P. syringaebecause it can recognize five ef-
fector families represented by alleles that are
rarely present in the same genetic background,
but that are distributed among nearly half
of theP. syringaestrains analyzed. Given that
ZAR1 is also required for the recognition of
AvrAC fromX. campestris( 37 ), this NLR ap-
pears to have particularly broad-spectrum
resistance that inhibits a diverse collection of
pathogens. Like AvrAC, theseP. syringaeef-
fectors may target and modify a diverse col-
lection of receptor-like cytoplasmic kinases,
leading to the downstream activation of
the ZAR1 resistome ( 38 – 40 ). Both CAR1 and
ZAR1 are conserved among 1135 sequenced
A. thalianaecotypes ( 41 ), which suggests
that they play a major role in immunity. It
will now be interesting to identify further
NLRs involved in recognition of other ef-
fectors, as they may offer insight into new
immune pathways.
Because of the prominence of ETI against
P. syringae, we hypothesize that ETI avoid-
ance will be a crucial virulence strategy of
successful pathogens, involving the loss and/or
modification of ETI-eliciting effectors ( 42 , 43 ).
In addition, meta-effector interactions will
also modulate ETI responses, as shown by the
loss of recognition of three effectors when
expressed in the PmaES4326 background
and the well-established interactions between
AvrRpt2 and AvrRpm1 ( 44 ). Finally, as dem-
onstrated by the differential recognition of
AvrE when encoded chromosomally versus
on a plasmid (see fig. S12), we hypothesize
that effector dosage must be regulated to
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ACKNOWLEDGMENTS
We thank all members of the Guttman and Desveaux labs
for helpful discussion and valuable input on this project;
H.Kim,L.Li,S.Park,andN.Hoffmannforhelpwithplant
assays; and D. Seto for crucial help with CRISPR/Cas9
mutagenesis.Funding:Supported by Natural Sciences and
Engineering Research Councilof Canada Discovery Grants
(D.S.G. and D.D.), a Natural Sciences and Engineering
Research Council of Canada Postgraduate Award (B.L.
and A.M.), Canada Research Chairs in Comparative Genomics
(D.S.G.) and Plant-Microbe Systems Biology (D.D.),
and the Center for the Analysis of Genome Evolution and
Function (D.S.G. and D.D.).Author contributions:B.L.,
M.M.D., D.D., and D.S.G. designed the project; M.M.D. and
R.N.D.A. assembled, annotated, and quality-controlled the
genomes, including hrp box promoter identification; M.M.D.
conceived and carried out the pipeline to identify, delimit,
and engineer representative effectors; B.L. and A.M. cloned
and delivered effectors intoP. syringae,performedWestern
analysis,andperformedgrowthandHRassays;B.L.and
A.M. screened effectors for ETI responses; B.L., M.M.D.,
A.M., and R.N.D.A. analyzed the data; B.L., M.M.D., D.D.,
and D.S.G. wrote the paper; and all authors reviewed and
agreed on the manuscript.Competing interests:The authors
declare no competing interests.Data and materials
availability:Accession numbers for all genome assemblies
used in this study are available in data S1 to S4. All data that
support the findings of our study are available in the
manuscript or supplementarymaterials. All strains with
International Collection of Microorganisms from Plants (ICMP)
designations are availablefrom ICMP at https://scd.
landcareresearch.co.nz/Search?collectionId=ICMP.SUPPLEMENTARY MATERIALS
science.sciencemag.org/content/367/6479/763/suppl/DC1
Materials and Methods
Figs. S1 to S15
Tables S1 to S8
References ( 45 – 65 )
View/request a protocol for this paper fromBio-protocol.20 March 2019; resubmitted 18 October 2019
Accepted 17 January 2020
10.1126/science.aax4079Laflammeet al.,Science 367 , 763–768 (2020) 14 February 2020 6of6
Fig. 4. ETI governs host specificity onA. thaliana.
TheP. syringaeisolate PmaYM7930 harbors one
additional ETI-eliciting effector (HopAR1, which
induces ETI via the RPS5 NLR) that is not present
in the closely related and highly virulent strain
PmaES4326. (A) Visual symptoms of representa-
tive wild-type Col-0 andrps5mutant plants
sprayed with PmaYM7930 compared to
Col-0 sprayed with PmaES4326 using a standard
no-flash setting. (B)Samephotosasin(A)
but with a green-pass filter to enhance the
difference between diseased and healthy tissue.
(CandD) In planta bacterialgrowth assays (C)
and plant disease scores based on the proportion
of yellow chlorotic plant tissue (D) illustrate
that the recognition of HopAR1 by RPS5 signifi-
cantly reduces bacterial growth and disease
symptoms. Letters above bars represent signif-
icance groups atP<0.05(Studentttests with
Holm-Bonferroni multiple test correction).
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