Download OPEN STOMATA1 opens the door to ABA signaling in Arabidopsis

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TRENDS in Plant Science
6 Steffenson, B.J. (1992) Analysis of durable resistance to stem rust in
barley. Euphytica 63, 153 – 167
7 Brueggeman, R. et al. (2002) The barley stem rust-resistance gene
Rpg1 is a novel disease-resistance gene with homology to receptor
kinases. Proc. Natl. Acad. Sci. U. S. A. 99, 9328 – 9333
8 Horvath, H. et al. (2003) Genetically engineered stem rust resistance
in barley using the Rpg1 gene. Proc. Natl. Acad. Sci. U. S. A. 100,
364 – 369
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9 Takasaki, T. et al. (2000) The S receptor kinase determines selfincompatibility in Brassica stigma. Nature 403, 913 – 916
10 Braun, D.M. et al. (1997) Interaction of the maize and Arabidopsis kinase
interaction domains with a subset of receptor-like protein kinases:
implications for transmembrane signaling in plants. Plant J. 12, 95
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OPEN STOMATA1 opens the door to ABA signaling in
Arabidopsis guard cells
Sarah M. Assmann
Biology Department, Penn State University, 208 Mueller Laboratory, University Park, PA 16802-5301, USA
Much of what we have learned in recent years concerning
the genes that encode abscisic acid (ABA) signaling
elements stems from characterization of Arabidopsis
mutants with regard to a few ‘classic’ ABA responses:
promotion of seed dormancy, inhibition of seed germination, regulation of root elongation, inhibition of stomatal
opening and promotion of stomatal closure [1]. Of these
responses, modulation of stomatal aperture is unique in
being the only response that does not involve irreversible
developmental changes. Rather, reversible changes in the
intracellular concentrations of the osmotically active solutes
Kþ, Cl2 malate22 and sucrose drive guard cell swelling or
shrinking, resulting in stomatal opening or closure,
respectively. Thus, one might anticipate that analysis of
guard cell ABA responses might reveal some mechanisms
that are largely specific to this specialized cell type.
Identification of the ost1 mutant
To identify such mechanisms, Anna-Chiara Mustilli,
Jérôme Giraudat and colleagues [2] used an ingenious
screen for guard cell effects on leaf temperature that had
earlier been applied to mutants of barley by Ilya Raskin
and Juanita A.R. Ladyman [3]. Given that transpirational
water loss through open stomata results in leaf cooling,
Jérôme Giraudat’s laboratory used remote infrared thermal imaging to screen an ethyl methane sulfonatemutagenized population for Arabidopsis mutants with
cooler leaves (Fig. 1). Two allelic mutants, ost1-1 and
ost1-2, as well as a third mutant, ost2, were observed to
have leaf temperatures , 18C cooler than wild-type plants,
while retaining a wild-type seed germination response to
Corresponding author: Sarah M. Assmann ([email protected]).
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ABA [4]. OST1 refers to Open STomata (stomata is Greek
for mouths).
Identification and characterization of the OST1 kinase
Positional cloning of the OST1 gene revealed that OST1 is
highly homologous to genes encoding serine –threonine
protein kinases [2]. OST1 expression is limited to guard
cells and vascular tissue. In a remarkable convergence of
genetic and biochemical approaches, the closest relative to
OST1 in the sequence databases is the ABA-activated
protein kinase (AAPK) of Vicia faba, a guard-cell protein
kinase initially identified by in gel kinase assays [5,6]. The
AAPK cDNA was cloned based on peptide sequence
obtained by de novo mass spectrometric sequence analysis
[7]. OST1 is 79% identical to AAPK and, like AAPK,
displays ABA-dependent autophosphorylation. ost1
mutant guard cells and V. faba guard cells expressing a
dominant negative form of AAPK both exhibit an inability
to engender stomatal closure in response to ABA. These
and other similarities between OST1 and AAPK (Table 1)
25
Ler
Leaf temperature (°C)
The plant hormone abscisic acid plays a crucial role in
plant responses to drought, salinity and cold. A recent
report shows that mutations in the OST1 gene, encoding a serine–threonine protein kinase, render Arabidopsis thaliana guard cells insensitive to abscisic acid, such
that stomata remain open in the presence of this
phytohormone.
24
23
ost1-1
22
ost1-2
21
Fig. 1. False-color infrared image of drought-stressed Ler wild type (top row),
ost1-1 (middle row), and ost1-2 Arabidopsis plantlets. Temperature scale for the
false color is indicated at the left of the panel. Reproduced, with permission, from
Ref. [1].
152
Update
TRENDS in Plant Science
Vol.8 No.4 April 2003
Table 1. Comparison of the properties of Arabidopsis thaliana OST1 and Vicia faba AAPK protein kinases
Property
OST1
AAPK
Refs
Molecular mass
Expressed in guard cells but not in epidermal cells or mesophyll cells
Kinase activity is dependent on prior in vivo ABA application
Kinase activity is not affected by kinetin, IAA, GA3 or 2,4-D
Kinase activity is Ca2þ-independent in vitro
Genetic ablation results in ABA insensitivity of stomatal closure
Genetic ablation results in ABA insensitivity of stomatal opening
Functions downstream of abi1-1
Functions upstream of reactive oxygen species and Ca2þ
Phosphorylates a single-stranded RNA-binding protein
42 kDa
Yes
Yes
?
Yes
Yes
Yes
Yes
Yes
?
48 kDa
Yes
Yes
Yes
Yes
Yes
No
?
?
Yes
[2,5,6]
[2,5]
[2,5,7]
[6]
[2,5]
[2,7]
[2,7]
[2]
[2]
[15]
Abbreviations: ABA, abscisic acid; GA3, gibberellin A3; IAA, indole-3-acetic acid.; 2,4-D, 2,4-dichlorophenoxyacetic acid; ?, data not available.
indicate that the two proteins are true orthologs. However,
one significant difference is that the ost1 mutants are also
insensitive to ABA-inhibition of stomatal opening [2],
whereas V. faba stomata expressing dominant negative
AAPK show normal ABA inhibition of stomatal opening
[7]. Whether these differences reflect true species difference in kinase function, or differences in the experimental
approaches used in the two studies (i.e. null versus
dominant negative mutants or the use of different ABA
concentrations) remains to be determined. OST1 also
bears homology to PKABA1 of wheat, a gene whose
transcript abundance in seeds increases strongly in
response to ABA and dehydration [8]; however, OST1
transcript levels are not upregulated by ABA.
Placing OST1 in a signaling pathway
The identification of an AAPK ortholog in the genetically
tractable model species Arabidopsis thaliana should
enable rapid progress to be made in elucidating the
AAPK/OST1 signaling pathway. A legion of signaling
elements has been implicated in ABA action in guard cells,
including enzymes, metabolites and ephemeral signals
such as transients in cytosolic Ca2þ, pH and reactive
oxygen species (ROS) (reviewed in Refs [9– 11]). By
employing a dye whose fluorescence increases under
oxidizing conditions, Mustilli and co-workers demonstrated that ost1-2 guard cells are deficient in ROS
production in response to ABA. Application of exogenous
H2O2 or Ca2þ restores stomatal closure and inhibits
stomatal opening in the ost1 mutants. Because ROS are
strong elicitors of plasma membrane Ca2þ channel activity
[12], one could hypothesize that ost1 mutants are deficient
in Ca2þ channel activation.
Disruption of ABA-induced ROS production and Ca2þ
channel activation in guard cells has already been
demonstrated in Arabidopsis plants harboring a dominant
mutation, abi1-1, in a type-2C protein phosphatase [13].
By assaying for ABA-dependent OST1 phosphorylation in
the abi1-1 dominant mutant background, Mustilli et al.
demonstrated that ABA stimulation of OST1 kinase
activity was prevented in these mutants, suggesting that
ABI1 acts upstream of OST1. Altogether, the results of the
epistasis assays suggest that the signal from ABA to OST1
is modulated by ABI1 and then proceeds via ROS to
regulation of cytosolic Ca2þ levels and ultimately to
changes in stomatal aperture. Given that the Kþ channels
that mediate Kþ uptake during stomatal opening and the
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anion channels that mediate Cl2 and malate22 loss during
stomatal closure are inhibited and activated by elevated
cytosolic Ca2þ, respectively, it would be useful to assess to
what extent altered ABA- and Ca2þ- regulation of these ion
channels is responsible for the ost1 mutant phenotype.
Besides protein kinases and protein phosphatases,
other diverse enzymes have been implicated in the
guard cell responses to ABA, including phospholipases C
and D, proteins involved in RNA processing, a syntaxin, a
farnesyl transferase, the small G proteins ROP10 and
AtRac1, and the heterotrimeric G protein a subunit, GPA1
[9– 11,14– 18]. The relative position of OST1 in the
ramifying signaling networks created by these proteins
remains to be identified. Another fruitful area for future
research concerns the role of other predicted protein
kinases with high homology to OST1. OST1 shares
67 – 80% amino acid identity with nine other putative
protein kinases of Arabidopsis, which have been given the
appellation OST1-kinase-like (OSKL) 2– 10 [2]. Apparently none of these family members has redundant
function with OST1 in guard cells because null mutation
of ost1 suffices to confer the guard-cell ABA-insensitive
phenotype. It will be interesting to ascertain whether
OSKL proteins function in mediating some of the other
‘classic’ ABA responses listed at the beginning of this
article or if they are involved in other, as yet unknown,
functions of this vital phytohormone.
Acknowledgements
Supported by NSF grants MCB 9874438 and MCB 0086315. I thank
members of Jérôme Giraudat’s laboratory for comments on the
manuscript.
References
1 Finkelstein, R.R. et al. (2002) Abscisic acid signaling in seeds and
seedlings. Plant Cell 14, S15 – S45
2 Mustilli, A. et al. (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
3 Raskin, I. and Ladyman, J.A.R. (1988) Isolation and characterization
of a barley mutant with abscisic acid-insensitive stomata. Planta 173,
73 – 78
4 Merlot, S. et al. (2002) Use of infrared thermal imaging to isolate
Arabidopsis mutants defective in stomatal regulation. Plant J. 30,
601 – 609
5 Li, J. and Assmann, S.M. (1996) An abscisic acid-activated and
calcium-independent protein kinase from guard cells of fava bean.
Plant Cell 8, 2359– 2368
6 Mori, I.C. and Muto, S. (1997) Abscisic acid activates a 48kilodalton protein kinase in guard cell protoplasts. Plant Physiol.
113, 833 – 839
Update
TRENDS in Plant Science
7 Li, J. et al. (2000) Regulation of abscisic acid-induced stomatal
closure and anion channels by guard cell AAPK kinase. Science
287, 300 – 303
8 Anderberg, R.J. and Walker-Simmons, M.K. (1992) Isolation of a wheat
cDNA clone for an abscisic acid-inducible transcript with homology to
protein kinases. Proc. Natl. Acad. Sci. U. S. A. 89, 10183 – 10187
9 Hetherington, A.M. (2001) Guard cell signaling. Cell 107, 711 – 714
10 Schroeder, J.I.S. et al. (2001) Guard cell signal transduction. Annu.
Rev. Plant Physiol. Plant Mol. Biol. 52, 627– 658
11 Blatt, M.R. (2000) Cellular signaling and volume control in stomatal
movements of plants. Annu. Rev. Dev. Biol. 16, 221– 241
12 Pei, Z.M. et al. (2000) Calcium channels activated by hydrogen
peroxide mediate abscisic acid signaling in guard cells. Nature 406,
731 – 734
13 Murata, Y. et al. (2001) Abscisic acid activation of plasma membrane
Ca2þ channels in guard cells requires cytosolic NAD(P)H and is
differentially disrupted upstream and downstream of reactive oxygen
Vol.8 No.4 April 2003
14
15
16
17
18
153
species production in abi1-1 and abi2-1 protein phosphatase 2C
mutants. Plant Cell 13, 2513– 2523
Fedoroff, N.V. (2002) RNA-binding proteins in plants: the tip of an
iceberg? Curr. Opin. Plant Biol. 5, 452 – 459
Li, J. et al. (2002) Modulation of an RNA-binding protein by abscisicacid-activated protein kinase. Nature 418, 793 – 797
Zheng, Z. et al. (2002) Plasma membrane-associated ROP10 small
GTPase is a specific negative regulator of abscisic acid responses in
Arabidopsis. Plant Cell 14, 2787– 2797
Lemichez, E. et al. (2001) Inactivation of AtRac1 by abscisic acid is
essential for stomatal closure. Genes Dev. 15, 1808– 1816
Wang, X-Q. et al. (2001) G protein regulation of ion channels and
abscisic acid signaling in Arabidopsis guard cells. Science 292,
2070– 2072
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doi:10.1016/S1360-1385(03)00052-9
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