Download Genes controlling expression of defense responses in

301
Genes controlling expression of defense responses in
Arabidopsis — 2001 status
Jane Glazebrook
In the past two years, the focus of studies of the genes
controlling expression of defense responses in Arabidopsis
has shifted from the identification of mutants to gene isolation
and the ordering of genes within branches of the signal
transduction networks. It is now clear that gene-for-gene
resistance can be mediated through at least three genetically
distinguishable pathways. Additional genes affecting salicylicacid-dependent signaling have been identified, and doublemutant analyses have begun to reveal the order in which they
act. Genes required for jasmonic-acid-dependent signaling and
for induced systemic resistance have also been identified.
sufficient speed, these responses are generally quite effective
in preventing disease. Consequently, there is considerable
interest in understanding the signal transduction networks
that control the activation of defense responses in plants.
One approach to this problem is to use the powerful
Arabidopsis genetic system to identify plant genes that
have crucial roles in regulating the activation of defense
responses. This article summarizes progress made using
this system since the previous review of this topic was
written [1]. A model describing the circuitry of the signal
transduction network is presented in Figure 1.
Addresses
Torrey Mesa Research Institute, 3115 Merryfield Row, San Diego,
California 92121, USA; e-mail: [email protected]
Gene-for-gene resistance is a particularly strong form of
plant disease resistance. Plants carry specific resistance (R)
genes that are able to recognize pathogens carrying corresponding avirulence (avr) genes. This reaction triggers a
rapid defense response that generally includes the programmed cell death of plant cells that are in contact with
the pathogen, a phenomenon called the hypersensitive
response (HR). Widespread difficulties in detecting direct
interactions between R and avr proteins have led to the
hypothesis that R proteins ‘guard’ plant proteins
(i.e. ‘guardees’) that are the targets of pathogen avr
proteins, triggering the HR and other responses when
avr–guardee interactions are detected [2]. The detection of
an in vivo complex containing an R protein, an avr protein,
and an unidentified plant protein is consistent with this
hypothesis [3]. This article does not discuss the extensive
literature concerning function of R proteins, but does
describe signaling downstream from R protein function.
Current Opinion in Plant Biology 2001, 4:301–308
1369-5266/01/$ — see front matter
© 2001 Elsevier Science Ltd. All rights reserved.
Abbreviations
ACD2 ACCELERATED CELL DEATH 2
avr
avirulence
coi1
coronatine insensitive 1
cpr1
constitutive PR 1
dnd1
defense no death 1
dth9
detachment 9
EDS1
ENHANCED DISEASE SUSCEPTIBILITY 1
ein2
ethylene insensitive2
ET
ethylene
HR
hypersensitive response
ISR
induced systemic resistance
JA
jasmonic acid
jar1
jasmonic acid resistant 1
LRR
leucine-rich repeat
LZ
leucine-zipper
MPK4 MITOGEN-ACTIVATED PROTEIN KINASE 4
nahG
salicylcate hydroxylase
NBS
nucleotide-binding site
NDR1 NON-RACE-SPECIFIC DISEASE RESISTANCE 1
NPR1
NON-EXPRESSOR OF PR 1
PAD4
PHYTOALEXIN DEFICIENT 4
PBS1
avrPphB Susceptible 1
PDF1.2 PLANT DEFENSIN1.2
PR-1
PATHOGENESIS-RELATED PROTEIN-1
R
resistance
RPP7
RESISTANCE TO PERONOSPORA PARASITICA 7
RPS5
RESISTANCE TO PSEUDOMONAS SYRINGAE 5
RPW8 RESISTANCE TO POWDERY MILDEW 8
SA
salicylic acid
SAR
systemic acquired resistance
SID1
SA INDUCTION DEFICIENT 1
sni1
suppressor of npr1-1 inducible
TIR
toll-interleukin 2 receptor
Introduction
Activation of the HR triggers a systemic resistance
response known as systemic acquired resistance (SAR).
This response includes the accumulation of the signal
molecule salicylic acid (SA) throughout the plant and the
consequent expression of a characteristic set of defense
genes, including PR-1. Plants expressing SAR are more
resistant to subsequent attack by a variety of otherwise
virulent pathogens. Many defense responses that are
characteristic of SAR also contribute to local resistance that
is mediated by some R genes, and to the local growth
limitation of moderately virulent pathogens. In a large
expression profiling experiment, at 48 hours after infection, the spectra of plant genes expressed in response to a
virulent Peronospora parasitica isolate and an isolate that
triggers gene-for-gene resistance were similar [4••].
Responses triggered by R genes generally occur much faster
than responses to virulent pathogens; this presumably
explains why R-gene-mediated resistance is so effective.
Plants defend themselves from attack by microbial
pathogens by activating a battery of defense responses
after infection. Provided that they are activated with
Some defense responses are activated by signal transduction
networks that require jasmonic acid (JA) and ethylene (ET)
302
Biotic interactions
Figure 1
A model describing the positions of
Arabidopsis genes in signal transduction
networks that control the activation of defense
responses. This figure is meant to provide a
rough outline of the sites of action of various
gene products in disease resistance signaling.
It does not account for all of the described
phenotypes of the various signaling mutants.
Boxes containing the word ‘resistance’
represent the numerous effectors of
resistance, whose effectiveness may vary
greatly according to the pathogen eliciting the
response. There are certainly interconnections
among the pathways shown in (a), (b), (c),
and (d), but many of these are not sufficiently
clear to allow them to be easily drawn.
(a) Three R-gene dependent pathways are
shown, one that requires NDR1 and PBS2, a
second that requires EDS1 and PAD4, and a
third for which the required genes have not
been reported. (b) In SA-dependent signaling,
CPR1, EDR1, and CPR5 tend to repress
pathway activation, and act upstream of
PAD4. The cpr6 mutation exerts a positive
effect upstream of PAD4, but it is difficult to
predict the function of the wild-type gene as
cpr6 is dominant. PAD4 and EDS1 promote
SA accumulation, as do SID2 and EDS5.
SA also promotes expression of PAD4 and
EDS1. PAD4 promotes expression of
resistance responses in an SA-independent
manner. Formation of lesions promotes SA
accumulation in a manner that is independent
of PAD4 and EDS1, but may require EDS5.
Downstream of SA, NPR1 mediates the
activation of expression of genes such as
PR-1, acting together with TGA transcription
factors in a manner that is inhibited by SNI1.
DTH9 also acts downstream of SA to activate
resistance responses. (c) JA and ET act
together to activate expression of genes such
(a) Gene-for-gene resistance
R gene
RPM1
RPS2
RPS5
NDR1
PBS2
R gene
RPW8
RPP2
RPP4
RPP5
RPP10
RPP14
RPS4
R gene
RPP7
RPP8
RPP13
(b) SA-dependent resistance
Systemic or pathogen-derived signal
DND1
EDR1
CPR5
DND2
cpr6
ACD2
CPR1
etc.
PAD4
EDS1
EDS1
PAD4
SID2
EDS5
Resistance
SA
(d) Induced systemic
JA
Systemic or pathogen-derived signal
Lesion formation
Camalexin
cpr6
JA (OPR3)
ET
COI1
JAR1
MPK4
EIN2
MPK4
PDF1.2 etc.
NPR1
DTH9
resistance
ISR
(c) Dependent resistance
Resistance
TGA factors
SNI1
PR-1 etc.
ET
Resistance
ISR1
NPR1
Resistance
Current Opinion in Plant Biology
as PDF1.2. This activation is promoted by
lesion formation and by cpr6. MPK4, COI1
and JAR1 are required to transduce the JA
signal, while EIN2 is required to transduce
the ET signal. Camalexin production requires
both PAD4 and COI1, and some lesion-mimic
mutations cause camalexin accumulation in
lesions. (d) ISR activation requires JA,
followed by ET. ISR1 affects sensitivity to ET.
NPR1 is required downstream of JA and ET
to activate resistance. The model does not
show the complicated interactions between
SA and JA signaling. For example, MPK4
inhibits SA signaling, possibly by promoting
JA signaling, but the nature of the
relationship between the promotion of JA
signaling and the inhibition of SA signaling is
not understood.
Mutations in PBS1, NDR1, EDS1, PAD4, and PBS2 block
gene-for-gene resistance that is mediated by some
R genes. The only R gene known to be affected by PBS1
mutations is RPS5 [5]. This observation raises the intriguing
possibility that PBS1 encodes the ‘guardee’ monitored by
RPS5. As RPS5 and the corresponding avr gene, avrPphB,
have both been isolated, it should be possible to test this
hypothesis in the near future when PBS1 has been isolated.
to require EDS1 belong to the subset of the NBS-LRR
class of R proteins with an amino-terminal domain similar
to the Drosophila Toll and mammalian interleukin 2
receptors (TIR-NBS-LRR) [6]. Although the effect of pad4
mutations on R-gene function has not been studied in great
detail, it seems that R genes that require EDS1 also require
PAD4 [7], an idea that is consistent with the extensive phenotypic similarities between pad4 and eds1 mutants
[8,9,10•,11•]. Similarly, R genes that require NDR1 also
require PBS2, although some R genes with an absolute
requirement for NDR1 are not completely blocked by
mutations in PBS2 and vice versa [5]. All of this evidence is
consistent with the hypothesis of Aarts et al. [6], who suggested that there may be two downstream pathways
triggered by R genes, with the structure of the R protein
determining which downstream factors are required.
R genes known to require NDR1 belong to the leucinezipper (LZ) subclass of nucleotide-binding site
(NBS)-leucine-rich repeat (LRR) genes. R proteins known
Other recent results have shown that the situation cannot be
this simple. Two R genes, RPP7 and RPP8, require neither
EDS1 nor NDR1 (although resistance is weakly suppressed
as signal molecules. Different pathogens are limited to different extents by SA-dependent responses and by
JA/ET-dependent responses. There appears to be considerable cross-talk between these signal transduction
networks, with at least some SA-dependent responses
limited by JA/ET-dependent responses and vice versa.
R-gene signal transduction
Genes controlling expression of defense responses in Arabidopsis — 2001 status Glazebrook
in an eds1 ndr1 double mutant), suggesting that there is at
least a third pathway for the transduction of R-gene signals
[12••]. RPP13-mediated resistance does not require EDS1,
PAD4, PBS2, or NDR1, is completely unaffected in an
eds1 ndr1 double mutant, and is not blocked by nahG [13•].
RPP7 has not been cloned, but RPP8 and RPP13 are
LZ-NBS-LRR proteins that are similar to each other [14,15].
Hence, these genes are exceptions to the rule that LZ-NBSLRR proteins require NDR1. Taken together, the data
support a model including at least three signal transduction
cascades acting downstream of R genes, one that is
NDR1-dependent, a second that is EDS1-dependent, and a
third for which no genetic components have yet been
defined. This conclusion rests on the assumption that the
mutant ndr1, eds1, pad4, and pbs2 alleles that were tested are
all null. Otherwise, the observed differences in genetic
requirements could reflect quantitative differences in the
signaling activities of the various alleles, rather than action of
different signaling pathways. The commonly-used ndr1,
pad4, and eds1 alleles are likely null, as they are deletions or
nonsense mutations that destabilize the mRNAs. The nature
of the pbs2 alleles will not be known until PBS2 is isolated.
What determines which pathway is used by a particular
R gene is not clear. For example, the R gene HRT is more
than 90% identical to RPP8 [16], yet HRT-mediated resistance is blocked by the transgene nahG [17], which
encodes an SA-degrading enzyme, whereas resistance
mediated by RPP8 or RPP13 is unaffected by nahG
[12••,13•]. Unfortunately, the effects of mutations in
NDR1, EDS1, PAD4, or PBS2 on HRT-mediated resistance
were not tested [17]. In another example, the resistance
mediated by the broad-spectrum R gene RPW8 requires
EDS1 and is blocked by nahG. The protein encoded by
RPW8 includes a coiled-coil domain but otherwise bears
little similarity to the LZ-NBS-LRR genes [18•]. Although
the pathway used by a particular R gene may be determined by the structural features of the R gene, factors in
addition to whether the R gene is of the LZ-NBS-LRR or
the TIR-NBS-LRR type are clearly important.
A few of the genes that mediate R-gene signal transduction
have been isolated. NDR1 encodes a protein with two predicted transmembrane domains [19]. It is possible that part
of the function of NDR1 is to hold R proteins close to the
membrane. EDS1 and PAD4 encode proteins with similarity to triacyl glycerol lipases [8,9]. The significance of
this is not known, but it is tempting to speculate that
EDS1 and PAD4 are involved in the synthesis or degradation of a signal molecule. Mutations in either gene can
cause a defect in SA accumulation and expression of both
genes can be induced by SA or pathogen infection, suggesting that they act in the SA pathway upstream of SA,
possibly as part of a signal amplification loop [8,9].
SA-dependent signaling
SA-dependent signaling is important for some gene-forgene resistance responses, for local responses that limit the
303
growth of virulent pathogens, and for SAR. SAR is a resistance response that is activated throughout the plant in
response to local infection by necrotizing pathogens. As
described above, EDS1 and PAD4 act upstream of SA to
promote SA accumulation. The ankyrin-repeat protein
NPR1; also known as NIM1 (NON-INDUCIBLE IMMUNITY 1) and SAI1 (SALICYLIC ACID INSENSITIVE 1)
acts downstream of SA to promote the expression of the
SAR-associated genes PR-1, BGL2, and PR-5.
Recently, additional genes that act in the SA signaling
pathway have been identified, double-mutant analyses
have placed some negative regulators in the pathway, and
NPR1-interacting factors have been identified. Mutations
in EDS5 (also known as SID1) and SID2 (also known as
EDS16 [20]) abolish the increase in SA levels observed in
infected plants. In fact, SA levels in these mutants are as
low as they are in plants carrying the nahG transgene,
which encodes an enzyme that degrades SA to catechol.
Hence, EDS5 and SID2 are suggested to function
upstream of SA accumulation [21]. A mutation in EDS4
causes reduced SA accumulation and reduced sensitivity to
SA, so EDS4 seems to function in SA signaling [22].
Interestingly, although eds5, sid2, eds4, eds1 and pad4 mutations (as well as the transgene nahG) reduce SA levels and
expression of PR-1, only pad4 and nahG reduce production
of the phytoalexin camalexin [21–23]. It appears, therefore,
that low SA production does not cause the camalexin defect
in these plants, and that pad4 and nahG may affect signaling
pathways in addition to the SA-dependent pathway.
Double-mutant analyses have been used to place some of
the regulatory factors in the SA signaling network in order.
A probable null mutation in EDR1 enhances resistance to
Pseudomonas syringae and Erisyphe cichoracearum, and causes
more rapid expression of defense-related genes, such as
PR-1, after infection [24,25••]. The resistance is blocked
by nahG and by eds1, pad4, or nim1 (allelic to npr1) mutations, indicating that the effect of the edr1 mutation
requires SA-dependent signaling [25••]. Presumably, the
mitogen-activated protein kinase (MAPK) kinase kinase
([MAPK]KK) encoded by EDR1 is involved in the negative regulation of SA signaling, acting upstream of pad4
and eds1 [25••]. A mutation in the MAPK gene MPK4
causes constitutively high SA accumulation, PR-1 expression
and resistance to P. syringae [26•]. These phenotypes are
blocked by nahG but not by the npr1 mutation [26•]. The
effects of other mutations were not tested. It does not
seem likely that MPK4 is a component of the EDR1 kinase
cascade, as the phenotypes of mpk4 are considerably more
severe than those of edr1 and are NPR1-independent.
Rather, MPK4 may impact SA signaling upstream of SA by
affecting the balance between SA-dependent and jasmonic
acid (JA)-dependent signaling, as discussed further below
[26•]. The mutations cpr1 and cpr6 cause constitutively
high SA levels, PR-1 expression, and enhanced disease
resistance. These phenotypes are blocked by mutations in
eds5 [27••], eds1 [10•], or pad4 [11•], placing the effects of
304
Biotic interactions
the cpr mutations on SA signaling upstream of PAD4.
Reminiscent of the situation with MPK4, however, the
disease resistance phenotypes of cpr1 and cpr6 are only partially blocked by mutations in npr1, demonstrating that
there must be SA-dependent, NPR1-independent resistance mechanisms [27••]. In dnd1 pad4 or dnd2 pad4
double mutants, the high SA and PR-1 expression levels
that are characteristic of dnd plants are retained but susceptibility to P. syringae is as high as it is in pad4 plants,
suggesting that there is also a PAD4-dependent, SA-independent component of resistance [11•].
Many mutants with constitutively high SA levels, defensegene expression, and disease resistance also display a
lesion-mimic phenotype. That is, they undergo spontaneous cell death in the absence of pathogen attack. Some
of the genes defined by these mutations may function as
negative regulators of hypersensitive cell death, whereas
others may merely perturb cellular metabolism in a way
that leads to cell death, which then triggers the SAR signal
transduction cascade. Indeed, some transgenes that perturb cellular metabolism, including the bacterio-opsin
proton pump [28] and antisense protoporphyrinogen oxidase [29], cause a lesion-mimic phenotype. The identities
of recently isolated lesion-mimic genes are consistent with
the idea that many lesion-mimic mutations disrupt cellular
metabolism. ACD2 encodes a protein that is similar to red
chlorophyll catabolite reductase, an enzyme responsible
for the proper catabolism of the porphyrin component of
chlorophyll [30]. Presumably, loss of this enzyme in acd2
mutants results in the accumulation of toxic catabolites
that trigger the lesions. This enzyme may also play a role
in coping with pathogen damage as overexpression of
ACD2 attenuates disease symptoms [30]. DND1 encodes a
cyclic nucleotide-gated ion channel [31]. This channel
could be involved in the regulation of ion flows that are
related to programmed cell death. Alternatively, the loss of
this function could simply perturb cellular homeostasis in
a manner that promotes cell death. (Recently, it has been
recognized that both dnd1 and dnd2 mutants are lesionmimics under certain growth conditions [11•,32].)
Double-mutant experiments involving some lesion-mimic
mutants have revealed interesting features of SA-dependent signal transduction. In cpr5 mutants, SA level and
PR-1 expression and disease resistance are partially
reduced by eds1 and pad4 mutations, and are completely
blocked by the eds5 mutation [10•,11•,27••]. This finding
shows that activation of SA accumulation does not
absolutely require EDS1 and PAD4, but that EDS5 may be
required. The SA and PR-1 expression phenotypes of dnd1
and dnd2 mutants are unaffected by PAD4, confirming the
observations that suggested that there is a PAD4-independent pathway to SA accumulation [11•,23]. However, pad4
blocks the enhanced resistance phenotypes of dnd1 and
dnd2, suggesting that there are resistance responses whose
expression requires a function of PAD4 other than its
activation of SA accumulation [11•]. The numerous lesion-
mimic mutants that have been isolated over the years may
prove to be useful in devising genetic screens to identify
components of the PAD4/EDS1-independent route to SA
accumulation. It would also be interesting to survey lesionmimic mutants to see if any of them activate SA
accumulation in ways that are independent of EDS5 or
SID2.
NPR1 is required downstream of SA to activate the expression of PR-1. Recent results suggest that NPR1 may
directly participate in the transcriptional control of
defense-gene expression. NPR1 concentrates in the nucleus
in response to SA, and nuclear localization is required for
the activation of PR-1 expression by NPR1 [33]. NPR1
also interacts with TGA-type transcription factors in yeast
two-hybrid assays [34–36] and promotes the binding of
TGA2 to its binding site [36]. As yet, no role for TGA
factors in activating defense-gene expression has been
detected, so the significance of the NPR1–TGA-factor
interaction is not certain. SA-inducible PR-1 expression is
restored in npr1 sni1 plants [37]. SNI1 is also a nuclearlocalized protein [37]. A reasonable model to explain these
results is that the activation of PR-1 expression requires
that repression of PR-1 must be released by SNI1, allowing
NPR1 to interact with transcription factors to activate gene
expression [37].
The existence of SA-dependent, NPR1-independent resistance mechanisms was suggested by the observation that
responses that are blocked by nahG are not necessarily
blocked by npr1. This logic is now somewhat questionable
as gene expression and camalexin accumulation are less
severely affected in the low-SA mutants eds5 and sid2 than
in plants containing nahG [21,38]. Nevertheless, the existence of an SA-dependent, NPR1-independent resistance
mechanism is still supported by the observation that eds5
completely blocks the enhanced disease resistance of cpr1,
cpr5, and cpr6 mutants, whereas npr1 blocks it only partially
[27••]. The dth9 mutation may define a component of this
pathway. Plants carrying the dth9 mutation express PR
genes normally in response to SA treatment but they fail to
develop resistance, demonstrating that DTH9 is required
for SAR but not for the gene expression changes known to
be characteristic of SAR [39••]. The dth9 mutant could now
be used as a tool to identify other factors that are required
for SA-dependent resistance.
Jasmonic acid/ethylene-dependent signaling
The role of JA and ET in the activation of disease resistance mechanisms was demonstrated by the observation
that expression of the PDF1.2 gene and other genes was
prevented by mutations that block JA signaling (i.e. coi1) or
ethylene signaling (i.e. ein2). JA and ET seem to be
required simultaneously, as PDF1.2 expression is not activated by either ET treatment of coi1 plants or JA treatment
of ein2 plants [40]. Furthermore, resistance to the fungal
pathogen Alternaria brassicicola is compromised in coi1
plants, and pretreatment with JA enhances resistance to
Genes controlling expression of defense responses in Arabidopsis — 2001 status Glazebrook
this pathogen [41], whereas ein2 plants display enhanced
susceptibility to the fungal pathogen Botrytis cinerea [42].
Mutations in OPR3 (12-OXOPHYTODIENOIC ACID
REDUCTASE 3), a gene encoding an enzyme that is
required for JA synthesis, were identified as a consequence
of their male-sterility phenotypes [43,44]. The opr3
mutants could be used to determine the effect of blocking
JA synthesis on defense responses. The coi1 mutation is
commonly considered to reflect the loss of JA signaling,
but COI1 encodes an F-box protein that presumably functions in targeting proteins for ubiquitin-dependent
degradation [45]. It is possible, therefore, that COI1 has
pleiotropic effects beyond its effect on JA signaling.
JA signaling is also involved in control of the formation of
lesions that are induced by ozone treatment. This observation is interesting in the context of disease resistance
signaling because ozone-induced cell death may be similar to hypersensitive cell death. The rcd1 (radical-induced
cell death1) mutant develops spreading lesions after ozone
treatments that do not induce lesions in wild-type plants
[46]. In this mutant, the HR lesions induced by infection
with a bacterial pathogen that triggers gene-for-gene
resistance are larger than those in wild-type plants [46].
The ozone-triggered cell death in rcd1 plants is inhibited
by JA and promoted by ethylene. Thus, JA and ET act in
opposition to each other during the formation of ozoneinduced lesions, whereas they both promote PDF1.2
expression [46]. In a related study, nahG plants showed
increased tolerance to ozone, JA treatment decreased SA
levels and increased ozone tolerance, and mutants
blocked in JA signaling (i.e. jar1) or synthesis (i.e. the
fatty-acid deficient 3 [fad3] fad7 fad8 triple mutant) showed
decreased tolerance [47].
There are many other examples of cross talk between SA
and JA/ET signaling. Many lesion-mimic mutants, including
cpr5, cpr6, acd2, dnd1, dnd2, and ssi1 (suppressor of SA insensitivity 1), display constitutively high expression of both
PR-1 and PDF1.2, suggesting that the SA and JA signaling
pathways may share common activating signals.
Expression of the JA-dependent gene PDF1.2 is, however,
strongly inhibited by SA, as demonstrated by observations
including increased rose-bengal-induced PDF1.2 expression in pad4 mutants [22], increased cpr6-dependent
PDF1.2 expression in cpr6 eds5, cpr6 pad4, and cpr6 eds1
double mutants [10•,11•,27••], enhanced PDF1.2 expression in sid2 plants infected with Erisyphe orontii [20], and
enhanced PDF1.2 expression in npr1 or nahG plants treated
with rose bengal or infected with Alternaria brassicicola
[48,49]. JA can also repress the expression of SA-induced
genes. For example, treatment with JA repressed the
ozone-induced accumulation of SA and expression of PR-1
[47]. The mpk4 mutation blocks the JA-inducible expression of PDF1.2 and causes the constitutive activation of
SA-dependent signaling, suggesting either that the block
in JA signaling relieves the suppression of SA signaling or
that the activation of SA signaling blocks JA signaling. The
305
former explanation is supported by the observation that
the activation of PDF1.2 expression is also blocked in nahG
mpk4 double mutants [26•]. Consistent with the idea that
JA and SA may sometimes act together rather than in
opposition to each other, some genes can be activated by
either JA or SA treatment [50].
Induced systemic resistance
Colonization of roots by certain rhizosphere bacteria confers a form of disease resistance called induced systemic
resistance (ISR). ISR occurs in nahG plants, so it is not an
SA-dependent phenomenon. No significant changes in
plant gene expression have been associated with ISR.
Nonetheless, progress has been made in the genetic dissection of this phenomenon. Several years ago, it was
found that ISR requires NPR1. This is interesting as it
implies that NPR1 is involved in perception of more than
one signal (i.e. SA and the ISR signal), and may respond
in one of two ways (by activating PR-1 expression or
by activating ISR). If NPR1 acts as an adaptor molecule
that brings the components of signaling complexes together,
this sort of dual role might be expected. ISR is blocked
by mutations that interfere with either ET or JA signaling.
As ethylene can induce ISR in jar1 mutants, it is thought
that the requirement for JA lies upstream of the requirement for ET in ISR signaling (for review, see [51]).
Some ecotypes fail to develop ISR, a trait that maps to a
single locus dubbed ISR1 [52]. The dominant ISR1
allele confers the ability to develop ISR [52].
Interestingly, the ISR trait cosegregates with high basal
resistance to a virulent strain of P. syringae, suggesting
that ISR and general resistance responses are intimately
related [52]. ISR1 also cosegregates with sensitivity to
ethylene, with the ISR1 allele correlating with increased
sensitivity to ethylene [53]. Simultaneous induction of
SAR and ISR resulted in enhanced resistance relative to
the induction of SAR alone, suggesting that ISR is an
example of a JA-dependent response that is not inhibited
by SA [54].
Genes not yet placed into pathways
Several interesting mutants have been described that
probably affect disease resistance signaling but that are not
yet characterized in sufficient detail to allow them to be
placed in particular positions in the network. Two genes,
HXC2 (Hypersensitivity to Xanthomonas campestris pv.
campestris 2) and RXC5 (Resistance to Xanthomonas
campestris 5), are required for resistance to a particular isolate of Xanthomonas campestris, implying that they may be
part of a gene-for-gene resistance mechanism [18•]. FLS2
(FLAGELLIN SENSITIVE 2) encodes a receptor-like
kinase that is required for responses to bacterial flagellin,
but the effects of fls2 mutations on resistance to bacterial
pathogens have not yet been reported [55,56]. Many other
mutants with enhanced disease resistance [57–60] or
enhanced susceptibility to various pathogens [5,20,61]
have also been described recently.
306
Biotic interactions
Conclusions and future prospects
A major area of progress in the past year has been in elucidating the relative sites of action of signaling components
in signal transduction cascades. On the basis of the different
requirements of various R genes for other genes to mediate resistance, R-gene-dependent signaling has been
resolved into at least three different pathways. New genes
have been defined in the SA signaling pathway, and
constitutive mutants have been useful in ordering components of the pathway.
There is still much work to be done to define genes that
are required for resistance. Although progress has been
made in defining the signal transduction cascades that control the expression of certain defense-related genes, there
is no clear correlation between gene expression patterns
and disease resistance (see [11•,27••] for examples). In the
future, the application of genome-wide expression profiling
to various mutants with defects in disease resistance
signaling should greatly facilitate both the elucidation of
signaling networks and the identification of genes with
roles in the regulation and execution of disease resistance
responses. The work of Maleck et al. [4••] demonstrates
the power of this sort of analysis. In the next year or two,
many more reports of genome-wide expression analysis
can be expected, as well as the isolation of many of the
genes that correspond to the interesting mutations
described in this review.
Acknowledgements
I apologize to scientists whose work I overlooked or was not able to include
because of length limitations.
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.
Glazebrook J: Genes controlling expression of defense responses
in Arabidopsis. Curr Opin Plant Biol 1999, 2:280-286.
2.
van der Biezen E, Jones JDG: Plant disease-resistance proteins
and the gene-for-gene concept. Trends Biochem Sci 1998,
23:454-456.
3.
Leister RT, Katagiri F: A resistance gene product of the nucleotide
binding site-leucine rich repeats class can form a complex with
bacterial avirulence proteins in vivo. Plant J 2000, 22:345-354.
4.
••
Maleck K, Levine A, Eulgem T, Morgan A, Schmid J, Lawton KA,
Dangl JL, Dietrich RA: The transcriptome of Arabidopsis thaliana
during systemic acquired resistance. Nat Genet 2000, 26:403-410.
Expression profiling was used to compare the patterns of gene expression
that are associated with various local and systemic resistance responses.
These responses were grouped on the basis of profile similarities, and
induced genes were grouped on the basis of expression-pattern similarities.
A promoter motif that may confer coordinate gene expression was identified.
This work sets the precedent for the use of expression profiling for studies
of signaling networks.
7.
Glazebrook J, Zook M, Mert F, Kagan I, Rogers EE, Crute IR,
Holub EB, Hammerschmidt R, Ausubel FM: Phytoalexin-deficient
mutants of Arabidopsis reveal that PAD4 encodes a regulatory
factor and that four PAD genes contribute to downy mildew
resistance. Genetics 1997, 146:381-392.
8.
Falk A, Feys BJ, Frost LN, Jones JDG, Daniels MJ, Parker JE: EDS1,
an essential component of R gene-mediated disease resistance
in Arabidopsis has homology to eukaryotic lipases. Proc Natl Acad
Sci USA 1999, 96:3292-3297.
9.
Jirage D, Tootle TL, Reuber TL, Frost LN, Feys BJ, Parker JE, Ausubel
FM, Glazebrook J: Arabidopsis thaliana PAD4 encodes a lipase-like
gene that is important for salicylic acid signaling. Proc Natl Acad
Sci USA 1999, 96:13583-13588.
10. Clarke JD, Aarts N, Feys BJ, Dong X, Parker JE: Induced disease
•
resistance requires EDS1 in the Arabidopsis mutants cpr1 and
cpr6 but is only partially EDS1-dependent in cpr5. Plant J 2001,
26:409-420.
The double-mutant analysis reported in this paper placed CPR1 and cpr6
upstream of EDS1 in SA-dependent signaling.
•11. Jirage D, Zhou N, Cooper B, Clarke JD, Dong X, Glazebrook J:
Constitutive salicylic acid-dependent signaling in cpr1 and cpr6
mutants requires PAD4. Plant J 2001, 26:395-407.
This double-mutant analysis placed CPR1 and cpr6 upstream of PAD4 in
SA-dependent signaling.
12. McDowell JM, Cuzick A, Can C, Beynon J, Dangl JL, Holub EB:
•• Downy mildew (Peronospora parasitica) resistance genes in
Arabidopsis vary in functional requirements for NDR1, EDS1,
NPR1, and salicylic acid accumulation. Plant J 2000, 22:523-529.
The R genes RPP7 and RPP8 were shown to function independently of
NDR1, EDS1, and NPR1, thus defining a possible third pathway for R-gene
function.
13. Bittner-Eddy PD, Beynon JL: The Arabidopsis downy mildew
•
resistance gene, RPP13-Nd, functions independently of NDR1
and EDS1 and does not require the accumulation of salicylic acid.
Mol Plant Microbe Interact 2001, 14:416-421.
The R gene RPP13 was found to act independently of EDS1 and NDR1,
confirming the existence of at least three alternative pathways for R-gene
function.
14. McDowell JM, Dhandaydham M, Long TA, Aarts MG, Goff S,
Holub EB, Dangl JL: Intragenic recombination and diversifying
selection contribute to the evolution of downy mildew resistance
at the RPP8 locus of Arabidopsis. Plant Cell 1998, 10:1861-1874.
15. Bittner-Eddy PD, Crute IR, Holub EB, Beynon JL: RPP13 is a simple
locus in Arabidopsis thaliana for alleles that specify downy
mildew resistance to different avirulence determinants in
Personospora parasitica. Plant J 2000, 21:177-188.
16. Cooley MB, Pathirana S, Wu HJ, Kachroo P, Klessig DF: Members of
the Arabidopsis HRT/RPP8 family of resistance genes confer
resistance to both viral and oomycete pathogens. Plant Cell 2000,
12:663-676.
17.
Kachroo P, Yoshioka K, Shah J, Dooner HK, Klessig DF: Resistance
to turnip crinkle virus in Arabidopsis is regulated by two host
genes and salicylic acid dependent but NPR1, ethylene, and
jasmonate independent. Plant Cell 2000, 12:677-690.
18. Xiao S, Ellwood S, Calis O, Patrick E, Li T, Coleman M, Turner JG:
•
Broad-spectrum mildew resistance in Arabidopsis thaliana
mediated by RPW8. Science 2001, 291:118-120.
The isolation of the broad-spectrum R gene RPW8 is reported. RPW8 is
structurally very different from known R genes, yet it requires EDS1to function.
19. Century KS, Shapiro AD, Repetti PP, Dahlbeck D, Holub E,
Staskawicz BJ: NDR1, a pathogen-induced component required
for Arabidopsis disease resistance. Science 1997, 278:1963-1965.
20. Dewdney J, Reuber TL, Wildermuth MC, Devoto A, Cui J, Stutius LM,
Drummond EP, Ausubel FM: Three unique mutants of Arabidopsis
identify eds loci required for limiting growth of a biotrophic fungal
pathogen. Plant J 2000, 24:205-218.
5.
Warren RF, Merritt PM, Holub E, Innes RW: Identification of three
putative signal transduction genes involved in R gene-specified
resistance in Arabidopsis. Genetics 1999, 152:401-412.
21. Nawrath C, Metraux JP: Salicylic acid induction-deficient mutants
of Arabidopsis express PR-2 and PR-5 and accumulate high
levels of camalexin. Plant Cell 1999, 11:1393-1404.
6.
Aarts N, Metz M, Holub E, Staskawicz BJ, Daniels MJ, Parker JE:
Different requirements for EDS1 and NDR1 by disease resistance
genes define at least two R gene-mediated signaling pathways in
Arabidopsis. Proc Natl Acad Sci USA 1998, 95:10306-10311.
22. Gupta V, Willits MG, Glazebrook J: Arabidopsis thaliana EDS4
contributes to salicylic acid (SA)-dependent expression of
defense responses: evidence for inhibition of jasmonic acid
signaling by SA. Mol Plant Microbe Interact 2000, 13:503-511.
Genes controlling expression of defense responses in Arabidopsis — 2001 status Glazebrook
23. Zhou N, Tootle TL, Tsui F, Klessig DF, Glazebrook J: PAD4 functions
upstream from salicylic acid to control defense responses in
Arabidopsis. Plant Cell 1998, 10:1021-1030.
24. Frye CA, Innes RW: An Arabidopsis mutant with enhanced
resistance to powdery mildew. Plant Cell 1998, 10:947-956.
25. Frye CA, Tang D, Innes RW: Negative regulation of defense
•• responses in plants by a conserved MAPKK kinase. Proc Natl
Acad Sci USA 2000, 98:373-378.
EDR1 is isolated and found to encode a MAPKKK. Double-mutant analysis
revealed that EDR1 acts upstream of EDS1 to suppress the activation of SA
signaling.
26. Petersen M, Brodersen P, Naested H, Andreasson E, Lindhart U,
•
Johansen B, Nielsen HB, Lacy M, Austin MJ, Parker JE et al.:
Arabidopsis MAP kinase 4 negatively regulates systemic acquired
resistance. Cell 2000, 103:1111-1120.
A mutation in MPK4 was found to block JA signaling and activate SA signaling.
These results demonstrate a role for a MAP kinase in defense signaling, and
illustrate cross-talk between JA-dependent and SA-dependent signaling pathways.
27.
••
Clarke JD, Volko SM, Ledford H, Ausubel FM, Dong X: Roles of
salicylic acid, jasmonic acid, and ethylene in cpr-induced
resistance in Arabidopsis. Plant Cell 2000, 12:2175-2190.
A detailed analysis of double and triple mutants reveals that CPR1 and cpr6
act upstream of EDS5, and detects complex interactions among SA, ET, and
JA signaling that affect disease resistance.
28. Mittler R, Shulaev V, Lam E: Coordinated activation of programmed
cell death and defense mechanisms in transgenic tobacco plants
expressing a bacterial proton pump. Plant Cell 1995, 7:29-42.
29. Molina A, Volrath S, Guyer D, Maleck K, Ryals J, Ward E: Inhibition of
protoporphyrinogen oxidase expression in Arabidopsis causes a
lesion-mimic phenotype that induces systemic acquired
resistance. Plant J 1999, 17:667-678.
30. Mach JM, Castillo AR, Hoogstraten R, Greenberg JT: The
Arabidopsis accelerated cell death gene ACD2 encodes red
chlorophyll catabolite reductase and suppresses the spread of
disease symptoms. Proc Natl Acad Sci USA 2001, 98:771-776.
31. Clough SJ, Fengler KA, Yu IC, Lippok B, Smith RK, Bent AF: The
Arabidopsis dnd1 ‘defense, no death’ gene encodes a mutated
cyclic nucleotide-gated ion channel. Proc Natl Acad Sci USA 2000,
97:9323-9328.
32. Yu IC, Fengler KA, Clough SJ, Bent AF: Identification of Arabidopsis
mutants exhibiting an altered hypersensitive response in genefor-gene resistance. Mol Plant Microbe Interact 2000, 13:277-286.
33. Kinkema M, Fan W, Dong X: Nuclear localization of NPR1 is
required for activation of PR gene expression. Plant Cell 2000,
12:2339-2350.
34. Zhang Y, Fan W, Kinkema M, Li X, Dong X: Interaction of NPR1 with
basic leucine zipper protein transcription factors that bind
sequences required for salicylic acid induction of the PR-1 gene.
Proc Natl Acad Sci USA 1999, 96:6523-6528.
35. Zhou JM, Trifa Y, Silva H, Pontier D, Lam E, Shah J, Klessig DF: NPR1
differentially interacts with members of the TGA/OBF family of
transcription factors that bind an element of the PR-1 gene
required for induction by salicylic acid. Mol Plant Microbe Interact
2000, 13:191-202.
36. Després C, DeLong C, Glaze S, Liu E, Fobert PR: The Arabidopsis
NPR1/NIM1 protein enhances the DNA binding activity of a
subgroup of the TGA family of bZIP transcription factors. Plant
Cell 2000, 12:279-290.
37.
Li X, Zhang Y, Clarke JD, Li Y, Dong X: Identification and cloning of
a negative regulator of systemic acquired resistance, SNI1,
through a screen for suppressors of NPR1-1. Cell 1999,
98:329-339.
38. Rogers EE, Ausubel FM: Arabidopsis enhanced disease
susceptibility mutants exhibit enhanced susceptibility to several
bacterial pathogens and alterations in PR-1 gene expression.
Plant Cell 1997, 9:305-316.
39. Mayda E, Mauch-Mani B, Vera P: Arabidopsis dth9 mutation
•• identifies a gene involved in regulating disease susceptibility
without affecting salicylic acid-dependent responses. Plant Cell
2000, 12:2119-2128.
The dth9 mutant was isolated and characterized. The phenotypes of dth9
suggest that DTH9 is required downstream of SA to activate SAR, but is not
307
required for the expression of known SAR-related genes. Thus, it appears to
define a new branch in the signal transduction network.
40. Penninckx IAMA, Thomma BPHJ, Buchala A, Metraux JP, Broekaert WF:
Concomitant activation of jasmonate and ethylene response
pathways is required for induction of a plant defensin gene in
Arabidopsis. Plant Cell 1998, 10:2103-2113.
41. Thomma BPHJ, Eggermont K, Penninckx IAMA, Mauch-Mani B,
Vogelsang R, Cammue BPA, Broekaert WF: Separate jasmonatedependent and salicylate-dependent defense-response pathways
in Arabidopsis are essential for resistance to distinct microbial
pathogens. Proc Natl Acad Sci USA 1998, 95:15107-15111.
42. Thomma BPHJ, Eggermont K, Tierens KFMJ, Broekaert WF:
Requirement of functional ethylene-insensitive 2 gene for efficient
resistance of Arabidopsis to infection by Botrytis cinerea. Plant
Physiol 1999, 121:1093-1101.
43. Sanders PM, Lee PY, Biesgen C, Boone JD, Beals TP, Weiler EW,
Goldberg RB: The Arabidopsis delayed dehiscence 1 gene
encodes an enzyme in the jasmonic acid synthesis pathway.
Plant Cell 2000, 12:1041-1061.
44. Stintzi A, Browse J: The Arabidopsis male-sterile mutant, opr3,
lacks the 12-oxophytodienoic acid reductase required for
jasmonate synthesis. Proc Natl Acad Sci USA 2000,
97:10625-10630.
45. Xie DX, Feys BF, James S, Nieto-Rostro M, Turner JG: COI1: an
Arabidopsis gene required for jasmonate-regulated defense and
fertility. Science 1998, 280:1091-1094.
46. Overmyer K, Tuominen H, Kettunen R, Betz C, Langebartels C,
Sandermann H, Kangasjarvi J: Ozone-sensitive Arabidopsis rcd1
mutant reveals opposite roles for ethylene and jasmonate
signaling pathways in regulating superoxide-dependent cell
death. Plant Cell 2000, 12:1849-1862.
47.
Rao MV, Lee HI, Creelman RA, Mullet JE, Davis KR: Jasmonic acid
signaling modulates ozone-induced hypersensitive cell death.
Plant Cell 2000, 12:1633-1646.
48. Penninckx IAMA, Eggermont K, Terras FFG, Thomma BPHJ,
De Samblancx GW, Buchala A, Metraux JP, Manners JM, Broekaert WF:
Pathogen-induced systemic activation of a plant defensin gene in
Arabidopsis follows a salicylic acid-independent pathway. Plant
Cell 1996, 8:2309-2323.
49. Clarke JD, Liu Y, Klessig DF, Dong X: Uncoupling PR gene
expression from NPR1 and bacterial resistance: characterization
of the dominant Arabidopsis cpr6-1 mutant. Plant Cell 1998,
10:557-569.
50. Schenk PM, Kazan K, Wilson I, Anderson JP, Richmond T,
Somerville SC, Manners JM: Coordinated plant defense responses
in Arabidopsis revealed by microarray analysis. Proc Natl Acad Sci
USA 2000, 97:11655-11660.
51. Pieterse CMJ, van Pelt JA, van Wees SCM, Ton J, Leon-Kloosterziel KM,
Keurentjes JJB, Verhagen BWM, Knoester M, van der Sluis I,
Bakker PAHM et al.: Rhizobacteria-mediated induced systemic
resistance: triggering, signalling and expression. Eur J Plant Pathol
2001, 107:51-61.
52. Ton J, Pieterse CMJ, Van Loon LC: Identification of a locus in
Arabidopsis controlling both the expression of rhizobacteriamediated induced systemic resistance (ISR) and basal resistance
against Pseudomonas syringae pv. tomato. Mol Plant Microbe
Interact 1999, 12:911-918.
53. Ton J, Davison S, van Wees SCM, van Loon LC, Pieterse CMJ: The
Arabidopsis ISR1 locus controlling rhizobacteria-mediated
induced systemic resistance is involved in ethylene signaling.
Plant Physiol 2001, 125:652-661.
54. van Wees SCM, de Swart EAM, van Pelt JA, van Loon LC, Pieterse CMJ:
Enhancement of induced resistance by simultaneous activation of
salicylate- and jasmonate-dependent defense pathways in
Arabidopsis thaliana. Proc Natl Acad Sci USA 2000, 97:87118716.
55. Gomez-Gomez L, Felix G, Boller T: A single locus determines
sensitivity to bacterial flagellin in Arabidopsis thaliana. Plant J 1999,
18:277-284.
56. Gomez-Gomez L, Boller T: FLS2: an LRR receptor-like kinase
involved in the perception of the bacterial elicitor flagellin in
Arabidopsis. Mol Cell 2000, 5:1003-1011.
308
57.
Biotic interactions
Silva H, Yoshioka K, Dooner HK, Klessig DF: Characterization of a
new Arabidopsis mutant exhibiting enhanced disease resistance.
Mol Plant Microbe Interact 1999, 12:1053-1063.
58. Rate DN, Cuenca JV, Bowman GR, Guttman DS, Greenberg JT: The
gain-of-function Arabidopsis acd6 mutant reveals novel regulation
and function of the salicylic acid signaling pathway in controlling cell
death, defenses, and cell growth. Plant Cell 1999, 11:1695-1708.
59. Stone JM, Heard J, Asai T, Ausubel FM: Stimulation of fungalmediated cell death by fumonisin B1 and selection of fumonisin
B1-resistant (fbr) Arabidopsis mutants. Plant Cell 2000,
12:1811-1822.
60. Vogel J, Somerville S: Isolation and characterization of powdery
mildew-resistant Arabidopsis mutants. Proc Natl Acad Sci USA
2000, 97:1897-1902.
61. Greenberg JT, Silverman FP, Liang H: Uncoupling salicylic aciddependent cell death and defense-related responses from
disease resistance in the Arabidopsis mutant acd5. Genetics
2000, 156:341-350.