Download Determinants of pathogenicity and avirulence in plant pathogenic

329
Determinants of pathogenicity 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: [email protected]
Current Opinion in Plant Biology 1998, 1:329–335
http://biomednet.com/elecref/1369526600100329
 Current Biology Ltd ISSN 1369-5266
Abbreviations
Avr
avirulence
HR
hypersensitive response
hrp
hypersensitive response and pathogenicity
pv
pathovar
R
resistance
Yop
Yersinia outer 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 [1]. The majority of these
pathogens are Gram-negative bacteria in the genera
Erwinia, Pseudomonas, Xanthomonas, and Ralstonia. 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 (hrp) genes which direct their ability
to elicit the HR in nonhosts or to be pathogenic (and
parasitic) in hosts [5•]. The hrp 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
hrp 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, bacterial
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 [1]. 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 bacterial
pathogens of both animals and plants, and are exemplified
by the type III system of Yersinia spp. [15,16•]. These
animal pathogens are primarily extracellular parasites,
and their Yops (Yersinia outer 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
hrp genes are universal components of type III secretion
systems, and these have been renamed hrc (HR and
conserved) and given the last-letter designation of their
Yersinia homolog (with the exception of hrcV) [17]. The
Hrc proteins enable protein movement across the bacterial
inner and outer membranes independently of the general
protein export (Sec) 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
Salmonella 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 hrp gene clusters in plant
pathogenic bacteria, and the hrp cluster in Ralstonia
solanacearum is carried on a megaplasmid [1]. The finding
of a plasmid-borne hrp gene cluster in Erwinia herbicola pv.
gypsophilae suggests that virulence may be acquired readily
by plant-associated bacteria [22•]. E. herbicola is a common
epiphyte that is usually benign, but strains classified
as E. herbicola pv. gypsophilae cause galls on gypsophila
and, like many plant pathogenic bacteria, can elicit the
HR in tobacco. A 150 kb plasmid carries phytohormone
biosynthetic genes and hrp genes, and the latter are
required for both gall formation and HR elicitation [22•].
The clustering of genes with related functions is also
consistent with the ability of some cloned hrp clusters
to enable nonpathogens like Escherichia coli to elicit
the HR. This has been reported for cosmids pHIR11
from Pseudomonas syringae pv. syringae, pCPP430 from
Erwinia amylovora [1], pPPY430 from P. syringae pv.
phaseolicola [23], and pCPP2156 from Erwinia chrysanthemi
[24••]. Although these cosmids support heterologous
HR elicitation, they do not enable E. coli to become
pathogenic. The basis for HR elicitation is best understood
with pHIR11. The cosmid carries a 25 kb set of hrp
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 hrp cluster, the
hrmA gene, which is avr-like in producing an avirulence
phenotype when expressed in a tobacco pathogen and in
being lethal when heterologously expressed inside tobacco
cells [25•]. 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 hrp+ cosmid [23,24••,26,27].
Hrp regulation
The hrp genes are expressed when bacteria are inoculated
into plants or are growing in apoplast-mimicking minimal
media but not usually in complex bacteriological media
[5•]. The Hrp regulatory systems in plant pathogenic
bacteria can be divided into two groups, which correspond
also to differences in hrp cluster composition [6•]. In the
group I Hrp systems of Erwinia and Pseudomonas, hrp
operons are activated by HrpL, a sigma factor [5•,13]. In
contrast, hrp transcription in the group II Hrp systems
of Xanthomonas (HrpX) and Ralstonia (HrpB) is activated
by an AraC homolog [5•]. Upstream activators of these
two factors have been described for P. syringae (HrpR
and HrpS) [5•,13], Xanthomonas campestris pv. vesicatoria
(HrpG) [28], and R. solanacearum (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 hrp 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 hrp
expression through genetic manipulation or appropriate
media has been experimentally useful, our knowledge of
Hrp regulation is still fundamentally incomplete regarding
the inventory of regulatory components, regulation in
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 [1]. Mutation of the
prototypical hrpN harpin gene in E. amylovora strain Ea321
strongly diminishes HR and pathogenicity phenotypes
[32•], but mutation of the hrpZ harpin gene in various P.
syringae 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 P. syringae harpin binds to cell walls and has
biological activity only with walled cells [35]. Second,
HrpW, another harpin discovered in both E. amylovora and
P. 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 P. syringae 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
Agrobacterium 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, secreted, virulence
factors
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 effectors 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 avr-like genes
in coevolving bacteria [1]. 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 P. 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 P. syringae pv. tomato avrE gene has
been found in E. amylovora and designated dspA 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 Ea321 [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 P. syringae AvrB and AvrPto proteins is
inside plant cells (e.g. see [10•]), and both proteins have
now been found to be secreted by an E. chrysanthemi Hrp
system functioning heterologously in E. coli [24••]. This
secretion is Hrp-dependent, and E. coli cells carrying the
E. chrysanthemi hrp genes also elicit an avrB-dependent HR
331
in appropriate test plants. A strong implication of this work
is that E. chrysanthemi, which is a host-promiscuous soft-rot
pathogen, also carries avr-like genes. The ability of the
cloned E. chrysanthemi Hrp system to secrete P. syringae Avr
proteins should promote searches for additional avr-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 in planta
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 (P. syringae pv. tomato) directs the
synthesis of syringolide elicitors of the HR [8]; AvrBs2 (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. vesicatoria)
is a homolog of AvrA (Salmonella typhimurium) and YopJ
(Yersina 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
that avr–R gene interactions may occur also in animal
pathogenesis [47•].
The primary sequences of the P. 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 P. 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 P. syringae avrE locus,
suggesting that DspE and AvrE have similar functions
332
Host–microbe interactions
Figure 1
avr
Horizontal
avr transfer
with other
pathogens?
Hypervariable
region with
many avr genes
Conserved
hrp/hrc
gene cluster
Region with
hrpW and
additional avr genes
Avr proteins
IM
OM
}
N RSTUV
J
C
Hrc proteins in Hrp
(type III) secretion
apparatus
HrpA pilus
CW
?
HrpZ
HrpW
} Harpins
Conclusions
PM
Plant R-gene
directed
surveillance
system
Plant
susceptibility
targets
}
Parasitism
[39••]. That dspE is essential for E. amylovora pathogenicity, whereas avrE contributes only quantitatively to the
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 P. syringae avr genes in E.
amylovora and E. chrysanthemi suggests that Hrp+ bacteria
in the field may be able to ‘sample’ a buffet of avr-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 P.
syringae avr genes are linked with transposable elements
or phage sequences ([52]; Kim JF et al., unpublished data),
and the hrp clusters in different strains of P. syringae,
although conserved in themselves, are bordered by a
hypervariable region enriched in avr genes and mobile
DNA elements (JR Alfano, AO Charkowski, A Collmer,
unpublished data).
Avr
proteins
e.g. AvrPto-Pto
HR and
defense
Current Opinion in Plant Biology
Proposed model for bacterial pathogenicity and coevolution with
plants, which is based on the injection by a conserved Hrp (type III)
secretion system of horizontally interchangeable bacterial Avr-like
proteins. A typical Pseudomonas syringae strain is depicted with
many avr genes linked to the hrp/hrc 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 that translocates Avr proteins
across the bacterial inner membrane (IM) and outer membrane
(OM). Extracellular Hrp proteins such 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 Avr-like proteins appears to be
necessary for parasitism, whereas a single Avr protein is sufficient for
betrayal to the defense system.
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 avr genes are linked to hrp gene clusters,
and certain hrp/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 avr 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 of pathogenicity and avirulence in plant pathogenic bacteria Collmer
understanding of the nature of bacterial parasitism and
plant defense.
Acknowledgements
I thank James R Alfano, David W Bauer, Steven V Beer, Amy O Charkowski,
Wen-Ling Deng, Derrick E Fouts, and Jihyun F Kim for critical review
of this manuscript. Work in my laboratory was supported by grants
MCB-9631530 from National Science Foundation (NSF) and 97-35303-4488
from the National Research Initiative Competitive Grants Program/USDA
(United States Department of Agriculture).
333
11.
•
Baker B, Zambryski P, Staskawicz B, Dinesh-Kumar SP:
Recognition and signaling in plant–microbe interactions.
Science 1997, 276:726-733.
A concisely presented big picture of plant–microbe interactions.
12.
Mudgett MB, Staskawicz BJ: Protein signaling via type III
secretion pathways in phytopathogenic bacteria. Curr Opin
Microbiol 1998, 1:109-114.
13.
Hutcheson SW: The hrp-encoded protein export systems of
Pseudomonas syringae and other plant pathogenic bacteria
and their role in pathogenicity. In Plant–Microbe Interactions, vol
3. Edited by Stacey G, Keen NT. New York: Chapman and Hall,
Inc.; 1997:145-179.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
2.
3.
Alfano JR, Collmer A: Bacterial pathogens in plants: life up
against the wall. Plant Cell 1996, 8:1683-1698.
Brown I, Mansfield J, Bonas U: hrp genes in Xanthomonas
campestris pv. vesicatoria determine ability to suppress papilla
deposition in pepper mesophyll cells. Mol Plant–Microbe
Interact 1995, 8:825-836.
Young SA, Wang X, Leach JE: Changes in the plasma
membrane distribution of rice phospholipase D during
resistant interactions with Xanthomonas oryzae pv. oryzae.
Plant Cell 1996, 8:1079-1090.
Bestwick CS, Brown IR, Bennett MHR, Mansfield JW:
Localization of hydrogen peroxide accumulation during the
hypersensitive reaction of lettuce cells to Pseudomonas
syringae pv. phaseolicola. Plant Cell 1997, 9:209-221.
This important study reveals that hydrogen peroxide, which has potential
defense-signaling and antimicrobial activities, accumulates in the plant cell
wall at the site of contact with bacteria expressing the hypersensitive response and pathogenicity (Hrp) system.
14.
He SY: Hrp-controlled interkingdom protein transport: learning
•
from flagellar assembly? Trends Microbiol 1997, 5:489-495.
A concise overview emphasizing the similarities between the Hrp and flagellar systems. This is based on the observation that eight of the Hrc proteins have homologs involved in flagellar-specific biogenesis and secretion
and that both systems produce extracellular (but quite different) surface appendages.
15.
16.
•
Cornelis GR, Wolf-Watz H: The Yersinia Yop regulon: a bacterial
system for subverting eukaryotic cells. Mol Microbiol 1997,
23:861-867.
A highly accessible overview of the prototypical type III secretion system.
17.
Bogdanove AJ, Beer SV, Bonas U, Boucher CA, Collmer A, Coplin
DL, Cornelis GR, Huang H-C, Hutcheson SW, Panopoulos NJ,
Van Gijsegem F: Unified nomenclature for broadly conserved
hrp genes of phytopathogenic bacteria. Mol Microbiol 1996,
20:681-683.
18.
Charkowski AO, Huang H-C, Collmer A: Altered localization
of HrpZ in Pseudomonas syringae pv. syringae hrp mutants
suggests that different components of the type III secretion
pathway control protein translocation across the inner and
outer membranes of Gram-negative bacteria. J Bacteriol 1997,
179:3866-3874.
19.
Lawrence JG, Roth JR: Selfish operons: horizontal transfer may
drive the evolution of gene clusters. Genetics 1996, 143:18431860.
20.
Groisman EA, Ochman H: Pathogenicity islands: bacterial
evolution in quantum leaps. Cell 1996, 87:791-794.
4.
••
5.
Lindgren PB: The role of hrp genes during plant-bacterial
•
interactions. Annu Rev Phytopathol 1997, 35:129-152.
A comprehensive review of Hrp systems, their genetics, component proteins,
regulation, interactions with Avr (avirulence) proteins. The discussion of the
role of the Hrp system in the suppression of plant defenses is particularly
important.
6.
•
Alfano JR, Collmer A: The type III (Hrp) secretion pathway of
plant pathogenic bacteria: trafficking harpins, Avr proteins, and
death. J Bacteriol 1997, 179:5655-5662.
A review of Hrp systems emphasizing type III secretion functions and the
delivery of Avr (avirulence) proteins into plant cells. Contains a useful comparison of group I and II hrp gene arrangements and a table of Hrc (hypersensitive response, conserved) protein homologs in Yersinia, Salmonella,
Shigella spp.
Finlay B, Falkow S: Common themes in microbial pathogenicity
revisited. Microbiol Mol Biol Rev 1997, 61:136-169.
21.
••
Hacker J, Blum-Oehler G, Muhldorfer I, Tschape H: Pathogenicity
islands of virulent bacteria: structure, function and impact on
microbial evolution. Mol Microbiol 1997, 23:1089-1097.
A definitive overview of pathogenicity islands, which deals exclusively with
animal pathogens and focuses on Pais (pathogenicity islands) encoding type
III secretion systems.
7.
Vivian A, Gibbon MJ: Avirulence genes in plant-pathogenic
•
bacteria: signals or weapons? Microbiology 1997, 143:693-704.
An excellent review and the most current general overview of avr genes.
The article has many good insights, for example, in the conceptually useful
division of avr genes into two groups: P. syringae-type and avrBs3-like.
Nizan R, Barash I, Valinsky L, Lichter A, Manulis S: The presence
of hrp genes on the pathogenicity-associated plasmid of the
tumorigenic bacterium Erwinia herbicola pv. gypsophilae. Mol
Plant–Microbe Interact 1997, 10:677-682.
A description of an exciting new Hrp system that may provide a useful case
study for portable pathogenicity.
8.
Leach JE, White FF: Bacterial avirulence genes. Annu Rev
Phytopathol 1996, 34:153-179.
23.
9.
Van den Ackerveken G, Bonas U: Bacterial avirulence proteins
as triggers of plant defense resistance. Trends Microbiol 1997,
5:394-398.
10.
•
Bonas U, Van den Ackerveken G: Recognition of bacterial
avirulence proteins occurs inside the plant cell: a general
phenomenon in resistance to bacterial diseases? Plant J 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. Mol
Plant–Microbe Interact 1997, 10:247-256.
Ham JH, Bauer DW, Collmer A: A cloned Erwinia chrysanthemi
Hrp (type III protein secretion) system functions in Escherichia
coli to deliver Pseudomonas syringae Avr signals to plant cells
and to secrete Avr proteins in culture. Proc Natl Acad Sci USA
1998, in press.
This paper describes the technically useful discovery of an Hrp system
that functions in E. coli 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. Mol Plant–Microbe Interact 1997,
10:580-588.
The functional cluster of P. syringae pv. syringae hrp genes carried on cosmid pHIR11 has provided a useful system for exploring the minimal requirements for bacterial elicitation of the hypersensitive response (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. tabaci, that expression of hrmA
in this pathogen renders it avirulent, and that expression of the gene inside
tobacco cells is lethal.
26.
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.
Plant Cell 1996, 8:1095-1105.
27.
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. coli is linked to
the activities of the hrp-encoded secretion system. Mol
Plant–Microbe Interact 1996, 9:252-260.
hypersensitive response and to bind to pectate. J Bacteriol
1998, in press.
This paper, which is a companion to Kim et al. [32•], additionally reports
that HrpW (but not the HrpZ harpin) binds to pectate and that sequences
hybridizing with the P. 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 Natl Acad Sci USA 1997, 94:3459-3464.
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. Mol
Plant–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 Ralstonia solanacearum hrp genes in
the presence of plant cells. Mol Microbiol 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 Erwinia carotovora
hrpNEcc gene, which encodes an elicitor of the hypersensitive
reaction. Mol 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.
31.
Cui Y, Madi L, Mukherjee A, Dumenyo CK, Chatterjee AK: The
RsmA– mutants of Erwinia carotovora subsp. carotovora
strain Ecc71 overexpress hrpNEcc and elicit a hypersensitive
reaction-like response in tobacco leaves. Mol Plant–Microbe
Interact 1996, 9:565-573.
32.
•
Kim JF, Beer SV: HrpW of Erwinia amylovora, a new harpin that
is a member of a proposed class of pectate lyases. J Bacteriol
1998, in press.
This discovery of a second harpin gene linked to the E. amylovora hrp gene
cluster is particularly significant because HrpW carries a domain that is
a member of a newly defined class (III) of pectate lyases, and the ability of hrpW to restore full hypersensitive response elicitation activity to an
E. amylovora hrpN mutant suggests that both harpins have similar functions.
33.
34.
•
Alfano JR, Bauer DW, Milos TM, Collmer A: Analysis of the role
of the Pseudomonas syringae pv. syringae HrpZ harpin in
elicitation of the hypersensitive response in tobacco using
functionally nonpolar deletion mutations, truncated HrpZ
fragments, and hrmA mutations. Mol Microbiol 1996, 19:715728.
Charkowski AO, Alfano JR, Preston G, Yuan J, He SY, Collmer
A: Pseudomonas syringae pv. tomato secretes a protein
via the Hrp (type III) pathway that has domains similar
harpins and pectate lyases and the capacity to elicit the plant
Hoyos ME, Stanley CM, He SY, Pike S, Pu X-A, Novacky A: The
interaction of HarpinPss with plant cell walls. Mol Plant–Microbe
Interact 1996, 9:608-616.
Fullner KJ, Lara JC, Nester EW: Pilus assembly by
Agrobacterium T-DNA transfer genes. Science 1996, 273:11071109.
38.
••
Gaudriault S, Malandrin L, Paulin J-P, Barny M-A: DspA, an
essential pathogenicity factor of Erwinia amylovora showing
homology with AvrE of Pseudomonas syringae, is secreted
via the Hrp secretion pathway in a DspB-dependent way. Mol
Microbiol 1997, 26:1057-1069.
This important report of an E. amylovora homolog of the P. syringae avrE
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. [16•], 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, dspEF, of
Erwinia amylovora and the avrE locus of Pseudomonas
syringae pathovar tomato. Proc Natl Acad Sci USA 1998,
95:1325-1330.
This important report characterizes the dspE and dspF genes of
E. amylovora, compares the completed sequences of AvrE and DspE, and
demonstrates that the P. syringae avrE locus in trans can restore pathogenicity to E. amylovora dspE mutants. This evidence that Hrp-dependent effector
loci can heterologously 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.
••
Bogdanove AJ, Bauer DW, Beer SV: Erwinia amylovora secretes
DspE, a pathogenicity factor and functional AvrE homolog,
through the Hrp (type III secretion) pathway. J Bacteriol 1998,
180: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
E. amylovora.
41.
••
Anderson DM, Schneewind O: An mRNA signal for the type III
secretion of Yop proteins by Yersinia enterocolitica. Science
1997, 278:1140-1143.
This landmark paper describes evidence signals that target two Yop proteins
to the prototypical type III pathway of Yersinia spp. reside in the cognate
mRNAs.
42.
Gabriel DW: Targeting of protein signals from Xanthomonas to
the plant nucleus. Trends Plant Sci 1997, 2:204-206.
43.
Yang Y, Yuan Q, Gabriel DW: Watersoaking function(s) of
XcmH1005 are redundantly encoded by members of the
Xanthomonas avr/pth gene family. Mol Plant–Microbe Interact
1996, 9:105-113.
44.
Swords KMM, Dahlbeck D, Kearney B, Roy M, Staskawicz BJ:
Spontaneous and induced mutations in a single open reading
frame alter both virulence and avirulence in Xanthomonas
campestris pv. vesicatoria avrBs2. J Bacteriol 1996, 4661-4669.
Determinants of pathogenicity and avirulence in plant pathogenic bacteria Collmer
45.
Hardt WE, Galan JE: A secreted Salmonella protein with
homology to an avirulence determinant of plant pathogenic
bacteria. Proc Natl Acad Sci USA 1997, 94:9887-9892.
46.
Monack DM, Mecsas J, Ghori N, Falkow S: Yersinia signals
macrophages to undergo apoptosis and YopJ is necessary for
this cell death. Proc Natl Acad Sci USA 1997, 94:10385-10390.
47.
Galan JE: ‘Avirulence genes’ in animal pathogens? Trends
•
Microbiol. 1998, 6:3-6.
This provocative commentary highlights the similarities between the hypersensitive response in plants and infection-limiting inflammatory responses in
animals, the presence of the AvrRxv/YopJ/AvrA family of effector proteins in
both plant and animal pathogens, and the potential therapeutic importance
of defense systems involving specific responsiveness of naı̈ve animals to
bacterial pathogens.
48.
•
Stevens C, Bennet MA, Athanassopoulos E, Tsiamis G, Taylor JD,
Mansfield JW: Sequence variations in alleles of the avirulence
gene avrPphE.R2 from Pseudomonas syringae pv. phaseolicola
lead to loss of recognition of the AvrPphE protein within bean
cells and gain in cultivar specific virulence. Mol Microbiol 1998,
in press.
A thorough study of the nonfunctional alleles of avrPphE that are found in all
races of P. 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 Arabidopsis induces
hypersensitive cell death. Plant J 1998, in press.
50.
•
Keith LW, Boyd C, Keen NT, Partridge JE: Comparison of
avrD alleles from Pseudomonas syringae pv. glycinea. Mol
Plant–Microbe Interact 1997, 10:416-422.
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 P. 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.
51.
Lorang JM, Shen H, Kobayashi D, Cooksey D, Keen NT: avrA and
avrE in Pseudomonas syringae pv. tomato PT23 play a role in
virulence on tomato plants. Mol Plant–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. FEBS Lett 1997, 415:4044.