View
223
Download
0
Category
Preview:
Citation preview
8/6/2019 aba signallingh
1/24
Annu. Rev. Plant Physiol. Plant Mol. Biol. 1998. 49:199222
Copyright c 1998 by Annual Reviews. All rights reserved
ABSCISIC ACID SIGNAL
TRANSDUCTION
Jeffrey Leung and J er ome GiraudatInstitut des Sciences Vegetales, Unite Propre de Recherche 40, Centre National de la
Recherche Scientifique, 1 Avenue de la Terrasse, 91190 Gif-sur-Yvette, France;e-mail: jerome.giraudat@isv.cnrs-gif.fr
KEY WORDS: guard cell, mutants, second messenger, seed development, stress tolerance
ABSTRACT
The plant hormone abscisic acid (ABA) plays a major role in seed maturation
and germination, as well as in adaptation to abiotic environmental stresses. ABA
promotes stomatal closure by rapidly altering ion fluxes in guard cells. Other
ABA actions involve modifications of gene expression, and the analysis of ABA-
responsive promoters has revealed a diversity of potential cis-acting regulatory
elements. The nature of the ABA receptor(s) remains unknown. In contrast,
combined biophysical, genetic, and molecular approaches have led to consid-
erable progress in the characterization of more downstream signaling elements.
In particular, substantial evidence points to the importance of reversible pro-
tein phosphorylation and modifications of cytosolic calcium levels and pH as
intermediates in ABA signal transduction. Exciting advances are being made in
reassembling individual components into minimal ABA signaling cascades at the
single-cell level.
CONTENTS
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
ABA SIGNAL TRANSDUCTION IN SEEDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
Seed Dormancy and Germination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200Reserve Accumulation and Acquisition of Desiccation Tolerance . . . . . . . . . . . . . . . . . . . 202
ABA SIGNAL TRANSDUCTION IN STRESS RESPONSE . . . . . . . . . . . . . . . . . . . . . . . . . . 206
Regulation of Gene Expression in Response to Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207Regulation of Stomatal Aperture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
CONCLUSION AND PERSPECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
199
1040-2519/98/0601-0199$08.00
8/6/2019 aba signallingh
2/24
200 LEUNG & GIRAUDAT
INTRODUCTION
The scientific origins of abscisic acid (ABA) have been traced to several in-
dependent investigations in the late 1940s, but it was only in the 1960s thatABA was isolated and identified (1). Mutants affected in ABA biosynthesis
are known in a variety of plant species (39, 146). The characterization of these
mutants, together with physicochemical studies, has enabled the pathway of
ABA biosynthesis to be elucidated in higher plants (134, 135, 146, 166). Two
ABA biosynthetic genes have been recently cloned (88, 135) and should provide
insights into the regulation and the sites of ABA biosynthesis.
A large body of evidence indicates that ABA plays a major role in adap-
tation to abiotic environmental stresses, seed development, and germination.
The present review focuses on our current knowledge concerning how the ABAsignal could be faithfully transduced to mediate these well-characterized phys-
iological and developmental processes. Physicochemical and molecular ge-
netic approaches have already provided fundamental insights into the diversity
of ABA perception sites and other downstream components that could couple
ABA stimuli to particular responses. It should be emphasized that many con-
cepts about the relative importance of these components in the ABA signaling
network are far from firmly established. Nonetheless, it is clear that the present
development of single-cell systems should allow some of these components to
be assembled into minimal signaling pathways within a cellular context.
ABA SIGNAL TRANSDUCTION IN SEEDS
Endogenous ABA content peaks during roughly the last two thirds of seed de-
velopment before returning to lower levels in the dry seed (121). ABA is thus
thought to regulate several essential processes occurring during the develop-
mental stages that follow pattern formation of the embryo. These processes
include the induction of seed dormancy, the accumulation of nutritive reserves,
and the acquisition of desiccation tolerance.
Seed Dormancy and Germination
Exogenous ABA can inhibit the precocious germination of immature embryos
in culture (121). Embryos of the ABA-biosynthetic mutants from maize exhibit
precocious germination while still attached to the mother plant or vivipary
(91). Seeds of the ABA biosynthetic mutants fromArabidopsis thaliana (71, 77)
andNicotiana plumbaginifolia (88) fail to become dormant. These observations
support that endogenous ABA inhibits precocious germination and promotes
seed dormancy.Thus far, our knowledge on the signaling elements that mediate the regulation
of seed dormancy and germination by ABA is primarily derived from genetic
analysis. As summarized in Table 1, mutations that alter the sensitivity to ABA
8/6/2019 aba signallingh
3/24
ABA SIGNALING 201
Table 1 Main characteristics of the various mutations known to affect ABA sensitivity
Species Mutation Dominancea Phenotype Gene productb Ref.
Hordeum cool ABA insensitivity 118
vulgare in guard cells
Zea mays vp1 R ABA insensitivity Seed-specific 92, 1 20
in seeds transcription
factor
rea R ABA insensitivity 144
in seeds
Craterostigma cdt-1 D Constitutive ABA Regulatory 31
plantagineum response in callus RNA or short
polypeptide
Arabidopsis abi1 SD ABA insensitivity Protein phos- 72, 78, 9 7thaliana phatase 2C
abi2 SD ABA insensitivity Protein phos- 72, 7 9
phatase 2C
abi3 R ABA insensitivity Seed-specific 38, 7 2,
in seeds transcription 102, 108
factor
abi4/5 R ABA insensitivity 27
in seeds
era1 R ABA hypersensitivityc subunit 24
of farnesyl
transferaseera2/3 R ABA hypersensitivityc 24
gca1/8 ABA insensitivityd 10
axr2 D Resistance to auxin, 163
ethylene, and ABAd
jar1 R Resistance to MeJa and 143
hypersensitivity to ABAc
jin4 R Resistance to MeJa 11
and hyper-sensitivity
to ABAc
bri1 R Resistance to brass- 22
inosteroids and hyper-sensitivity to ABAd
sax R Hypersensitivity to auxin f
and ABAe
Nicotiana plum- iba1 R Resistance to auxin, Molybdenum 13, 8 0
baginifolia (aba1)g cytokinin, and ABAc cofactor
biosynthesis
aDominance of the mutant alleles over wild-type; D, dominant; SD, semidominant; R, recessive.bMolecular function of the product of the wild-type gene.cSensitivity of seed germination to exogenous ABA.dSensitivity of seedling (root) growth to exogenous ABA.eSensitivities of root growth and stomatal closing to exogenous ABA.fM. Fellner and G. Ephritikhine, personal communication.gThe IBA1 locus was renamed ABA1 when it was found that iba1 and other alleles are in fact ABA-deficient, as
a result of a defect in the biosynthesis of a molybdenum cofactor that is required for multiple enzymatic activitiesincluding ABA aldehyde oxidase (the last step in ABA biosynthesis).
8/6/2019 aba signallingh
4/24
202 LEUNG & GIRAUDAT
in seeds have been described in maize and in A. thaliana, and several of the
corresponding genes have been cloned.
The maize vp1 (viviparous1) (119) and rea (red embryonic axis) (144) muta-
tions lead, like ABA biosynthetic mutations in this species, to vivipary. How-ever, vp1 (120) and rea (144) embryos do not have reduced ABA content but
rather exhibit a reduced sensitivity to germination inhibition by exogenous
ABA in culture. As is discussed below, the VP1 gene encodes a seed-specific
transcription factor (92).
The A. thaliana ABA-insensitive ABI1 to ABI5 loci were all identified by
selecting for seeds capable of germinating in the presence of ABA concen-
trations (310 M) that are inhibitory to the wild type (27, 72). The abi1
(72), abi2 (72), and abi3 (72, 101, 108) mutants also exhibit, like A. thaliana
ABA-deficient mutants, a marked reduction in seed dormancy. These threeloci thus seem to mediate the inhibitory effects of endogenous ABA on seed
germination. The ABI1 (78, 97) and ABI2 (79) genes encode homologous pro-
tein serine/threonine phosphatase 2C (PP2C) (12, 79). The ABI3 gene is the
ortholog of the maize VP1 gene mentioned above (38). It is presently unclear
whether the ABI1, ABI2, and ABI3 loci act in the same or in partially distinct
ABA signaling cascade in seeds (28, 110).
Mutations in the ERA1 to ERA3 (Enhanced Response to ABA) loci of A.
thaliana were identified by a lack of seed germination in the presence of low
concentrations of ABA (0.3 M) that are not inhibitory to the wild type (24).
The era1 mutations also markedly increase seed dormancy. The ERA1 gene
encodes the subunit of a farnesyl transferase, which may possibly function as a
negative regulator of ABA signaling by modifying signal transduction proteins
for membrane localization (24). The exact relationships between ERA1 and the
above-mentioned ABIloci are unknown.
Reserve Accumulation and Acquisition
of Desiccation Tolerance
The accumulation of nutritive reserves and the acquisition of desiccation tol-
erance are associated with the expression of specific sets of mRNAs (60, 121).
Transcripts encoding either storage proteins or late-embryogenesis-abundant
(LEA) proteins thought to participate in desiccation tolerance can be preco-
ciously induced by exogenous ABA in cultured embryos (60, 121). The char-
acterization of ABA-deficient mutants in A. thaliana (70, 77, 96, 101, 112) and
in maize (91, 109) supports a contribution of endogenous ABA in the develop-
mental expression of these genes in seeds.
The functional dissection of such ABA-responsive promoters, conducted pri-marily in transient expression systems, has identified several cis-acting elements
involved in ABA-induced gene expression.
8/6/2019 aba signallingh
5/24
ABA SIGNALING 203
Table 2 cis-acting promoter elements involved in the activation of gene
expression by ABA
Gene (Species) Element Sequencea Ref.
Rab16 Motif I GTACGTGGC 141
(Oryza sativa) Motif III GCCGCGTGGC 107
Em Em1a ACACGTGGC 44
(Triticum aestivum) Em1b ACACGTGCC 44
HVA22 ABRE3 GCCACGTACA 138
(Hordeum vulgare) CE1 TGCCACCGG 138
HVA1 ABRE2 CCTACGTGGC 139
(Hordeum vulgare) CE3 ACGCGTGTCCTC 139
C1 Sph CGTGTCGTCCATGCAT 65(Zea mays)
CDeT27-45 AAGCCCAAATTTCACA- 104
(Craterostigma GCCCGATAACCG
plantagineum)
aThe ACGT core in G-box-type ABREs is underlined.
CIS-ACTING PROMOTER ELEMENTS A first category of elements is exemplified
by the related motif I of the Rab16LEA gene from rice (107, 141) and Em1a
motif of the wheat Em LEA gene from wheat (44). These sequences share
a G-box ACGT core motif (Table 2) and have been designated ABREs, for
ABA Response Elements. ABRE-related sequence motifs are present in many
other ABA-inducible genes, although their function in ABA signaling often
remains speculative in the absence of experimental tests. For instance, only
a subset of the multiple ABRE-like motifs present in certain promoters are
indeed required for ABA regulation (20, 48, 117, 138, 139). A second type of
cis-acting element has been identified in the maize C1 gene, a regulator of
anthocyanin biosynthesis in seeds. The ABRE-like motifs present in C1 are
not major determinants of the ABA responsiveness of this gene, whereas the
distinct sequence motif designated as Sph element (Table 2) is essential for
ABA induction of the C1 promoter (65).
Multimerized copies of ABREs (107, 141, 155) or of the Sph element (65)
can confer ABA responsivity to minimal promoters. Single copies of these
elements were, however, not sufficient for ABA response, suggesting that mul-
timerization substituted for other aspects of the native promoter contexts of
these elements. In fact, the smallest promoter units designated ABRCs shown
to be both necessary and sufficient for ABA induction of gene expression appearto consist of (at least) two essential cis-elements. ABRCs can be composed of
two G-boxes as in the case of the wheatEm promoter, where both the Em1a and
8/6/2019 aba signallingh
6/24
204 LEUNG & GIRAUDAT
Em1b G-box motifs (Table 2) contribute to the activity of the 75-bp Complex
I (44, 155). In contrast, several other ABRCs consist of an ABRE and of a
cis-element not related to G-boxes. The G-box-type motif I and the distinct
motif III are essential for the ABA responsiveness of a 40-bp fragment of thericeRab16B promoter (107). In the barleyHVA22 promoter, ABRC1 is a 49-bp
fragment that comprises ABRE3 and the Coupling Element CE1 located 20-bp
downstream (138). In the barley HVA1 promoter, the 22-bp long ABRC3 con-
sists of the coupling element CE3 directly upstream of ABRE2 (139). These
results indicate that a diversity of ABRCs participate in ABA signaling in
seeds.
TRANS-ACTING FACTORS The nature of the transcription factors that mediate
ABA regulation of seed genes via the various above-mentioned cis-elementsremains largely unknown. Most proteins that bind DNA sequences with ACGT
core motifs contain a basic region adjacent to a leucine-zipper motif (bZIP-
proteins). Several plant bZIP-proteins that bind to ABREs in vitro and/or are in-
ducible (at the mRNA level) by ABA have been cloned (44, 68, 74, 100, 103, 105).
However, few of these genes are known to be expressed in seeds, and thus far
none has been unambiguously shown to be involved in ABA signaling in vivo
(83).
In contrast, genetic and molecular evidence supports that the maize VP1
and A. thaliana ABI3 proteins are homologous transcription factors essential
for ABA action in seeds. The vp1 (91, 109) and abi3 (28, 101, 108, 112) mu-
tants are similarly altered in sensitivity to ABA, and in various other aspects
of seed maturation including the developmental expression of storage proteins
and LEA genes. The maize VP1 (92) and A. thaliana ABI3 (38) genes, as
well as the rice OsVP1 (47) and Phaseolus vulgaris PvALF(19) genes, encode
homologous proteins (Figure 1). These genes are all exclusively expressed
in seeds (19, 47, 92, 112). However, when ectopically expressed in transgenic
A. thaliana plants, ABI3 rendered vegetative tissues hypersensitive to ABA and
Figure 1 Schematic diagram of the architecture of the VP1/ABI3-like proteins. The maize VP1
(92), A. thaliana ABI3 (38), rice OsVP1 (47), and Phaseolus vulgaris PvALF (19) proteins display
a similar and novel structural organization. They contain, in particular, four domains of high aminoacid sequence identity: the A domain located in the large acidic () N-terminal region and three
basic domains designated as B1, B2, and B3 in order from the N terminus. The sizes of these
proteins range from 691 (VP1) to 752 (PvALF) amino acids.
8/6/2019 aba signallingh
7/24
ABA SIGNALING 205
activated expression of several otherwise seed-specific genes in leaves when ex-
ogenous ABA was supplied (110, 112). Similarly, in transient expression stud-
ies, VP1 (65, 92, 155), OsVP1 (47, 48), and PvALF (18, 19) could trans-activate
various seed-specific promoters. When investigated, these trans-activationswere found to require the acidic N-terminal domain of the relevant VP1/ABI3-
like protein (48, 92), and domain-swapping experiments showed that the acidic
N-terminal regions of VP1 (92) and PvALF (19) are indeed functional transcrip-
tional activation domains. Thus, taken together, the above-mentioned results
indicate that VP1/ABI3-like proteins act as transcription factors.
The maize vp1 (56, 91, 109) and A. thaliana abi3 (70, 77, 101, 108, 112) mu-
tants exhibit a larger spectrum of seed phenotypes than the ABA-deficient
mutants in the corresponding species. A possible explanation would be that the
physiological roles of VP1 and ABI3 are not strictly confined to ABA signal-ing and rather that these proteins integrate ABA and other seed developmental
factors. This model is also consistent with the observation that VP1/ABI3-
like proteins can substantially trans-activate seed promoters in the absence
of (added) ABA (19, 48, 92, 110), and that, within a given promoter, the cis-
elements involved in VP1 responsiveness are partially separable from those
involved in ABA responsiveness (65, 155).
In the wheat Em promoter, the ABRE Em1a (Table 2) that is essential for
ABA induction also mediates transactivation by VP1 and synergism between
ABA and VP1 (155). The conserved domain B2 of VP1 is required for Em
transactivation (54), but its exact role in this process remains unclear (54, 131).
Furthermore, no specific binding of VP1 to G-box-like ABREs has been ob-
served thus far (54, 92, 145), suggesting that VP1 is a coactivator protein that
physically interacts with G-box-binding proteins (91, 155). Unlike for Em,
the combined response of the maize C1 promoter to ABA and VP1 is less
than additive , and transactivation of the C1 promoter by VP1 is mediated ex-
clusively by the sequence CGTCCATGCAT located within the Sph promoter
element (Table 2) (65). The conserved basic domain B3 of VP1 is essential
for C1 transactivation, and the isolated B3 domain binds specifically to the
above-mentioned sequence element in vitro (145). VP1 is, however, likely to
interact with other DNA-binding proteins involved in ABA regulation of C1,
because the entire Sph element is required for responsiveness of C1 to both
ABA and VP1 (65). The VP1/ABI3-like proteins thus appear to be multifunc-
tional transcription factors that integrate ABA and other regulatory signals of
seed maturation, most likely by interacting with distinct trans-acting factors
that remain to be identified.
UPSTREAM SIGNALING ELEMENTS Genetic studies in A. thaliana have iden-
tified a few other intermediates in the ABA regulation of gene expression in
seeds. The already mentioned abi4 and abi5 mutants share with abi3 mutants
8/6/2019 aba signallingh
8/24
206 LEUNG & GIRAUDAT
a decreased ABA sensitivity in germinating seeds, as well as a reduced devel-
opmental expression of certain LEA genes. These three loci have thus been
proposed to act in a common regulatory pathway of seed maturation (27). In
addition, the recent analysis of abi3 fus3 and abi3 lec1 double mutants in-dicates that ABI3 acts synergistically with the FUSCA3 (FUS3) and LEAFY
COTYLEDON1 (LEC1) genes to control multiple elementary processes dur-
ing seed maturation, including sensitivity to ABA and accumulation of storage
protein mRNAs (111). The ABI4, ABI5, FUS3, and LEC1 gene products are
currently unknown.
Studies on barley aleurone protoplasts indicate that reversible protein phos-
phorylation is likely to be implicated in the regulation of gene expression by
ABA in seeds. ABA rapidly stimulated the activity of a MAP kinase, and
this stimulation appeared to be correlated with the induction of the Rab16
mRNA (69). Okadaic acid (an inhibitor of the PP1 and PP2A classes of ser-
ine/threonine protein phosphatases) inhibited the induction of the HVA1 (73)
and Rab16 (51) mRNAs by ABA, and phenylarsine oxide (an inhibitor of ty-
rosine protein phosphatases) blocked the induction ofRab16(51). The protein
kinase and phosphatases revealed by these studies remain to be isolated.
Finally, suggestive evidence indicates that modifications of intracellular cy-
tosolic free Ca2+ levels ([Ca]i) and cytoplasmic pH (pHi) may act as intracellular
second messengers in the ABA regulation of gene expression in seeds. In barley
aleurone protoplasts, ABA triggers an increase in pHi (153) and a decrease in
[Ca]i (35, 158). However, the exact contribution of these alterations in [Ca]i and
pHi to the regulation of gene expression by ABA could not be clearly established
(152, 153).
ABA SIGNAL TRANSDUCTION IN STRESS RESPONSE
During vegetative growth, endogenous ABA levels increase upon conditions
of water stress, and ABA is an essential mediator in triggering the plant re-
sponses to these adverse environmental stimuli (166). As is discussed below,
substantial evidence supports that the increased ABA levels limit water loss
through transpiration by reducing stomatal aperture. ABA is also involved
in other aspects of stress adaptation. For instance, ABA-deficient mutants of
A. thaliana are impaired in cold acclimation (87) and in a root morphogenetic
response to drought (drought rhizogenesis) (154). The role of ABA in signaling
stress conditions has also been extensively documented by molecular studies
showing that ABA-deficient mutants are affected in the regulation of numer-
ous genes by drought, salt, or cold (39, 60, 140). It should, however, be notedthat the adaptation to these adverse environmental conditions also involves
ABA-independent pathways (140). Finally, ABA is involved in the induction
of gene expression by mechanical damage (116).
8/6/2019 aba signallingh
9/24
ABA SIGNALING 207
Regulation of Gene Expression in Response to Stress
PROMOTER STUDIES The regulation of gene expression by ABA in vegetative
tissues appears to involve several signaling pathways. The ABA induction of
distinct genes exhibit differential requirements for protein synthesis (164). In
addition, several types ofcis-acting elements are involved in ABA-induced gene
expression in vegetative tissues. Many of the LEA genes that are abundantly
expressed in desiccating seeds are also responsive to drought stress and ABA in
vegetative tissues (60, 121). For several of these genes, the G-box-type ABREs
already described (Table 2) are required for ABA induction both in seeds and
in vegetative tissues (20, 44, 48, 107, 117).
In contrast, ABRE-like motifs are not involved in the ABA regulation of other
stress-inducible genes such as theA. thaliana RD22 (63) and the Craterostigmaplantagineum CDeT27-45 genes (104). The distinct sequence motif shown in
Table 2 is essential for ABA responsiveness of CDeT27-45 (104). In this
case, as for ABREs (see section on ABA Signal Transduction in Seeds), the
corresponding physiological trans-acting factors are presently unknown. Genes
that are induced by ABA and encode other types of potential transcription
factors include the A. thaliana homeobox gene ATHB-7(142) and several myb
homologues from A. thaliana (151) and Craterostigma (62). The role of these
genes in ABA signaling has, however, not been tested experimentally.
POTENTIAL INTERMEDIATES Studies performed on various model systems
have identified several potential signaling intermediates in the ABA activation
of gene expression in response to stress.
The resurrection plant Craterostigma plantagineum can tolerate extreme de-
hydration. However, in vitro propagated callus derived from this plant has a
strict requirement for exogenously applied ABA in order to survive a severe
dehydration (31). This property has been exploited for isolation of dominant
mutants by activation tagging, in which high expression of resident genes acti-
vated by insertion of a foreign promoter would confer desiccation tolerance to
the transformed cells without prior ABA treatments. One gene was identified
(CDT-1), whose high expression did confer the expected phenotypes in calli
and led to constitutive expression of several ABA- and dehydration-inducible
genes (31). The function of the CDT-1 gene is not immediately obvious, be-
cause it encodes a transcript with no large open reading frame. It is possible
that the biologically active product of CDT-1 is a regulatory RNA or a short
polypeptide (31).
The ABA regulation of gene expression in vegetative tissues is likely to
involve reversible protein phosphorylation events. Several stress- and ABA-inducible mRNAs that encode protein kinases have been identified (5759). In
epidermal peels ofPisum sativum, the ABA-induced accumulation of dehydrin
mRNA was reduced by K-252a (an inhibitor of serine/threonine protein kinases)
8/6/2019 aba signallingh
10/24
208 LEUNG & GIRAUDAT
and also by okadaic acid and cyclosporin A (an inhibitor of the PP2B class of
serine/threonine protein phosphatases) (53).
The involvement of particular protein phosphatases 2C was revealed by the
cloning of the ABI1 (78, 97) and ABI2 (79) genes ofA. thaliana. Their corre-sponding mutants show decreased sensitivities to the ABA inhibition of seedling
growth, and defects in various morphological and molecular responses to ap-
plied ABA in vegetative tissues (28, 41, 72). Despite the highly similar archi-
tecture of the ABI1 and ABI2 proteins (79), their functions are unlikely to
be completely redundant for all ABA actions. The abi1-1 and abi2-1 muta-
tions lead to identical amino acid substitutions at equivalent positions in the
ABI1 and ABI2 proteins, respectively (79), but have (at least quantitatively)
different effects on some of the responses to exogenous ABA or water stress
(25, 33, 41, 124, 142, 154).The identification of stress- and ABA-inducible mRNAs that code for a Ca2+-
binding membrane protein in rice (30) and for a phosphatidylinositol-specific
phospholipase C in A. thaliana (55), respectively, provides circumstantial ev-
idence for the possible involvement of Ca2+ in ABA signaling in vegetative
tissues. In addition, ABA has been shown to induce increases in [Ca]i and pHiin cells of corn coleoptiles and of parsley hypocotyls and roots (32).
SINGLE-CELL SYSTEMS Ambitious attempts are being made to reconstruct
minimal stress and ABA signaling pathways by reassembling candidate com-
ponents in single-cell systems.
In maize leaf protoplasts, the barley HVA1 promoter fused to reporter genes
could be activated by applied ABA or by various stress conditions (136). In the
absence of these stimuli, expression of the reporter gene could be induced by
treating the protoplasts with 1 mM Ca2+ plus Ca2+ ionophores, or by overex-
pressing constitutively active forms of the normally Ca2+-dependent ATCDPK1
and ATCDPK1a protein kinases fromA. thaliana. Overexpressing the catalytic
domain of the wild-type A. thaliana ABI1 PP2C inhibited the activation of
HVA1 by ABA or by ATCDPK1 (136).
Microinjection experiments were conducted in hypocotyl cells of the tomato
aurea mutant to investigate the signaling cascade that mediates ABA activa-
tion of the A. thaliana rd29A (165) and kin2/cor6.6 (157) promoters fused to
the GUS reporter gene. This work supports the existence of a minimal linear
cascade in which (a) ABA triggers a transient accumulation of cyclic ADP-
ribose (cADPR) (for information on cADPR, see 3), (b) cADPR induces a
release of Ca2+ from internal stores, and (c) a K-252a-sensitive kinase acting
downstream of Ca
2+
is required for activation of rd29A and kin2 (Y Wu &N-H Chua, personal communication). Microinjection of the mutant abi1-1
protein inhibited ABA-, cADPR-, and Ca2+-induced gene expression, and these
8/6/2019 aba signallingh
11/24
ABA SIGNALING 209
effects could be reversed by an excess of wild-type ABI1 protein (Y Wu, A
Himmelbach, E Grill & N-H Chua, personal communication).
These two sets of results illustrate that combining genetics with single-cell
analyses represents a promising approach for deciphering possible epistaticrelationships between candidate components in ABA signaling cascades.
Regulation of Stomatal Aperture
The stomatal pore is defined by a pair of surrounding guard cells. The closing
and opening of the pore result from osmotic shrinking and swelling of these
guard cells, respectively. In conditions of water stress, the increase in cellular
ABA (45) or in apoplastic ABA at the surface of guard cells (161) is thought
to provoke a reduction in turgor pressure of the guard cells, which in turn
leads to stomatal closure to limit transpirational water loss. Applied ABA
inhibits the opening and promotes the closure of stomatal pores (8, 16, 159). The
enhanced transpiration of ABA-biosynthetic mutants (71, 77, 88) and transgenic
plants expressing an anti-ABA antibody (7) provides evidence for such a role
of endogenous ABA.
Unlike most other cells in higher-plant tissues, guard cells at maturity lack
plasmodesmata (162). This property has afforded a single-cell system, directly
accessible in planta, to investigate how ABA is perceived and transduced to
trigger integrated physiological responses.
ABA PERCEPTION Available evidence suggests that guard cells possess at least
two sites of ABA perception involved in the regulation of stomatal aperture,
one located at the plasma membrane and a second located intracellularly.
Stomatal closure can be triggered by extracellular application of ABA. How-
ever, the protonated form of the weak acid ABA (ABAH) readily permeates
the lipid bilayer of the cell membrane (64). Guard cells ofCommelina commu-
nis appear to have, in addition, a significant carrier-mediated uptake of ABA
(86, 133). The requirement for an extracellular ABA receptor is indicated by
the failure of ABA to inhibit the stomatal opening when microinjected directly
into the cytosol of Commelina guard cells (5). In this species, extracellular
ABA was, however, less effective in regulating stomatal aperture at high than
at low external pH (that favors passive uptake of ABAH), providing indirect
evidence for an intracellular ABA receptor (5, 106, 113, 133). This conclusion
is also supported by the reports that stomatal closure could be triggered in Com-
melina by ABA released inside guard cells from caged ABA microinjected into
the cytosol (2), and by ABA microinjected into guard cells in the presence of
1 M extracellular ABA (133).ABA closes stomatal pores by inducing net efflux of both K+ and Cl from
the vacuole to the cytoplasm, and from the cytoplasm to the outside of guard
8/6/2019 aba signallingh
12/24
210 LEUNG & GIRAUDAT
cells (85, 86). Tracer flux studies, measuring rate of loss of86Rb+ from isolated
Commelina guard cells, also indicate multiple actions of ABA, both inside and
outside the cell. It has been proposed that internal receptors regulate tonoplast
ion channels for release of vacuolar ions, whereas external receptors mediatethe stimulation of ion efflux at the plasma membrane (85, 86).
ION CHANNELS REGULATED BY ABA Electrophysiological studies, either by
whole cell impalement or by patch clamping of the plasma membrane of guard
cell protoplasts or of isolated vacuoles, have identified a number of ionic chan-
nels present in these guard cell membranes. The nature of the tonoplast ion
channels involved in the regulation of stomatal aperture by ABA is not yet
clearly identified (159). Attention is thus focused here on ion transport mech-
anisms across the plasma membrane of guard cells.
ABA causes a depolarization of the plasma membrane (149). Two types of
anion channels, which may reflect different states of a single channel protein
(26, 132), have been identified in the plasma membrane of guard cells (50, 129).
One of these types of anion channels (S-type) shows slow and sustained acti-
vation properties and is active over a wide range of voltage. Inhibitor studies
(132) and the recent demonstration that ABA activates S-type anion channels
(43, 114) indicate that these channels can account for the membrane depolariza-
tion and prolonged anion efflux required for ABA-mediated stomatal closure.
Inactivation of the plasma membrane H+-ATPase by ABA may, however, also
contribute to the membrane depolarization during stomatal closure (40).
The guard cell plasma membrane contains two types of K+ channels. The
inward-rectifying K+ channel is responsible for K+ influx to the guard cell
and is inhibited by ABA (76, 130, 149). The outward-rectifying K+ channel
is active only at membrane potentials positive to the equilibrium potential for
K+, and mediates K+ efflux required for stomatal closure (14, 130, 149). The
ABA-induced membrane depolarization mentioned above will thus activate
outward-rectifying K
+
channels, and the magnitude of this outward K
+
currentis further enhanced by ABA in a largely voltage-independent manner (14, 76).
SECOND MESSENGERS One of the earliest responses of guard cells to ABA is
an increase in the concentration of cytosolic free Ca2+ ([Ca]i) (2, 34, 61, 88a, 89,
128). The origin of the Ca2+ required to elevate [Ca]i in response to ABA is
unclear (90), but evidence favoring influx across the plasma membrane (128) or
release from internal stores mediated by inositol 1,4,5-trisphosphate (17, 37, 75)
and/or cyclic ADP-ribose (3, 90) has been presented. Experimental elevation
of [Ca]i is sufficient to induce stomatal closure (37) and mimics several of theabove-mentioned effects of ABA on ion channels in the plasma membrane of
guard cells. Increased [Ca]i activates the S-type anion channel and inhibits
8/6/2019 aba signallingh
13/24
ABA SIGNALING 211
the inward-rectifying K+ channel (50, 127). These results thus provide strong
evidence for the involvement of Ca2+-coupled signal transduction cascade(s)
in the regulation of guard cell turgor by ABA.
Several observations, however, indicate that Ca2+-independent signalingpathways are also involved. The extent of ABA-induced increase in [Ca]iis variable, and stomata could even close in response to ABA without any
detectable change in [Ca]i (2, 34, 89). It remains, however, unclear whether
ABA always produces a local increase in [Ca]i that does not systematically
translate into global cytoplasmic change (89), or whether ABA can produce
stomatal closure using only Ca2+-independent signaling pathways (2). In any
case, the fact that the outward-rectifying K+ channel is insensitive to increases
in [Ca]i (76, 127) indicates that other second messengers must be involved in
the regulation of plasma membrane ion channels by ABA.
In particular, ABA increases the cytoplasmic pH (pHi) of guard cells (15, 61).
Studies in which guard cells were loaded with pH buffers or pHi was experi-
mentally modified support that the ABA-induced cytoplasmic alkalinization can
account for ABA activation of the outward-rectifying K+ channel and also con-
tribute to ABA inactivation of the inward-rectifying K+ channel (15, 42, 76, 98).
The ABA regulation of ion channels in guard cells thus appears to involve, at
least, both Ca2+-dependent and pHi-dependent signaling pathways.
DOWNSTREAM SIGNALING ELEMENTS How the ABA-induced Ca2+ and pHisignals are then decoded and relayed by other cellular components in the trans-
duction chain is not clear. The recent report that pHi regulates the outward-
rectifying K+ channel in isolated membrane patches from Vicia faba guard cells
suggests that the signaling elements acting downstream of the pH i signal are
membrane associated (98). The Ca2+ signal is possibly relayed by particular
protein kinases and phosphatases. Both Ca2+-dependent protein kinases (115)
and phosphatases (4, 23, 84) have been inferred to be potential candidates in
modulating the activity of various ion channels in guard cells, but their precise
roles in ABA signaling have not been clearly established at present.
There is nonetheless incontrovertible evidence that protein phosphorylation
is a critical component in the ABA signal transduction controlling stomatal
closure. In Vicia faba and Commelina, stomatal closure induced by ABA
was abolished by kinase inhibitors while enhanced by inhibitors of the protein
phosphatases PP1 and/or PP2A (125). These protein phosphatases, whose
actions are Ca2+-independent, have been implicated in the regulation of the
inward-rectifying K+ channel (82, 148), outward-rectifying K+ channel (148),
and S-type anion channel (114, 125) in Vicia faba and Commelina guard cells.Recent reports described two protein kinases whose activities are rapidly
stimulated by ABA in protoplasts from guard cells but not mesophyll cells in
8/6/2019 aba signallingh
14/24
212 LEUNG & GIRAUDAT
Vicia faba (81, 99). Although they have a similar apparent molecular weight,
the two protein kinases are probably distinct because they differ in their ability
to autophosphorylate and in their substrate specificity in vitro (81, 99). The
cellular targets of these kinases are not yet known, but because their activitiesare detectable minutes after induction by physiological concentrations of ABA,
these kinases may be involved in the modulation of ion channel activities that
are essential for rapid stomatal responses.
Genetic dissection of ABA signaling in guard cells has been rather limited
despite an initial and promising report of the barley mutant cool, which dis-
played excessive transpiration and ABA-insensitive guard cells (118). Mutants
with specific impairment in ABA-induced stomatal closure have not yet been
identified in A. thaliana.
However, among the gca1-gca8 (Growth Control by ABA) mutants that wereisolated based on their reduced sensitivities to the inhibition of seedling growth
by exogenous ABA, the gca1 and gca2 mutants display a disturbed stomatal
regulation (10). The abi1-1 and abi2-1 mutants mentioned earlier also have
ABA-insensitive guard cells as part of their pleiotropic phenotypes (114, 122).
What are the roles of the Ca2+-independent (12) ABI1 and ABI2 protein
phosphatases 2C (PP2Cs) in stomatal regulation by ABA? Taking advantage of
the fact that the abi1-1 mutation is dominant (72, 78), the mutant A. thaliana
gene was introduced into the diploid tobacco Nicotiana benthamiana to facil-
itate electrophysiological measurements (6). Voltage clamp studies found that
the mutant abi1-1 transgene had no detectable effect on anion channels, but de-
creased ABA sensitivity of both the inward- and outward-rectifying K+channels
in the guard cell plasma membrane (6, 43). ABA sensitivity of these K+ chan-
nels and ABA-induced stomatal closure could be partially reestablished in the
abi1-1 transgenic plants by adding protein kinase inhibitors (6). Recent and im-
portant advances made in single-cell techniques have permitted measurements
of ion channel activities in A. thaliana guard cells, which had previously posed
technical difficulties because of their small sizes (114, 123). In patch-clamp
studies ofA. thaliana guard cell protoplasts, the ABA response of S-type anion
channels was impaired in the abi1-1 mutant. However, consistent with the re-
versibility of the effects of the abi1-1 transgene in tobacco (6), treatment with
the protein kinase inhibitor K-252a could partially restore ABA regulation of
the anion channel and ABA-induced stomatal closure in the A. thaliana abi1-1
mutant (114). The abi2-1 mutation also suppressed the ABA activation of S-
type anion channels, but curiously, this inhibition could not be counterbalanced
by K-252a treatments (114). In addition, the abi2-1 mutation also diminished
the background activity of inward-rectifying K
+
channels (114), an effect ob-served neither in the abi1-1 A. thaliana mutant(114)norintheabi1-1 transgenic
tobacco plants (6). The differential effects of the abi1 and abi2 mutations on
8/6/2019 aba signallingh
15/24
ABA SIGNALING 213
the background activity of inward-rectifying K+ channels, and the differential
sensitivity of these mutants to K-252a, thus indicate that, although the ABI1
and ABI2 proteins are homologous PP2Cs, the functions of these proteins in
stomatal regulation, as in other physiological responses, are nonredundant.The apparent discrepancies between the above-mentioned results on abi1-1
transgenic tobacco and abi1-1 mutant A. thaliana plants may be indicative of
the functional flexibility of ABI1 in maintaining an integrated ABA regulation
of both K+ and anion channels, depending on the imposed (environmental or
experimental) conditions. This would be analogous to the relative importance
of the ABA-induced increase in [Ca]i, which is subjected to both experimental
(89) and environmental (2) influences. A more serious problem in terms of
formulating a unified model of stomatal regulation, however, is how many
species-dependent differences exist in ABA signaling. For instance, various
pharmacological studies indicate that PP1/PP2A protein phosphatases (distinct
from the ABI1 and ABI2 PP2Cs) act as positive regulators of anion currents inA.
thaliana guard cells (114), but as negative regulators in Vicia, Commelina, and
N. benthamiana (43, 125). Therefore, it seems that detailed comparative studies
with different species would be necessary before a fuller understanding of the
signaling cascades that mediate stomatal regulation by ABA would emerge.
ABA SIGNALING TO GUARD CELL NUCLEUS As described above, stomatal
guard cells have been used extensively as a cellular system to explore the trans-duction pathways that target the ABA signal to ion channels during stomatal
closing. These responses are rapid and are thus thought to be operationally
distinct from long-term ABA responses that require RNA and protein synthesis
(166). However, hormone induction of gene expression at the transcriptional
level can also take place within minutes, as has been observed in the case of
auxin (93). The relevance of ABA in gene activation in guard cells has only
been explored recently. Several promoters or mRNAs have been shown to be
induced in guard cells by exogenous ABA treatment (53, 110, 137, 147, 157).
These studies demonstrate that guard cells are competent to relay ABA signals
to the nucleus. Initial attempts are being made to analyze whether identical or
distinct signaling pathways mediate the ABA regulations of ion channels and
gene expression in guard cells (53). It also remains to be established whether
there are gene products directly implicated in controlling stomatal movements
that are regulated by ABA transcriptionally.
CONCLUSION AND PERSPECTIVES
In recent years, the research into the molecular and physiological action of
ABA has benefited tremendously from cross-fertilization of different expertise
8/6/2019 aba signallingh
16/24
214 LEUNG & GIRAUDAT
and ideas. Studies with optically pure ABA analogues revealed that the stereo-
chemical requirements of ABA are not identical in all ABA responses (21, 156),
indicating that higher plants may contain several types of ABA receptors. Ev-
idence for ABA perception sites located both at the cell surface and intracel-lularly has come from analyses on the regulation of ion fluxes in guard cells.
Whether this model is applicable to other cell types, and to more long-term
responses involving changes in gene expression, are important questions to be
explored (for instance, see 36). In addition, even though locations of these
reception sites can be detected physiologically, we have no firm idea whether
they are functionally distinct, and if so, what physiological relevance they may
harbor. Cloning genes of ABA receptors and creating corresponding mutants
would thus constitute a major step toward dissecting these initial events of ABA
action.Besides reception sites, we also need to improve further our understand-
ing of the more downstream events in the transduction cascades triggered by
ABA signals. Recent advances point to the central role of reversible protein
phosphorylation in mediating several of the physiological responses to ABA.
Much work, however, is clearly needed to identify the pertinent kinases and
phosphatases, and cellular targets of their action.
The ectopic expression of the seed-specific ABI3 gene in leaves resulted in
activation of several otherwise seed-specific genes. This observation and the
pleiotropic phenotypes of some ABA response mutants raise the question of
the degree of similarity between signaling cascades in the different tissues (or
cell types in the same tissue). It might be possible that many core components
of the ABA signaling network exist in different tissues. A simple model would
be that these central cascades are then regulated by cell-specific factors such
as the VP1/ABI3-like proteins. In this light, ABA-stimulated protein kinases
with apparent specificity to guard cells may play similar deterministic roles in
activating a branch of the ABA pathway in this cell type.
Isolation of additional mutants by judicious genetic screens (taking into ac-
count the potential for redundancy in signaling components) will no doubt
enrich our knowledge about regulatory points in ABA action, which might
include modulation in the activity of synthetic enzymes and transport of the
hormone (150). These two latter points have not been the subject of intense
investigations. Furthermore, mutations that simultaneously alter the respon-
siveness to ABA and other hormones or developmental clues, will permit a
fuller appreciation of cross-talk between ABA and other cellular signals. Sev-
eral mutants of this type have already been isolated in A. thaliana (Table 1).
The eventual cloning of the genes identified by mutational analyses will revealtheir molecular identity and, in some cases, suggest possible functions based on
homologies with known proteins. Genes with unexpected structural features
8/6/2019 aba signallingh
17/24
ABA SIGNALING 215
(such as CDT-1 in C. plantagineum) will divulge novel signal transduction
mechanisms, to the delight of researchers with iconoclastic tendencies.
The isolation of genes responsive to ABA, coupled with promoter analyses,
are vital to our understanding of how combinatorial and synergistic actions ofcis-acting elements and trans-acting factors are involved in the versatile control
of the output response. Other approaches, such as biochemical purification and
those based on interaction cloning, will undoubtedly complement the more
established technologies in unraveling additional signaling elements or even
signaling complexes.
Investigators with a pioneering spirit have already attempted to reconstruct
particular signaling cascades by reassembling individual components suspected
to play the relevant roles. These are much welcome advances, since these
cellular systems will complement the more established guard cell model. It iscertain that continuation of such a multidisciplinary approach will yield ever
deeper insight not only into ABA signal transduction itself but also into how
ABA and other signaling molecules jointly coordinate physiological responses.
ACKNOWLEDGMENTS
Work in the authors laboratory is supported by the Centre National de la
Recherche Scientifique and grants from the International Human Frontier
Science Program (RG-303/95), the European Community BIOTECH program
(BIO4-CT96-0062), and the Groupement de Recherches et dEtudes sur les
Genomes (Decision 9-95).
Visit the Annual Reviews home page at
http://www.AnnualReviews.org.
Literature Cited
1. Addicott FT, Carns HR. 1983. History and
introduction. InAbscisic Acid, ed.FT Ad-dicott, pp. 121. New York: Praeger Sci.2. Allan AC, Fricker MD, Ward JL, Beale
MH, Trewavas AJ. 1994. Two transduc-tion pathways mediate rapid effects ofabscisic acid in Commelina guard cells.Plant Cell 6:131928
3. Allen GJ, Muir SR, Sanders D. 1995.Release of Ca2+ from individual plantvacuoles by both InsP3 and cyclic ADP-ribose. Science 268:73537
4. Allen GJ, Sanders D. 1995. Calcineurin,a type 2B protein phosphatase, modu-lates the Ca2+-permeable slow vacuolarion channel of stomatal guard cells. PlantCell 7:147383
5. Anderson BE, Ward JM, Schroeder JI.
1994. Evidence for an extracellular recep-
tion site for abscisic acid in Commelinaguard cells. Plant Physiol. 104:1177836. Armstrong F, Leung J, Grabov A, Brear-
ley J, Giraudat J, Blatt MR. 1995. Sen-sitivity to abscisic acid of guard cell K+
channels is suppressed by abi1-1, a mu-tantArabidopsis gene encoding a putativeprotein phosphatase. Proc. Natl. Acad.Sci. USA 92:952024
7. Artsaenko O, Peisker M, zur Nieden U,Fiedler U, Weiler EW, et al. 1995. Expres-sion of a single chain Fv antibody againstabscisic acid creates a wilty phenotype intransgenic tobacco. Plant J. 8:74550
8. Assmann SM. 1993. Signal transductionin guard cells. Annu. Rev. Cell Biol.9:34575
8/6/2019 aba signallingh
18/24
216 LEUNG & GIRAUDAT
9. Deleted in proof10. Benning G, Ehrler T, Meyer K, Leube M,
Rodriguez P, Grill E. 1996. Genetic anal-ysis of ABA-mediated control of plant
growth. Proc. Workshop Abscisic AcidSignal Transduction Plants, Madrid, pp.34. Madrid: Juan March Found.
11. Berger S, Bell E, Mullet JE. 1996. Twomethyl jasmonate-insensitive mutantsshow altered expression of AtVsp in re-sponse to methyl jasmonate and wound-ing. Plant Physiol. 111:52531
12. Bertauche N, Leung J, Giraudat J. 1996.Protein phosphatase activity of abscisicacid insensitive 1 (ABI1) protein fromArabidopsis thaliana. Eur. J. Biochem.241:193200
13. Bitoun R, Rousselin P, Caboche M. 1990.A pleiotropic mutation results in cross-resistance to auxin, abscisic acid and pa-clobutrazol. Mol. Gen. Genet. 220:23439
14. Blatt MR. 1990. Potassium channel cur-rents in intact stomatal guard cells: rapidenhancement by abscisic acid. Planta180:44555
15. Blatt MR, Armstrong F. 1993. K+ chan-nels of stomatal guard cells: abscisicacidevoked control of the outward recti-fier mediated by cytoplasmic pH. Planta191:33041
16. Blatt MR, Thiel G. 1993. Hormonal con-trol of ion channel gating. Annu. Rev.Plant Physiol. Plant Mol. Biol. 44:54367
17. Blatt MR, Thiel G, Trentham DR. 1990.Reversible inactivation of K+ channelsof Vicia stomatal guard cells followingthe photolysis of caged inositol 1,4,5-trisphosphate. Nature 346:76669
18. Bobb AJ, Chern M-S, Bustos MM. 1997.Conserved RY-repeats mediate transacti-vation of seed-specific promoters by thedevelopmental regulator PvALF. Nucleic
Acids Res. 25:6414719. Bobb AJ, Eiben HG, Bustos MM. 1995.PvAlf, an embryo-specific acidic tran-scriptional activator enhances gene ex-pression from phaseolin and phytohemag-glutinin promoters. Plant J. 8:33143
20. Busk PK, Jensen AB, Pages M. 1997.Regulatory elements in vivo in the pro-moter of the abscisic acid responsive generab17 from maize. Plant J. 11:128595
21. Chandler JW, Abrams SR, Bartels D.1997. The effect of ABA analogs on
callus viability and gene expression inCraterostigma plantagineum. Physiol.Plant. 99:46569
22. Clouse SD, Langford M, McMorris TC.
1996. A brassinosteroid-insensitive mu-tant inArabidopsis thaliana exhibits mul-tiple defects in growth and development.Plant Physiol. 111:67178
23. Cousson A, Cotelle V, Vavasseur A. 1995.Induction of stomatal closure by vanadateor a light/dark transition involves Ca2+-calmodulin-dependent protein phospho-rylations. Plant Physiol. 109:49197
24. Cutler S, Ghassemian M, Bonetta D,Cooney S, McCourt P. 1996. A proteinfarnesyl transferase involved in abscisicacid signal transduction in Arabidopsis.Science 273:123941
25. deBruxelles GL,Peacock WJ,Dennis ES,Dolferus R. 1996. Abscisic acid inducesthe alcohol dehydrogenase gene in Ara-
bidopsis. Plant Physiol. 111:3819126. Dietrich P, Hedrich R. 1994. Intercon-version of fast and slow gating modesof GCAC1, a guard cell anion channel.Planta 195:3014
27. Finkelstein RR. 1994. Mutations at twonew Arabidopsis ABA response loci aresimilar to the abi3 mutations. Plant J.5:76571
28. Finkelstein RR, Somerville CR. 1990.Three classes of abscisic acid (ABA)insensitive mutations of Arabidopsis de-fine genes that control overlapping sub-sets of ABA responses. Plant Physiol.
94:11727929. Deleted in proof30. Frandsen G, Muller-Uri F, Nielsen M,
Mundy J, Skriver K. 1996. Novel plantCa2+-binding protein expressed in re-sponse to abscisic acid and osmotic stress.J. Biol. Chem. 271:34348
31. Furini A, Koncz C, Salamini F, BartelsD. 1997. High level transcription of amember of a repeated gene family con-fers dehydration tolerance to callus tissueofCraterostigma plantagineum.EMBO J.16:3599608
32. Gehring CA, Irving HR, Parish RW. 1990.Effects of auxin and abscisic acid on cy-tosolic calcium and pH in plant cells.Proc. Natl. Acad. Sci. USA 87:964549
33. Gilmour SJ, Thomashow MF. 1991. Coldacclimation and cold-regulated gene ex-pression in ABA mutants of Arabidopsisthaliana. Plant Mol. Biol. 17:123340
34. Gilroy S, Fricker MD, Read ND, Tre-wavas AJ. 1991. Role of calcium in signaltransduction of Commelina guard cells.Plant Cell 3:33344
35. Gilroy S, Jones RL. 1992. Gibberellic
acid and abscisic acid coordinately reg-ulate cytoplasmic calcium and secretoryactivity in barley aleurone protoplasts.Proc. Natl. Acad. Sci. USA 89:359195
8/6/2019 aba signallingh
19/24
ABA SIGNALING 217
36. Gilroy S, Jones RL. 1994. Perception ofgibberellin and abscisic acid at the exter-nal face of the plasma membrane of bar-ley (Hordeum vulgare L.) aleurone proto-
plasts. Plant Physiol. 104:11859237. Gilroy S, Read ND, Trewavas AJ. 1990.Elevation of cytoplasmic calcium bycaged calcium or caged inositol trispho-sphate initiates stomatal closure. Nature343:76971
38. Giraudat J, Hauge BM, Valon C, Smalle J,Parcy F, Goodman HM. 1992. Isolation ofthe Arabidopsis ABI3 gene by positionalcloning. Plant Cell 4:125161
39. Giraudat J, Parcy F, Bertauche N, Gosti F,Leung J, et al. 1994. Current advances inabscisic acid action and signaling. PlantMol. Biol. 26:155777
40. Goh C-H, Kinoshita T, Oku T, Shi-mazaki K-I. 1996. Inhibition of bluelightdependent H+ pumping by abscisicacid in Vicia guard-cell protoplasts. PlantPhysiol. 111:43340
41. Gosti F, Bertauche N, Vartanian N, Girau-dat J. 1995. Abscisic aciddependent andindependent regulation of gene expres-sion by progressive drought in Arabidop-sis thaliana. Mol. Gen. Genet. 246:1018
42. Grabov A, Blatt MR. 1997. Parallel con-trol of the inward-rectifier K+ channel by
cytosolic free Ca2+ and pH in Vicia guardcells. Planta 201:8495
43. Grabov A, Leung J, Giraudat J, Blatt MR.1997. Alteration of anion channel kineticsin wild-type and abi1-1 transgenic Nico-tiana benthamiana guard cells by abscisicacid. Plant J. 12:20313
44. Guiltinan MJ, Marcotte WR, QuatranoRS. 1990. A plant leucine zipper proteinthat recognizes an abscisic acid responseelement. Science 250:26771
45. Harris MJ, Outlaw WH Jr. 1991. Rapidadjustment of guard-cell abscisic acid
levels to current leaf-water status. PlantPhysiol. 95:1717346. Deleted in proof47. Hattori T, Terada T, Hamasuna S. 1994.
Sequence and functional analyses of therice gene homologous to the maize Vp1.Plant Mol. Biol. 24:80510
48. Hattori T, Terada T, Hamasuna S. 1995.Regulation of the Osem gene by abscisicacid and the transcriptional activator VP1:analysis of cis-acting promoter elementsrequired for regulation by abscisic acidand VP1. Plant J. 7:91325
49. Deleted in proof50. Hedrich R, Busch H, Raschke K. 1990.
Ca2+ and nucleotide dependent regula-tion of voltage dependent anion channels
in the plasma membrane of guard cells.EMBO J. 9:388992
51. Heimovaara-Dijkstra S, Nieland TJF, vander Meulen RM, Wang M. 1996. Ab-
scisic acidinduced gene-expression re-quires the activity of protein(s) sensitiveto the protein-tyrosine phosphatase in-hibitor phenylarsine oxide. Plant GrowthRegul. 18:11523
52. Deleted in proof53. Hey SJ, Bacon A, Burnett E, Neill SJ.
1997. Abscisic acid signal transductionin epidermal cells of Pisum sativum L.Argenteum: Both dehydrin mRNA accu-mulation and stomatal responses requireprotein phosphorylation and dephospho-rylation. Planta 202:8592
54. Hill A, Nantel A, Rock CD, QuatranoRS. 1996. A conserved domain of theViviparous-1 gene product enhances theDNA binding activity of the bZIP proteinEmBP-1 and other transcription factors.J. Biol. Chem. 271:336674
55. Hirayama T, Ohto C, Mizoguchi T, Shi-nozaki K. 1995. A gene encoding aphosphatidylinositol-specific phospholi-pase C is induced by dehydration and saltstress inArabidopsis thaliana. Proc. Natl.Acad. Sci. USA 92:39037
56. Hoecker U, Vasil IK, McCarty DR. 1995.Integrated control of seed maturation and
germination programs by activator andrepressor functions of Viviparous-1 ofmaize. Genes Dev. 9:245969
57. Holappa LD, Walker-Simmons MK.1995. The wheat abscisic acid-responsiveprotein kinase mRNA, PKABA1, is up-regulated by dehydration, cold tempera-ture, and osmotic stress. Plant Physiol.108:120310
58. Hong SW, Jon JH, Kwak JM, Nam HG.1997. Identification of a receptor-like pro-tein kinase gene rapidly induced by ab-scisic acid, dehydration, high salt, and
cold treatments in Arabidopsis thaliana.Plant Physiol. 113:12031259. Hwang IW, Goodman HM. 1995. AnAra-
bidopsis thaliana root-specific kinase ho-molog is induced by dehydration, ABA,and NaCl. Plant J. 8:3743
60. Ingram J, Bartels D. 1996. The molecularbasis of dehydration tolerance in plants.Annu. Rev. Plant Physiol. Plant Mol. Biol.47:377403
61. Irving HR, Gehring CA, Parish RW. 1992.Changes in cytosolic pH and calcium ofguard cells precede stomatal movements.
Proc. Natl. Acad. Sci. USA 89:17909462. Iturriaga G, Leyns L, Villegas A,
Gharaibeh R, Salamini F, Bartels D. 1996.
8/6/2019 aba signallingh
20/24
218 LEUNG & GIRAUDAT
A family of novel myb-related genesfrom the resurrection plant Craterostigmaplantagineum are specifically expressedin callus and roots in response to ABA or
desiccation. Plant Mol. Biol. 32:7071663. Iwasaki T, Yamaguchi-Shinozaki K, Shi-nozaki K. 1995. Identification of a cis-regulatory region of a gene in Arabidop-sis thaliana whose induction by dehydra-tion is mediated by abscisic acid and re-quires protein synthesis.Mol. Gen. Genet.247:39198
64. Kaiser WM, Hartung W. 1981. Uptakeand release of abscisic acid by iso-lated photoautotrophic mesophyll cell,depending on pH gradients. Plant Phys-iol. 68:2026
65. Kao C-Y, Cocciolone SM, Vasil IK, Mc-Carty DR. 1996. Localization and inter-action of the cis-acting elements for ab-scisic acid, VIVIPAROUS1, and light ac-tivation of the C1 gene of maize. PlantCell 8:117179
66. Deleted in proof67. Deleted in proof68. Kim SY, Chung H-J, Thomas TL. 1997.
Isolation of a novel class of bZIP tran-scription factors that interact with ABA-responsive and embryo-specification ele-ments in the Dc3 promoter using a mod-ified yeast one-hybrid system. Plant J.
11:12375169. Knetsch MLW, Wang M, Snaar-Jagalska
BE, Heimovaara-Dijkstra S. 1996. Ab-scisic acidinduces mitogen-activatedpro-tein kinase activation in barley aleuroneprotoplasts. Plant Cell 8:106167
70. Koornneef M, Hanhart CJ, HilhorstHWM, Karssen CM. 1989. In vivo inhi-bition of seed development and reserveprotein accumulation in recombinants ofabscisic acid biosynthesis and respon-siveness mutants inArabidopsis thaliana.Plant Physiol. 90:46369
71. Koornneef M, Jorna ML, Brinkhorst-vander Swan DLC, Karssen CM. 1982. Theisolation of abscisic acid (ABA) deficientmutants by selection of induced rever-tants in nongerminating gibberellin sen-sitive lines of Arabidopsis thaliana (L.)Heynh. Theor. Appl. Genet. 61:38593
72. Koornneef M, Reuling G, Karssen CM.1984. The isolation and characteriza-tion of abscisic acid-insensitive mutantsof Arabidopsis thaliana. Physiol. Plant.61:37783
73. Kuo A, Cappelluti S, Cervantes-Cervan-
tes M, Rodriguez M, Bush DS. 1996.Okadaic acid, a protein phosphatase in-hibitor, blocks calcium changes, gene ex-pression, and cell death induced by gib-
berellin in wheat aleurone cells. PlantCell 8:25969
74. Kusano T, Berberich T, Harada M, SuzukiN, Sugawara K. 1995. A maize DNA-
binding factor with a bZIP motif is in-duced by low temperature. Mol. Gen.Genet. 248:50717
75. Lee YS, Choi YB, Suh S, Lee J, AssmannSM, et al. 1996. Abscisic acidinducedphosphoinositide turnover in guard cellprotoplasts of Vicia faba. Plant Physiol.110:98796
76. Lemtiri-Chlieh F, MacRobbie EAC.1994. Role of calcium in the modulationof Vicia guard cell potassium channelsby abscisic acid: a patch clamp study. J.Membr. Biol. 137:99107
77. Leon-Kloosterziel KM, Gil MA, RuijsGJ, Jacobsen SE, Olszewski NE, et al.1996. Isolationand characterization of ab-scisic acid-deficient Arabidopsis mutantsat two new loci. Plant J. 10:65561
78. Leung J, Bouvier-Durand M, Morris P-C,Guerrier D, Chefdor F, Giraudat J. 1994.Arabidopsis ABA-response gene ABI1:features of a calcium-modulated proteinphosphatase. Science 264:144852
79. Leung J, Merlot S, Giraudat J. 1997.The Arabidopsis ABSCISIC ACID- INSENSITIVE 2 (ABI2) and ABI1 genesencode redundant protein phosphatases
2C involved in abscisic acid signaltransduction. Plant Cell 9:75971
80. Leydecker M-T, Moureaux T, KraepielY, Schnorr K, Caboche M. 1995. Molyb-denum cofactor mutants, specifically im-paired in xanthine dehydrogenase activityand abscisic acid biosynthesis, simultane-ously overexpress nitrate reductase. PlantPhysiol. 107:142731
81. Li JX, Assmann SM. 1996. An abscisicacid-activated and calcium-independentprotein kinase from guard cells of Favabean. Plant Cell 8:235968
82. Li WW, Luan S, Schreiber SL, AssmannSM. 1994. Evidence for protein phos-phatase 1 and 2A regulation of K+ chan-nels of two types of leaf cells. Plant Phys-iol. 106:96370
83. Lu GH, Paul A-L, McCarty DR, Ferl RJ.1996. Transcription factor veracity: IsGBF3 responsible for ABA-regulated ex-pression of Arabidopsis Adh? Plant Cell8:84757
84. Luan S, Li WW, Rusnak F, Assmann SM,Schreiber SL. 1993. Immunosuppressantsimplicate protein phosphatase regulationof K+ channels in guard cells. Proc. Natl.Acad. Sci. USA 90:22026
85. MacRobbie EAC. 1995. Effects of ABAon 86Rb+ fluxes at plasmalemma and
8/6/2019 aba signallingh
21/24
ABA SIGNALING 219
tonoplast of stomatal guard cells. PlantJ. 7:83543
86. MacRobbie EAC. 1995. ABA-inducedion efflux in stomatal guard cells: mul-
tiple actions of ABA inside and outsidethe cell. Plant J. 7:56576
87. Mantyla E, Lang V, Palva ET. 1995. Roleof abscisic acid in drought-induced freez-ing tolerance, cold acclimation, and accu-mulation of LTI78 and RAB18 proteinsin Arabidopsis thaliana. Plant Physiol.107:14148
88. Marin E, Nussaume L, Quesada A,Gonneau M, Sotta B, et al. 1996. Molecu-lar identification of zeaxanthin epoxidaseof Nicotiana plumbaginifolia, a gene in-volved in abscisic acid biosynthesis and
corresponding to ABA locus ofArabidop-sis thaliana. EMBO J. 15:23314288a. McAinsh MR, Brownlee AM, Hethering-
ton AM. 1990. Abscisic acid-induced el-evation of guard cell cytosolic Ca2+ pre-cedes stomatal closure. Nature 343:18688
89. McAinsh MR, Brownlee C, Hethering-ton AM. 1992. Visualizing changes incytosolic-free Ca2+ during the responseof stomatal guard cells to abscisic acid.Plant Cell 4:111322
90. McAinsh MR, Brownlee C, Hetherington
AM. 1997. Calcium ions as second mes-sengers in guard cell signal transduction.Physiol. Plant. 100:1629
91. McCarty DR. 1995. Genetic control andintegration of maturation and germina-tion pathways in seed development.Annu. Rev. Plant Physiol. Plant Mol. Biol.46:7193
92. McCarty DR, Hattori T, Carson CB,Vasil V, Lazar M, Vasil IK. 1991.The viviparous-1 developmental gene ofmaize encodes a novel transcriptional ac-tivator. Cell 66:895905
93. McClure BA, Hagen G, Brown CS, GeeMA, Guilfoyle TJ. 1989. Transcription,organization, and sequence of an auxin-regulated gene cluster in soybean. PlantCell 1:22939
94. Deleted in proof95. Deleted in proof96. Meurs C, Basra AS, Karssen CM, van
Loon LC. 1992. Role of abscisic acid inthe induction of desiccation tolerance indeveloping seeds ofArabidopsis thaliana.Plant Physiol. 98:148493
97. Meyer K, Leube MP, Grill E. 1994. A pro-tein phosphatase 2C involved in ABA sig-nal transduction in Arabidopsis thaliana.Science 264:145255
98. Miedema H, Assmann SM. 1996. A
membrane-delimited effect of internal pHon the K+ outward rectifier ofVicia fabaguard cells. J. Membr. Biol. 154:22737
99. Mori IC, Muto S. 1997. Abscisic acid
activates a 48-kilodalton protein kinasein guard cell protoplasts. Plant Physiol.113:83339
100. Nakagawa H, Ohmiya K, Hattori T. 1996.A rice bZIP protein, designated OSBZ8,is rapidly induced by abscisic acid. PlantJ. 9:21727
101. Nambara E, Keith K, McCourt P, NaitoS. 1995. A regulatory role for the ABI3gene in the establishment of embryo mat-uration inArabidopsis thaliana.Develop-ment121:62936
102. Nambara E, Naito S, McCourt P. 1992. A
mutant ofArabidopsis which is defectivein seed development and storage proteinaccumulation is a new abi3 allele. PlantJ. 2:43541
103. Nantel A, Quatrano RS. 1996. Charac-terization of three rice basic/leucine zip-per factors, including two inhibitors ofEmBP-1 DNA binding activity. J. Biol.Chem. 271:31296305
104. Nelson D, Salamini F, Bartels D.1994. Abscisic acid promotes novelDNA-binding activity to a desiccation-related promoter of Craterostigma plan-
tagineum. Plant J. 5:45158105. Oeda K, Salinas J, Chua N-H. 1991. A to-bacco bZip transcription activator (TAF-1) binds to a G-box-like motif conservedin plant genes. EMBO J. 10:1793802
106. Ogunkami AB, Tucker DJ, Mansfield TA.1973. An improved bioassay for abscisicacid and other antitranspirants. New Phy-tol. 72:27778
107. Ono A, Izawa T, Chua N-H, Shi-mamoto K. 1996. The rab16B promoterof rice contains two distinct abscisicacid-responsive elements. Plant Physiol.
112:48391108. Ooms JJJ, Leon-Kloosterziel KM, Bar-tels D, Koornneef M, Karssen CM.1993. Acquisition of desiccation toler-ance and longevity in seeds of Arabidop-sis thaliana. A comparative study usingabscisic acidinsensitive abi3 mutants.Plant Physiol. 102:118591
109. Paiva R, Kriz AL. 1994. Effect of abscisicacid on embryo-specific gene expressionduring normal and precocious germina-tion in normal and viviparous maize (Zeamays) embryos. Planta 192:33239
110. Parcy F, Giraudat J. 1997. Interactions be-tween the ABI1 and the ectopically ex-pressedABI3 genesin controllingabscisicacid responses in Arabidopsis vegetative
8/6/2019 aba signallingh
22/24
220 LEUNG & GIRAUDAT
tissues. Plant J. 11:693702111. Parcy F, Valon C, Kohara A, Misera
S, Giraudat J. 1997. The ABSCISIC ACID-INSENSITIVE 3 (ABI3), FUSCA
3 (FUS3) and LEAFY COTYLEDON 1(LEC1) loci act in concert to controlmultiple aspects of Arabidopsis seed de-velopment. Plant Cell. 9:126577
112. Parcy F, Valon C, Raynal M, Gaubier-Comella P, Delseny M, Giraudat J. 1994.Regulation of gene expression programsduring Arabidopsis seed development:roles of the ABI3 locus and of endoge-nous abscisic acid. Plant Cell 6:156782
113. Paterson NW, Weyers JDB, Brook RA.1988. The effect of pH on stomatal sensi-
tivity to abscisic acid. Plant Cell Environ.11:8389114. Pei Z-M, Kuchitsu K, Ward JM, Schwarz
M, Schroeder JI. 1997. Differential ab-scisic acid regulation of guard cell slowanion channels in Arabidopsis wild-typeand abi1 and abi2 mutants. Plant Cell9:40923
115. Pei Z-M, Ward JM, Harper JF, SchroederJI. 1996. A novel chloride channel in Viciafaba guard cell vacuoles activated by theserine/threonine kinase, CDKP. EMBO J.15:656474
116. Pena-Cortes H, Fisahn J, Willmitzer L.
1995. Signals involved in wound-inducedproteinase inhibitor II gene expressionin tomato and potato plants. Proc. Natl.Acad. Sci. USA 92:410613
117. Pla M, Vilardell J, Guiltinan MJ, MarcotteWR, Niogret M-F, et al. 1993. The cis-regulatory element CCACGTGG is in-volvedin ABA and water-stress responsesof the maize gene rab28. Plant Mol. Biol.21:25966
118. Raskin I, Ladyman JAR. 1988. Isola-tion and characterization of a barley mu-tant with abscisic acid-insensitive stom-
ata. Planta 173:7378119. Robertson DS. 1955. The genetics ofvivipary in maize. Genetics 40:74560
120. Robichaud C, Sussex IM. 1986. The re-sponse ofviviparous-1 and wild-type em-bryos ofZea mays to culture in the pres-ence of abscisic acid. J. Plant Physiol.126:23542
121. Rock CD, Quatrano RS. 1995. The roleof hormones during seed development. InPlant Hormones, ed. PJ Davies, pp. 67197. Dordrecht: Kluwer
122. Roelfsema MRG, Prins HBA. 1995. Ef-
fect of abscisic acid on stomatal openingin isolated epidermal strips ofabi mutantsof Arabidopsis thaliana. Physiol. Plant.95:37378
123. Roelfsema MRG, Prins HBA. 1997. Ionchannels in guard cells of Arabidopsisthaliana. Planta 202:1827
124. Savoure A, Hua X-J, Bertauche N, Van
Montagu M, Verbruggen N. 1997. Ab-scisic acid-independent and abscisic acid-dependent regulation of proline biosyn-thesis following cold and osmotic stressesinArabidopsis thaliana.Mol. Gen. Genet.254:1049
125. Schmidt C, Schelle I, Liao Y-J, SchroederJI. 1995. Strong regulation of slow an-ion channels and abscisic acid signalingin guard cells by phosphorylation and de-phosphorylation events. Proc. Natl. Acad.Sci. USA 92:953539
126. Deleted in proof
127. Schroeder JI, Hagiwara S. 1989. Cytoso-lic calcium regulates ion channels in theplasma membrane of Vicia faba guardcells. Nature 338:42730
128. Schroeder JI, Hagiwara S. 1990. Repeti-tive increases in cytosolic Ca2+ of guardcells by abscisic acid activation of non-selective Ca2+-permeable channels. Proc.Natl. Acad. Sci. USA 87:93059
129. Schroeder JI, Keller BU. 1992. Two typesof anion channel currents in guard cellswith distinct voltage regulation. Proc.Natl. Acad. Sci. USA 89:502529
130. Schroeder JI, Raschke K, Neher E. 1987.Voltage dependence of K+ channels inguard cells protoplasts. Proc. Natl. Acad.Sci. USA 84:410812
131. Schultz TF, Spiker S, Quatrano RS. 1996.Histone H1 enhances the DNA bindingactivity of the transcription factor EmBP-1. J. Biol. Chem. 271:2574245
132. Schwartz A, Ilan N, Schwarz M, Scheaf-fer J, Assmann SM, Schroeder JI. 1995.Anion-channel blockers inhibit S-typean-ion channels and abscisic responses inguard cells. Plant Physiol. 109:65158
133. Schwartz A, Wu W-H, Tucker EB, Ass-
mann SM. 1994. Inhibition of inwardK+ channels and stomatal response byabscisic acid: an intracellular locus ofphytohormone action. Proc. Natl. Acad.Sci. USA 91:401923
134. Schwartz SH, Leon-Kloosterziel KM,Koornneef M, Zeevaart JAD. 1997. Bio-chemical characterization of the aba2 andaba3 mutants in Arabidopsis thaliana.Plant Physiol. 114:16166
135. Schwartz SH, Tan BC, Gage DA, ZeevaartJAD, McCarty DR. 1997. Specific oxida-tive cleavage of carotenoids by VP14 of
maize. Science 276:187274136. Sheen J. 1996. Ca2+-dependent proteinkinases and stress signal transduction inplants. Science 274:19002
8/6/2019 aba signallingh
23/24
ABA SIGNALING 221
137. Shen LM, Outlaw WH, Epstein LM.1995. Expression of an mRNA with se-quence similarity to pea dehydrin (Ps-dhn1) in guard cells of Vicia faba in re-
sponse to exogenous abscisic acid. Phys-iol. Plant. 95:99105
138. Shen QX, Ho T-HD. 1995. Functionaldissection of an abscisic acid (ABA)-inducible gene reveals two independentABA-responsive complexes each con-taining a G-box and a novel cis-acting el-ement. Plant Cell 7:295307
139. Shen QX, Zhang PH, Ho T-HD. 1996.Modular nature of abscisic acid (ABA)response complexes: composite promoterunits that are necessary and sufficient forABA induction of gene expression in bar-
ley. Plant Cell 8:110719140. Shinozaki K, Yamaguchi-Shinozaki K.1996. Molecular responses to droughtand cold stress. Curr. Opin. Biotechnol.7:16167
141. Skriver K, Olsen FL, Rogers JC, MundyJ.1991. Cis-acting DNAelements respon-sive to gibberellin and its antagonist ab-scisic acid. Proc. Natl. Acad. Sci. USA88:726670
142. Soderman E, Mattson J, Engstrom P.1996. The Arabidopsis homeobox geneATHB-7 is induced by water deficit andby abscisic acid. Plant J. 10:37581
143. Staswick PE, Su WP, Howell SH. 1992.Methyl jasmonate inhibition of rootgrowth and induction of a leaf protein aredecreased in anArabidopsis thaliana mu-tant. Proc. Natl. Acad. Sci. USA 89:683740
144. Sturaro M, Vernieri P, Castiglioni P,Binelli G, Gavazzi G. 1996. The rea (redembryonic axis) phenotype describes anew mutation affecting the response ofmaize embryos to abscisic acid and os-motic stress. J. Exp. Bot. 47:75562
145. Suzuki M, Kao CY, McCarty DR.
1997. The conserved B3 domain ofVIVIPAROUS1 has a cooperative DNAbinding activity. Plant Cell 9:799807
146. Taylor IB. 1991. Genetics of ABA syn-thesis. In Abscisic Acid Physiology andBiochemistry, ed. WJ Davies, HG Jones,pp. 2337. Oxford: Bios Sci.
147. Taylor JE, Renwick KF, Webb AAR,McAinsh MR,Furini A, et al. 1995. ABA-regulated promoter activity in stomatalguard cells. Plant J. 7:12934
148. Thiel G, Blatt MR. 1994. Phosphatase an-tagonist okadaic acid inhibits steady state
K
+
currents in guard cells of Vicia faba.Plant J. 5:72733149. Thiel G, MacRobbie EAC, Blatt MR.
1992. Membrane transport in stomatal
guard cells: the importance of voltagecontrol. J. Membr. Biol. 126:118
150. Trejo CL, Clephan AL, Davies WJ. 1995.How do stomata read abscisic acid sig-
nals? Plant Physiol. 109:80311151. Urao T, Yamaguchi-Shinozaki K, Urao S,Shinozaki K. 1993. An Arabidopsis mybhomolog is induced by dehydration stressand its gene product binds to the con-served MYB recognition sequence. PlantCell 5:152939
152. Van der Meulen RM, Visser K, WangM. 1996. Effects of modulation of cal-cium levels and calcium fluxes on ABA-induced gene expression in barley aleu-rone. Plant Sci. 117:7582
153. van der Veen R, Heimovaara-Dijkstra S,
Wang M. 1992. Cytosolic alkalinizationmediated by abscisic acid is necessary, butnot sufficient, for abscisic acidinducedgene expression in barley aleurone proto-plasts. Plant Physiol. 100:699705
154. Vartanian N, Marcotte L, Giraudat J.1994. Drought rhizogenesis in Arabidop-sis thaliana. Differentialresponses of hor-monal mutants. Plant Physiol. 104:76167
155. Vasil V, Marcotte WR Jr, Rosenkrans L,Cocciolone SM, Vasil IK, et al. 1995.Overlap of Viviparous1 (VP1) and ab-scisic acid response elements in the Em
promoter: G-box elements are sufficientbut not necessary for VP1 transactivation.Plant Cell 7:151118
156. Walker-Simmons MK, Anderberg RJ,Rose PA, Abrams SR. 1992. Opticallypure abscisic acid analogstools for re-lating germination inhibition and gene ex-pression in wheat embryos. Plant Physiol.99:5017
157. Wang H, Cutler AJ. 1995. Promot-ers from kin1 and cor6.6, two Ara-bidopsis thaliana low-temperature- andABA-inducible genes, direct strong -
glucuronidase expression in guard cells,pollen and young developing seeds. PlantMol. Biol. 28:61934
158. Wang M, Van Duijn B, Schram AW. 1991.Abscisic acid induces a cytosolic calciumdecrease in barley aleurone protoplasts.FEBS Lett. 278:6974
159. Ward JM, Pei Z-M, Schroeder JI. 1995.Roles of ion channels in initiation of sig-nal transduction in higher plants. PlantCell 7:83344
160. Deleted in proof161. Wilkinson S, Davies WJ. 1997. Xylem
sap pH increase: a droughtsignal receivedat the apoplastic face of the guard cell thatinvolves the suppression of saturable ab-scisic acid uptake by the epidermal sym-
8/6/2019 aba signallingh
24/24
222 LEUNG & GIRAUDAT
plast. Plant Physiol. 113:55973162. Wille AC, Lucas WJ. 1984. Ultrastruc-
tural and histochemical studies on guardcells. Planta 160:12942
163. Wilson AK, Pickett FB, Turner JC, EstelleM. 1990. A dominant mutation in Ara-bidopsis confers resistance to auxin, ethy-lene and abscisic acid. Mol. Gen. Genet.222:37783
164. Yamaguchi-Shinozaki K, Shinozaki K.1993. The plant hormone abscisic acidmediates the drought-induced expressionbut not the seed-specific expression of
rd22, a gene responsive to dehydrationstress in Arabidopsis thaliana. Mol. Gen.Genet. 238:1725
165. Yamaguchi-Shinozaki K, Shinozaki K.
1993. Characterization of the expressionof a desiccation-responsive rd29 gene of Arabidopsis thaliana and analysis of itspromoter in transgenic plants. Mol. Gen.Genet. 236:33140
166. Zeevaart JAD, Creelman RA. 1988.Metabolism and physiology of abscisicacid.Annu. Rev. Plant Physiol. Plant Mol.Biol. 39:43973
Recommended