E., Jackson, R.B. and Pei, Z.M. (2007) Coupling
diurnal cytosolic Ca2+ oscillations to the CAS-IP3 pathway in
Arabidopsis. Science 315, 1423–1426.
Adjobo-Hermans, M.J., Goedhart, J. and Gadella Jr., T.W.
(2006) Plant G protein heterotrimers require dual lipidation
motifs of Ga and Gc and do not dissociate upon activation. J.
Cell Sci. 15, 5087–5097.
Obrdlik, P., El-Bakkoury, M., Hamacher, T., Cappellaro, C.,
Vilarino, C., Fleischer, C., Ellerbrok, H., Kamuzinzi, R., Ledent,
V., Blaudez, D., Sanders, D., Revuelta, J.L., Boles, E., Andre, B.
and Frommer, W.B. (2004) K+ channel interactions detected by
a genetic system optimized for systematic studies of membrane
protein interactions. Proc. Natl. Acad. Sci. USA 101, 12242–
12247.
Pandey, S. and Assmann, S.M. (2004) The Arabidopsis putative
G protein-coupled receptor GCR1 interacts with the G protein a
subunit GPA1 and regulates abscisic acid signaling. Plant Cell
16, 1616–1632.
Taylor, A.R. and Assmann, S.M. (2001) Apparent absence of a
redox requirement for blue light activation of pump current in
broad bean guard cells. Plant Physiol. 125, 329–338.
Merlot, S., Gosti, F., Guerrier, D., Vavasseur, A. and Giraudat,
J. (2001) The ABI1 and ABI2 protein phosphatases 2C act in a
negative feedback regulatory loop of the abscisic acid signalling
pathway. Plant J. 25, 295–303.