Abscisic acid inhibits type 2C protein phosphatases via the PYR/PYL family of START proteins

Abstract

Type 2C protein phosphatases (PP2Cs) are vitally involved in abscisic acid (ABA) signaling. Here, we show that a synthetic
growth inhibitor called pyrabactin functions as a selective ABA agonist. Pyrabactin acts through PYRABACTIN RESISTANCE 1 (PYR1), the founding member of a family of START proteins called PYR/PYLs, which are necessary for both pyrabactin and ABA signaling
in vivo. We show that ABA binds to PYR1, which in turn binds to and inhibits PP2Cs. We conclude that PYR/PYLs are ABA receptors
functioning at the apex of a negative regulatory pathway that controls ABA signaling by inhibiting PP2Cs. Our results illustrate
the power of the chemical genetic approach for sidestepping genetic redundancy.

Full text

DOI: 10.1126/science.1173041
, 1068 (2009); 324Science et al.Sang-Youl Park,
Proteins
Phosphatases via the PYR/PYL Family of START
Abscisic Acid Inhibits Type 2C Protein
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33. S.-Y. Park et al., Science 324, 1068 (2009); published
online 30 April 2009 (10.1126/science.1173041).
34. D. M. Priest et al., Plant J. 46, 492 (2006).
35. A. Christmann, E. W. Weiler, E. Steudle, E. Grill, Plant J.
52, 167 (2007).
36. This study was funded by the Deutsche
Forschungsgemeinschaft CH-182/5, European Union
Marie-Curie-Program MEST-CT-2005-020232, and the
Fonds der Chemischen Industrie. We thank J. Berger,
C. Heidersberger, and C. Kornbauer for technical
assistance. We are grateful to D. Weinfurtner, Chemistry
Department TUM, and K. Berendzen, University of
Tübingen, for help in conducting the ITC and FACS
analysis, respectively. The cytokinin reporter and the
At5g46790 cDNA clone were kindly provided by J. Sheen,
Harvard University, and the Arabidopsis Biological
Resource Center, USA, respectively. We thank F. Assaad
for critical reading of the manuscript. This contribution
is dedicated to Hubert Ziegler, who passed away on
17 April 2009.
Supporting Online Material
www.sciencemag.org/cgi/content/full/1172408/DC1
Materials and Methods
Figs. S1 to S7
References
27 October 2008; accepted 17 April 2009
Published online 30 April 2009;
10.1126/science.1172408
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Abscisic Acid Inhibits Type 2C
Protein Phosphatases via the
PYR/PYL Family of START Proteins
Sang-Youl Park,
1
*Pauline Fung,
2
*Noriyuki Nishimura,
4
†Davin R. Jensen,
8
†
Hiroaki Fujii,
1
Yang Zhao,
2
Shelley Lumba,
2
Julia Santiago,
5
Americo Rodrigues,
5
Tsz-fung F. Chow,
2
Simon E. Alfred,
2
Dario Bonetta,
6
Ruth Finkelstein,
7
Nicholas J. Provart,
2,3
Darrell Desveaux,
2,3
Pedro L. Rodriguez,
5
Peter McCourt,
2
Jian-Kang Zhu,
1
Julian I. Schroeder,
4
Brian F. Volkman,
8
Sean R. Cutler
1,9,10,11
‡
Type 2C protein phosphatases (PP2Cs) are vitally involved in abscisic acid (ABA) signaling.
Here, we show that a synthetic growth inhibitor called pyrabactin functions as a selective ABA
agonist. Pyrabactin acts through PYRABACTIN RESISTANCE 1 (PYR1), the founding member of a
family of START proteins called PYR/PYLs, which are necessary for both pyrabactin and ABA
signaling in vivo. We show that ABA binds to PYR1, which in turn binds to and inhibits PP2Cs.
We conclude that PYR/PYLs are ABA receptors functioning at the apex of a negative regulatory
pathway that controls ABA signaling by inhibiting PP2Cs. Our results illustrate the power of the
chemical genetic approach for sidestepping genetic redundancy.
Abscisic acid (ABA), identified in plants
in the 1960s, is a small molecule that
functions to inhibit growth and to regu-
late plant stress responses. Genetic analyses have
identified many factors involved in ABA signal-
ing (1), including the group A type 2C protein
phosphatases (PP2Cs), which negatively regulate
ABA signaling at an early step in the pathway
(2), and the SNF1-related kinase 2 (SnRK2
kinases), which are positive regulators (3–5).
Several (+)-ABA (1) binding proteins have been
reported (6–8) (Fig. 1A); however, their roles are
not fully understood, and they do not appear to
work via the genetically defined signaling path-
way (9) or bind to the nonnatural but bioactive
stereoisomer (–)-ABA (2)(10–12). The genetic
dissection of ABA perception has not identified
proteins resembling receptors, which suggests
that the ABA receptor(s) may be functionally
redundant or may be required for viability (13).
We therefore pursued a chemical genetic strategy
(14), because chemicals can bypass redundancy
by inducing phenotypes not revealed by single-
locus mutations (15). For example, an antagonist
with low selectivity can perturb the function of an
entire protein family, whereas a selective agonist
can illuminate the function of one member of
normally redundant receptors, as we describe
here with pyrabactin (3) (Fig. 1A), a synthetic
seed germination inhibitor (14). The analysis of
analogs revealed that pyrabactin’s activity re-
quires its pyridyl nitrogen, because the analog
apyrabactin (4) is biologically inactive (fig. S1)
(16). Further investigation of pyrabactin’s action
revealed reduced sensitivity in ABA-insensitive
mutants, but not ABA biosynthesis or gibberellic
acid–perception mutants (fig. S2), which suggests
it is an agonist of ABA signaling that inhibits ger-
mination in response to environmental stress (17).
Aside from ABA analogs, no synthetic agonists
of this stress pathway are known. Microarray
analyses of the ABA and pyrabactin responses of
seeds and seedlings revealed that, in seeds, both
compounds induce highly correlated transcrip-
tional responses (r= 0.98; Fig. 1B; table S1).
Three unrelated germination inhibitors (18) failed
to induce ABA-like effects (fig. S2), which
demonstrates that an indirect germination effect
1
Department of Botany and Plant Sciences, University of
California at Riverside, Riverside, CA 92521, USA.
2
Department
of Cell and Systems Biology, University of Toronto, 25 Willcocks
Street, Toronto, ON, M5S 3B2, Canada.
3
Centre for the Analysis
of Genome Evolution and Function, University of Toronto, 25
Willcocks Street, Toronto, ON, M5S 3B2, Canada.
4
Division of
Biological Sciences, Cell and Developmental Biology Section,
University of California at San Diego, 9500 Gilman Drive, La
Jolla, CA 92093, USA.
5
Instituto de Biología Molecular y Celular
de Plantas, Universidad Politécnica de Valencia, Avenida de los
Naranjos, Edificio CPI, 8E, ES-46022 Valencia, Spain.
6
Faculty
of Science, University of Ontario Institute of Technology, 2000
Simcoe Street North, Oshawa, ON, L1H 7K4, Canada.
7
Depart-
ment of Molecular, Cellular, and Developmental Biology, Uni-
versity of California at Santa Barbara, Santa Barbara, CA
93106, USA.
8
Department of Biochemistry, Medical College of
Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI
53226, USA.
9
Center for Plant Cell Biology, University of
California at Riverside, Riverside, CA 92521, USA.
10
Institute
for Genome Biology, University of California at Riverside,
Riverside, CA 92521, USA.
11
Department of Chemistry,
University of California at Riverside, Riverside, CA 92521, USA.
*These authors contributed equally to the work described.
†These authors contributed equally to the work described.
‡To whom correspondence should be addressed. E-mail:
sean.cutler@ucr.edu
Fig. 1. Pyrabactin is a seed-selective
ABA agonist. (A) Structures of mole-
cules described in this study. (B)ATH1
microarray comparison of pyrabactin
and ABA effects on seeds and seedlings.
The axes plot log
2
-transformed values
for probe responses to pyrabactin
(yaxis) or ABA (xaxis), relative to
controlsamples.ThePearsoncorre-
lation coefficient (r)foreachcompar-
ison is shown within the graph. Probes
selected for analyses were those
significantly responsive to either ABA
or pyrabactin. Germination-responsive
transcripts were removed for seed
analyses. Detailed methods are pro-
vided in (16).
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is not sufficient to account for pyrabactin’s
agonistactivityinseeds.Incontrasttoseedre-
sponses, seedling ABA and pyrabactin responses
show poorer correlation (r=0.72)(Fig.1C),and
few ABA-responsive genes significantly respond
to pyrabactin (table S1). Thus, pyrabactin affects
some, but not all, of the pathways regulated by
ABA and is therefore a selective agonist.
Selective agonists have a long history as re-
agents for receptor identification, so we used
pyrabactin for genetic dissection and isolated 12
PYRABACTIN RESISTANCE 1 (Pyr1)mutantal-
leles. Pyr1, isolated by map-based cloning, en-
codes a member of the cyclase subfamily of the
START domain superfamily. START proteins
share a conserved hydrophobic ligand–binding
pocket (19–21). There are 13 genes in the
Arabidopsis genome having marked similarity to
Pyr1 (fig. S1), which we have named Pyl1 to
Pyl13 (for PYR1-Like). This 14-member gene
family (Fig. 4B) has been independently identified
as ABI1-interacting proteins (called “regulatory
component of ABA receptor,”RCAR1 through
RCAR14) by Ma et al.(22), and their role in ABA
signaling is characterized there. The pyr1 mutant
alleles we isolated are predicted to produce a
variety of defects in PYR1 (fig. S3). Gene
expression databases (23–27) show that Pyr and
Pyl mRNAs are expressed at high levels in seeds
and guard cells and respond to ABA (Fig. 2A),
which is consistent with a role in ABA signaling.
However, all of the pyr1 mutants isolated,
including putative null alleles, reduce pyrabactin,
but not ABA, sensitivity. To examine if functional
redundancy might obscure a role for Pyr1 in ABA
signaling, we isolated Pyl1,Pyl2,andPyl4
insertion alleles and constructed multilocus
mutants. Triple (pyr1;pyl1;pyl4) and quadruple
(pyr1;pyl1;pyl2;pyl4) mutant lines display strong
ABA insensitivity (Fig. 2B), which can be reversed
by introducing PYR1- or PYL4-expressing trans-
genes (fig. S3). The quadruple mutant is less
sensitive to (+)-ABA as measured by seed ger-
mination, root growth, and quantitative reverse
transcription polymerase chain reaction (qRT-PCR)
(Fig. 2C), and SnRK2 kinase assays (Fig. 2D and
fig. S4). Collectively, our genetic data show that
Pyr1 and Pyls are necessary for multiple ABA
responses in vivo and illustrate the differential
behavior that natural and synthetic agonists can
display in genetic screens.
Because PYR1 is predicted to be a ligand-
binding START protein necessary for pyrabactin
activity, we hypothesized that pyrabactin might
Fig. 2. Pyr/Pyls are necessary for ABA signaling. (A)Pyr1and
Pyl1 to Pyl4 expression levels. Plots were made with the eFP
browser (24); these heat maps show normalized ATH1 microarray
expression values (divided by 100) according to the color scales
shown; the right color scale is for the guard cell data only. (B)
Genes for Pyr/Pyl act redundantly in ABA signaling. Genotypes
shown were germinated on medium containing 0.9 mM (+)-ABA
and were documented 7 days post imbibition. (C) Pyr/Pyls are
required for normal ABA-induced gene expression in seedlings.
Shown are qRT-PCR results for the ABA-responsive gene RD29. L,
Ler; C, Col; and Q, quadruple mutant. (D) The Pyr/Pyls are
required for normal SnRK2 kinase activity. In-gel kinase assays
were conducted on extracts made from control or ABA-treated
plants of the genotypes shown; extracts from two separate
quadruple (Q) mutant lines are shown. Red arrow corresponds to
SnRK2.2 and 2.3; blue to SnRK2.6 (OST1), as described (37).
Fig. 3. PYR/PYLs bind to group A PP2Cs in response to ABA. (A)
Wild-type and mutant PYR1-PP2C interactions. PYR1 and three
different pyrabactin-insensitive substitution mutants were
constructed as binding-domain (BD) fusion proteins and were
tested for their interactions with activation domain (AD)–fused
HAB1 with the yeast two-hybrid assay by using the compounds
shown at top (top panel). In the two bottom panels, AD fusions
of HAB1, ABI1, ABI2, or ABI2
G168D
were tested for ABA- and
pyrabactin-induced interactions with BDPYR1. (B)ABApro-
motes PYR1 to PP2C interactions in planta. Total protein
extracts (input) were made from N. benthamiana leaves
transformed with the indicated constructs and/or treatments,
immunoprecipitated with antibody against HA-agarose, and
immunodetected with antibodies against green fluorescent
protein (GFP) or HA. YFP-PP2Cs migrate at ~100 kD and HA-
PYR1 at ~25 kD. (C) ABA-orfeome analysis of ABI1 interactions.
Shown are subsets of an ABA-orfeome queried with ABI1;
autoactivators are circled and ABA-dependent interactors are
indicated with arrows. (D) Reconstitution of ABA responses in
vitro. Pull-down assays using glutathione S-transferase (GST)–
HAB1 (~80 kD) and His
6
-tagged PYR1 (~25 kD) were conducted
with purified recombinant proteins. GST-ABI1 and ABI2 tests
were done with protein in crude lysates. Pyrabactin (10 mM,
PyrA) was used in (A), 10 mM(+)-ABAwasusedin(A)and(D),
and 100 mM ABA (mixed stereoisomers) in (B) and (C).
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promote a protein-protein interaction between
PYR1 and a downstream effector. To test this, ~2
million prey cDNA clones were screened against
PYR1 yeast two-hybrid bait on medium contain-
ing pyrabactin. This revealed that the PP2C HAB1
(28,29) interacts with PYR1 in response to pyrabac-
tin and (+)-ABA, but not the inactive analog apyr-
abactin or several other plant hormones (Fig. 3A
and fig. S4). HAB1 resides in the group A
subfamily of plant PP2Cs, which contains nine
partially redundant members that negatively regulate
ABA signaling (30,31). To examine the relevance
of the yeast two-hybrid data, we next performed a
series of control experiments. First, we investigated
the effects of defective PYR1 and PP2C amino acid
substitution mutants on ABA-responsive interac-
tions in the yeast two-hybrid assay (32). PYR1
S152L
and PYR1
P88S
, isolated because they cause pyr-
abactin insensitivity in planta, reduce ABA-induced
PYR1-PP2C interactions (Fig. 3A); similarly, the
dominant ABA-insensitive ABI2
G168D
mutant (Fig.
3B) disrupts the PYR1-ABI2 interaction. Second,
we investigated in planta interactions by coexpress-
ing a panel of yellow fluorescent protein (YFP)–
tagged PP2Cs and hemagglutinin (HA)-tagged
PYR1 constructs in N. benthamiana. Coimmuno-
precipitation experiments performed on ABA or
mock-treated plants recapitulated the PYR1-PP2C
interactions observed in yeast and also showed that
PYR1 does not interact with ABI1
G180D
, encoded
by abi1-1 (Fig. 3C). Third, we used PYR1 and
ABI1 as bait in a yeast two-hybrid screen against a
set of 258 ABA-responsive open reading frames
and observed that interactions between PYR1 and
group A PP2C proteins occur only in the presence
of ABA (Fig. 3D and table S2). Fourth, we success-
fully reconstituted the ABA-induced PYR1-PP2C
interactions in vitro using recombinant proteins (Fig.
3E). Thus, we conclude that ABA promotes a bio-
logically meaningful interaction between PYR1 and
group A PP2Cs.
Given the redundancy observed in our genetic
analyses, it is likely that other PYR/PYLs interact
with PP2Cs in response to ABA. We therefore used
the yeast two-hybrid assay to explore interactions of
HAB1 and PP2CA (AHG3) (ABA-hypersensitive
germination gene 3) with a panel of 12 PYR/PYLs,
which shows that (+)-ABA promotes interactions
between PYR1, PYL1 to PYL4, and HAB1 (Fig.
4A). We next used this PYR/PYL panel to examine
ligand response selectivities, which shows that these
five (+)-ABA–responsive PYR/PYLs do not all bind
HAB1 in response to nonnatural agonists. For
example, PYL2, PYL3, and PYL4 respond to both
(+)-ABA and (–)-ABA (Fig. 4A), which makes
these proteins candidates for the dual-stereoisomer
receptors predicted by earlier studies (12). Consistent
with this hypothesis, our quadruple mutant has great-
ly reduced (–)-ABA sensitivity (fig. S3). Ligand-
selective interactions are also observed for pyrabactin,
which promotes interactions between HAB1 and
PYR1,PYL1,orPYL3(Fig.4A).Ofthese,only
Pyr1 is highly transcribed in seeds (Fig. 2B), which
likely explains why mutations in Pyr1 cause the
seeds to be insensitive to pyrabactin. Last, we
observe that PYL12 interacts with PP2CA (AHG3)
in response to ABA (fig. S4). Thus, at least 6 of the
14 PYR/PYLs confer ABA responsiveness to yeast.
We hypothesize that the entire PYR/PYL gene
family participates in ABA-promoted interactions, as
these six genes are distributed across the PYR/PYL
phylogenetic tree (Fig. 4B). The specificity with
which pyr/pyl genes control which ligands trigger
ABA signaling suggests that the ABA pathway may
be dissected using selective PYR/PYL agonists.
Given the role of PYR/PYLs in controlling
ligand responses, we explored whether (+)-ABA
bindsPYR1byusing
15
N-labeled PYR1 and
PYR1
P88S
in heteronuclear single quantum coher-
ence (HSQC) nuclear magnetic resonance (NMR)
experiments, which probe chemical shifts of protein
amide–NH bonds in response to ligands (33).
Because START proteins contain a conserved
ligand-binding cavity (34), binding should selec-
tively perturb residues lining this cavity. Addition of
(+)-ABA altered the HSQC signals for many
PYR1 and PYR1
P88S
residues (Fig. 4C and figs.
S5 and S6), which showed that ABA binds PYR1
and likely induces a conformational change. We
also investigated the PYR1-HAB1 interaction in
the presence of ABA, because our NMR exper-
iments showed that PYR1
P88S
is not defective
in ABA binding. Addition of unlabeled HAB1
caused peak broadening for PYR1, but not
PYR1
P88S
, HSQC signals (fig. S7), which localized
the PYR1
P88S
defect to binding HAB1 after ABA
perception. Because PP2Cs are negative regulators
of ABA signaling, we hypothesized that ABA-
promoted PYR/PYL–PP2C interactions would
inhibit phosphatase activity. To test this, we
examined the effects of (+)-ABA on PP2C enzyme
Fig. 4. PYR1 is an ABA-binding protein that regulates PP2C activity. (A) PYR/PYL proteins determine
selectivities for responses to different ligands. A panel of PYR/PYL genes were constructed as BD fusions
and tested in yeast for interactions with HAB1 and AHG3 in response to (+)-ABA, (–)-ABA, pyrabactin, or
apyrabactin (all 10 mM), dimethyl sulfoxide (DMSO) (carrier solvent, 1%). Shown are results for five
PYR/PYLs that interact with HAB1 in response to ABA. (B) ABA response activity is distributed throughout
the PYR/PYL family. Shown is a neighbor-joining tree of the PYR/PYL family. (Middle) Ligand selectivity
data derived from yeast two-hybrid experiments. (+)-ABA–responsive PYR/PYLs are colored red, Arabidopsis
Genome Initiative (AGI) annotations are shown at right; PYL9 colored red, on the basis of data from Ma et al.
(22). (C) ABA binds to PYR1 and PYR1
P88S
. Shown are subregions of HSQC spectra for
15
N-labeled PYR1
and PYR1
P88S
in response to increasing amounts of ABA. Arrows indicate amide protons whose chemical
environments shift in response to ABA. (D) PYR1 inhibits PP2C activity in the presence of ABA. Initial
reaction velocities of recombinant GST-HAB1 were tested in the presence of PYR1 or PYR1
P88S
,and
differing ABA concentrations using the colorimetric substrate p-nitrophenyl phosphate (pNPP). The mea-
sured IC
50
values are 125 nM for PYR1 and 50 mMforPYR1
P88S
.(E) Hypothesized model for PYR/PYL
control of ABA signaling. We propose the following model: In the absence of ABA (left), PYR/PYL proteins
are not bound to PP2Cs, and therefore, PP2C activity is high, which prevents phosphorylation and activation
of SnRK2s and downstream factors (DFs). In the presence of ABA, PYR/PYLs bind and inhibit PP2Cs. This
allows accumulation of phosphorylated downstream factors and ABA transcriptional responses. The regu-
lation of SnRK2s by PYR/PYLs may be indirect or may involve other factors.
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kinetics using recombinant HAB1, PYR1, or
PYR1
P88S
. These experiments show that (+)-ABA
acts as a potent and saturable inhibitor of phos-
phatase activity in the presence of PYR1 [median
inhibitory concentration (IC
50
) = 125 nM], but not
PYR1
P88S
(IC
50
=50mM) (Fig. 4D and fig. S8).
Similarly, ABA displays saturable inhibition of
HAB1 PP2C activity in the presence of recombi-
nant PYL4 (fig. S8). Thus, PYR/PYLs regulate
PP2Cs in response to ABA, which defines an un-
precedented mechanism for ligand-mediated regu-
lation of PP2C activity.
Collectively, we have shown that PYR1 binds
(+)-ABA, PYR/PYLs bind to and inhibit PP2Cs
in response to (+)-ABA, and PYR/PYLs control
which ligands trigger PP2C interactions. We con-
clude that the PYR/PYLs are a family of ABA
receptors. However, the precise site of ABA bind-
ing remains unclear, because the ABA-binding
sitemaybesharedwiththePP2C.Discriminat-
ing between these receptor and co-receptor mod-
els will require structural studies of cocrystallized
PYR/PYLs, PP2Cs, and ligands. Note that the
PYR/PYLs interact directly with PP2Cs, which
are core components of the ABA signaling path-
way. Because SnRK2 activity is decreased in the
PYR/PYL quadruple mutant, we propose a hypo-
thetical model (Fig. 4D) for ABA action in which
ABA and PYR/PYLs inhibit PP2Cs, which in turn
relieves repression of positive factors, such as the
SnRK2s. Consistent with this model, we observed
interaction of SnRK2.2 with PP2CA (AHG3),
AHG1, and ABI1 when we used the yeast two-
hybrid assay (fig. S4). This suggested that the low
SnRK2 activity observed in the PYR/PYL quadru-
ple mutant may be a direct consequence of PP2C-
SnRK2 interactions. Understanding of the role of
PP2Cs in ABA signaling has been complicated by
observations from abi1-1 and abi2-1 mutations.
Their dominant phenotypes suggest that they en-
code hypermorphic proteins (35), but they paradox-
ically reduce, but do not abolish, PP2C activity
(36). Our data show that these mutants do not bind
PYR1 in response to ABA. We therefore hypoth-
esize that ABA normally lowers wild-type PP2C
activity through PYR/PYL proteins, but abi PP2Cs
escape this and disrupt signaling because of their
residual activity. Consistent with this model, a sec-
ond site mutation that abolishes abi1-1’s catalytic
activity suppresses its dominant ABA-insensitive
phenotype (36).
The redundancy in the Pyr/Pyl gene family,
typical of many plant genes, has kept these genes
from emerging as factors necessary for ABA
response. We leveraged pyrabactin’s selectivity
for a subset of the PYR/PYL family to bypass the
genetic redundancy that masks ABA phenotypes
in single mutants. Thus, our results demonstrate
the power of synthetic molecules to expose phe-
notypes for otherwise redundant genes.
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38. We thank N. Raikhel, J. Bailey-Serres, and C. Larive for
comments; K. Nito and T. Demura for materials. Supported by
CRC (S.R.C., P.M., D.D.); NSERC (S.R.C., D.D., N.J.P.);
NSF awards IOS0820508 (S.R.C.) and MCB0417118 (J.I.S.);
NIH awards R01GM060396 (J.I .S.), 01GM59138 ( J.-K.Z.),
and U54GM074901 (B.V.). S.R.C. thanks J. R. Coleman for
early support. Patent application “Control of plant stress
tolerance, water use efficiency and gene expression using
novel ABA receptor proteins and synthetic agonists”
inventors:S.R.Cutler,S.Y.Park,N.Nishimura,J.I.
Schroeder, and A. Defries. Author contributions are as
follows.Microarrays:P.F.andN.J.P.;pyr1mutants:P.F.;
mutant analyses: P.F. and T.F.C.; yeast two-hybrid assays,
biochemistry, and Pyr/Pyl genetics: S.Y.P.; pyr1
complementation and allele sequencing: Y.Z.; in planta pull-
downs: N.N. and J.I.S.; PYL12-AHG3 and SnRK2-PP2C
interactions: R.F.; HSQC experiments: D.J. and B.V.; SnRK2
assays: H.F. and J.-K.Z.; GFP-PYR1 localization: S.E.A.; PP2C
assays: S.Y.P., J.S., A.R., and P.R.; EMS seed: D.B.; ABA
orfeome: S.L., D.D., and P.M.; project conception, positional
cloning, phone calls, and writing: S.R.C.
Supporting Online Material
www.sciencemag.org/cgi/content/full/1173041/DC1
Materials and Methods
Figs. S1 to S8
Tables S1 and S2
References
27 October 2008; accepted 16 April 2009
Published online 30 April 2009;
10.1126/science.1173041
Include this information when citing this paper.
Understanding the Spreading Patterns
of Mobile Phone Viruses
Pu Wang,
1,2
Marta C. González,
1
César A. Hidalgo,
1,2,3
Albert-László Barabási
1,4
*
We modeled the mobility of mobile phone users in order to study the fundamental spreading
patterns that characterize a mobile virus outbreak. We find that although Bluetooth viruses can
reach all susceptible handsets with time, they spread slowly because of human mobility, offering
ample opportunities to deploy antiviral software. In contrast, viruses using multimedia messaging
services could infect all users in hours, but currently a phase transition on the underlying call
graph limits them to only a small fraction of the susceptible users. These results explain the lack of
a major mobile virus breakout so far and predict that once a mobile operating system’smarketshare
reaches the phase transition point, viruses will pose a serious threat to mobile communications.
Lacking a standardized operating system,
traditional cellphones have been relatively
immune to viruses. Smart phones, however,
canshareprogramsanddatawitheachother,
representing a fertile ground for virus writers (1–4).
Indeed, since 2004 more than 420 smart phone
viruses have been identified (2,3), the newer
ones having reached a state of sophistication that
took computer viruses about two decades to achieve
(2). Although smart phones currently represent less
than 5% of the mobile market, given their reported
fast annual growth rate (4) they are poised to be-
come the dominant communication device in the
near future, raising the possibility of virus break-
outs that could overshadow the disruption caused
by traditional computer viruses (5).
1
Center for Complex Network Research, Departments of Physics,
Biology, and Computer Science, Northeastern University,
Boston, MA 02115, USA.
2
Center for Complex Network Research
and Department of Physics, University of Notre Dame, Notre
Dame, IN 46556, USA.
3
Center for International Development,
Kennedy School of Government, Harvard University, Cambridge,
MA 02139, USA.
4
Department of Medicine, Harvard Medical
School, and Center for Cancer Systems Biology, Dana Farber
Cancer Institute, Boston, MA 02115, USA.
*To whom correspondence should be addressed. E-mail:
barabasi@gmail.com
www.sciencemag.org SCIENCE VOL 324 22 MAY 2009 1071
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