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Biotechnology Advances 22 (2004) 363 – 382
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Research review paper
Plants as models for the study of human pathogenesis
David S. Guttman *
Department of Botany, University of Toronto, 25 Willcocks St., Toronto, ON, Canada M5s 3B2
Received 25 September 2003; accepted 20 November 2003
Abstract
There are many common disease mechanisms used by bacterial pathogens of plants and humans.
They use common means of attachment, secretion and genetic regulation. They share many virulence
factors, such as extracellular polysaccharides and some type III secreted effectors. Plant and human
innate immune systems also share many similarities. Many of these shared bacterial virulence
mechanisms are homologous, but even more appear to have independently converged on a common
function. This combination of homologous and analogous systems reveals conserved and critical
steps in the disease process. Given these similarities, and the many experimental advantages of plant
biology, including ease of replication, stringent genetic and reproductive control, and high
throughput with low cost, it is proposed that plants would make excellent models for the study of
human pathogenesis.
D 2003 Elsevier Inc. All rights reserved.
Keywords: Plant pathogens; Animal pathogens; Virulence; Innate immunity; Type III secretion; Extracellular
polysaccharides
1. Introduction
The discovery that very different human pathogens rely on a fairly small set of
common mechanisms to cause disease sparked an intellectual revolution that has had a
profound influence on how we view, study and combat infectious agents (Finlay and
Falkow, 1989, 1997). Yet, for many years, these advances did not substantially influence
the study of plant pathogenesis. Medical microbiology and plant pathology seemed
worlds apart in their intellectual traditions and scientific methods, with even the
* Correspondence. Tel.: +1-416-978-6865; fax: +1-416-978-5878.
E-mail address: [email protected] (D.S. Guttman).
0734-9750/$ - see front matter D 2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.biotechadv.2003.11.001
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D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
terminology used by the two disciplines acting more to polarize than unify. This is not
hard to understand, after all, how could pathogenesis in humans and plants have any
significant similarities when these very divergent hosts are so radically different in their
genetic systems, cell structures, and immune systems? However, despite the many very
real differences, recent studies are finding numerous and significant areas of similarity
among the mechanisms used by bacterial pathogens of plants and humans. These
similarities include the use of common toxins, secretion systems, mechanisms of
adhesion, invasion and regulation (Buttner and Bonas, 2003; Cao et al., 2001; Lugtenberg et al., 2002; Staskawicz et al., 2001).
Why is it significant that plant and human pathogens share common mechanisms for
causing disease? There are three reasons that the study of human pathogenesis could
benefit tremendously from knowledge of plant pathology. First, virulence factors that are
conserved across hosts ranging from humans to plants are likely to be evolutionary
constrained, and perhaps essential virulence mechanisms. These constrained systems are
logical targets for antimicrobials and vaccines.
Second, the comparative study of human and plant pathogenesis has identified
numerous virulence factors and mechanisms that are not related by evolutionary descent,
yet which have evolved common functions. These analogous systems are perhaps our best
window into the complex interactions that occur during pathogenesis. They may reveal
processes so critical to eukaryotic cellular function, that different pathogens have
independently evolved similar mechanisms to exploit them in highly dissimilar hosts.
By identifying these critical steps in pathogenesis, we will be in a better position to
control and treat infectious agents.
Finally, plants provide tremendous benefits for experimental studies of pathology.
They can be grown easily, inexpensively and rapidly. Thousands of plants can be
grown and propagated with negligible costs and regulation, while animal models
typically cost thousands of dollars per animal, and impose a tremendous maintenance and regulatory burden. Plants can be reproductively and genetically manipulated in ways that are unavailable for most animal systems. For instance, they
can be self-fertilized, clonally propagated and hybridized to other species. Plants
can be assayed in very large numbers providing a level of statistical rigor not
available in animal systems. Finally, there are extensive genomic and proteomic
resources available for a number of model plants, most notably Arabidopsis
thaliana.
In this review, I will describe some of the common mechanisms used by bacterial
pathogens of humans and plants. I will almost exclusively focus on Gram-negative
bacterial pathogens, although there are certainly commonalities with other bacterial, viral,
and even fungal pathogens. This is not meant to be a fully comprehensive assessment of all
the similarities between pathogenesis in these divergent hosts, but instead a critical review
of the major underlying themes. The goal of the review is to illustrate that there are as
many commonalities between plant and human pathogenesis as there are among
mechanisms used by different human pathogens. Given these similarities, and the
numerous advantages inherent in plant experimental biology, plant pathology has the
potential to provide new and valuable insight into the essential mechanisms used by
bacteria that attack humans.
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
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2. Attachment
Before anything else, bacterial pathogens must first come in contact with and
recognize their eukaryotic hosts. Both plant and human pathogens interact with their
hosts’ cells through pili and fimbriae (Sauer et al., 2000; Soto and Hultgren, 1999).
There are a number of distinct types of pili and fimbriae, the most relevant for this
review being those produced by the type III secretion system (TTSS, discussed below)
and the type IV pilus (Tfp) (Wall and Kaiser, 1999). Pseudomonas syringae, the
causal agent of a wide range of important bacterial spot, speck and blight diseases in
plants, uses both systems for adhesion. Evidence of the role played by these
attachment systems in host colonization can be seen with Tfp mutants. These strains
are washed from leaves more readily, and are less efficient at initiating colonization in
the laboratory (Romantschuk and Bamford, 1986; Romantschuk et al., 1993; Suoniemi
et al., 1995). Both TTSS and Tfp mutants grow to lower densities that wildtype
strains in the field (Hirano et al., 1999; Roine et al., 1998), although in the former
case it is not clear if the growth deficit is due to reduced attachment ability, or if the
defective TTSS can no longer be used to acquire resources for growth or to repel
eukaryotic competitors.
The role of pili in human pathogenesis, particularly with respect to Escherichia
coli, has been well established and heavily reviewed (Finlay and Falkow, 1997;
Mulvey, 2002; Nougayrede et al., 2003). In general, Tfp are essential for bacterial
motility and virulence (Wall and Kaiser, 1999). Their exact function in virulence is
not entirely clear, although they contribute to adhesion, bacterial movement and even
signal transduction. The Tfp can both bring a pathogen into contact with an
appropriate eukaryotic cell, and maintain that contact, thereby facilitating the transfer
of other virulence factors such at TTSS effectors (see below) (Wall and Kaiser, 1999).
In Pseudomonas aeruginosa, an opportunistic human pathogen that is a major cause
of hospital infections, and the leading cause of death for Cystic Fibrosis (CF)
patients, the Tfp is responsible for 90% of the adherence function in human lung
cells, while non-piliated strains are 10-fold less virulent in mouse models (Hahn,
1997).
Although it is clear that bacterial pathogens of both plants and animals rely on a variety
of attachment mechanism in the disease process, it is likely that these same attachment
mechanisms are also important for commensal and beneficial bacteria. Antimicrobial
treatments that target common factors and kill bacteria indiscriminately will invariably
open up the infected host to secondary infections. Consequently, although attachment
mechanisms are shared between pathogens of both plants and animals, they would make
poor therapeutic targets.
3. Secretion systems
Three classes of secretion systems have been implicated in virulence, and are
widely distributed and highly conserved among bacterial pathogens of plants and
humans (Fig. 1).
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Fig. 1. Bacterial secretion systems directly implicated in virulence. The type I secretion system forms a
continuous channel that spans both bacterial membranes, enabling the secretion of a variety of toxins, proteases
and lipases into the environment. The type III section system (TTSS) forms a direct connection between the
bacterial and eukaryotic host cell. TTSS effectors are directly injected into the host cytosol via the TTSS pilus.
The type IV secretion system permits the movement of DNA or protein – DNA complexes between bacterial cells,
or directly into the cytosol of a eukaryotic hosts. PM, plasma membrane; OM, outer bacterial membrane; PPS,
periplasmic space; IM, inner bacterial membrane.
3.1. Type I secretion
The type I secretion system is the simplest of the major secretion apparatuses. It
consists of three major proteins that form a continuous channel that spans the bacterial
inner membrane, the periplasmic space, and the bacterial outer membrane (Binet et al.,
1997; Thanassi and Hultgren, 2000). This system depends on ABC protein mediated
transporters, and secretes a variety of toxins, proteases and lipases into the environment. It
is exemplified by E. coli hemolysin secretion (Gentschev et al., 2002), a toxin that causes
the lysis of red blood cells. Serratia marcescens is an opportunistic human pathogen that is
one of the leading causes of hospital-related infections. It secretes hemoprotein HasA
through the type I secretion system to sequester iron, an element essential for both
bacterial and host enzymatic reactions (Letoffe et al., 1994). The plant pathogens Erwinia
amylovora, the causal agent of fire blight, and Erwinia chrysanthemi uses a type I
secretion system to secrete metalloproteases, which have been shown to be required for
leaf colonization (Gentschev et al., 2002; Zhang et al., 1999).
3.2. Type III secretion
The type III secretion system (TTSS) is directly responsible for many of the most
devastating diseases known to plants and humans (Cornelis and Van Gijsegem, 2000;
Galan and Collmer, 1999; He, 1998; Hueck, 1998, Lee, 1997; Lee and Schneewind, 1999;
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
367
Lindgren, 1997; Mecsas and Strauss, 1996). This specialized protein secretion system is
required for pathogenesis in a wide range of human pathogens (e.g. E. coli, P. aeruginosa,
Salmonella enterica, Shigella spp. and Yersinia spp.) and plant pathogens (e.g. P. syringae,
Xanthomonas spp. and Erwinia spp.). It is used to strategically inject proteins (effectors)
directly from the bacterial cell into the cytoplasm of its host. The TTSS is a complex
apparatus that spans both bacterial membranes and the periplasmic space. It is encoded by
20 or more genes, many of which are highly conserved among both plant and human
pathogens. These genes are typically clustered in a so-called ‘pathogenicity island’ on the
bacterial genome or an accessory plasmid. The TTSS is believed to have evolved from the
flagellar system, as a number of the inner membrane components of the TTSS show
similarity to proteins of the flagellar basal body (Aizawa, 2001).
TTSS effectors interact with host proteins both inside and outside the host cell to
modulate the host response. In many cases, effectors are necessary for pathogenesis and
are directly responsible for determining host specificity. They have been shown to interfere
with signal transduction, cause cytoskeletal changes, and to have a direct cytotoxic effect
(Hueck, 1998). They also are known to induce immune responses in resistant animal and
plant hosts. Given their prominent role in determining the course and fate of pathogen –
host interactions, it is reasonable to consider them as one of the pathogen’s primary
offensive weapons.
As more TTSS effectors are discovered and functionally characterized, it is becoming
clear that they can be divided into functional classes, and that these classes are shared
among human and plant pathogens. In most cases, it appears that the common mechanism
is not due to homology, but rather analogy—similarity due to convergent evolution. In
other words, there has been convergence of function in TTSS effectors of plant and human
pathogens. The fact that pathogens have independently evolved similar mechanisms to
attack very different hosts strongly supports there being important commonalities among
the disease processes of humans and plants.
Some examples of conserved effector function include two classes of cysteine
proteases. The YopJ class of cysteine proteases was originally isolated from Yersinia
pestis, the causal agent of bubonic and pneumonic plague. This protein induces
apoptosis and inhibits the host immune response by disrupting the MAPK and NFnB signaling pathways (Orth, 2002). The catalytic residues of YopJ have been identified
and shown to be necessary for Yersinia virulence (Orth et al., 2000). Proteins with the
same conserved catalytic residues as YopJ have been found in the S. enterica, the causal
agent of gastroenteritis and typhoid fever, and in the plant pathogens P. syringae, E.
amylovora, Ralstonia solanacearum and Xanthomonas campestris (Staskawicz et al.,
2001; Ciesiolka et al., 1999; Lavie et al., 2002). These conserved catalytic residues have
been shown to be required for the induction of the defense response in plants resistant
to X. campestris, the causal agent of bacterial spot of pepper and tomato (Orth et al.,
2000).
YopT is the second class of TTSS-dependent cysteine protease. This Yersinia protein
cleaves Rho GTPases, which are regulators of the eukaryotic actin cytoskeleton. Cleavage
and release of Rho GTPases from the plasma membrane causes the disruption of actin
stress fibers (Aepfelbacher and Heesemann, 2001; Aepfelbacher et al., 2003), thereby
disrupting the phagocytotic uptake of the pathogen by the host immune cells. YopT shares
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invariant residues with P. syringae TTSS effectors AvrPphB, AvrPpiC2, HopPtoC and
HopPtoN, in addition to proteins from E. coli, Haemophilus spp., Pasteurella multocida,
and Chlamydia muridarum (Buttner and Bonas, 2003; Collmer et al., 2002; Shao et al.,
2002). These residues are essential for the YopT protease activity as well as the induction
of the AvrPphB-mediated hypersensitive reaction (HR) defense response in resistant plants
(Shao et al., 2002).
Another class of shared virulence factors are the ADP-ribosyltransferase toxins. Some
of these are TTSS-dependent, such as P. aeruginosa’s ExoS and ExoT, and P. syringae’s
HopPtoO (HopPtoS1), HopPtoS2 and HopPtoS3 (Collmer et al., 2002; Guttman et al.,
2002). The injection of ExoS into the host cells elicits a cytotoxic and antiphagocytic
response, corresponding to the disruption of the actin microfilament structure (FrithzLindsten et al., 1997). ExoT enables P. aeruginosa to inhibit epithelial wound repair by
causing actin cytoskeleton collapse, cell rounding and cell detachment (Kazmierczak and
Engel, 2002). The function of the P. syringae ADP-ribosyltransferase toxins has not been
determined. There are also a number of other notable human pathogens that secrete ADPribosyltransferases, although not in a TTSS-dependent manner. These include the P.
aeruginosa exotoxin A, Corynebacterium diphtheriae diphtheria toxin, Vibrio cholera
cholera toxin, and Bordetella pertussis pertussis toxin (Okazaki and Moss, 1994).
A final class of TTSS-dependent virulence factors shared by plant and human
pathogens are pore-forming molecules. Shigella flexneri, the causal agent of bacterial
dysentery, effectors IpaB and IpaC (Blocker et al., 1999), Yersinia effectors YopB and
YopD (Tardy et al., 1999), and P. syringae effector HrpZ (Lee et al., 2001) create pores in
synthetic lipid bilayers. Although these proteins are not homologous, they appear to be
another example of functional convergence. It is not clear if the goal of pore formation in
the eukaryotic plasma membrane is to assist in the translocation of other effectors, or to
cause the leakage of nutrients.
3.3. Type IV secretion
The type IV secretion system has two distinct claims to fame. It encodes the well
known conjugative pilus used by bacteria to exchange plasmids. It has also been
intensively investigated for its ability to move DNA – protein complexes and toxins into
eukaryotic cells (Foulongne et al., 2002). The plant pathogen Agrobacterium tumefaciens
has the best-studied type IV secretion system. It delivers gall-forming (oncogenic) DNA –
protein complexes into plant cells through its VirB type IV system (Lai and Kado, 2000).
Human pathogens, such as Haemophilus influenzae, S. flexneri, and Legionella pneumophila also use a type IV system (Finlay and Falkow, 1997; Foulongne et al., 2002). B.
pertussis delivers the pertussis toxin via a type IV system (Weiss et al., 1993), while
Helicobacter pylori uses its type IV system to inhibit phagocytosis (Ernst, 2000). Type IV
systems have also been found in P. syringae (Sexton and Vogel, 2002) (Stavrinides and
Guttman, unpublished data) and X. campestris pv. vesicatoria (Ojanen-Reuhs et al., 1997),
where type IV mutant strains exhibit reduced aggregation in the laboratory, but still retain
wildtype virulence toward their tomato host.
Secretion systems represent the most significant evolutionary and functional link
between plant and animal pathogens. The TTSS in particular is both evolutionary
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
369
conserved and necessary for pathogenesis among both plant and animal pathogens. Given
their central role in pathogenesis, secretion systems have justifiably attracted attention as
potential targets for development of vaccines and therapeutics (Kauppi et al., 2003; Muller
et al., 2001). Whereas we know a tremendous amount about the secretion systems, we
know much less about the effectors that travel those systems. These molecules are on the
front line during a bacterial attack. They come into more intimate contact with the host
than any other pathogen-derived molecule, often being directly injected into the host cell.
Unfortunately, we know very little about the natural genetic variation underlying these
effectors. What we do know is that their distribution among strains of a species is
extraordinarily variable, largely due to the action of horizontal gene transfer. What we do
not know is the extent and importance of allelic variation in these important molecules,
and the specific causal or correlative associations between this variation and disease. These
studies are a necessary step in the development of widely effective and durable treatments.
4. Exopolysaccharides and biofilms
Exopolysaccharides (EPS) are vital virulence determinants of many pathogens. They
play their most important role as key components of the slime layer, or glycocalyx, of
bacterial biofilms (Costerton et al., 1999; Davey and O’TooleG, 2000; Donlan, 2001;
Dunne, 2002; Miller and Bassler, 2001; Pulcini, 2001). Biofilms are highly structured
polymer matrices produced by sessile bacteria, which provide for the free flow of water,
nutrients, and signaling molecules, in addition to protection from dehydration, antimicrobials, environmental extremes and predators. EPS are required for P. aeruginosa biofilm
production, and biofilms are necessary for the colonization of the lungs of CF patients
(Costerton, 2001; Lyczak et al., 2000). Vibrio cholerae EPS mutants are unable to form
biofilms and have reduced virulence (Watnick and Kolter, 1999). It is also believed that V.
cholerae biofilm mutants do not survive as well in their natural aquatic environment
(Watnick and Kolter, 1999). X. campestris produces biofilms upon infection of its plant
host. Strains that were defective in biofilm formation had smaller lesions due to a reduced
ability to spread through the leaf vasculature (Dow et al., 2003). Although P. syringae has
not been shown to produce biofilms via microscopic examination or in vitro adhesion
assays, it does produce copious amount of the EPS alginate. P. syringae alginate mutants
were found to be significantly impaired in their ability to colonize leaves, form less severe
lesion, and reach lower population densities than wildtype strains (Keith et al., 2003;
Osman et al., 1986; Yu et al., 1999).
Membrane-derived oligosaccharides are an important class of EPS. These glucans are
produced by homologs of the mdoGH locus, and are secreted into the periplasmic space of
many Gram-negative bacteria. They are important components of the bacterial cell
envelope, and have been found to play a role in a wide range of functions, from
osmoregulation to virulence. P. syringae mutants deficient for these glucans lose the
ability to cause disease on susceptible hosts and do not induce a defense response on
resistant hosts (Mukhopadhyay et al., 1988). In E. chrysanthemi periplasmic glucan
mutants were complete avirulent, and unable to grow in planta (Page et al., 2001). In P.
aeruginosa mdoH mutants were severely reduced in their virulence toward Caenorhabdi-
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tis elegans (Mahajan-Miklos et al., 1999). MdoGH homologues are also found in Shigella
spp., S. enterica, and V. cholerae (Bohin, 2000).
EPS and biofilms pose more of a challenge than opportunity for the development of
antimicrobials, as they are notoriously non-immunogenic due to their unpredictable
structures. They also render many bacteria resistant to treatment and environmental
perturbations (Drenkard and Ausubel, 2002). As a consequence, therapies that disrupt
the production of EPS and biofilms are rapidly gaining in significance. One of the most
exciting areas of research is the use of bacteriophage to penetrate biofilms and directly
attack pathogens (Davies, 2002; Hughes et al., 1998). Bacteriophages are natural predators
of bacteria, and some carry genes that enable them to effectively depolymerize biofilms
(Adams and Park, 1956). This work is still in the very early stages, and faces the daunting
task of dealing with bacterial resistance, but has been shown to be effective under
laboratory conditions (Hughes et al., 1998; Corbin et al., 2001; Doolittle et al., 1995).
5. Regulatory pathways and regulators
Plant and human pathogens use conserved regulatory mechanisms to respond to their
environments and induce virulence-associated genes. The most important class of these
systems is the two-component regulators. These systems are composed of a transmembrane sensor and a cytosolic response regulator. Upon receiving an extracellular signal, the
sensor will cause a change in the associated response regulator. The now active response
regulator will then induce or repress the appropriate genetic systems either directly, as a
transcriptional regulator, or indirectly via a signal transduction cascade. The most
important of this class of molecules is the global-activator two-component system
(gacA/gacS), which controls a wide range of virulence phenotypes in both plant and
human pathogens. It is near the top of the signal transduction cascade that is responsible
for the production of extracellular enzymes, toxins and antibiotics in the human pathogens
P. aeruginosa (Reimmann et al., 1997) and V. cholerae (Wong et al., 1998), and plant
pathogens P. syringae (Bender et al., 1999), and Erwinia carotovora (Cui et al., 2001). It
also controls alginate production in P. aeruginosa (Reimmann et al., 1997), swarming in P.
syringae (Kinscherf and Willis, 1999), and cell invasion in S. enterica (Johnston et al.,
1996).
Quorum sensing (which is under gac control in many species) is another very important
regulatory system that has been implicated in virulence in nearly every bacterial pathogen
studied (Miller and Bassler, 2001; Donabedian, 2003). Quorum sensing systems in Gramnegative bacteria typically consist of an autoinducer, which produces a free diffusible
molecule, and a receptor/transcriptional activator protein, which monitors the concentration of the autoinducer. As the bacterial population grows, the level of autoinducer in the
environment increases. When the concentration of the autoinducer reaches a critical
threshold level, the receptor protein is activated, which induces or represses a range of
target genes. In general, bacterial pathogens use quorum sensing to ensure that virulence
genes are only expressed after their population has reached a critical size. This unified
attack strategy makes it more difficult for the host to mount an effective defense. Quorum
sensing has been shown to directly control biofilm formation, the expression of virulence
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
371
genes, swarming, conjugation, and the production of secondary metabolites. Its role in
pathogenesis, has been extensively reviewed (Miller and Bassler, 2001; Donabedian,
2003; Smith and Iglewski, 2003; Von Bodman et al., 2003), and therefore will not be
further discussed here.
There are a number of regulatory elements that are shared among pathogens of plant
and humans. The most important of these are the alternative sigma factors, such as j54,
which is encoded by the rpoN locus (Cao et al., 2001). This protein controls the regulatory
cascade that induces the expression of the toxin coronatine and the TTSS in P. syringae
(Hendrickson et al., 2000a,b). It also induces pili and alginate production in P. aeruginosa
(Hendrickson et al., 2001), and capsular polysaccharide synthesis in Klebsiella pneumoniae (Arakawa et al., 1995).
Another shared regulatory element is the periplasmic disulfide bond-forming enzyme
DsbA. This protein has been implicated in the virulence of a wide range of bacterial
pathogens (Cao et al., 2001). In P. aeruginosa DsbA is required for proper secretion of
TTSS effectors (Ha et al., 2003). DsbA is also required for the secretion of the pertussis
toxin by B. pertussis (Stenson and Weiss, 2002). Cell-to-cell transmission of the S. flexneri
in hosts was disrupted in DsbA mutant strains (Yu et al., 2000). DsbA mutants of E.
carotovora produced greatly reduced levels of pectate lyase, their primary virulence factor
(Vincent-Sealy et al., 1999).
Finally, the periplasmic protein DegP acts both as a chaperone and serine protease,
depending on the environmental conditions. Its primary function is to degrade misfolded
proteins in the periplasm. It also been shown to be necessary for virulence in P. syringae,
P. aeruginosa, Yersinia and S. enterica (Cao et al., 2001; Pallen and Wren, 1997; Yorgey et
al., 2001).
Common regulatory mechanisms present a unique opportunity for the development of
antimicrobials. These systems are often highly constrained, and as such present widespread and durable targets for therapeutics and vaccines. They also often control multiple
components of virulence. The challenge will be to design strategies that specifically target
these high level controls in pathogens, while avoiding the scorched earth indiscriminate
killing of all bacteria, including commensal and beneficial species. If these regulatory
systems could be selectively disrupted or turned off then many dangerous pathogens
would effectively be turned into harmless commensals.
6. Common disease-associated genes in plant and animal hosts
Comparative genomic analyses have also found a remarkable number of human
disease-associated genes in diverse eukaryotes. Rubin et al. (2000) (also see
mips.gsf.de/proj/thal/db/tables/tables_frame.html) mined the human, Drosophila melanogaster, C. elegans, Saccharomyces cerevisia, and A. thaliana genomes for human
disease-associated genes. Of 289 total genes, D. melanogaster shared 230 (BLAST evalue < 10 6), C. elegans shared 209, S. cerevisia shared 119, and A. thaliana shared a
remarkable 172 genes with humans. These genes span a wide spectrum of functions, from
those involved in cancer, to immune and neurological disorders. Examples of human
disease-associated genes that have greater protein similarity to A. thaliana proteins (as
372
Associated disease
Cancer
Ataxia telangiectasia
Acute myeloid leukemia
Breast cancer
Gene
AT
DEK
BRCA1
Putative function
NCBI
OMIM #
BLASTP e-valuea
D.m.
145
17
06
C.e.
S.c.
A.t.
2e 49
1e + 00
9e 11
4e 92
2e + 00
5e 05
1e
2e
2e
168
29
20
Breast cancer
Tumor suppressor
Tumor suppressor
Type 1 hereditary nonpolyposis
colon cancer
Xeroderma pigmentosum
Xeroderma pigmentosum
BRCA2
CDKN2A
CDKN1C
MSH3
lipid-mediated signaling
oncogene
component of RNA
polymerase II holoenzyme
homologous recombination
cyclin-dependent inhibitor
cyclin-dependent inhibitor
MutS homolog
208,900
125,264
113,705
1e
1e
2e
600,185
600,160
600,856
600,887
1e 03
5e 08
1e + 00
4e 67
2e + 00
1e 08
8e 08
2e 64
9e + 00
6e 05
1e + 00
1e 126
4e
9e
7e
7e
38
12
19
134
XPF
XP7
nucleotide excision repair
nucleotide excision repair
278,760
278,780
8e
1e
93
64
1e
8e
49
23
1e
6e
60
47
1e
7e
146
89
Neurological
Dentatorubral-pallidoluysian atrophy
Parkinson disease
Retinitis pigmentosa
Spinocerebellar ataxia
Spinal muscular atrophy
Stargardt macular dystrophy
DRPLA
UCHL1
RPGR
SCA7
SMN1
ABCA4
atrophin family protein
ubiquitin C-terminal hydrolase
GTPase regulator
nuclear matrix associated
assembly factor for snRNPs
ATP-binding cassette
125,370
191,342
312,610
164,500
600,354
601,691
2e
7e
9e
2e
5e
1e
10
41
34
03
07
119
4e
1e
1e
2e
1e
2e
03
24
19
02
04
95
9e 04
1e 07
9e 09
5e 06
1e + 00
4e 14
3e
4e
1e
8e
5e
3e
37
42
43
10
10
168
Cardiovascular
Tangier disease
ABCA1
ATP-binding cassette
600,046
1e
127
1e
103
7e
2e
181
13
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
Table 1
Human disease-associated genes with highest protein similarity to A. thaliana
CKN1
DNMT3B
transcription-coupled repair
methyltransferase
216,400
602,900
5e
9e
CREBBP
TCOF
PEX1
CREB binding protein
early embryonic development
peroxisome biogenesis
600,140
154,500
302,136
Endocrine
Congenital adrenal hyperplasia
Hyperinsulinemic hypoglycemia
Hypothyroidism
CYP21A2
ABCC8
TRH
cortisol biosynthesis
ATP-binding cassette
Thyrotropin-releasing hormone
Renal
Williams – Beuren syndrome
ELN
Immune
Chronic granulomatous disease
MHC class II deficiency
14
07
4e
1e
13
01
1e 14
2e + 00
2e
1e
43
08
330e 01
7e 06
3e 81
5e + 00
5e 05
2e 67
1e + 00
5e 01
7e 91
6e
4e
4e
07
15
125
201,910
600,509
275,120
1e 43
1e 160
6e + 00
2e 34
1e 160
1e + 00
1e 08
1e 176
1e + 00
6e
7e
2e
46
188
07
elastin
130,160
1e + 00
1e + 00
1e + 00
6e
44
CYBB
MHC2TA
NADPH oxidase
MHC class II transactivator
306,400
600,005
5e
2e
36
02
4e
9e
34
03
6e
4e
06
03
1e
9e
61
07
Metabolic
Galactokinase deficiency
GALK1
galactokinase
604,313
9e
15
2e
14
3e
10
4e
25
Other
Ehlers – Danlos syndrome
Epidermolytic palmoplantar keratoderma
Osteogenesis imperfecta
Spondyloepiphyseal dysplasia congenita
Vohwinkel syndrome, ichthyosis
COL3A1
KRT9
COL1A1
COL2A1
LOR
collagen synthesis
keratin synthesis
collagen synthesis
collagen synthesis
Loricrin-component of epidermis cell envelope
120,180
144,200
120,150
120,140
152,445
2e 06
6e 22
7e 11
5e 18
1e + 00
1e + 00
7e 09
1e + 00
1e + 00
1e + 00
1e
8e
8e
2e
2e
35
81
35
36
60
1e 06
5e 26
9e 11
9e 20
1e + 00
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
Malformation syndromes
Cockayne syndrome
Immunodeficiency-centromeric
instability-facial anomalies
Rubinstein – Taybi syndrome
Treacher – Collins syndrome
Zellweger syndrome, neonatal
adrenoleukodystrophy, infantile
Refsum disease
D.m.—D. melanogaster, C.e.—C. elegans, S.c.—S. cerevisia, and A.t.—A. thaliana.
a
BLASTP data from Rubin et al. (2000) and mips.gsf.de/proj/thal/db/tables/tables_frame.html.
373
374
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
determined by BLASTP e-value) than to homologous proteins in yeast, fly or worm are
presented in Table 1. This analysis clearly shows that the cellular processes required for
life are not fundamentally different in plants and animals.
7. Common pathogens of plants and animals
An appreciation that most human diseases are zoonotic (transmitted from animals to
humans) (Cleaveland et al., 2001; Taylor et al., 2001) has led to an increased interest in
identifying and understanding the determinants of host specificity. A surprising outcome
of this work is the discovery of pathogens that can attack both plants and animals.
Examples of these cross-kingdom pathogens include Erwinia spp., a well-known cause of
a variety of wilt diseases in plants, including bacterial fire blight of apples and pears.
Recently, it has also been isolated from human wounds and abscesses (Cao et al., 2001).
Burkholderia cepacia, the causal agent of soft rot in onion, can cause life-threatening
infections in immunocompromised and CF patients (Cao et al., 2001). But by far the beststudied cross-kingdom pathogen is P. aeruginosa (Cao et al., 2001; Staskawicz et al.,
2001; Mahajan-Miklos et al., 1999; Hendrickson et al., 2001; Yorgey et al., 2001; Choi et
al., 2002; Hogan and Kolter, 2002; Jander et al., 2000; Mahajan-Miklos et al., 2000;
Plotnikova et al., 2000; Pukatzki et al., 2002; Rahme et al., 1995, 1997, 2000; Tan et al.,
1999). Recent work has shown that this pathogen can cause disease in such diverse hosts
as mammals, insects (Jander et al., 2000), worms (Mahajan-Miklos et al., 1999), amoeba
(Pukatzki et al., 2002), fungi (Hogan and Kolter, 2002), and plants (Plotnikova et al.,
2000; Rahme et al., 1995, 1997). Many of the genetic systems necessary for human
pathogenesis are also required for virulence in these diverse hosts (Cao et al., 2001;
Staskawicz et al., 2001; Rahme et al., 2000). For example, the TTSS P. aeruginosa
effector exoU is a highly cytotoxic molecule that is required for in vitro killing of epithelial
cells and virulence in mouse models (Allewelt et al., 2000; Hauser et al., 1998). ExoU is
also required for virulence in the social amoeba Dictyostelium discoideum (Pukatzki et al.,
2002), the caterpillar Galleria mellonella (Miyata et al., 2003), and S. cerevisiae when
introduced on a yeast vector (Rabin and Hauser, 2003). Cross-kingdom pathogens present
tremendous opportunities for deciphering the mechanisms of host adaptation. What factors
and processes are required for the exploitation of one host, but not others? Identification of
these commonalities and differences will clearly define those virulence factors that are
essential for human disease, and provide clear direction for the development of targeted
therapeutics.
8. Innate immune response
The animal immune system has often been viewed as being wholly unique and unlike
anything found in plants. In actuality, a number of similarities have been found between
the innate immune response of plants and humans. The animal innate immune system
poses the first challenge to invading microbial pathogens. This system detects pathogenassociated molecular patterns (PAMPs) that are usually highly conserved within and
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
375
among a wide range of pathogenic species. Arrays of pattern recognition receptors (PRRs)
recognize these PAMPs. An important family of PRRs is the cell-surface Toll-like
receptors (Jebanathirajah et al., 2002; Takeda and Akira, 2003; Young, 2000), which
are characterized by an extracellular leucine-rich-repeat (LRR) domain and a cytoplasmic
TIR (Drosophila Toll/Interleukin-1 receptor) domain. The binding of specific pathogenassociated molecular patterns to these receptors results in the activation of a signal
transduction cascade through NF-nB, which modulates the animal immune response and
the production of inflammatory effectors such as cytokines (Staskawicz et al., 2001;
Aderem and Ulevitch, 2000). The animal innate immune response also includes the
activation of intracellular nucleotide-binding oligomerization domain (NOD) proteins
(Inohara and Nunez, 2003; Inohara et al., 2002). These proteins require a C-terminal
LRR, and an N-terminal effector-binding domain (EBD) that binds specific signals and
activate NF-nB. The human genome is estimated to carry approximately 30 Nod proteins
with varied N-terminal receptor domains that differ in their specificity (Staskawicz et al.,
2001; Dangl and Jones, 2001) (Fig. 2).
Plant innate immune systems also use LRR proteins analogous to the human Toll-like
and NOD receptors. Plants also have the ability to mount a direct defense response, and
to send systemic signals that can induce an acquired resistance, effectively blocking
subsequent pathogen attack (Maleck and Dietrich, 1999). Because plants have no
specialized and distinct immune system, each cell has the ability to respond to a
pathogen attack. This defense capacity is maintained by a very large repertoire of
rapidly evolving resistance (R) genes that constitute the plant’s innate immune system
Fig. 2. A comparison of animal and plant innate immunity proteins. Animal membrane bound Toll-like receptors
have an extracellular leucine-rich-repeat (LRR) domain and a cytoplasmic TIR (Drosophila Toll/Interleukin-1
receptor) domain. Animal intracellular nucleotide-binding oligomerization domain (NOD) proteins have a Cterminal LRR domain, a NOD domain, and a variable N-terminal effector-binding domain (EFD). Plant
intracellular resistance proteins have a C-terminal LRR domain, a NOD domain, and an N-terminal TIR domain.
376
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
(Dangl and Jones, 2001). The majority of these R genes have cytoplasmic NOD
domains, C-terminal leucine-rich repeats (LRR), and N-terminal TIR domains (Dangl
and Jones, 2001; Sessa and Martin, 2000), making them analogous to the human Tolllike and NOD proteins. The number of LRRs is extremely large and accounts for much
of the flexibility in the innate immune system (Kajava, 1998; Kobe and Kajava, 2001;
Parniske et al., 1997). For example, A. thaliana has approximately 150 NB-LRR genes
(Anonymous, 2000).
The finding that plants and animals use analogous proteins to recognize invading
pathogens should not have been such a dramatic surprise. We now understand that many
plant and animal pathogens are evolutionary cousins that have the ability and propensity to
share genetic material. This review has illustrated many of these shared mechanisms and
strategies. Are these common tactics a cause or consequence of analogous immune
responses in divergent hosts? Given the greater evolutionary potential of bacterial
pathogens (due to their rapid generation time and ability to exchange genetic material
horizontally), it seems likely that their use of common mechanisms reflects the common
molecular and cellular environment presented by their respective hosts.
9. Conclusion
Bacterial pathogens of humans and plants share numerous mechanisms for causing
disease. There are common mechanisms for adhesion, secretion, and regulation. There are
homologous and analogous toxins and virulence molecules. There are remarkable
similarities between the plant and human innate immune systems and disease-associated
genes. Finally, there are pathogens that are quite adept at attacking both plants and
animals. Given these similarities, it is reasonable to expect that what is learned about plant
pathogenesis may have direct relevance to human pathogenesis. But given the extensive
divergence between plants and humans, there must be some compelling reason to use plant
pathogens as models for human pathogenesis. The compelling reason is that the study of
plant pathogenesis provides many practical and scientific advantages. Plants provide the
means to significantly increase experimental replication and throughput. They provide
vastly greater reproductive control and tractability. They can be genetically manipulated
with ease. All of these benefits come along with dramatically reduced experimental costs
and regulation.
The identification of virulence factors conserved across a broad range of hosts will help
identify evolutionarily constrained, and perhaps essential virulence mechanisms. These are
logical targets for the development of antimicrobials and therapeutics. Perhaps more
importantly, identifying analogous virulence factors, not related by evolutionary descent,
but with common functions, will reveal mechanisms fundamental to bacterial pathogenesis. If divergent pathogens have independently evolved similar virulence mechanisms in
highly dissimilar hosts, then it is logical to assume that the pathogens have found a way to
exploit common eukaryotic processes. Identifying these critical steps in pathogenesis will
not only put us in a better position to control and treat infectious agents, it may also help us
realize that leaves and roots aside, we may not be as different from plants as we like to
think.
D.S. Guttman / Biotechnology Advances 22 (2004) 363–382
377
Acknowledgements
I would like to acknowledge Dr. Pauline Wang and an anonymous reviewer who
provided significant insight and a valuable critique of this review, and Prof. Bernie
Glick for his encouragement. DSG is supported by grants from the Natural Sciences
and Engineering Research Council of Canada and the Canadian Foundation for
Innovation.
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