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Determinants of pathogenic@ and avirulence in plant
pathogenic bacteria
Alan Collmer
Many plant pathogenic bacteria possess a conserved protein
secretion system that is thought to transfer Avr (avirulence)
proteins, with potential activities in both parasitism and
defense elicitation, into plant cells. avr genes may be acquired
horizontally by these bacteria, and avr gene compositions
are highly variable. In the past year, heterologous expression
experiments have revealed that the products of avr genes
can be interchanged among different genera of bacteria
with retention of secretion, pathogenicity, and avirulence
activities, suggesting mechanisms for rapid coevolution of
these parasites with changing plant hosts.
Addresses
Department of Plant Pathology, Cornell University, Ithaca, NY
14853-4203,
USA; e-mail: arc2@cornelLedu
Current Opinion in Plant Biology 1998, 1:329-335
http://biomednet.com/elecref/1369526600100329
0 Current Biology Ltd ISSN 1369-5266
Abbreviations
Avr
HR
hrP
Pv
R
YOP
avirulence
hypersensitive response
hypersensitive response and pathogenicity
pathovar
resistance
Yefsiniaouter protein
Introduction
The most common bacterial pathogens of plants colonize the apoplast, and from this location outside the
walls of living cells they incite a variety of diseases
in most cultivated
plants [l]. The majority of these
pathogens
are Gram-negative
bacteria in the genera
Etwitria, Pseudomotras, Xatrthomonas, and Ralstotzia. Most
are host-specific and will elicit the hypersensitive
response
(HR) in nonhosts. The HR is a rapid, programmed death
of plant cells in contact with the pathogen. Some of the
defense responses associated with the HR are localized
at the periphery of plant cells at the site of bacterial
contact, but what actually stops bacterial growth has not
been established
[2,3,4”]. Pathogenesis
in host plants,
in contrast, involves prolonged
bacterial multiplication,
spread to surrounding tissues, and the eventual production
of macroscopic symptoms characteristic
of the disease.
Although these bacteria are diverse in their taxonomy
and pathology, they all possess hypersensitive
response
and pathogenicity
(/rq) genes which direct their ability
to elicit the HR in nonhosts or to be pathogenic
(and
parasitic) in hosts [5*]. The /rq genes encode a type
III protein-secretion
system that appears to be capable
of delivering Avr (avirulence)
proteins across the walls
and plasma membranes
of living plant cells [6*]. The
/Irp genes encode a type III protein secretion system
that appears to be capable of delivering
Avr proteins
across the walls and plasma membranes
of living plant
cells [6*]. Bacterial type III protein secretion systems are
characterized by an ability to inject effector proteins into
host cells, a membrane translocation apparatus containing
several flagellum biogenesis
homologs, and a lack of
processed amino-terminal
signal peptides on the secreted
proteins. The Avr proteins are so named because they
can betray the parasite to the resistance (R) gene-encoded
surveillance system of plants, thereby triggering the HR
[7*,8]. But Avr proteins also appear to be key to parasitism
in ‘compatible’ host plants, where the parasite proteins are
undetected
and the HR is not triggered. Thus, bacteria1
avirulence and pathogenicity
are inter-related
phenomena
and explorations
of HR elicitation
are furthering
our
understanding
of parasitic mechanisms.
Proteins
delivered
by the Hrp system are not the
only molecular weapons
in pathogen
arsenals; toxins,
phytohormones,
and enzymes that degrade the plant cell
wall often contribute
significantly
to the full expression
of symptoms [l]. The Hrp system and its protein traffic,
however, appear to underlie
basic parasitism, and this
article will focus on that aspect of pathogenesis.
The
succession of publications
in 1996 providing evidence
that Avr proteins
indeed
function
inside plant cells
following delivery by the Hrp system has been extensively
reviewed [5*-7*,9,10*,11’,12,13,14*]. This article will focus
on more recent reports concerning
the operation
and
ubiquity of Hrp systems, novel extracellular Hrp proteins,
and the secretion,
virulence
functions,
and potential
interchangeability
of Avr proteins.
Hrp systems
Type III protein secretion systems are present in bacteria1
pathogens of both animals and plants, and are exemplified
by the type III system of Yersitzia spp. [15,16’]. These
animal pathogens
are primarily
extracellular
parasites,
and their Yops (Yetzritziaouter proteins) are secreted and
translocated directly into host cells in a contact-dependent
manner [16*]. A similar host-contact
dependency
may
operate in most plant pathogenic
bacteria. Nine of the
hip genes are universal components
of type III secretion
systems, and these have been renamed /ITC (HR and
conserved) and given the last-letter designation
of their
Yenitzia homolog (with the exception
of ArcI’) [17]. The
Hrc proteins enable protein movement across the bacterial
inner and outer membranes
independently
of the genera1
protein export (Set) pathway [18]. In contrast to the Hrc
proteins, the Hrp proteins may be peripheral components
330
Host-microbe
interactions
of the Hrp secretion system and are more likely to perform
type III secretion functions
that are extracellular
and
specific to protein transfer across the plant cell wall and
plasma membrane (discussed below).
The genes encoding
type III secretion
systems are
usually clustered, and the emerging concept that genes
with related functions
in virulence
are often grouped
on plasmids or in horizontally
acquired pathogenicity
islands has important implications
throughout pathogenic
microbiology
[ 19,20,21*“]. Some pathogenicity
islands
govern key steps in pathogenesis,
such as the entry of
Sa/moneI/a into epithelial cells, and differences
in codon
usage and GC content between genes in the island and
those in the rest of the genome provide part of the
evidence that these islands are obtained by horizontal
transfer from other bacteria. There is some evidence
for horizontal acquisition
of /tq gene clusters in plant
pathogenic
bacteria, and the /rip cluster in Ralstonia
solanaceanrm is carried on a megaplasmid
[l]. The finding
of a plasmid-borne Rq gene cluster in Erwinia herbicola pv.
gypsophilae suggests that virulence may be acquired readily
by plant-associated
bacteria [Z?]. E. herbicola is a common
epiphyte
that is usually benign,
but strains classified
as E. herbicola pv. gypsop/riiae cause galls on gypsophila
and, like many plant pathogenic
bacteria, can elicit the
HR in tobacco. A 150kb plasmid carries phytohormone
biosynthetic
genes and /Ilp genes, and the latter are
required for both gall formation and HR elicitation [Z?‘].
The clustering of genes with related functions
is also
consistent
with the ability of some cloned /r7p clusters
to enable nonpathogens
like Escheridia co/i to elicit
the HR. This has been reported for cosmids pHIRl1
from Pseudomonas syringae pv. syfingae, pCPP430 from
Erwinia amylovora [l], pPPY430
from R syringae pv.
phaseolicola [23], and pCPP2156 from Erwinia chysanthemi
[24”].
Although
these cosmids
support
heterologous
HR elicitation,
they do not enable E. coii to become
pathogenic. The basis for HR elicitation is best understood
with pHIR11. The cosmid carries a 25 kb set of /lq
genes that is intact and functional, as revealed by DNA
sequencing and the ability to direct secretion of the HrpZ
harpin (a protein of unknown function that appears to be
targeted to the plant cell wall, as discussed below) [6*].
The cosmid also carries, adjacent to the /rrp cluster, the
/zrmA gene, which is aerr-like in producing an avirulence
phenotype when expressed in a tobacco pathogen and in
being lethal when heterologously
expressed inside tobacco
cells (25’1. The concept that the minimal requirement
for
bacterial elicitation of the HR is a functional Hrp system
and an avr gene whose product is recognized
by the
R-gene surveillance system of the test plant is supported
by experiments
in which the HR is observed only when
an appropriate, heterologous avr gene is supplied in trans
of the /Ilp+ cosmid [~23,24~*,26,27].
Hrp regulation
The /rtp genes are expressed when bacteria are inoculated
into plants or are growing in apoplast-mimicking
minimal
media but not usually in complex bacteriological
media
[SO]. The Hrp regulatory systems in plant pathogenic
bacteria can be divided into two groups, which correspond
also to differences in /zrp cluster composition
[6*]. In the
group I Hrp systems of Erwinia and Pseudomonas, /IQ
operons are activated by HrpL, a sigma factor [5*,13]. In
contrast, Aq transcription
in the group II Hrp systems
of Xanthomonas (HrpX) and Rafstonia (HrpB) is activated
by an AraC homolog [5*]. Upstream activators of these
two factors have been described
for E! syringae (HrpR
and HrpS) [5*,13], Xanthomonas campestris pv. vesicatoria
(HrpG) [28], and R. solanaceanrm (PrhA) [29”]. The recent
discovery of PrhA is particularly significant because this
putative outer membrane
protein, which appears to act
at the top of the Hrp regulatory hierarchy, is required
specifically for induction of irk genes in the presence of
plant cells and for full virulence in Arabidopsis [29”]. In
the host-promiscuous
pathogen E. carotovora, production
of the hrpN-encoded
harpin is activated by the quorum
(cell density) sensing signal, N-(3-oxohexanoyl)-L-homoserine lactone, and negatively regulated by RsmA, which
are two global regulators that similarly control exoenzyme
production
(30*,31]. Although
the ability to alter irq
expression through genetic manipulation
or appropriate
media has been experimentally
useful, our knowledge of
Hrp regulation is still fundamentally
incomplete regarding
in
the inventory
of regulatory components,
regulation
planta, and the presumed contact-dependent
activation of
Avr protein transfer.
Extracellular
Hrp proteins
Two classes of extracellular Hrp proteins have now been
defined - harpins and pilins. Harpins are glycine-rich
proteins that lack cysteine, are secreted in culture when
the Hrp system is expressed,
and possess heat-stable
HR elicitor activity when infiltrated into the leaves of
tobacco and several other plants [l]. Mutation
of the
prototypical /IqN harpin gene in 8. amylovora strain Ea321
strongly diminishes
HR and pathogenicity
phenotypes
[32*], but mutation of the h$.Z harpin gene in various l?
syritzgae strains has little or no effect on Hrp phenotypes
[33,34’]. The natural function of harpins and the basis
for their ability to elicit an apparent programmed
cell
death when artificially introduced
into the apoplast of
plants is unknown.
Two lines of evidence,
however,
point to a site of action in the plant cell wall. First,
purified J? syringae harpin binds to cell walls and has
biological activity only with walled cells [35]. Second,
HrpW, another harpin discovered in both E. amyhora and
/? syringae, has an amino-terminal
half that is harpin-like
but a carboxy-terminal
half that is homologous to a newly
defined class of pectate lyases found in fungal and bacterial
pathogens [32*,34*]. Elicitor activity resides in the harpin
Determinants of pathogenicity and avirulence in plant pathogenic bacteria Collmer
domain, and the pectate lyase domain, although lacking
enzymatic activity, binds specifically to pectate [34’]. The
second class of extracellular
Hrp proteins is represented
by the I? srringae HrpA pilin, which is a subunit of an Hrp
pilus that is 6-8 nm in diameter and is formed on bacteria
in an Hrp-dependent
manner [36”]. The Hrp pilus is
required for pathogenicity
and elicitation of the HR, and
a similar structure is important
for T-DNA
transfer in
Agrobactetium tumefaciens [37]. Whether
these structures
promote the transfer of bacterial macromolecules
into
plant cells by serving as conduits, guides, or attachment
factors is not known.
Avr proteins as swappable,
factors
secreted, virulence
A current
model for plant-bacterium
interaction
and
coevolution
based on Hrp delivery of Avr proteins into
plant cells (Figure 1) proposes firstly that Avr-like proteins
are the primary effecters of parasitism,
secondly that
conserved Hrp systems are capable of delivering many,
diverse Avr-like proteins into plant cells, and thirdly
that genetic changes in host populations
that reduce
the parasitic benefit of an effector protein or allow its
recognition by the R-gene surveillance
system will lead
to a proliferation
of complex arsenals of m/r-like genes
in coevolving bacteria [l]. There are still many gaps in
this picture. For example, the physical transfer of Avr
proteins into plant cells has never been observed, the
virulence functions of ‘Avr’ proteins are unknown, and it
is likely that previous searches for avr genes in various
bacteria have yielded incomplete inventories of the genes
encoding effector proteins. Recent progress, however, has
been made in each area.
Avr proteins had not previously been reported outside of
the cytoplasm of living li syringae and Xanthomonas spp.
cells [8,23], but it now appears that the Hrp systems
of Erwinia spp. can secrete Avr proteins in culture. A
homolog of the R syringae pv. tomato avrE gene has
been found in E. amyhora
and designated
a?pA in
strain CFBP1430 and dspE in strain Ea321 [38**,39**].
The dsp (disease specific) genes are required for the
pathogenicity
of E. amylovora, but not for HR elicitation.
A protein of the size expected for DspA is secreted in a
Hrp- and DspB-dependent
manner by CFBP1430 (DspB
is a potential chaperone, required for DspA secretion)
[38”]. Specific antibodies
were used to demonstrate
unambiguously
that DspE is efficiently
secreted in a
Hrp-dependent
manner by strain Ea.Ql [40”].
Nothing is known of the localization or expected site of
action of AvrE. There is strong evidence, however, that the
site of action of the I! syringae AvrB and AvrPto proteins is
inside plant cells (e.g. see [lo*]), and both proteins have
now been found to be secreted by an E. chysant/emi Hrp
system functioning
heterologously
in E. co/i [24**]. This
secretion is Hrp-dependent,
and E. co/i cells carrying the
E. chrrysanthemihlp genes also elicit an avrB-dependent
HR
331
in appropriate test plants. A strong implication of this work
is that E. chysanthemi, which is a host-promiscuous
soft-rot
pathogen, also carries mr-like genes. The ability of the
cloned E. chysanthemi Hrp system to secrete E! syringae Avr
proteins should promote searches for additional ar-like
genes by providing
an assay that can be performed
in culture and is independent
of the requirement
for
test plants that happen
to have a corresponding
R
gene, and it will enable
direct investigation
of Avr
targeting signals and secretion mechanisms.
For example,
chaperone-independent
targeting information in two Yop
proteins has been shown to reside in the mRNA encoding
the amino terminus of the protein [41**]. The involvement
of similar signals in Avr secretion is suggested by the need
for continued protein (but not mRNA) synthesis inplanta
for Avr signal delivery, which would be consistent with a
cotranslational secretion process [23].
The biochemical activities or parasite-promoting
functions
of Avr proteins remain unclear, although several of those
known make measurable
contributions
to virulence [8].
Members of the AvrBs3 family in Xanthomonas spp. are
targeted to the plant cell nucleus [10’,42], and some of
these have been shown recently to redundantly
direct
the production of watersoaking symptoms associated with
virulence [43]. AvrD (I! syringae pv. tomato) directs the
synthesis of syringolide elicitors of the HR [8]; AvrBsZ (X.
campestris pv. vesicatoria) shows similarity to A. tumefaciens
agrocinopine
synthase, which enables crown gall tumors
to produce a specialized carbon source for utilization by A.
tumefaciens [44]; and AvrRxv (X. campestris pv. vesicatoh)
is a homolog of AvrA (Salmonella typhimurium) and YopJ
(Yenina spp.), proteins which travel the type III pathway
in animal pathogens and trigger apoptosis in macrophages
[45,46]. This last observation
has led to the suggestion
may occur also in animal
that avr-R gene interactions
pathogenesis [47*].
The primary sequences
of the t! syringae Avr proteins
reveal little about their potential function, but interestingly, when heterologously
expressed in plants, three of
them have produced necrosis in test plants lacking the
cognate R gene [26,48*,49]. A key question is whether
this results from interaction of abnormally high levels of
the bacterial protein with plant virulence targets or from
interaction with cross-reacting
R-gene products. Further
evidence suggesting
that some avr genes in I! syringae
are beneficial to the bacteria in host plants was found in
recent studies of avrD and avrPphE; highly conserved,
nonfunctional
alleles of these genes have been retained
in pathogens whose hosts would recognize the functional
Avr product [48’,50’].
Avr-like genes may function heterologously
to support
pathogenesis as well as HR elicitation. The pathogenicity
of an E. amylovora dspE mutant can be restored (at least
partially) by a plasmid carrying the li syringae avrE locus,
suggesting that DspE and AvrE have similar functions
332
Host-microbe
interactions
[39**]. That dspE is essential for E. amylowora pathogeniconly quantitatively
to the
ity, whereas azwE contributes
virulence
of P syringae pv. tomato [51], suggests that
there is less redundancy
in the E. amylovora virulence
system. This would be consistent
with a more recent
acquisition of the Hrp system by E. amylovora and/or with
a slower coevolution with its perennial hosts [39”]. The
heterologous
functioning
of II syringae avr genes in E.
amylovora and E. uitysanthemi suggests that Hrp+ bacteria
in the field may be able to ‘sample’ a buffet of mr-like
genes from diverse sources during their coevolution with
changing plant populations.
Many avr genes have been
thought to be potentially mobile because of their presence
on plasmids [7*,8]. Recent observations
with P syringae
highlight the apparent mobility of avr genes. Several E!
syringae aw genes are linked with transposable
elements
or phage sequences ([52]; Kim JF et at’., unpublished
data),
and the /r@ clusters in different strains of li syringae,
although conserved
in themselves,
are bordered by a
hypervariable
region enriched in avr genes and mobile
DNA elements (JR Alfano, A0 Charkowski, A Collmer,
unpublished
data).
Figure 1
Conclusions
Plant R-gene
directed
surveillance
system
Plant
susceptibility
targets
V
AW
a
/
Parasitism
F
proteins
Y
vwvvw.
HRand
defense
/
Current Opmon in Plant Biology
Proposed model for bacterial pathogenicity
and coevolution
with
plants, which is based on the injection by a conserved Hrp (type Ill)
secretion system of horizontally
interchangeable
bacterial Avr-like
proteins. A typical Pseudomonas
syringae strain is depicted with
many avr genes
linked to the hrplhrc
gene cluster
in a region
containing
mobile genetic elements and also carried on plasmids.
The Hrp secretion apparatus is capable of delivering the products
of avr genes introduced from other pathovars or even other genera
of plant pathogenic
bacteria. Widely conserved
Hrc proteins are core
components
of a secretion apparatus
across the bacterial inner membrane
(OM).
Extracellular
Hrp proteins
such
that translocates
Avr proteins
(IM) and outer membrane
as the HrpA pilus protein
and
possibly the HrpZ and HrpW harpins are proposed to contribute
to the subsequent
transfer of Avr proteins across the plant cell
wall (CW) and plasma membrane (PM). Inside plant cells, the
recognition
of a single Avr protein by the R-gene surveillance
system
triggers the hypersensitive
response (HR) and plant defenses that
lead to resistance. Avr proteins are also proposed to interact with
putative susceptibility
targets that produce unknown changes in
plant metabolism favoring growth of the parasite in the apoplast.
The collective
contribution
of several
necessary for parasitism, whereas
betrayal to the defense system.
Avr-like
proteins
a single Avr protein
appears
to be
is sufficient
for
A fundamental
characteristic
of the prevalent
bacterial
plant pathogens
in the genera Erwinia, Pseudomonas,
Xanthomonas, and Ralstonia is the possession of a conserved
type III protein secretion pathway and a variable collection
of genes encoding
Avr (effector) proteins that appear
to be injected into plant cells via this pathway. As is
consistent with emerging concepts related to pathogenicity
islands, some aw genes are linked to h7p gene clusters,
and certain hp/avr ensembles are functional in that they
enable nonpathogens
to elicit the HR in appropriate test
plants. The evidence that Avr proteins can be delivered
by the Hrp system into plant cells remains indirect, but
it is strengthened
by the demonstration
that Avr proteins
can travel the Hrp pathway [24”]. This general model
demands both rigorous proof and answers to several new
questions.
How has the type III system been adapted
in plant pathogens
to permit the transfer of effector
proteins across the plant cell wall? What signals target
effector proteins to the Hrp pathway and then to specific
locations on or in plant cells? What is the inventory of
effector proteins delivered into plant cells? What do these
effector proteins do in plant cells to promote parasite
growth in the apoplast? What genetic mechanisms underlie
the high degree of variability in avr gene composition
of these pathogens? To what degree is host specificity
controlled by differences
in Hrp secretion mechanisms,
Hrp regulation, the virulence functions of Avr-like effector
proteins, the distribution
of interacting aur and R genes,
or the operation of ancillary virulence
systems? What
similarities will emerge between the effector proteins and
host targets of plant and animal pathogens? The answers
to these questions will further elucidate the determinants
of pathogenicity
and avirulence
and lead to a deeper
Determinants
understanding
plant defense.
of the
nature
of bacterial
parasitism
of pathogenicity
and
Acknowledgements
1thank James R Alfano, David
W Bauer, Steven V Beer, Amy 0 Charkowski,
Wee-Ling Deng, Derrick E Fouts, and Jihyun F Kim for critical review
of this manuscript. Work in my laboratory was supporred by grants
hlCB-9631530 from National Science Foundation (NSF) and 97-35303-4488
from the National Research Initiative Competitive Grants Program/USDA
(United States Department of Agriculture).
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10.
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Bonas U, Van den Ackerveken G: Recognition of bacterial
avirulence proteins occurs inside the plant cell: a general
phenomenon in resistance to bacterial diseases7 Plant I 1997,
12:1-7.
A particularly accessible and insightful account of the evidence that Avr
proteins act inside plant cells following delivery by the Hrp system.
22.
.
24.
..
Puri N, Jenner C, Bennet M, Stewart R, Mansfield J, Lyons N,
Taylor J: Expression of avrPphB, an avirulence gene from
Pseudomonas syringae pv. phaseolicola. and the delivery
of signals causing the hypersensitive reaction in bean. MO/
P/ant-Microbe Interact 1997, 10:247-256.
Ham JH, Bauer DW, Collmer A: A cloned Erwinia chrysanthemi
Hrp (type Ill protein secretion) system functions in fscherichia
co/i to deliver pseudomonas syringae Avr signals to plant cells
and to secrete Avr proteins in culture. Proc Nat/ Acad Sci USA
1998, in press.
This paper describes the technically useful discovery of an Hrp system
that functions in E. co/i to secrete heterologous Avr (avirulence) proteins,
334
Host-microbe
interactions
it demonstrates that the two P. syringae Avr proteins with the strongest
evidence for a site of action inside plant cells actually travel the Hrp pathway.
25.
.
Alfano JR, Kim H-S, Delaney TP, Collmer A: Evidence that the
pseudomonas
syringae pv. syringae hrp-linked hrmA gene
encodes an Avr-like protein that acts in a hrp-dependent
manner within tobacco cells. MO/ Plant-Microobe lnreract 1997,
10:580-588.
The functional cluster of /? sy’ingae pv. syringae hrp genes carried on cosmid pHlRl1 has provided a useful system for exploring the minimal requirements for bacterial elicitation of the hvoersensitive resoonse (HR). This work
characterizes hrmA, the avr-like component of the system that is essential for
HR elicitation in tobacco. It shows that DNA hybridizing with hrmA is lacking
from the tobacco pathogen, P. syringae pv. t.&aci, that expression of hrmA
in this pathogen renders it avirulent, and that expression of the gene inside
tobacco cells is lethal.
28.
Gopalan S, Bauer DW, Alfano JR, Loniello AO, He SY, Collmer A:
Expression of the pseudomonas
syringae avirulence
protein AvrB in plant cells alleviates its dependence
on the
hypersensitive
response and pathogenicity
(Hrp) secretion
system in eliciting genotype-specific
hypersensitive
cell death.
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Pirhonen MU, Lidell MC, Rowley DL, Lee SW, Jin S, Liang Y,
Silverstone S, Keen NT, Hutcheson SW: Phenotypic
expression
of pseudomonas
syringae avr genes in E. w/i is linked to
the activities of the hrp-encoded
secretion system. MO/
Plant-Microbe
Interact 1996, 9:252-260.
hypersensitive
response and to bind to pectete. J Bacterioiol
1998, in press.
This paper, which is a companion to Kim et a/. [32’1, additionally reports
that HrpW (but not the HrpZ harpin) binds to pectate and that sequences
hybridizing with the t? syringae hrpW gene are present in Xanthomonas
spp. and R. solanacearum.
Both papers discriminate the lethal activity of
active pectate lyases and harpins on the basis of the requirement of plant
metabolism for the lethal action of harpins. Both papers discriminate the
lethal activity of active pectate lyases and harpins on the basis of the requirement of plant metabolic activities, such as protein synthesis, for the lethal
action of harpins.
35.
36.
..
Roine E, Wei W, Yuan J, Nurmiaho-Lassila E-L, Kalkkinen N,
Romantschuk M, He SY: Hrp pilus: an hrp-dependent bacterial
surface appendage produced by Pseudomonas syringae pv.
tomato DC3000. Proc Nat/ Acad Sci USA 1997, 9413459-3464.
This breakthrough paper reports that hrpA encodes a subunit for a Hrp pilus
and that the pilus is required for Hrp phenotypes.
37.
28.
Wengelnik K, Van den Ackerveken G, Bonas U: HrpG, a key hrp
regulatory
protein of Xanthomonas
campestris
pv. vesicatoria
is homologous
to two-component
response regulators.
MO/
P/ant-Microbe
interact 1996, 9:704-712.
29.
..
Marenda M, Brito B, Callard D, Genin S, Barberis P, Boucher C,
Arlat M: PrhA controls a novel regulatory
pathway required for
the specific induction of Ralsfonia solanacearum
hrp genes in
the presence of plant cells. MO/ Microbial 1998, 27:437-453.
This breakthrough study provides evidence for the specific induction of hrp
genes in the presence of co-cultivated
plant cells and for a novel regulatory component involved in receiving plant signals. The paper describes the
discovery of the prhA gene, prhA mutant phenotypes in virulence and hypersensitive response assays, phylogenetic and structural relationships of PrhA
and known TonB-dependent receptors, and the effects of prhA mutation and
plant cell co-cultivation on the expression of various hrp operons.
30.
.
Mukherjee A, Cui Y, Liu Y, Chatterjee AK: Molecular
characterization
and expression
of the frwinia
carofovora
hrpA& gene, which encodes an elicitor of the hypersensitive
reaction. MO/ Plant-Microbe
interact 1997, 10:462-471.
A particularly thorough study of the environmental and regulatory factors
affecting expression of a harpin gene, which reveals overlapping control of
harpin and exoenzyme production by two global regulators.
Hoyos ME, Stanley CM, He SY, Pike S, Pu X-A, Novacky A: The
interaction
of Harpinpsr with plant cell walls. MO/ P/ant-Microbe
Interact 1996. 9:608-616.
Fullner KJ, Lara JC, Nester EW: Pilus assembly
Agrobacterium T-DNA transfer genes. Science
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Gaudriault S, Malandrin L, Paulin J-P, Barny M-A: DspA, an
essential pathogenicity
factor of frwinia amy/ovora showing
homology
with AvrE of Pseudomonas syringae, is secreted
via the Hrp secretion pathway in a DspB-dependent
way. MO/
Microbial 1997, 26:1057-l 089.
This important report of an f. amylovora homolog of the R syringae avrf
gene has particularly detailed information on the regulation of these dsp
genes and provides evidence suggesting that DspB is a chaperone required
for DspA secretion. Customized chaperones are required for the secretion
of Yop effector proteins by Yersinia spp. [I 6.1, but their importance in the
secretion of Avr-like proteins is unclear.
39.
..
Bogdanove AJ, Kim JF, Wei Z, Kolchinsky P, Charkowski AO,
Conlin AK, Collmer A, Beer SV: Homology
and functional
similarity of a hrp-linked
pathogenicity
operon, dspfE of
frwinia amyfovora and the avrf locus of Pseudomonas
syringae pathovar tomato. Proc Nat/ Acad Sci USA 1998,
95:1325-l 330.
This important
report characterizes
the dspf
and dspF genes of
E. amvlovora. comoares the comofeted seauences of AvrE and DsDE. and
demonstrates that ihe P. syringae ‘avrf locusin trans can restore pathogonicity to E. amylovora dspE mutants. This evidence that Hrp-dependent effector
loci can hsterologouhly
support pathogenicity across bacterial genera has
important implications for the evolution of plant pathogenic bacteria because
it suggests that pathogens can recruit from each other in the development
of their virulence factor arsenals.
40.
31.
Cui Y, Madi L, Mukherjee A, Dumenyo CK, Chatterjee AK: The
RsmA- mutants of fnvinia carofovora subsp. carofovora
strain Ecc71 overexpress
hrp&,
and elicit a hypersensitive
reaction-like
response in tobacco leaves. MO/ P/ant-Microbe
interact 1996, 9:565-573.
Kim JF, Beer SV: HrpW of Erwinia amy/ovora, a new harpin that
is a member of a proposed class of pectete lyares. J Bacterial
1998, in press.
This discovery of a second harpin gene linked to the f. amylovora hrp gene
cluster is particularly significant because HrpW carries a domain that is
a member of a newly defined class (Ill) of pectate lyases, and the ability of hrpW to restore full hypersensitive response elicitation activity to an
E. amy/ovora hrpN mutant suggests that both harpins have similar functions.
32.
.
33.
Alfano JR, Bauer DW, Milos TM, Collmer A: Analysis of the role
of the Pseudomonas syringae pv. syringae Hrpi! harpin in
elicitation of the hypersensitive
response in tobacco using
Boadanove AJ. Bauer DW. Beer SV: Erwinia amvlovora
secretes
D&E, a pathdgenicity
factor and functional
AviE homolog,
through the Hrp (type Ill secretion)
pathway. J Bacterial 1998,
160:in press.
Through the use of specific antibodies, this paper provides definitive evidence for the Hrp-dependent
secretion of DspE, an Avr-like protein, by
f. amylovora.
..
41.
..
Anderson DM, Schneewind 0: An mRNA signal for the type Ill
secretion of Yop proteins by Yersinia enferow/ifica.
Science
1997, 276:1140-l
143.
This landmark paper describes evidence signals that target two Yop proteins
to the prototypical type Ill pathway of Yersinia spp. reside in the cognate
mRNAs.
42.
Gabriel DW: Targeting of protein signals from Xanfhomonas
the plant nucleus. Trends P/ant Sci 1997, 2:204-206.
43.
Yang Y, Yuan 0, Gabriel DW: Wetersoaking
function(s)
of
XcmH1005
are redundantly
encoded by members of the
Xanfhomonas avr/pfh gene family. Mel Plant-Microbe
Interact
1996. 9:105-l 13.
44.
Swords KMM, Dahlbeck 0, Kearney B, Roy M, Staskawicz BJ:
Spontaneous
and induced mutations
in a single open reading
frame alter both virulence and avirulence
in Xanfhomonas
campesfris pv. vesicaforia avrBs2. J Bacterial 1996, 4661-4669.
functionally nonpolar deletion mutations, truncated HrpZ
fragments, and hrmA mutations. MO/ Microbial 1996, 19:715728.
34.
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Charkowski AO, Alfano JR, Preston G, Yuan J, He SY, Collmer
A: Pseudomonas syringae pv. tomato secretes a protein
via the Hrp (type Ill) pathway that has domains similar
harpins and pectate lyases and the capacity to elicit the plant
to
Determinants of pathogenic@ and avirulence in plant pathogenic bacteria Collmer
45.
Hardt WE, Galan JE: A secreted S&none//e
protein with
homology to an avirulence determinant
of plant pathogenic
bacteria. Proc Nat/ Acad Sci USA 1997, 94:9887-9892.
A6.
Monack DM. Mecsas J. Ghori N. Falkow S: Yersinia sianals
macrophages
to undergo apoptosis
and YopJ is necessary for
this cell death. Proc Nat/ Acad Sci USA 1997, 94:10385-i
0390.
Trends
Galan JE: ‘Avirulence genes’ in animal pathogens?
47.
Microbial 1998, 6:3-6.
.
This orovocative commentarv hiahliahts the similarities between the hvoersensitive response in plants and%f&tion-limiting
inflammatory respons&
in
animals, the presence of the AvrRxvPlopJIAvrA family of effector proteins in
both plant and animal pathogens, and ihe potential iherapeutic importance
of defense systems involving specific responsiveness
of ntiive animals to
bacterial pathogens.
Stevens C, Bennet MA, Athanassopoulos
E, Tsiarnis G, Taylor JD,
Mansfield JW: Sequence variations
in alleles of the avirulence
gene 8vrPphE.W from pseudomonas syringae pv. phaseolicole
lead to loss of recognition
of the AvrPphE protein within bean
cells and gain in cultivar specific virulence. MO/ Microbial 1998,
in press.
A thorough study of the nonfunctional alleles of aMphE that are found in all
races of /? syringae pv. phaseolicola, including the effects of alleles when
heterologously expressed in differential bean cultivars.
49.
335
McNellis TW, Mudgett MB, Li K, Aoyama T, Horvath D, Chua
N-H, Staskawicz BJ: Glucocorticoid-inducible
expression
of a
bacterial avirulence gene in transgenic
Arebidopsis induces
hypersensitive cell death. Plant I 1998, in press.
Keith LW, Boyd C, Keen NT, Partridge JE: Comparison of
avrLJ alleles from pseudomonas syringae pv. g/ycinea. MO/
Plant-Microbe interact 1997, lo:41 6-422.
This latest chapter in a long series of important studies of avrD, the only
avr gene directing a known biochemical activity, reveals that multiple races
of /? syringae pv. glycinea possess avrD alleles that are nonfunctional with
respect to syringolide elicitor production, but are highly conserved (although
the regions flanking them are polymorphic). These results suggest that avr
genes are both beneficial and highly mobile in plant pathogens.
50.
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51.
Lorang JM, Shen H, Kobayashi D, Cooksey D, Keen NT: evrA and
avrE in pseudomonas syringae pv. fomefo PT23 play a role in
virulence on tomato plants. MO/ P/ant-Microbe interact 1994,
7:508-515.
52.
Hanekamp T, Kobayashi D, Hayes S, Stayton MM: Avirulence
gene D of pseudomonas syringae pv. tomato may have
undergone
horizontal
gene transfer. FEES Lett 1997, 415:40-
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44.