Download Microbial recognition and activation of plant defense systems

R E V I E W S
Microbial recognition and activation
of plant defense systems
William P. Lindsay, Christopher J. Lamb and Richard A. Dixon
p
lants respond to the threat
of disease by the induction of defense response
proteins. These proteins inhibit
pathogen ingress via such
mechanisms as digestion of
fungal cell walls, fortification
of plant cell walls, and biosynthesis of antimicrobial compounds (phytoalexins). Here,
we review understanding of
the interactions between plants
and pathogenic microbes, with
an emphasis on those events
linking perception of microbial
ingress to defense induction.
Molecules released or generated during
microbial entry (elicitors) are recognized
by components of plant cells, ultimately
resulting in the induction of a battery of
plant defense responses. The molecular
mechanisms underlying these signaling
systems, as well as the plant defense
responses they control, are becoming
increasingly well characterized.
expression of an avr gene may
be direct or indirect; see left
panel of centerfold.
General elicitors, which may
function in nonhost resistance
to a broad range of potential
pathogens, represent the vast
majority of those described,
and often are structural components of the microbial'~cell.
W.P. Lindsay and R.A. Dixon are in the Plant
They include various oligosacBiology Division, Samuel Roberts Noble Foundation, charides, polysaccharides, proPO Box 2180, Ardmore, OK 73402, USA;
teins and glycoproteins4, some
C.J. Lamb is in the Plant Biology Laboratory,
of which may be released
Salk Institute for Biological Studies,
10010 North Torrey Pines Road, La Jolla,
from invading fungal strucCA 92037, USA.
tures as early as the stage of
germ tube formation s. Of parPlant resistance and pathogen avirulence
ticular interest are the elicitins, peptides produced by
Interactions between specific plant cultivars and de- Phytopbtbora spp., which are potent inducers of
fined races of potentially pathogenic microbes are hypersensitive cell death, both in initially infected
categorized as compatible (host susceptible and patho- cells and at a distance from the infection site6,7. Fatty
gen virulent) or incompatible (host resistant and acid elicitors (e.g. arachidonic acid) of Phytopbtbora
pathogen avirulent). Incompatibility often involves infestans are also under intensive investigation,
localized tissue necrosis in the so-called hypersensitive primarily because their oxidation by lipoxygenase
responseL The genetics of many plant-pathogen inter- produces signal molecules similar to those involved
actions are described by the gene-for-gene hypothesis2, in mammalian inflammatory responses s.
such that a dominant resistance gene will confer
Various low molecular weight compounds such
resistance to a particular race only if the pathogen as glutathione, the concentration of which increases
expresses the corresponding avirulence (avr) gene. in plants in response to infection or elicitation9, may
This genetic relationship implies molecular recog- function as endogenous elicitors. Treatment with
nition between the products (direct or indirect) of the glutathione induces rapid, selective transcription of
paired resistance and avr genes, epistatic to the action a number of defense genes ~°, suggesting a role in
of other resistance-avr gene interactions 3. The left localized cellular signaling. Other compounds of host
panel of the centerfold shows the possible ways in origin, such as salicylic acid, methyl iasmonate, jaswhich microbial avr genes and plant disease resistance monic acid, ethylene, the peptide systemin and cell
genes may influence the outcome of the interaction wall oligogalacturonide fragments, have been implibetween plant and microbe.
cated in the 'long-distance' signaling that underlies
systemic acquired resistance (SAR) or wound-induced
Avirulence genes and microbial elicitors
systemic accumulation of proteinase inhibitors. The
The genetic characterization of avr genes has revealed role of systemin in the proteinase inhibitor response
the dominant nature of avirulence2, and implicated has been elegantly confirmed by the suppression of
avr gene products in plant perception of attack.
systemic wound signaling in plants expressing an
Efforts to isolate the underlying signals have led to antisense systemin precursor gene H. The SAR rethe characterization of a group of molecules collec- sponse, in which plants locally exposed to avirulent
tively referred to as elicitors4. General elicitors are pathogens develop resistance throughout the plant to
those produced by all members of a pathogen taxon normally virulent pathogens, has been considered in
and eliciting a defense response in all genotypes of detail elsewhere~2.
a plant species. In contrast, race-specific elicitors
Several race-cultivar-specific elicitors have been
are produced only by pathogen biotypes expressing isolated from microbial cells, and cloning of avr genes
a given avr gene, and affect only those plant (which either directly encode or indirectly lead to
cultivars expressing the complementary resistance production of these molecules) has recently been
gene. Generation of these race-specific elicitors via achieved. A functional Cladosporium fulvum avr9
© 1993 Elsevier Science Publishers Ltd (UK) 0966 842X/93/$06.00
TRENDS
IN MICROBIOLOGY
181
VOL.
1
NO.
5
AUGUST
1993
Microbial recognition and,
William P. Lindsay, Chris1
E iE", IE~
TRENDS
JOLRNmLS
f
Microbial cell
f
C
Microbial
metabolite
Plant cell
product
[]
J
J
f
Plasma membrane
Plant cell
-"7
[]
\
Releas
inv
systerT
resi~
vascu
Plant cell
metabolite
Inductio
genes (e
possibl
of defer
path~
,ation of plant defense systems
J. Lamb and Richard A. Dixon
August 199!
Modification
of cell wall
components
/~
K
÷
CI-
H202
Plasma
membrane
h,..=
y
t
1
Cytoplasm
C a 2+ H +
(
J
Modification of cytoplasmic
transcription factors
(e.g.H-box~,~1r~,
\
Nucleus
Activation of nuclear
transcription factors
e
.=);
I
g
Defense gene
R E V I E W S
gene is essential for induction of the hypersensitive
response in an incompatible interaction with tomato
plants expressing the corresponding resistance gene
Cf9 (Ref. 13). A race-specific polypeptide elicitor of
C. fulvum has been purified, and the corresponding
gene cloned by oligonucleotide screening. Pathovars
virulent on Cf9 plants do not contain an allele
of the avr9 gene TM, and transformation of such
strains with the avr9 gene results in avirulence on Cf9
plants. Although avr9 is dispensable for normal
growth of C. fulvum, it is highly expressed in planta
in compatible interactions, suggesting a function in
pathogenesis.
The avrBs3 gene of Xanthomonas campestris and
the avrXa7 and avrXalO genes of Xanthomonas
oryzae contain multiple copies (17.5 repeats in avrBs3)
of a 102 bp direct repeat 1s,16. It is possible that the
amino acid sequence encoded by these repeats is a
structural feature associated with recognition. In rice,
resistance to X. oryzae harboring these avr genes
depends on light. The timing of the hypersensitive response depends on the particular avr gene expressed
by the pathogen 16, suggesting that there are differences
in signal transduction in different avr-resistance gene
combinations.
The avrD gene of Pseudomonas syringae determines
hypersensitivity in soybean plants that contain the
Rpg4 resistance gene ly. The elicitor molecule conditioned by avrD is not the direct gene product, but
the product of the enzymatic conversion of a normal
cellular constituent catalysed by the activity encoded
by avrD. The avrD gene may be part of an operon that
is induced by an unidentified plant factor and that
may function in bacterial multiplication in planta TM.
Thus, this operon may have a similar role to the
hypersensitive response and pathogenicity (hrp) genes
characterized in a number of phytopathogenic bacteria 19. Some hrp genes have recently been shown to
be homologous to the pathogenicity determinants of
animal pathogenic bacteria involved in secretion2°.
Others, such as hrpN of Erwinia amylovora, encode
harpins, which are bacterial cell-envelope-associated
elicitors of the hypersensitive response 21.
The functions of avr genes in microbial cells remain
unknown, although involvement in normal microbial
growth and in pathogenesis in compatible plants have
been suggested2z. A selective advantage for the pathogen is implied by the observations that microbes
harboring certain avr genes predominate in the pathogen population over those deleted in avr functions23.
For example, the durability of a pepper resistance
gene in the field reflects a contribution to fitness of
X. campestris by the corresponding avrBs2 gene24;
avrBs2 is somehow needed for full virulence of the
pathogen on susceptible hosts.
Plant defense genes: signals for activation
The induction of defense-related proteins as a result
of interaction with an incompatible race of pathogen
or treatment with elicitor molecules is generally
brought about by defense gene expression (see Refs
25 and 26 for reviews). Exceptions to this rule include
TRENDS
IN MICROBIOLOGY
184
the activation of pre-existing callose synthase 27 and
H 2 0 z production 2s, neither of which require increased
protein synthesis.
Rapid activation of defense genes appears critical
in determining whether a compatible or incompatible
interaction results 3. This concept has been elegantly
confirmed using isogenic lines of both host plant,
Arabidopsis tbaliana, and pathogen, Pseudomonas
syringae pv. maculicola. Induction of phenylalanine
ammonia-lyase (PAL) transcripts and those of a novel
defense gene designated ELI3 is strictly dependent
on the interaction of dominant alleles of the RPM1
resistance gene and avrRpinl avirulence gene29.
However, recent evidence indicating that defense
gene transcripts can be induced by pathogenic bacteria lacking functional hrp genes suggests there may
be two distinct signal pathways controlling the activation of classical defense genes and the induction of
a macroscopic hypersensitive response 3°, although
resistance gene products may function in both pathways. Likewise, superimposition of several signal
pathways may also operate for activation of SAR
(Refs 12, 31).
Evidence has been presented for the existence of
several potential signaling pathways by which plant
cells respond to microbial elicitor (summarized in
the right panel of the centerfold). Removal of extracellular Ca 2÷ inhibits phytoalexin accumulation
in soybean, carrot and parsley cells 32-34. Treatment
with inhibitors of Ca z* membrane transport blocks
elicitation of phytoalexin accumulation in carrot
cells33 and the phytoalexin biosynthetic enzyme
chalcone synthase (CHS) in soybean cells 35. Moreover, rapid influx of Ca z+and H ÷, and efflux of K÷and
CI- (Ref. 36) coincides with increases of calmodulinbinding proteins in elicited parsley protoplasts 37.
Thus Ca 2÷may function in elicitor signaling3s, but the
molecular mechanisms underlying transient ion
fluxes in elicited cells are not understood. Investigation of other components of animal signal systems
suggests that heterotrimeric G proteins and cyclic
nucleotide triphosphates are unlikely to have a role in
parsley cell elicitation 37.
The Ca z÷ concentration in animal cells controls a
variety of responses via its effects on the activity of
an array of CaZ+-dependent protein kinases. Several
studies have demonstrated the potential involvement
of reversible protein phosphorylation in plant signal
transduction. Dietrich et al. 39 describe the specific
phosphorylation of about 16 proteins in vivo in
parsley cells in response to elicitor, a response that
decreases when cells are grown in lower levels of
Ca 2÷. However, treatment of the cells with the protein
kinase inhibitors K252a or staurosporine does not
prevent phytoalexin induction, although the protein
phosphatase inhibitor okadaic acid is effective37.
These results contrast with those from tomato cells, in
which kinase inhibitors block elicitor-induced changes
in protein phosphorylation and defense induction4°.
Several groups have approached elicitor signaling
from the perspective of the transcriptional activation
of defense genes, in an attempt to trace the signal path-
VOL.
I
No.
5
AUGUST
1993
. f
R E V I E W S
way back to the initial stimulus. For example, in our
laboratories we have studied the signals involved in
the activation of genes encoding isoflavonoid phytoalexin biosynthetic enzymes in legumes. The importance of the flavonoid biosynthetic pathway in normal
plant development, and in interactions between legumes and symbiotic nitrogen-fixing bacteria, makes
this a useful model for examination of the integration
of developmental and environmental signals.
In bean (Phaseolus vulgaris), isoforms of PAL and
CHS, two key regulatory enzymes in flavonoid
biosynthesis, are encoded by small gene families2s.
Transcription of chslS, a representative member of
the chs gene family, rapidly increases following penetration of bean hypocotyls by an incompatible race
of Colletotrichurn lindemuthianum, or treatment of
bean cells with a C. lindemuthianum elicitor41.
Functional analysis of nested 5' deletions of the
chs15 promoter and in vitro footprinting studies have
delineated sequences in the 335 bp proximal region
of the promoter as important in elicitor induction4z.
A silencer comprising three conserved boxes (consensus sequence GGTFAA) is located between positions -326 and -173. These boxes are strongly footprinted and coincide with DNase I hypersensitive
sites in vivo 43"44. However, promoter fragments containing just 13 0 bp upstream of the transcription start
site confer elicitor inducibility. This region contains
both a G-box (CACGTG; Ref. 45) and an H-box
(CCTACCN7CT; Ref. 46). Two other copies of the
latter are present further upstream and the core
consensus CCTACC is highly conserved among the
promoters of other genes encoding enzymes of the
phenylpropanoid pathway. The G- and H-boxes are
both necessary for elicitor regulation of chslS in vivo,
and further evidence of their importance comes from
assay of chs15 promoter activity in a plant in vitro
transcription system47. Thus, depletion of soybean
whole-cell extracts by pre-incubation with the G-box
and H-box cis elements abolishes subsequent transcription of the chs15 promoter. This demonstrates
both the functional relevance of these sequences and
also the dependence of promoter activity on binding
of trans-acting factors that have specific affinities for
G- and H-boxes.
G-boxes are present in many plant promoters 4s,
and this family of cis elements has been implicated in
gene regulation by light, abscisic acid and tissuespecific signals49. A number of basic leucine zipper
(bZIP) trans-acting factors with affinity for variants
of the core sequence have been isolateds°. Three
'common plant regulatory' factors (CPRF-1, -2 and -3),
which interact with G-boxes involved in UV regulation of the parsley chs gene, have been identifiedSL
Transcription of the CPRF-1 gene is induced by light,
suggesting a role in chs photoregulation. It will be
interesting to determine the role of these factors in
elicitor regulation, since expression of chs is suppressed
by elicitor in parsley, a species that does not make
flavonoid-derived phytoalexins.
Bean nuclear proteins that bind cis elements involved
in chs15 regulation have been isolated by sequence-
TRENDS
IN MICROBIOLOGY
185
specific DNA affinity chromatography. Thus, purified SBF-1 is specific for the conserved element of
the silencer regions2, and distinct H-box-binding
activities (KAP-1 and KAP-2) have been isolated46.
KAP-1 also binds to the chs15 G-box in vitro, but not
to the related sequence from the parsley gene (W.P.
Lindsay, C.J. Lamb and R.A. Dixon, unpublished).
These data suggest that KAP-1, a protein with 97 kDa
subunits, may bind to more than one functional cis
element (as recently observed for GT-2, a factor
involved in light regulation of phytochrome gene expressionS3), and this phenomenon may be important
for the integration of environmental and developmental regulation.
The DNA-binding characteristics of both bean Hbox factors (KAP-1 and KAP-2) and SBF-1 are affected
by dephosphorylation. Binding of SBF-1 is abolished
by phosphatase treatments2, whereas for KAP-1 and
KAP-2 the mobility of the H-box/factor binding complex is increased, suggesting conformational changes46.
Further studies are needed to address the relevance of
these observations to gene activation, and the potential
role of kinases and phosphatases that may modulate the activity of these factors. Increased defense
gene transcription can be detected within 5-10 min of
elicitor treatment in bean cells41. Thus signal transduction may involve relatively few steps and the factors required for gene activation are probably present
prior to the activation stimulus. Consistent with this
hypothesis, KAP-1 and KAP-2 are present in the cytoplasm and appear to be translocated to the nucleus
following elicitation46 (illustrated in the right panel of
the centerfold).
This emerging mechanism for plant defense gene
elicitation is strikingly similar to that for the activation of interferon-stimulated genes in animal cells
by the phosphorylation of pre-existing cytoplasmic
transcription factors leading to nuclear translocation,
and hence target gene activations4. A second emerging
parallel with animal systems pertains to the elicitoror pathogen-induced oxidative burst 2s, which closely
resembles early events in macrophage activation and
egg protection following fertilization. Recent data
suggest that the elicitor-induced oxidative burst in
plants not only provides H202 at the cell surface for
rapid toughening of the cell wall by crosslinking of
structural proteins ss, but may also be involved in signal
pathways that result in phytoalexin biosynthesiss6.
This latter point is a matter of controversy, howevers7.
Interestingly, activation of nuclear factor ~B (NF-~B),
which interacts with an immunoglobulin gene enhancer element, appears to be mediated by various
stimuli that act through the generation of activated
oxygen speciessS.
Establishment of SAR depends on new protein
synthesissg, and treatment of plant cells with putative
SAR signal molecules apparently enhances defense
gene activation by elicitors31. Thus signal pathway
components responsible for activation of defense
genes may be systemically induced, thereby allowing
more effective induction in response to normally virulent pathogens (see right panel of centerfold).
VOL.
1
NO.
5
AUGUST
1993
REVIEWS
Prospscts
The next few years will almost certainly see the molecular cloning
of plant disease resistance
genes.
Recently, genes responsible for resistance of Arubidopsis tbaliana and of tomato to Pseudomonas syringae
have been isolated on inserts of specific yeast artificial
chromosome
clones60T61. Moreover, the development
of A. thulium as a model system for studying plantpathogen
interactions
opens up powerful
new approaches to dissecting signal pathways
for defense
induction.
In one such approach,
the promoter
of
a defense gene drives the expression
of alcohol
dehydrogenase,
which is lethal when expressed in the
presence of ally1 alcoho1‘j2. By screening mutagenized
plants for insensitivity to ally1 alcohol and subsequent
screening
to eliminate
cis mutations,
it should be
possible to isolate plants deficient in components
of
the signal transduction
pathway. Experiments
with
an A. tbaliuna PAL promoter
have yielded several
truns-acting
PAL regulation
mutants (B. Kraft, pers.
commun.).
Likewise, screening
of A. thaliuna for
mutants deficient in SAR will provide a system for the
isolation of genes involved in systemic signaling and
establishment
of acquired resistance62.
The identification
of elicitors involved
in racespecific interactions14,37 provides a means to analyse
the molecular
interactions
occurring
between these
molecules and their presumed cognate receptors in
plant cells. Two groups have reported detailed biochemical studies on elicitor-receptor
interactions63T64,
though in both cases the elicitor used was a general,
rather than race-specific, oligoglucan from fungal cell
walls. It is anticipated
that similar studies with racespecific elicitors may lead to the biochemical identification of the products of corresponding
plant resistance genes. By a concerted application
of molecular,
genetic and biochemical approaches,
the mechanisms
by which resistance genes modulate
plant defenses
should soon emerge.
Acknowledgements
We thank the Samuel Roberts Noble Foundation for support. C.J.L.
also thanks the US Dept of Agriculture and National Science
Foundation for grants. W.P.L. is a Noble Foundation/Salk Institute
Post-doctoral Fellow in Plant Biology.
References
1 Klement, Z. (1982) in Phytoputhogenic Prokaryotes (Vol. 2)
(Mount, M.S. and Lacy, G.H., eds), pp. 149-177, Academic
Press
2 Flor, H.H. (1942) Phytopathology 32,653-669
3 Lamb, C.J. et al. (1989) Cell 56,215-224
4 Dixon, R.A. (1986) Biol. Rev. 61,239-291
5 Waldmiiller, T. et al. (1992) Plantu 188,498-505
6 Nespoulous, C., Huet, J-C. and Pernollet, J-C. (1992) Planta 186,
551-557
7 Devergne, J-C. et al. (1992) Plant Physiol. 99,843-847
8 Bostock, R.M. et al. (1992) Plant Physiol. 100,1448-1456
9 Edwards, R., Blount, J. and Dixon, R.A. (1991) Planta 184,
403-409
IO Wingate, V.P.M., Lawton, M.A. and Lamb, C.J. (1988) Plant
Physiol. 87,206-210
11 McGurl, B. et al. (1992) Science 255, 1570-1573
12 Enyedi, A.J. et al. (1992) Cell 70, 879-886
TRENDSINMICROBIOI_OGY
186
13 De Wit, P.J.G.M. and Spikman, G. (1982) Physiol. Plant Puthol.
21, I-II
14 van den Ackerveken, G.F.J.M., van Kan, J.A.L. and De Wit,
P.J.G.M. (1992) Plant]. 2,359-366
15 Bonas, U., Stall, R.E. and Staskawicz, B. (1989) Mol. Gen. Genet.
318,127-136
16 Leach,J.E. et al. (1993) in Advances in Molecular Genetics of
Plant-Microbe lnteructions (Nester, E.W. and Verma, D.P.S.,
eds), pp. 221-230, Kluwer Academic Press
17 Keen, N.T. and Buzzell, R.I. (1991) Theor. Appl. Genet. 81,
133-138
18 Keen, N.T. and Dawson, W.O. (1992) in Plant Gene Research:
Genes Involved in Plant Defense (Boiler, T. and Meins, F., eds),
ib
pp. 85-114, Springer-Verlag
19 Willis, D.K., Rich, J.J. and Hrabak, E.M. (1991) Mol.
Plant-Microbe Interact. 4,132-138
20 Gerin, S. et al. (1993) in Advances in Molecular Genetics of
Plant-Microbe interactions (Nester, E.W. and Verma, D.P.S.,
eds), pp. 259-266, Kluwer Academic Press
21 Wei, Z-M. et al. (1992) Science 257,85-88
22 Keen, N.T. (1990) Annu. Rev. Genet. 24,447-463
23 Grant, M.W. and Archer, S.A. (1983) Phytoputhology 73,
547-551
24 Keamey, B. and Staskawicz, B.J. (1990) Nature 356,385-387
25 Dixon, R.A. and Harrison, M.J. (1990) Adv. Genet. 28, 165-234
26 Dixon, R.A. (1992) in Molecular Plant Pathology: A Practical
Approach (Vol. I) (Gurr, S.J., McPherson, M.J. and Bowles, D.J.,
eds), pp. 191-201, Oxford University Press
27 KGhle, H. et al. (1985) Plant Physiol. 77,544-581
28 Sutherland, M.W. (1991) Pbysiol. Mol. Plant Putbol. 39,79-93
29 Kiedrowski, S. et al. (1992) EMBO 1. 11,4677-1684
30 Jakobek, J.L. and Lindgren, P.B. (1993) Plant Cell 5,49-56
31 Kauss, H. et al. (1992) Plant]. 2,655&O
32 Stib, M.R. and Ebel, J. (1987) Arch. Biochem. Biophys. 257,
416-423
33 Kurosaki, F., Tsurusawa, Y. and Nishi, A. (1987) Phytochemistry
26,1919-1923
34 Scheel, D. et al. (1989) in Signal Molecules in Plants and
Plant-Microbe Interactions (Lugtenberg, B.J.J., ed.), pp.
211-218, Springer-Verlag
35 Ebel, J. and Scheel, D. (1992) in Plant Gene Research: Genes
involved in Plant Defense (Boller, T. and Meins, F., eds), pp.
153-205, Springer-Verlag
36 Renelt, A. et al. (1993) J. Exp. Bot. 44,257-268
37 Sacks, W.R. et al. (1993) in Advances in Molecular Genetics of
Plant-Microbe Interactions (Nester, E.W. and Verma, D.P.S.,
eds), pp. 485-495, Kluwer Academic Publishers
38 VGgeli, U., &geli-Lange, R. and Chappell, J. (1992) Plant
Physiol. 100,1369-1376
39 Dietrich, A., Mayer, J.E. and Hahlbrock, K. (1990) 1. Biol.
Chem. 265,6360-6368
40 Felix, G. et al. (1991) Proc. Nut1Acad. Sci. USA 88,8831-8834
41 Lawton, M.A. and Lamb, C.J. (1987) Mol. Cell. Biol. 7,335-341
42 Dixon, R.A. et al. (1993) in Advances in Molecular Genetics of
Plant-Microbe Interactions (Nester, E.W. and Verma, D.P.S.,
eds), pp. 447-509, Kluwer Academic Publishers
43 Lawton, M.A., Clouse, S.D. and Lamb, C.J. (1990) Plant Cell
Rep. 8,561-564
44 Lawton, M.A. et al. (1991) Plant Mol. Biol. 16,235-249
45 Giuliano, G. et al. (1988) Proc. NutI Acad. Sci. USA 85,
7089-7093
46 Yu, L.M., Lamb, C.J. and Dixon, R.A. Plant]. (in press)
47 Arias, J., Dixon, R.A. and Lamb, C.J. (1993) PIant Cell 5,485496
48 Williams, M.E., Foster, R. and Chua, N-H. (1992) Plant Cell 4,
485-496
49 Salinas, J., Oeda, K. and Chua, N-H. (1992) P/ant Cell 4,
1485-1493
50 Katagiri, F. and Chua, N-H. (1992) Trends Genet. 8,22-27
VOL.
1
No.5
AUGUST
1993
R E V I E W S
51 Weisshaar, B. et al. (1991) EMBO J. 10, 1777-1786
52 Harrison, M.J. et al. (1991) Pro& Natl Acad. Sci. USA 88,
2515-2519
53 Dehesh,K. et al. (1992) EMBO J. 11, 4131-4144
54 Schindler, C. et al. (1992) Science 257, 809-815
55 Bradley,D.J., Kjellbom,P. and Lamb, C.J. (1992) Cell 70, 21-30
56 Apostol,I., Heinstein, P.F. and Lon, P.S. (1989)Plant Physiol.
90, 109-116
57 Davis, K. et al. (1993) Phytochemistry 32, 607-611
58 Schreck,R., Rieber, P. and Baeuerle,P.A. (1991)EMBO J. 10,
2247-2258
59 M&raux, J.P. et aL (1990) Science 250, 1004-1006
60 Dangl,J. et aL (1993) in Advances in Molecular Genetics of
Plant-Microbe Interactions (Nester, E.W. and Verma, D.P.S.,
eds), pp. 405-415, KluwerAcademicPublishers
61 Martin, G. et al. (1993) in Advances in Molecular Genetics of
Plant-Microbe Interactions (Nester, E.W. and Verma, D.P.S.,
eds), pp. 451-455, KluwerAcademicPublishers
62 de Maagd, R.A. et al. (1993) in Advances in Molecular Genetics
of Plant-Microbe Interactions (Nester, E.W. and Verma, D.P.S.,
eds), pp. 445-449, Kluwer AcademicPublishers
63 Cheong,J-J. and Hahn, M.G. (1991) Plant Cell 3, 137-147
64 Cosio, E.G., Frey, T. and Ebel,J. (1992) Eur. J. Biocbem. 204,
1115-1123
Calmodulin-activated bacterial
adenylate cyclases as virulence factors
Mich61e Mock and Agnes Ullmann
mong
the
various Bordetella pertussis and Bacillus anthraeis toxin, which are involved in
mechanisms known to
each produce a virulence-associated,
localized tissue damage, and
account for the pathocalmodulin-dependent adenylate cyclase
an ACT (for review, see Refs 1
genesis of bacterial infection,
toxin, which generates increased levels of
and 2).
a particular class of toxirts
cyclic AMP in eukaryotic cells. The two
Bacillus
anthracis,
the
has been shown to adt by dis- proteins share sequence similarities in their causative agent of anthrax,
rupting the control of levels
catalytic domains. The remaining regions
secretes three proteins - proof cyclic AMP (cAMP), an
display different structural and functional
tective antigen (PA, 85 kDa),
ubiquitous regulatory molorganizations that account for the
lethal factor (LF, 83 kDa)
ecule that plays an integrating
differences both in interaction of the two
and an adenylate cyclase,
function in many of the meta- toxins with target cells and in the resulting termed edema factor (EF, 89
bolic processes of the cell.
disease symptoms.
kDa) - that form two distinct
There are two ways in
anthrax toxins. These toxins
M. Mock is in the Dept of Bacteriology and
which bacterial toxins interare organized in an original
Mycology, and A. Ullmann is in the Dept of
fere with cAMP regulation: (1)
example of the A-B type toxin
Biochemistry and Molecular Genetics,
model 3,4. PA is the common
they act directly on an adenyInstitut Pasteur, 28 rue du Docteur Roux,
late cyclase of the host cell and
B domain which mediates the
7 5 7 2 4 Paris Cedex 15, France.
disrupt its normal regulation,
cellular entry of two different
or (2) they are themselves adenylate cyclases that
A moieties, EF or LF. The combination of PA and
elicit the formation of high and unregulated levels EF forms edema toxin or ACT, whereas PA and LF
of cAMP in t h e host. In this latter class, only two
represent lethal toxin s. As might be expected from
such bacterial adenylate cyclase toxins (ACTs) have
their names, intradermal injection of edema toxin
been identified and characterized, and they are
causes edema formation whereas intravenous inproduced by taxonomically distant organisms: the
jection of lethal toxin causes rapid death of sensitive
Gram-negative Bordetella pertussis and the Gramanimals. These two toxins are responsible for the
positive Bacillus anthracis.
pathological effects of anthrax (for review, see
Bordetella pertussis is the etiological agent of
Ref. 6).
whooping cough and, like the other members of
ACTs produced by B. pertussis and B. anthracis
the genus Bordetella, it produces a large number
represent a particular class of bacterial adenylate
of virulence determinants. Some of them, such as cyclases: they are extracellular enzymes and their
filamentous hemagglutinin, pertactin and fimbriae,
most fascinating property is that both are activated
are involved in adhesion of the bacteria to cells of
by a eukaryotic protein, calmodulin 7,s. This obserthe respiratory tract. Once an infection has been
vation (made a decade ago) has raised intriguing
established, B. pertussis then releases several toxins
questions about the origins of these toxins and has
responsible for systemic manifestations of the
stimulated intensive efforts to study their structure
disease: pertussis toxin, which has a wide range
and function, and their interactions with eukaryotic
of activities, dermonecrotic toxin and tracheal cytotarget cells.
A
© 1993 Elsevier Science Publishers Ltd (UK) 0966 842X/93/$06.00
TREN.S,N
M,CROB
OLOG
187
VOL
1
NO.
5
AUGUST
1993