Download Fungal and oomycete effectors – strategies to subdue a host

Can. J. Plant Pathol. (2011), 33(4): 425–446
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Fungal and oomycete effectors – strategies to subdue a host
SHAWKAT ALI1,2 AND GUUS BAKKEREN1
1
Agriculture and Agri-Food Canada, Pacific Agri-Food Research Center, Summerland, BC V0H 1Z0, Canada
Department of Botany, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; current address: Horticulture R&D Center,
Agriculture and Agri-Food Canada, St-Jean-sur-Richelieu Québec J3B 3E6, Canada
2
(Accepted 4 september 2011)
Abstract: Molecular studies focusing on the interface between microbes and plant hosts have provided major insights into the basis
underlying pathogenesis, symbiosis and plant defence and resistance mechanisms. A more recent focus on microbes, facilitated by the
generation of complete genome sequences, has uncovered the sheer number of protein effectors microbes deliver in this interface as well as
inside host cells to manipulate the plant immune system. Although studies on the characterization and roles of bacterial effectors are further
advanced, in this review we focus on the current knowledge of fungal and oomycete effectors and their roles. Examples are given of effectors
disarming plant defence enzymes, such as the apoplastic effector AVR2 from Cladosporium fulvum which inhibits the tomato defence cysteine
protease. Other effectors interfere with the perception by the host of microbes exposing molecular determinants such as Phytophthora
infestans INF1 protein. Many effectors alter gene expression induced by the host during defence, exemplified in fungi by Ustilago maydis
Pit2 suppressing maize defence genes. Effectors recognized by resistance gene products, either directly or indirectly, and eliciting defence,
represent the classical avirulence genes and almost 50 have now been cloned from fungi and oomycetes. Evolutionary adaptations and arms
races have produced diversification in both pathogen and host, and in pathogens, are the cause of breaking crop resistance in agricultural
settings. Molecular insight provides valuable information for applications. For example, some effectors are crucial for pathogenesis, thereby
revealing targets for disease control and others interact with host resistance gene products and could be used to screen germplasm for novel
sources of disease resistance. Variation among effectors will likely yield diagnostic tools for pathogen race identification. The study of model
systems is providing insight into avenues by which other, major plant diseases can potentially be controlled.
Keywords: avirulence genes, disease resistance, effector proteins, microbe–plant interactions
Résumé: Les études moléculaires ciblant l’interface entre les microorganismes et les plantes hôtes ont ouvert de nouvelles perspectives sur les
bases sous-jacentes de la pathogenèse, de la symbiose ainsi que des mécanismes de défenses et de résistance. Une nouvelle approche des
microorganismes, facilitée par la génération de séquences entières de génomes, a permis de découvrir l’importance du nombre de protéines
effectrices que les microorganismes sécrètent dans cette interface ainsi que dans les cellules hôtes afin de modifier le système immunitaire de
la plante. Bien que les études sur la caractérisation et les rôles des effecteurs microbiens soient avancées, dans cet exposé, nous mettons
l’accent sur les connaissances actuelles relatives aux effecteurs fongiques et à ceux des oomycètes, ainsi que sur leurs rôles. On y présente des
exemples d’effecteurs neutralisant les enzymes de défense de plantes, comme l’effecteur apoplastique AVR2 de Cladosporium fulvum qui
inhibe chez la tomate les mécanismes de défense dépendant de la protéase à cystéine. D’autres effecteurs modifient la perception que l’hôte a
de microorganismes exposant des déterminants moléculaires comme la protéine INF1 de Phytophthora infestans. Un grand nombre
d’effecteurs altèrent l’expression du gène induite durant la phase de défense, phénomène illustré chez les champignons par la protéine
Pit-2 d’Ustilago maydis qui inactive les gènes de défense du maïs. Les effecteurs reconnus, directement ou indirectement, par les produits
géniques de résistance suscitant la défense sont les gènes classiques d’avirulence et presque 50 ont à ce jour été clonés à partir de
champignons et d’oomycètes. Les adaptations évolutives et les « courses aux armements » ont induit la diversification tant chez l’hôte que
chez les agents pathogènes et, chez ces derniers, elles sont la cause de l’effondrement de la résistance des cultures commerciales. Les
perspectives moléculaires fournissent des données précieuses en vue d’applications utiles. Par exemple, certains effecteurs sont essentiels à la
Correspondence to: G. Bakkeren. E-mail: [email protected]
ISSN: 0706-0661 print/ISSN 1715-2992 online © 2011 The Canadian Phytopathological Society
http://dx.doi.org/10.1080/07060661.2011.625448
S. Ali and G. Bakkeren
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pathogenèse, déterminant de ce fait des cibles de lutte contre les maladies des plantes; d’autres interagissent avec les produits géniques de la
résistance de l’hôte et pourraient être utilisés pour analyser les germoplasmes afin d’y déceler de nouvelles sources de résistance aux maladies.
La variation observée chez les effecteurs contribuera de toute évidence au développement d’outils de diagnostic permettant l’identification de
races pathogènes. L’étude de modèles ouvre de nouvelles voies grâce auxquelles d’autres maladies importantes pourront être traitées.
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Mots clés: gènes d’avirulence, interactions microorganismes plantes, protéines effectrices, résistance aux maladies
Introduction
Among the myriad of microbes larger organisms
encounter, there are a few that have adapted to overcome the many barriers and defence mechanisms of these
organisms to become pathogens. The more tools such
pathogens have at their disposal, the more virulent they
are on a given host. Over the last 10 years, tremendous
progress has been made in the field of plant–microbe
interactions and the discovery of many pathogen and host
genes involved in these interactions has provided some
insight into the molecular mechanisms that decide the outcome of such encounters i.e. plant disease or host defence
and resistance. Pathogens flood their hosts with numerous proteins and compounds to counteract the defence
and ensure proliferation. The host, on its part, resists and
the uncovering of the increasingly astonishing ways each
interacting organism tries to decide the outcome of this
delicate balance in their favour, promises novel strategies
to help plants boost their defence responses. Although the
focus of this paper is on fungal and oomycete effectors,
many recent discoveries into their roles have emerged
from work on bacterial systems, some of which will be
highlighted because they might direct research on fungal
and oomycete effectors.
Strategies used by pathogens to infect host plants
Pathogens use different strategies to enter their host
plants. Fungi and oomycetes enter either through natural openings or directly through the plant epidermal
cells by mechanical and chemical means, or expand their
hyphae on the top of, within or between the plant cells
(Jones & Dangl, 2006). Pathogens need to adhere to the
plant before penetrating the plant cuticle. Fungal hyphae
and spores use mucilaginous and adhesive substances at
their tips, and the intermolecular forces between plant
and pathogen are responsible for the close contact. Most
rust fungi enter plants through stomata by developing
appressoria over the stomata to penetrate into the cavity
below, while ascomycetes such as the rice blast fungus
Magnaporthe grisea and the powdery mildews, such as
Blumeria graminis f. sp. hordei (Bgh), penetrate the cuticle of plants directly by means of an appressorium. For
direct penetration of the cuticle, M. grisea uses turgor
pressure in its melanized appressorium while Bgh penetrates cell walls by the combined activity of cellulases and
turgor pressure (Pryce-Jones et al., 1999; Talbot, 2003).
Other pathogens secrete cell wall-degrading enzymes,
such as cutinases, cellulases, pectinases and wax degrading enzymes for penetration of their host. After gaining
entrance to the host plants, pathogens require additional
‘tools’ to neutralize the defence reaction of the host and to
gain access to nutrients.
Resistance in plants to pathogens
Resistance in plants to pathogens is often categorized in
three types; nonhost, race-nonspecific and race-specific
resistance. Nonhost resistance is defined by the resistance
of an entire plant species against a specific non-adapted
pathogenic microbe and is the most common and durable
form of disease resistance exhibited by plants. It is the
result of both preformed and inducible defence mechanisms and seems to be under complex genetic control
which can involve multiple defence factors that individually may segregate within host species without compromising overall resistance (Heath, 2002; Mysore &
Ryu, 2004). Race-nonspecific resistance is known as general, quantitative or partial resistance of a plant against a
species of an adapted pathogen. It is generally durable and
is mostly controlled by genes with incremental/additive
effects, such as combinations of wheat leaf rust resistance gene Lr34 and several ‘slow-rusting’ genes (Singh
& Rajaram, 1992). Race-specific resistance is qualitative,
usually controlled by dominant avirulence (Avr) genes
in the pathogen and corresponding dominant resistance
(R) genes in a specific plant genotype or cultivar of an
otherwise susceptible host species. This resistance is of
the ‘gene-for-gene interaction’ type and is often easily
overcome by the pathogen.
Preformed physical barriers can include waxy cuticular surface layers of the leaves and thick and stable cell
walls that most microbes are not equipped to penetrate.
Even after successful penetration, the pathogen needs to
overcome the biochemical barriers that include low pH
of the apoplast, broad-spectrum antimicrobial compounds
and ‘defence’ enzymes that degrade microbial cell walls
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Fungal and oomycete effectors
(Felle, 1998; Dangl & Jones, 2001; Huckelhoven, 2007).
Cell wall components such as actin microfilaments play
an important role in defence against fungal penetration
and whose disruption results in loss of nonhost resistance
against several nonhost fungi (Kobayashi et al., 1997;
Heath, 2002; Mysore & Ryu, 2004). Biochemical barriers
consist of many peptides, proteins and non-proteinaceous
secondary antimicrobial metabolites which may determine the host range of some pathogens (Broekaert et al.,
1995; Morrissey & Osbourn, 1999), and fungitoxic substances on the surface of leaves in sufficient concentration to inhibit the germination of spores (Agrios,
2005). Phytoanticipins, preformed antimicrobial compounds such as tannins and fatty acid-like dienes, often
present in high concentration in cells of young fruits,
leaves or seeds, or saponins, are responsible for resistance to pathogens that lack the corresponding detoxifying
enzymes (Osbourn et al., 1996; Pedras & Ahiahonu,
2005). Other classes of antimicrobial compounds are
proteases, such as cysteine proteases secreted into the
apoplast by tomato and potato (Lucas, 1998; Kruger et al.,
2002) and protease inhibitors, such as a cysteine protease
inhibitor produced by pearl millet (Joshi et al., 1999).
Some of the above-mentioned defence systems are
preformed or could be kicked into higher gear, or
represent inducible mechanisms only activated in the
host when attacked. Examples are induced physical
defences, such as the formation of cell wall depositions (papilla) directly under the penetration sites of
pathogens; this can stop up to 90% of the penetration attempts in the Bgh/Arabidopsis nonhost interaction
(Collins et al., 2003). The production of phytoalexins
is often induced upon attack (Hammerschmidt, 1999).
Pathogenesis-related (PR) defence proteins such as chitinases and β-1,3 glucanase, (per-)oxidases, proteinase
inhibitors, thionins and lipid-transfer proteins, are induced
in response to pathogen attack and the accumulation of
these proteins is usually associated with the acquisition
of systemic resistance in plants against a wide range
of other pathogens. These are specific families of proteins, currently in 17 classes (van Loon et al., 2006)
although a larger group of ‘‘inducible defence-related proteins’’, including phenylalanine ammonia lyase, one of
the key enzymes involved in the synthesis of aromatic
compounds, like phytoalexins, is often included in the PR
proteins.
Pathogen-associated molecular patterns
In addition to preformed and inducible physical and biochemical barriers, plants also have evolved a surveillance
system to recognize various pathogen surface-exposed
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and cytoplasmic molecules known as pathogen (microbe)
associated molecular patterns: PAMPs or MAMPs (Shiu
& Bleecker, 2003). These are highly conserved molecules
of microbes that are perceived by host receptors (PAMP
recognition receptors or PRRs) at an early stage of infection. Recognition results in induction of PAMP-triggered
immunity (PTI). Examples of surface-exposed PAMPs
that have been shown to be capable of triggering PTI
are bacterial flagellin (Felix et al., 1999), lipopolysaccharides (LPS; Meyer et al., 2001; Erbs & Newman, 2003),
lipooligosaccharides from Gram-negative bacteria, chitin
from cell walls of higher fungi (Bartnicki-Garcia, 1968;
Ren & West, 1992), invertase from Saccharomyces cerevisiae (Basse et al., 1992), and 1,3-1,6-hepta-β-glucoside
from the cell walls of Phytophthora (Sharp et al., 1984a,
1984b). Examples of cytoplasmic PAMPs that induce
host defence are cold shock protein (CSP) and elongation factor Tu (Ef-Tu; Felix & Boller, 2003; Kunze et al.,
2004).
Pathogen effectors and their delivery during
plant–microbe interactions
Pathogens secrete a wide range of so-called ‘effectors’,
small molecules and proteins that can modify host cell
structures or functions. Effectors from several groups of
pathogens such as bacteria, oomycetes and fungi can enter
plant cells (Huang et al., 2003; Chisholm et al., 2006;
Kamoun, 2007). Bacteria use the Type II, Type IV and
Type III Secretion System (T3SS) to deliver 20 to hundreds of protein effectors into the host plant (Lindeberg
et al., 2006; Cunnac et al., 2009). Oomycetes and fungi
likely secrete even more protein effectors; computational
analysis of their genomes has revealed hundreds (Kamper
et al., 2006; Tyler et al., 2006; Jiang et al., 2008; Mueller
et al., 2008; Haas et al., 2009; Schirawski et al., 2010),
even more than 1700 of potential effectors for rust fungi
(Duplessis et al., 2011).
Predicted effectors from oomycetes, in addition to a
N-terminal signal peptide (SP), carry a host targeting signal (HTS) next to the SP that contains conserved RXLR
and a dEER motifs that can target them into host cells in
the absence of the pathogen (Rehmany et al., 2005; Tyler
et al., 2006; Whisson et al., 2007; Jiang et al., 2008; Dou
et al., 2008b; Haas et al., 2009; Kale et al., 2010), while
effectors from fungi have an N-terminal SP and in some
cases an RXLR-like motif (Dean et al., 2005; Kamper
et al., 2006; Kale et al., 2010; Schirawski et al., 2010).
Many examples exist where predicted effectors with a
SP are shown to be delivered into the host cytoplasm
(Kemen et al., 2005; Catanzariti et al., 2006; Mosquera
et al., 2009; Doehlemann et al., 2009; Khang et al.,
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S. Ali and G. Bakkeren
2010) and some can do so through N-terminal sequences
without the help of the pathogen (Rafiqi et al., 2010; Kale
et al., 2010). Recently, Kale et al. (2010) showed that the
conserved RXLR motif from oomycetes and the RXLRlike motif from other fungi bind specifically to phospholipids, in particular phosphatidylinositol-3-phosphate
(PI3P) on the surface of the plasma membrane and likely
enter the cell through lipid raft-mediated endocytosis;
results on lipid-binding assays have been controversial
(Gan et al., 2010). In a search for possible equivalent
motifs in other fungi, Godfrey et al. (2010) showed that
small secreted proteins from haustoria-forming fungal
pathogens share a conserved Y/F/WxC motif in addition
to an N-terminal secretion signal although no function has
yet been ascribed to these motifs.
Roles of effectors in infection
Breaking the physical barrier
After landing on the surface of plants, pathogens either
enter their host through natural openings (stomata and
hydathodes) or penetrate the surface directly. Bacteria
use natural openings or wounds to enter the apoplast
for colonization. Pathogenic bacteria sense compounds
released during photosynthesis from stomata and move
towards them. The PRR of the guard cells can sense
PAMPs such as bacterial flagellin or lipopolysaccharides
or lipooligosaccharides, which induces the closure of
stomata (Gohre & Robatzek, 2008). Pathogenic bacteria, such as Pseudomonas syringae, produce coronatine,
a phytotoxin that mimics jasmonic acid (JA) to interfere
with salicylic acid (SA) and abscisic acid (ABA) signalling for reopening stomata to get access to the host
apoplast (Melotto et al., 2006). It is not yet known whether
MAMP-associated defence pathways also close stomata
to eukaryotic pathogens and whether these pathogens,
such as cereal rusts, use effectors in a similar way to overcome this hurdle. Powdery mildew fungi, such as Bgh,
penetrate the cuticle of plant cells directly by forming an
appressorium. It has been shown that effectors Avra10 and
Avrk1 increase the penetration efficiency of Bgh on susceptible barley cultivars but the exact mechanism is not
yet known (Ridout et al., 2006). After getting into the
plant apoplast, the next physical barrier is the plant cell
wall. To promote nutrient leakage from the cytosol into the
apoplast, lytic enzymes that degrade the cell wall locally
are secreted. At the same time, pathogens secrete effector
molecules to suppress the host defence that is activated by
danger-associated molecular patterns (DAMPs) generated
from degrading cell wall molecules and signalling damage to the plant integrity and hence potential danger of
infection (Jha et al., 2007).
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Disarming plant defence enzymes
Plants produce antimicrobial enzymes such as proteases,
hydrolases, glucanases and chitinases that can degrade the
cell wall of invading pathogenic fungi in the apoplast,
without detrimental effect to the plant itself (Lucas, 1998).
This has a dual role in defence; the degradation of cell
walls can attenuate fungal growth while on the other hand
the molecules released from degraded cell walls serve
as elicitors for the induction of host defence. Pathogens
use effector molecules either to stop the delivery of
these antimicrobial enzymes and compounds by preventing their secretion, or by inhibiting their activity after they
are secreted (Bent & Mackey, 2007), and/or by inhibiting
downstream signalling. Bacteria secrete plant cell walldegrading enzymes locally in order to construct the T3SS
and use effectors such as HopP1 to suppress defence
induced by DAMPs in the N. benthamiana apoplast (Gust
et al., 2007; Oh et al., 2007). HopP1 either sequesters
or processes the fragments that are produced during cell
wall degradation so that they cannot function as DAMPs
(Gohre & Robatzek, 2008).
Examples from fungi are represented by the apoplastic
fungus Cladosporium fulvum which secretes effector
AVR2, a cysteine protease inhibitor, during infection that
binds directly to RCR3, a tomato cysteine protease, to protect the fungus from the deleterious effect of the enzyme.
AVR2 also promotes virulence for other fungal pathogens
that cause diseases in tomato, such as Verticillium dahliae
and Botrytis cinerea, when expressed heterologously in
Arabidopsis (van Esse et al., 2008). Similarly, C. fulvum
effector AVR4 binds to chitin of fungal cell walls to protect it from tomato host chitinases (van den Burg et al.,
2006). AVR4 can also protect chitin against plant chitinases in the cell wall of other fungi, such as Trichoderma
viride and Fusarium solani f. sp. phaseoli (van den Burg
et al., 2006). In this way, AVR4 not only protects the
fungi from hydrolysis by plant chitinases but also keeps
chitin fragments from eliciting PTI (Libault et al., 2007).
Effector ECP6 from C. fulvum has a Lys-M domain that
binds to carbohydrates including chitin and protects the
pathogen from plant chitinases or may be involved in
scavenging of chitin fragments that are released during
cell wall degradation by plant chitinases, thus preventing
them from inducing PTI (Bolton et al., 2008).
The oomycete pathogen, P. infestans, is known to
secrete a suite of Cysteine and Kazal family protease
inhibitors. The tomato papain-like protease, PIP1, which
is induced by SA, is blocked by the EPIC2B inhibitor
of P. infestans (Tian & Kamoun, 2005; Tian et al.,
2007; van Esse et al., 2008). PIP1 is related to RCR3,
and AVR2 from C. fulvum apparently can also inhibit
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Fungal and oomycete effectors
PIP1 and two other plant cysteine proteases, aleurain
and TDI65 (Rooney et al., 2005; Shabab et al., 2008;
van Esse et al., 2008). P. infestans EPIC1 and EPIC2B
can also bind and inhibit RCR3, similar to AVR2, but
unlike AVR2, these EPICs do not elicit an HR on Cf2/Rcr3pimp tomato plants, suggesting that P. infestans
evolved stealthy effectors that can inhibit tomato proteases without activating defence responses (Song et al.,
2009). These findings show that effectors from different pathogens can target the same apoplastic enzymes
to increase pathogen fitness in the host (Shabab et al.,
2008). Recently, another apoplastic tomato and potato
papain-like cysteine protease, C14, was shown to be targeted by EPIC1 and EPIC2B (Kaschani et al., 2010).
Other P. infestans effectors, EPI1 and EPI10, target the
subtilisin-like serine protease of tomato P69B, a PR protein (Tian et al., 2004; Tian & Kamoun, 2005; Tian
et al., 2007). AVRP123, a secreted protein from the flax
rust fungus M. lini, also shows similarity to Kazal serine
protease inhibitors (Catanzariti et al., 2006). The soybean
pathogen, P. sojae, secretes glucanase inhibitor proteins,
GIP1 and GIP2, that target endo-β-1,3-glucanaseA of the
host plant to protect the pathogen during infection and
also to prevent PTI induced by oligoglucosides (Rose
et al., 2002) as discussed next.
Suppression of receptor activation
The recognition of PAMPs by plant PRRs leads to the activation of defence responses against the pathogen. It was
proposed in the beginning that resistance induced by
PRRs is only basic and not as strong as that induced by
resistance genes, but it is clear now, at least in the case
of FLS2 (the flagellin receptor), that the contribution of
PRRs towards overall resistance is huge (He et al., 2006;
de Torres et al., 2006; Hann & Rathjen, 2007). It is therefore not surprising that pathogens have evolved to target
with their effectors these signalling pathways in order to
reduce resistance responses. Insight into the molecular
mechanisms involved has emerged over the last few years.
For example, several effectors target the PRRs directly
to jam all the downstream resistance responses (Jamir
et al., 2004; Jones & Dangl, 2006; Blocker et al., 2008)
such as P. syringae AVRPto and AVRPtoB which can
also suppress nonhost HR in N. benthamiana induced by
FLG22 or P. infestans INF1 (Hann & Rathjen, 2007).
Several oomycete RXLR effectors can suppress host
cell immunity in a similar manner. AVR3A from
P. infestans can block an HR induced by INF1 (Bos
et al., 2009) as can several random P. infestans effectors
(Oh et al., 2009). Similarly, AVR1B from P. sojae
also suppresses programmed cell death triggered by
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the mouse protein, BAX, in plants and yeast (Dou
et al., 2008a) and M. oryzae AvrPiz-t can also suppress
mouse BAX protein-mediated programmed cell death in
tobacco leaves (Li et al., 2009). ATR1 and ATR13 from
Hyaloperonospora arabidopsidis increased the virulence
of P. syringae DC3000 on susceptible Arabidopsis when
these effectors were delivered by the P. syringae T3SS,
and ATR13 seemed to target PTI by suppressing callose
accumulation and ROS production (Sohn et al., 2007).
In the genus Phytophthora, effectors such as P. infestans IpiO1 bind to a membrane-spanning lectin receptor
kinase and seem to prevent downstream defence signalling (Bouwmeester et al., 2011).
Suppression of R gene-triggered resistance
Pathogenic fungi such as Fusarium oxysporum f. sp.
lycopersici (Fol) can avoid host defences by evolving
effectors that can suppress R gene-triggered resistance
(Houterman et al., 2008). AVR3 and AVR2 are required
for full virulence on tomato plants but they are also recognized by tomato lines that have resistance genes I-3
or I-2, respectively, triggering an HR and cause arrest
of the pathogen (Huang & Lindhout, 1997; Rep et al.,
2004, 2005). In contrast, AVR1 is a small cysteine-rich
secreted protein that is recognized by resistance gene I
or I-1 but is not required for virulence. It seems generally true that effectors that contribute to full virulence are
maintained in the species and tellingly, AVR3 is present
in all Fol strains analysed, while AVR1 is present only
in Fol strains that are virulent on I-3 lines. Houterman
et al. (2008) showed that AVR1 actually suppresses the
resistance triggered by I-2 and I-3, as the transformation
of Avr1 to Fol strains that were avirulent on I-2 or I-3
became virulent on these lines. It was proposed that Fol
strains acquired Avr1 as a mechanism of partial functional
redundancy so that they can avoid the consequences of
losing Avr3 and probably Avr2 that are required for full
virulence (Stergiopoulos & de Wit, 2009). In the flax rust
fungus, M. lini, the interaction of AvrL567 in strain CH589 with the flax cultivar ‘Barnes’ that contains the L7
gene is inhibited by an inhibitor gene and thus results in a
lower virulence reaction (Lawrence et al., 2010). P. infestans secreted effector SNE1 can suppress programmed
cell death resulting from the interaction of several Avr-R
protein interactions (Kelley et al., 2010).
Alteration of the plant defence transcriptome
Bacterial effectors have been identified many years before
fungal and oomycete effectors and it is therefore not surprising that a better understanding of the impact of such
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S. Ali and G. Bakkeren
effectors on the host transcriptome during infection has
come from the study of bacteria–plant interactions. For
example, the perception of PAMPs such as bacterial flagellin changes the expression of at least 1000 genes in
Arabidopsis (Zipfel et al., 2004). Microbes have evolved
to become pathogens in part because their effectors inhibit
the activation of defence genes. Effectors can have an indirect effect on transcription by dephosphorylating MAPK
signalling components in order to suppress the defence
response triggered by the recognition of PAMPs by PRRs
(reviewed in Gohre & Robatzek, 2008). Another mechanism by which effectors have been shown to affect
the plant transcriptome is by causing changes in RNA
stability. For example, P. syringae HopU1, a mono-ADPribosyltransferase, acts on glycine-rich RNA-binding proteins such as Arabidopsis AtGRP7 and AtGRP8 which are
RNA chaperones and thus, HopUI changes the plant transcriptome by reducing transcript levels and hence affects
the production of defence response proteins (Fu et al.,
2007). Some pathogen effectors act as transcription factors and can induce the expression of host genes for the
benefit of the pathogen. For example, the AVRBs3 family of effectors from X. campestris pv. vesicatoria has
plant nuclear localization signals and binds to a ‘upa-box’
(upregulated by AVRBs3) that is found in the promoter of
Upa20, a master regulator of cell size inducing hypertrophy, and several other host genes, ensuring proper nutrient
supply for pathogen multiplication (Szurek et al., 2002;
Gurlebeck et al., 2006; Kay et al., 2007); however, in
resistant plant cultivars, the promoter of Bs3 also carries
a ‘upa-box’ and, thus, binding of the AVRBs3 effector
induces transcription of Bs3 and cell death (Kay et al.,
2007). This shows that under selection pressure, plants
can evolve to recognize effectors and use them for their
own defence.
Several studies in fungal and oomycete pathosystems have reported on changes in host transcriptomes:
in compatible interactions, initial defence reactions supposedly triggered by recognition of PAMPs, are the
result of the upregulation of genes in defence pathways;
at later infection stages, these are suppressed and this
effect is proposed to be caused by suites of secreted
pathogen effectors. Such a scenario has been demonstrated in the smut fungus Ustilago maydis–maize interaction (Doehlemann et al., 2008). In this latter system,
effectors likely even reprogram specific developmental
pathways in the host to create the best niche for their
own development, depending on the age and part of
the plant infected (Skibbe et al., 2010). One particular
effector, Pit2, was deduced to suppress defence genes
(Doehlemann et al., 2011). In the M. oryzae–rice interaction, changes in the host transcriptome were noted
430
including upregulation of susceptibility genes and downregulation of defence genes and these effects on gene
transcription were attributed to the actions of secreted
effectors, although no particular effector could be identified to cause specific effects (Mosquera et al., 2009).
Killing of host cells
Necrotrophic pathogens produce several phytotoxins in
addition to cell wall hydrolysing enzymes in order to
kill the host tissue for colonization during infection.
Some necrotrophic fungi produce proteinaceous effectors,
also called host selective toxins (HST), that are required
for infection on susceptible host plants that have the
corresponding dominant receptor gene (Wolpert et al.,
2002). This represents a situation opposite to the classical gene-for-gene interaction in which a dominant gene is
required for disease resistance rather than susceptibility.
Two wheat snecrotrophs, Stagonospora nodurum (Sn) and
Pyrenophora tritici-repentis (Ptr), produce several hostspecific peptide effectors, such as PtrTOXA, SnTOX1,
SnTOX2 and SnTOX4, that, when recognized by their
corresponding dominant susceptibility genes in wheat
(TsN1, Snn1, Snn2 and Snn4), cause disease (Liu et al.,
2004, 2006; Friesen et al., 2007; Abeysekara et al., 2009;
de Wit et al., 2009; Manning et al., 2009). PtrTOXA, the
best-studied effector, has an N-terminal secretion signal,
followed by an RGD domain (Arg-Gly-Asp-containing
loop) for host targeting and a C-terminal domain with
effector function (Sarma et al., 2005; Manning et al.,
2007). After entering the host cell, PtrTOXA was reported
to enter the chloroplast and bind to ToxABP1, thereby
interfering with functions of the chloroplast and affecting
photosystem I and II function in a light-dependent manner (Manning et al., 2007). PtrTOXA is an ortholog to
SnTOX1 and both effectors are recognized by the same
wheat susceptibility gene, Tsn1 (Liu et al., 2006).
Some pathogenic bacteria, oomycetes and fungi produce NEP1-like (necrosis-inducing protein) proteins
(NLPs), effectors that are toxic to only dicotyledonous
plants, possibly because of the different molecular
compositions of dicot and monocot cell membranes
(Pemberton & Salmond, 2004; Gijzen & Nurnberger,
2006; Ottmann et al., 2009). The common hepta-peptide
motif, GHRHDWE, and two conserved cysteine residues
make NLPs structurally similar to actinoporins, cytolytic
toxins from marine organisms. Hemibiotrophs, such as P.
infestans and P. sojae, produce NLPs such as NPP1 and
PiNPP1, in the late necrotrophic phase which could
contribute to disease development with their cytolytic
activities (Qutob et al., 2002; Kanneganti et al., 2006).
Fungal and oomycete effectors
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Suppression of host defence by symbionts
Symbiotic microorganisms also secrete effectors into host
plants to suppress host defence responses, a condition
essential for their lifestyle. Many rhizobial strains use
the T3SS to deliver effectors into host cells to suppress
host defence responses in a similar way that pathogenic
bacteria do (Deakin & Broughton, 2009). This could
indicate that this ancient delivery system is used for different purposes or that symbiotic organisms have evolved
their effectors such as to overcome most host defence
responses. In the fungal kingdom, the genome of Laccaria
bicolor, an ectomycorrhizal fungus, revealed more than
3000 predicted secreted proteins of which 10% were small
secreted proteins (SSPs) of the effector-type (Martin et al.,
2008; Martin & Selosse, 2008). Some of these SSPs are
homologous to rust fungus ‘haustoria-expressed secreted
proteins’ (HESPs) and are differentially expressed during infection. One effector, MiSSP7, was recently shown
to localize to host cell nuclei upon cell entry where it
altered host transcription and was shown to be necessary
for promoting symbiosis and evading the host defence
(Plett et al., 2011).
Gene-for-gene or R-Avr gene interaction
Adapted pathogens secrete effector molecules into the
host plant to surmount physical barriers, neutralize
preformed antimicrobial compounds, and overcome PTI.
As a result, during co-evolution of plant host and adapted
pathogen, plants have developed sophisticated recognition
systems, usually encoded by R genes to recognize these
effectors or their action and trigger defence responses;
this induced resistance has been called effector-triggered
immunity (ETI) and leads to a rapid and enhanced defence
response in the host plant often including an HR (Jones
& Dangl, 2006). These effectors represent avirulence
(AVR) factors since they activate the plant defence system and make the pathogen unable to cause disease when
the plant has the ‘recognizing’ R gene. This genetically
superimposed R and Avr interaction has been known for
many years as ‘gene-for-gene’ resistance, or host/cultivarlevel resistance, as particular cultivars of the host with
a certain R gene product recognize an Avr gene product
from a particular race of the pathogen. This concept was
formulated after genetic studies on two fungal pathosystems: the Melampsora lini–flax (Flor, 1942) and the
Ustilago tritici–wheat interactions (Oort, 1944). As mentioned, some Avr genes encode effectors that suppress host
defence or carry out other essential functions and, thus,
act as virulence factors when the plant does not have the
‘recognizing’ resistance gene. ETI represents the qualitative, secondary layer of resistance and has led to an
431
evolutionary arms race between the pathogen and the plant
in which the pathogen either mutates or discards effectors
to avoid recognition by the host or develops new effectors
to suppress ETI, while the plant develops new R genes to
recognize the mutated or new effectors (Jones & Dangl,
2006; de Wit, 2007; Bent & Mackey, 2007)
Avr (-triggering effector) genes
Because, until recently, the term avirulence gene/factor
featured prominently in the literature, we will discuss
these below but would prefer the term ‘Avr-triggering
effector genes’. The first Avr gene was cloned from a
bacterium in 1984 (Staskawicz et al., 1984), which was
followed by the cloning and characterization of more than
40 bacterial Avr genes in the following decades (van ’t
Slot & Knogge, 2002; Mudgett, 2005). The cloning and
characterization of Avr genes from fungi and oomycetes
lagged behind because of these organisms’ larger genome
sizes and sometimes inefficient transformation systems.
The first Avr gene from a fungus was isolated from C. fulvum in 1991 (van Kan et al., 1991) and the first oomycete
Avr gene was cloned from P. sojae and P. infestans in
2004 (Shan et al., 2004). In the following years, most
fungal Avr genes were cloned from ascomycetes, although
more recently, a few have been reported from a basidiomycete, the flax rust fungus, M. lini (Catanzariti et al.,
2006; Table 1).
Avr genes isolated and characterized to date differ
among one another both in sequence and function, and
those from different plant pathogens do not seem to
share many common features, other than the ones already
mentioned. Exceptions are found among some family
members, e.g. in Pseudomonas and Xanthomonas species
(Lahaye & Bonas, 2001; Deslandes et al., 2003). And it is
true that among genomes from related species, sometimes
even among genera, homologous effectors are found, but
these, or not all of these, have proven avirulence functions.
Most fungal and oomycete Avr genes characterized to date
encode small proteins (28–311 amino acids), except the
ACE1 of M. grisea (see below) and most have a secretion signal/protein transport motif at the N-terminus (Ellis
et al., 2006), although there are exceptions (see further).
Fungal Avr (-triggering effector) genes
Cladosporium fulvum
This fungus is an apoplastic pathogen of the ascomycete
subgroup that reproduces asexually and causes leaf mold
of tomato (de Wit et al., 1997; Joosten & de Wit, 1999;
Thomma et al., 2005). Four avirulence genes have been
cloned: Avr2, Avr4, Avr4E and Avr9, which all encode
small secreted cysteine-rich effector proteins that are
C. fulvum
C. fulvum
C. fulvum
C. fulvum
C. fulvum
C. fulvum
C. fulvum
C. fulvum
C. fulvum
R. secalis
R. secalis
R. secalis
B. graminis
B. graminis
AVR4
AVR4E
AVR9
ECP1
ECP2
ECP4
ECP5
ECP6
ECP7
NIP1
NIP2
NIP3
AVRa10
AVRk1
M. lini
AVRL567
(A, B and
C)
C. fulvum
Organism
AVR2
Protein
150 (127)
177
286
115 (?)
109 (?)
– (100)
82 (60)
119 (101)
115 (98)
222 (199)
165 (143)
96 (65)
121 (101)
63 (28)
135 (86)
78 (58)
1
3
4
9 (8)
7 (6)
6
10
6
6
8
4
8
6
6
8
8
23
–
–
17
16
–
22
18
17
23
22
23
10
23
18
20
Length aa
residues Number of
(mature) cysteines
SP length (aa)
Table 1. Effector proteins of fungal and oomycete plant pathogens.
Probably in
apoplast
Probably in
cytoplasm
Probably in
cytoplasm
Cytoplasm
> 30 paralogs in Bgh and other f. sp.
No N-terminal SP, induces HR
> 30 paralogs in Bgh and other f. sp.
No N-terminal SP, induces HR
Unknown, induces HR,functional
RXLR like motif
Probably in
apoplast
Apoplast
Probably in
apoplast
Apoplast
Apoplast
Apoplast
Apoplast
Apoplast
Apoplast
Apoplast
Apoplast;
fungal cell
wall chitin
Apoplast
Protein
localization
Non-specific toxin/induces necrosis
in several plants species
Induces HR
Induce necrosis,
LysM-domains; chitin-binding,
ortholog found in different
pathogen and nonpathogenic sp.
Unknown
non-specific toxin/induces necrosis
and plasma-membrane H+
ATPase
Non-specific toxin/induces necrosis
in several plant spp.
Induces HR
Induces HR, Carboxypeptidase
inhibitor
Induces HR, Tumor-necrosis factor
receptor
induces HR,
Induces HR in the presence of
Tomato Rcr3, Protease inhibitor
Induces HR, Chitin-binding,
orthologs in some other fungi
Biological activity/homology
Unknown
Unknown
Unknown
Not required for full
virulence
Not required for full
virulence
Disruption leads to
reduced virulence
Disruption leads to
reduced virulence
Unknown
Unknown
Knock-down leads
to reduced
virulence
Unknown
Not required for
virulence
Unknown
Unknown
Inhibits Rcr3 and
other proteases
Protects against
chitinases
Role in virulence/
pathogenicity
L5, L6 and
L7
Mlk1
Mla10
Unknown
Unknown
Unknown
Rrs-1
Cf-Ecp4
Cf-Ecp5
Unknown
Cf-Ecp2
Cf-Ecp1
Hcr9-4E
Cf-9
Cf-4
Cf-2
Correspon-ding R
gene
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Dodds et al., 2004;
Kale et al., 2010;
Rafiqi et al., 2010
Ridout et al., 2006
Rohe et al., 1995;
Stergiopoulos &
De Wit, 2009
Rohe et al., 1995;
Stergiopoulos & de
Wit, 2009
Ridout et al., 2006
Bolton et al., 2008
Rohe et al., 1995
Stergiopoulos et al.,
2010
Joosten et al., 1994;
Stergiopoulos
et al., 2010; van
den Burg et al.,
2006
Westerink et al., 2004
van den Ackerveken
et al., 1993
van den Ackerveken
et al., 1993
van den Ackerveken
et al., 1993
Bolton et al., 2008
Bolton et al., 2008
de Jonge &
Thomma, 2009
References
S. Ali and G. Bakkeren
432
not cloned yet
85 (66)
M. lini
M. oryzae
M. oryzae
M. oryzae
M. oryzae
M. oryzae
M. oryzae
M. oryzae
M. oryzae
M. oryzae
M. oryzae
M. oryzae
M. oryzae
AVRP4
Avr-Pita1
AVR-Pita2
AVR-Pita3
PWL1
PWL2
PWL3
PWL4
ACE1
AVR1CO39
AVR-Pia
AVR-Pii
AVR-Pii
144 (124)
AVRLm6
6
1
205 (183)
L. maculans
3
113 (92)
3
3
2
–
43
0
0
2
2
8
8
8
7
11
1
AVRM. oryzae
Pik/km/kp
AVRLm1
L. maculans
70 (51)
70 (51)
4035
138 (117)
137 (116)
145 (124)
147 (124)
226 (?)
224 (?)
223 (176)
95 (67)
117 (94)
M. lini
AVRP123
314 (286)
M. lini
AVRM
20
22
21
19
19
19
–
–
21
21
21
23
16
16
16
28
23
28
Unknown,functional RXLR
like motif
Induces HR
Induces HR
Induces HR
Induces HR
Induces HR
Glycine-rich hydrophilic
protein
Glycine-rich hydrophilic
protein
Hybrid polyketide
synthase/nonribosomal
peptide synthetase
Unknown
Glycine-rich hydrophilic
protein
Glycine-rich hydrophilic
protein
Homology to
metallo-proteases
Homology to
metallo-proteases
Induces HR, Kazal Serine
protease inhibitor
Induces HR, cysteine-knotted
peptide
Homology to
metallo-proteases, RXLR
like motif
Unknown, induces HR,RXLR
like motif
Probably in
cytoplasm
Probably in
cytoplasm
Probably in
cytoplasm
Probably in
cytoplasm
Probably in
cytoplasm
Probably in
apoplast
Unknown
Probably in
apoplast
Probably in
apoplast
Not secreted
Biotrophic
interfacial
complex
Cytoplasm
Probably in
cytoplasm
Probably in
apoplast
Probably in
cytoplasm
Probably in
cytoplasm
Cytoplasm
Cytoplasm
Required for full
virulence
Unknown
Unknown
Unknown
Unknown
Unknown
Rlm6
Rlm1
Pik
Pii
Pii
Pia
Pi-CO39(t)
Pi33
Unknown
Unknown
Unknown
Unknown
(Continued)
Fudal et al., 2007;
Kale et al., 2010
Gout et al., 2006
Yoshida et al., 2009
Yoshida et al., 2009
Yoshida et al., 2009
Yoshida et al., 2009
Farman et al., 2002
Bohnert et al., 2004
Kang et al., 1995
Kang et al., 1995;
Khang et al.,
2010
Sweigard et al.,
1995; Khang
et al., 2010
Kang et al., 1995
Unknown
Unknown
Khang et al., 2008
Catanzariti et al.,
2006; Kale et al.,
2010; Rafiqi
et al., 2010
Catanzariti et al.,
2006
Catanzariti et al.,
2006
Orbach et al., 2000;
Kale et al., 2010;
Khang et al.,
2010
Khang et al., 2008
Unknown
Pi-ta
Pi-ta
P, P1, P2 and/or
P3
P4
M
Non-functional
Non-functional
Unknown
Probably not
required for
virulence on rice
Probably not
required for
virulence on rice
Unknown
Not required for
virulence on rice
Unknown
Unknown
Unknown
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Fungal and oomycete effectors
433
P. sojae
P. sojae
P. sojae
P. infestans
M. fijiensis
M. fijiensis
MfAVR4
MfECP2
F. oxysporum f. sp.
lycopersici
H.
arabidopsidis
H.
arabidopsidis
P. sojae
AVR1a
AVR3a
AVR3c
AVR3a
AVR1-b1
ATR13
ATR1NdWsB
AVR1
(SIX4)
AVR2
(SIX3)
161 (142)
121 (100)
121 (98)
111 (91)
220 (200)
147 (126)
4
10
–
–
1
–
1
–
187 (168)
138 (117)
–
6
19
21
23
20
20
21
21
19
15
17
19
3 (2)
311 (296)
242 (184)
20
21
21
8
6 or 8
284 (189)
F. oxysporum f. sp.
lycopersici
F. oxyspo232 (172)
rum f. sp.
lycopersici
163 (144)
F. oxysporum f. sp.
lycopersici
AVR3
(SIX1)
AVR4
(SIX2)
8
L. maculans 143 (122)
Organism
Length aa
residues Number of
(mature) cysteines SP length (aa)
AVRLm4-7
Protein
Table 1. (Continued.)
Induces HR, suppresses
BAX-induced cell death, RXLR
domain
RXLR domain
RXLR domain
Induces HR, RXLR domain
Induces HR, suppresses
INF1-induced HR, RXLR domain,
interact with CMPG1 ubiquitin
E3 ligase
Chitin-binding peritrophin-A,
induces HR or necrosis in
Cf4 tomato
Induces HR or necrosis in
CfEcp2 tomato
Induces HR, RXLR domain
induces HR, RXLR domain
Unknown
Induces HR, unknown, functional
RXLR like motif
Unknown
Unknown
Unknown
Biological activity/homology
Probably in
apoplast
Probably in
apoplast
Cytoplasm
Unknown
Probably
cytoplasm
Promote virulence
by interacting
with host cell
target causing
necrosis
References
Rps1a
Rps3a
Rps3c
R3a
Rps1b and Rpsk1
RPP13
RPP1WsB and
RPP1Nd
I or I-1
I-2
Unknown
I-3
Ecp2
Stergiopoulos et al.,
2010
Stergiopoulos et al.,
2010
Qutob et al., 2009
Qutob et al., 2009
Dong et al., 2009
Armstrong et al.,
2005; Bos et al.,
2006, 2009, 2010
Shan et al., 2004;
Dou et al., 2008a,b
Allen et al., 2004;
Sohn et al., 2007
Houterman et al.,
2007;
Stergiopoulos & de
Wit, 2009
Houterman et al.,
2007;
Stergiopoulos & de
Wit, 2009; Kale
et al., 2010
Houterman et al.,
2007;
Stergiopoulos & de
Wit, 2009
Rehmany et al. 2005;
Sohn et al., 2007
Rep et al., 2004;
Stergiopoulos & de
Wit, 2009
Rlm4 and/or Rlm7 Parlange et al., 2009
Correspon-ding R
gene
Protect fungi
Cf4 and Hcr9
against chitinases
Unknown
Unknown
Unknown
Required for full
virulence
Suppress host
defense
Suppress host
defense
Suppression of
I-2 and
I-3 resistance
Required for full
virulence
Probably not
required for
virulence
Required for full
virulence
Required for full
virulence
Role in virulence/
pathogenicity
Cytoplasm
Cytoplasm
Xylem
Xylem
Xylem
Probably in
apoplast
Xylem
Protein
localization
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S. Ali and G. Bakkeren
434
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Fungal and oomycete effectors
recognized in tomato by Cf2, Cf4, Cf4E and Cf9 resistance proteins, respectively (Table 1; de Wit et al., 1997;
Joosten & de Wit, 1999; Thomma et al., 2005). The
virulence function of AVR2 and AVR4 has been discussed earlier. AVR4E is a 101 amino acid (aa) protein
but does not have a known virulence function (Westerink
et al., 2004). Several field isolates that overcome Cf4Emediated resistance reveal point mutations in AVR4E or
a complete loss of the Avr4E gene, indicating that loss
of this effector does not affect fitness of the pathogen
significantly (Stergiopoulos et al., 2007). Avr9 encodes a
28 aa mature protein with six cysteine residues after it is
processed by plant and fungal proteases at its C- and Ntermini (van Kan et al., 1991; van den Ackerveken et al.,
1993). Structurally, AVR9 is similar to carboxy-peptidase
inhibitor but no definitive function has been identified so
far (van Kan et al., 1991; van den Ackerveken et al., 1993;
van den Hooven et al., 2001). All natural strains of C.
fulvum that overcome Cf9 resistance lack Avr9, suggesting that it is not required for full virulence (Stergiopoulos
et al., 2007). C. fulvum deleted for Avr9 is fully virulent on tomato plants, but the heterologous expression of
Avr9 in tomato plants makes it more susceptible to C.
fulvum strains lacking Avr9, indicating some (redundant)
virulence function (Marmeisse et al., 1993; de Wit et al.,
2009).
Besides Avr effectors, four extracellular cysteine-rich
proteins (ECP), such as ECP1, ECP2, ECP4 and ECP5,
have been cloned and characterized from C. fulvum that
induce an HR in tomato lines containing the corresponding Cf-Ecp resistance genes (Lauge et al., 2000; de
Kock et al., 2005). Ecp6 and Ecp7 have been cloned
but the corresponding tomato lines that recognize these
genes have not yet been identified (Bolton et al., 2008).
ECPs are present in all strains of C. fulvum and are
secreted during infection. They contain an even number of cysteine residues that are most likely involved in
disulphide bridge formation and secondary structure formation to protect them from apoplastic proteases (Luderer
et al., 2002). Three of the ECPs, ECP1, ECP2 and
ECP6, have virulence functions on host plants, based
on data showing that deleting or suppressing expression
of these genes reduced virulence (Lauge et al., 1997;
Bolton et al., 2008). Orthologs for AVR4 and ECP6 have
been identified in several fungal species because of the
presence of CMB14 and lysin motif (LysM, carbohydratebinding) domains in these proteins (Bolton et al., 2008).
Orthologs of AVR4 and ECP2 have been identified in
Mycosphaerella fijiensis that causes black Sigatoka disease of banana (Stergiopoulos et al., 2010). The M.
fijiensis ortholog of AVR4 induces an HR in tomato
lines containing the corresponding Cf4 gene and binds
435
to chitin of fungal cell walls to protect against cell wall
degradation, similar to C. fulvum AVR4 (Stergiopoulos
et al., 2010). Similarly, M. fijiensis ECP2 is a functional
ortholog of C. fulvum ECP2 and is recognized by Cf2 of
tomato to induce an HR, while in the absence of Cf2, it
promotes virulence on tomato plants (Stergiopoulos et al.,
2010).
Rynchosporium secalis
The imperfect fungus R. secalis causes leaf scald disease
on barley by secreting low molecular-weight toxic proteins. The genes for three of these effectors, designated
as necrosis-inducing proteins NIP1, NIP2 and NIP3, have
been cloned (Table 1). They encode small secreted proteins with toxicity in a genotype non-specific manner on
barley and related cereal plant species (Hahn et al., 1993;
Rohe et al., 1995; Steiner-Lange et al., 2003). Mature
NIP1 is a 60 aa protein with 10 cysteine residues that
are involved in intramolecular disulphide bond formation.
NIP1 triggers specific defence responses without an HR
on barley cultivars that have the corresponding resistance
gene, Rrs1 (Lehnackers & Knogge, 1990). The injection
of NIP1 into leaves of barley and other cereal plant species
causes scald-like lesion formation (Wevelsiep et al., 1991;
van’t Slot et al., 2007). A nip1 disruption mutant of R.
secalis is slightly less virulent than the wild type on susceptible plants, demonstrating a (minor) role in virulence
(Knogge & Marie 1997). Virulent strains of R. secalis
overcome Rrs1 resistance of barley either by a point mutation in the ORF that results in a single aa substitution or
by jettison of the Nip1 gene (Rohe et al., 1995). It has
been shown that NIP1 interacts with a single plasma membrane receptor (different from Rrs1) that is involved both
in necrosis and defence induction (van’t Slot et al., 2007).
A field population study of this pathogen showed a positive diversifying selection on the Nip1 locus, as three out
of the 14 isoforms gained virulence on Rrs1 barley lines
and a high deletion frequency was observed in the Nip1
locus compared with Nip2 and Nip3 (de Wit et al., 2009).
The deletion frequency of Nip1 was higher than the occurrence of the point mutation that gains virulence indicating
a reduced fitness penalty for the loss of the Nip1 gene.
Nip2 encodes a 109 aa protein with a predicted secretion signal of 16 aa and a mature protein with 6 cysteine
residues, while Nip3 encodes a 115 aa protein with a predicted secretion signal of 17 aa and a mature protein with
8 cysteine residues (de Wit et al., 2009).
Magnaporthe oryzae
M. oryzae (formerly known as M. grisea) is a filamentous
ascomycete fungus that causes rice blast disease,
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S. Ali and G. Bakkeren
destructive to rice production worldwide, but can also
cause disease in many other members of graminaceous
plants (Kato et al., 2000; Couch & Kohn, 2002). More
than 40 resistance genes have been identified in rice controlling the blast fungus and several of them have been
extensively used in resistant rice lines in the past few
decades (Bryan et al., 2000; Chen et al., 2006). These
resistant rice lines are overcome quickly in the field by the
emergence of new races of the pathogen through various
mechanisms in the pathogen, such as deletion of Avr genes
from the genome (Yoshida et al., 2009), changes in their
gene expression (Kang et al., 2001; Fudal et al., 2005) or
point mutations in their genes (Orbach et al., 2000) resulting in escaping recognition by R genes (Kolmer, 1989;
Leach et al., 2001; McDonald & Linde, 2002). Eight
cultivar- and species-specific Avr genes have been cloned
and characterized from M. oryzae: Avr-Pita, Avr1-CO39,
Ace1, Pwl1, Pwl2, AvrPiz-t, Avr-Pia, Avr-Pii and Avrpik/km/kp (Table 1; Valent et al., 1991; Farman & Leong,
1998; Orbach et al., 2000; Bohnert et al., 2004; Collemare
et al., 2008; Li et al., 2009; Miki et al., 2009; Yoshida
et al., 2009).
The AVR-Pita effector shows similarity to fungal metalloproteases of the deuterolysin family and is not required
for full virulence on rice plants (Jia et al., 2000; Orbach
et al., 2000). Avr-Pita encodes a 223 aa protein that
is predicted to be secreted and needs to be processed
into a 176 aa active form (AVR-Pita 176) to interact
directly with the LRR (leucine-rich repeat) domain of
the PITA resistance protein and trigger PITA-mediated
defence responses, as assayed by yeast 2-hybrid assays, in
vitro binding analyses and direct expression in rice cells
(Jia et al., 2000). In certain strains of M. oryzae, AvrPita undergoes spontaneous mutations in the laboratory
and also under field conditions, such as deletion, point
mutation, and the insertion of transposons, all resulting
in overcoming Pita resistance in rice cultivars (Orbach
et al., 2000; Kang et al., 2001; Zhou et al., 2007; Khang
et al., 2008). Avr-Pita is located close to the telomere
of chromosome 3 in the genome of M. oryzae and this
may be responsible for the genetic instability of this
gene. Avr-Pita was renamed Avr-Pita1 after identification
of Avr-Pita2 and Avr-Pita3 (Khang et al., 2008). AvrPita2 is recognized by the rice Pita gene and elicits the
defence response while Avr-Pita3 is not recognized by
Pita (Khang et al., 2008).
Avr1-Co39 was isolated from M. oryzae isolate 40915-8 pathogenic on weeping lovegrass and specifies avirulence on rice cultivar CO39 that contains the resistant
gene, Pi-CO39(t) in a gene-for-gene manner (Valent et al.,
1991; Chauhan et al., 2002). Isolates virulent on rice
cultivar ‘CO-39’, lack Avr1-CO39 in most of the cases
436
(Farman et al., 2002). It has been shown that Avr1-CO39
is a species-specific rather than a cultivar-specific type
of Avr gene (Zheng et al., 2011). The Pwl genes stop
this pathogen from causing disease on weeping lovegrass
and finger millet in a species-specific manner, but they
still can infect rice (Kang et al., 1995; Sweigard et al.,
1995). PWL effectors are small glycine-rich secreted proteins that are evolving fast and belong to a gene family
designated as PWL1-PWL4. Pwl2 strains virulent on
weeping lovegrass appear due to spontaneous mutations,
predominantly by genetic rearrangement and large deletions (Kang et al., 1995; Sweigard et al., 1995). In the
three homologs of Pwl2, identified by homology searches,
only Pwl1 is the functional homolog while Pwl3 and Pwl4
are not functional; however, Pwl4 is functional only when
expressed under the control of the Pwl1 or Pwl2 promoter, while Pwl3 is not functional in that case (Kang
et al., 1995). Three new Avr genes, Avr-Pia, Avr-Pii and
Avr-Pik/km/kp, have been isolated from M. oryzae by
association genetics, i.e. by looking for polymorphisms
among 1032 predicted secreted proteins in field isolates
(Yoshida et al., 2009).
An Avr gene from a different class, Ace1, encodes a
4035 aa polyketide synthase (PKS) fused to a nonribosomal peptide synthetase (NRPS); these are two different
classes of enzymes that are probably involved in the
production of a secondary metabolite that triggers Pi33mediated resistance in rice cultivars (Bohnert et al., 2004).
M. grisea genome analysis revealed that Ace1 is present in
a cluster of 15 genes of which 14 encode enzymes such as
a second PKS-NRPS (Syn2), two enoyl reductases (Rap1
and Rap2) and a putative Zn(II)(2)Cys(6) transcription
factor (BC2) which probably all play a role in secondary
metabolism (Collemare et al. 2008). Ace1 and all other
genes in the cluster are specifically expressed during penetration into the host plant, defining an infection-specific
gene cluster, which suggests that Ace1 might have a role
in virulence; however, an Ace1 disruption mutant did not
show any reduction in virulence (Bohnert et al., 2004;
Fudal et al., 2005).
Leptosphaeria maculans
This ascomycete fungus causes blackleg (phoma stem
canker) disease on oilseed rape (Brassica napus). Genetic
analysis of the interaction has revealed at least nine
avirulence genes designated as AvrLm1–AvrLm9 that are
recognized by corresponding resistance genes Rlm1–Rlm9
of the host (Balesdent et al., 2002; Rouxel & Balesdent,
2005; Yu et al., 2005; Fitt et al., 2006). Seven of
these nine genes are present in two unlinked clusters
(AvrLm1-2-6 and AvrLm 3-4-7-9) while the remaining two
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Fungal and oomycete effectors
are individual genes (Balesdent et al., 2002). AvrLm1,
AvrLm6 and AvrLm4-7 have been cloned by a map-based
strategy and all encode small putative secreted proteins
that have no similarity to sequences in public databases
(Table 1; Gout et al., 2006; Fudal et al., 2007; Parlange
et al., 2009). Both AvrLm1 and AvrLm6 are located in
a gene-poor, AT-rich, non-coding heterochromatin-like
region as solo genes in stretches of 269 and 131 kb,
respectively, which contain a number of degenerated
nested copies of long terminal repeat (LTR) retrotransposon (Gout et al., 2006; Fudal et al., 2007). Also, both
AvrLm1 and AvrLm6 are single copy genes that encode
small proteins of 205 and 144 aa, respectively, with an
N-terminal secretion signal but no other conserved motif
or any similarity to each other. The expression of both
genes is strongly induced during early leaf infection but
they are also expressed in culture media at relatively low
levels when not in contact with the host (Gout et al.,
2006; Fudal et al., 2007). AvrLm1 has only one cysteine
residue and is likely taken up by the host cell (Gout et al.,
2006), while AvrLm6 contains six cysteine residues that
make disulphide bridges that could provide stability in
the apoplastic environment (Fudal et al., 2007). Strains
virulent on Rlm1 oilseed rape cultivars lack the AvrLm1
gene. Repeat-induced point (RIP) mutation, a common
mechanism to inactivate genes in ascomycetes, and deletion were also responsible for gain of virulence on Rlm1
cultivars (Fudal et al., 2009). Similar to the AvrLm1 and
AvrLm6 loci, the isolated AvrLm4-7 gene was located to
a 238 kb genetic locus having multiple LTR retrotransposons; this gene induces a resistance response in plants
having either Rlm7 or Rlm4. Within the locus, this single
gene is embedded in a gene-poor, AT-rich, recombinationdeficient 60 kb isochor (Parlange et al., 2009). It encodes
a small cysteine-rich secreted protein of 143 aa with a predicted N-terminal secretion signal of 21 aa. Like other
effectors from this fungus, the expression of AvrLm4-7
is induced in the plant but is also expressed at low levels in culture media (Parlange et al., 2009). The partial or
complete loss of AvrLm4-7 in field isolates is responsible
for gain of virulence on both Rlm4 and Rlm7 cultivars,
while a point mutation that changes a glycine to an arginine residue, can overcome recognition by Rlm4 (Parlange
et al., 2009).
Fusarium oxysporum
F. oxysporum f. sp. lycopersici (Fol) infects tomato roots
via wounds or by direct penetration after which it colonizes xylem vessels. Several small cysteine-rich proteins
were identified from xylem sap during Fol infection, and
three of them induced resistance in an R gene-specific
437
manner. They are coded for by avirulence gene SIX
(secreted in xylem) 1 to 4 (Table 1; Rep et al., 2004;
Houterman et al., 2007, 2008, 2009). Fusarium genome
comparisons revealed that all the SIX genes are located
on the same chromosome that is absent from nonpathogenic isolates. The transfer of this chromosome to
non-pathogenic strains converted them to pathogens (Ma
et al., 2010). The function of three of these genes in
virulence has been discussed in the previous section.
Blumeria graminis
Powdery mildews are biotrophic ascomycete fungi that
cause diseases on various mono- and dicotyledonous plant
species, including food and feed crops and ornamental
plants (Bushnell, 2002). They are obligate biotrophs that
need a living host for growth and reproduction and produce intracellular feeding structures, the haustoria, in the
epidermis of their host plants (Yarwood, 1957; Glawe,
2008). Bgh causes powdery mildew on barley and is
the most thoroughly studied powdery mildew fungus.
It interacts with its host in a ‘gene-for-gene’ manner
(Both et al., 2005; Zhang et al., 2005). Genetic analysis revealed that there are more than 85 dominant or
semi-dominant mildew (Ml) resistance genes that recognize different Bgh races including 28 highly similar
genes at the Mla locus on barley chromosome 5 (Jensen
et al., 1980). Seven Mla genes at this locus have been
cloned and they all encode closely related intracellular
coiled-coil, nucleotide-binding site, leucine-rich repeat
(CC-NBS-LRR) type R proteins that recognize different Bgh avirulent effector proteins (Halterman et al.,
2001; Halterman & Wise, 2004). Two of these effectors,
AVRa10 and AVRk1, that are recognized by the barley R proteins, Mla10 and Mlk1, respectively, inducing
an HR, have been cloned; however, they are also virulence factors in that they promote infection on susceptible barley cultivars (Table 1; Ridout et al., 2006).
Both Avr genes belong to multi-gene families that have
more than 30 paralogs and they also have orthologs
in other forma speciales that are pathogenic on other
grasses (Ridout et al., 2006). The predicted AVRa10 and
AVRk1 effectors lack an N-terminal secretion signal or
host targeting sequence, but it has been shown by fluorescence microscopy that the Mla10 protein is present
intracellularly, both in the cytoplasm and the nucleus of
invaded barley cells. Assuming a necessary interaction
with effectors, this means that AVRa10 is secreted from
the fungus by non-endomembranous pathways and taken
up by the cell by an as yet unknown mechanism (Bieri
et al., 2004; Shen et al., 2007). Using a novel approach
to gene silencing, Nowara et al. (2010) showed that by
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S. Ali and G. Bakkeren
expressing double-stranded or antisense RNA sequences
in barley but targeted to the Bgh-specific Avra10 and
Avrk1 effector genes, they could reduce fungal development in hosts lacking the corresponding Mla10 resistance
gene, confirming their role in virulence. Interestingly, no
clear release from R-gene recognition could be seen by
silencing the Avra10 gene using this system which they
termed ‘Host-Induced Gene Silencing’ or HIGS. Besides
AVRa10 and AVRk1, two other Avr genes, Avra22 and
Avra12, have been mapped recently (Skamnioti et al.,
2008).
Melampsora lini
Another obligate biotrophic fungus, but one that belongs
to the basidiomycetes, is M. lini that causes flax rust
disease not only in flax but also in other species of
the genus Linum. A number of flax R proteins have
been analysed; these are highly polymorphic cytoplasmic
(Toll/Interleukin1 Receptor) TIR-NBS-LRR proteins that
recognize effector proteins that are delivered into flax cells
during colonization. This interaction triggers an HR in a
‘gene-for-gene’ manner, arresting growth of the fungus
(Lawrence et al., 2007). The genetic analysis of M. lini
with its host plant flax (the basis of the original ‘genefor-gene’ theory by Flor in 1942) revealed at least 30
Avr genes and corresponding R genes (Ellis et al., 1997).
Several Avr genes have been cloned from M. lini, mainly
from four different loci: AvrL567, AvrM, AvrP123 and
AvrP4 (Table 1). These genes encode haustoria-expressed
secreted proteins (HESPs), suggesting that they have virulence functions, but elicit defence responses in hosts that
have the corresponding R genes (Catanzariti et al., 2006).
The AvrL567 locus has a cluster of three polymorphic
Avr genes: AvrL567A, AvrL567B and AvrL567C. All three
encode 127 aa mature proteins after cleavage of a 23 aa
SP and are recognized directly by the L5, L6 and L7 proteins inside the cell (Dodds et al., 2004). The mature
AVR proteins are highly polymorphic with at least one
or more aa substitutions in the exposed surface of the
proteins, suggesting functional interactions (Ellis et al.,
2007; Wang et al., 2007). Some of the isolates harbouring
these AVR proteins became virulent by defeating matching plant resistance genes, indicating that genes in the
AvrL567 locus are under positive diversifying selection
(Dodds et al., 2006). The analysis of six flax rust isolates revealed 12 members of the AvrL567 effector gene
family, including the three previously isolated AvrL567AC genes. Seven of these caused avirulence while five
belonged to virulent strains and could no longer be recognized by L5, L6 or L7 (Dodds et al., 2006). The
AvrM gene family is a small family that consists of five
438
avirulence paralogs, AvrMA to AvrME, and one virulent
one, avrM, which is not recognized by any known flax
R protein (Catanzariti et al., 2006). These AvrM proteins
do not have any known homologs and are highly variable
both in sequence and size due to deletions or insertions of
DNA, or to premature termination of the protein because
of the location of stop codons (Catanzariti et al., 2006).
AvrP123 is a small cysteine-rich protein that contains
the characteristic CX7CX6YX3CX2-3C signature of the
Kazal serine protease inhibitor family, suggesting its role
as an inhibitor of host proteases. This is similar to the
function of C. fulvum AVR2 that inhibits the Rcr3 cysteine
protease in the tomato apoplast (Catanzariti et al., 2006).
AvrP4 also encodes a small cysteine-rich protein of 67 aa
after cleavage of a 28 aa SP. The 28 aa C-terminal part of
AvrP4 has 6 cysteine residues with a spacing consensus of
a typical ‘cysteine-knotted’ peptide, similar to the C. fulvum AVR9 protein (van den Hooven et al., 2001). AvrP4
is expressed only in planta while AvrM is expressed both
in planta and in vitro (Stergiopoulos & de Wit, 2009).
Agroinfiltration of AvrP4 and AvrM genes in flax plants
with matching resistance genes results in a HR indicating that the produced effectors are functional in host cells
and are, therefore, likely translocated into the host during infections. This is in agreement with the predicted
cytoplasmic location of P and M resistance proteins in
flax (Anderson et al., 1997). Using a yeast 2-hybrid system, the AvrM-A and -D effectors were shown to interact
directly with the M resistance protein resulting in fast
necrosis, and that a globular C-terminal domain, variable
among alleles, is important for this interaction (Catanzariti
et al., 2010).
Oomycete Avr (-triggering effector) genes
Oomycetes belong to the kingdom Stramenopila and are
evolutionarily related to algae and include some wellknown plant pathogens. Many effector genes have been
isolated, some of which have proven avirulence functions
(Table 1).
Phytophthora infestans
P. infestans causes late blight disease in potato and
tomato and was responsible for the ‘Irish Famine’ in
1840. Many effectors have been identified and isolated
from P. infestans such as INF1 ‘elicitin’, a secreted protein that elicits hypersensitive cell death and is used in
many current functional assays (Kamoun et al., 1997;
Kamoun, 2006; Haas et al., 2009). However, only one
race-specific avirulence gene, Avr3a has been cloned.
Avr3a encodes a cytoplasmic RXLR effector and when
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Fungal and oomycete effectors
transiently expressed lacking its SP, in potato cells, R3amediated cell death ensues illustrating its recognition in
the cytoplasm and avirulence function (Armstrong et al.,
2005). The mature secreted protein is 147 aa in length
and allelic polymorphic residues distinguishing functionality were identified in various pathogen races: one allelic
variant Avr3AK80/I103 (abbreviated as Avr3A KI ) causes
avirulence on potato plants expressing R3a with the lysine
residue at position 80 being critical for recognition, while
the virulent allele has two aa substitutions, glutamate at
position 80 and methionine at position 103 (designated
as Avr3AE80/M103 or Avr3AEM ). In the absence of R3a,
Avr3A KI strongly suppresses P. infestans INF1 elicitininduced cell death in plants, thereby illustrating a major
virulence function. The virulent allele Avr3a EM cannot induce an R3a-mediated HR but can still (weakly)
suppress INF1-induced cell death suggesting that this
effector has various parts with distinct functions that
likely act through amino acid interactions with host proteins (Bos et al., 2006). These two distinct activities
have been separated through saturated high-throughput
mutant screens revealing important aa residues (Bos et al.,
2009). Deletions or mutations in the C-terminal residue
at tyrosine 147 maintained the R3A-mediated HR activity while it caused the loss of the ability to suppress
INF1-induced cell death. Molecular analysis of its cell
death-suppressing function showed that Avr3AKI interacts
and stabilizes the host ubiquitin E3-ligase CMPG1 which
is required for INF1-induced cell death (Bos et al., 2010).
The effector Avr3A likely targets CMPG1, interfering
with the ubiquitin proteasome system to block signal
transduction upon pathogen perception (Gilroy et al.,
2011). P. infestans Avr3a is located in a region syntenic to a part in the genome of another oomycete,
Hyaloperonospora arabidopsidis which harbours avirulence effector ATR1NdWsB (see below), suggesting that
this locus is ancient in these oomycetes.
The P. infestans genome sequence revealed many predicted secreted effectors and targeted searches for function revealed novel Avr activities for some effectors
interacting with specific resistance genes found in certain potato species to induce hypersensitive cell death
(Oh et al., 2009). Another secreted effector, SNE1, likely
is delivered into the plant host nucleus where it orchestrates the suppression of the effects of cell death-inducing
effectors secreted during the biotrophic phase (Kelley
et al., 2010).
Phytophthora sojae
P. sojae causes root and stem rot of soybean, resulting
in huge damage to soybean production in North America
439
(Shan et al., 2004). Four avirulence genes, designated
Avr1b-1, Avr1a, Avr3a and Avr3c, have been cloned from
this pathogen; they are recognized by soybean resistance genes Rps1b, Rps1a, Rps3a and Rps3c, respectively
(Table 1; Shan et al., 2004; Dong et al., 2009; Qutob et al.,
2009). Avr1b-1 was cloned by map-based cloning and is
predicted to encode a secreted protein of 138 aa with
a RXLR motif. Interestingly, this gene requires another
gene, Avr1-b2, at the locus for accumulation of Avr1b1 mRNA. Virulent P. sojae isolates such as P6497 and
P9073, harbour a complete Avr1b-1 gene but cannot accumulate Avr1b-1 mRNA like avirulent strains (Shan et al.,
2004). In addition to recognition by Rps1B, Avr1B-1 is
also recognized in plants having the Rpsk1 resistance
gene resulting in limited cell death and showing that such
effectors might have multiple targets (Kamoun, 2006).
As R proteins from the Rps1 gene clusters are cytoplasmic, it is assumed that Avr1-B is recognized inside the
host cytoplasm (Bhattacharyya et al., 1997; Shan et al.,
2004).
P. sojae Avr1a, Avr3a and Avr3c were cloned through
a combination of genetic mapping, transcript profiling,
and functional analysis (Qutob et al., 2009). All P. sojae
strains, whether virulent or avirulent on Rps1 plants,
contain the Avr1a gene which is present in four nearly
identical copies of 5.2 kb. Virulence was attributed to differences in transcription of Avr1a in some strains whereas
in other virulent strains, two of the multiple copies of
the fragments containing Avr1a were additionally deleted
(Qutob et al., 2009). Avr3a and four other predicted ORFs
were also found in four duplicated copies, of a fragment
of 10.8 kb. Transcriptional silencing of these four Avr3a
copies was found to be responsible for avoiding recognition by Rps3 plants in some P. sojae virulent strains, while
in other virulent strains, three copies were deleted and the
fourth one was transcriptionally silenced (Qutob et al.,
2009). The Avr3c gene was located to a 33.7 kb fragment
together with eight other predicted ORFs and three nearidentical copies of this fragment are present in a tandem
array in the P. sojae genome (Dong et al., 2009). Avr3c
virulent strains avoid recognition by Rps3A through specific mutations in the effector and subsequent sequence
exchange between two copies of Avr3c in the arrays. This
example illustrates ways by which pathogens can evolve
their effector repertoire as to avoid recognition by host R
genes (Dong et al., 2009).
As for P. infestans, the release of the P. sojae genome
sequence has allowed the large-scale analysis of candidate
effectors predicted to be secreted through their RXLRdEER motif. Among 169 recently tested effectors, most
could suppress cell death triggered by BAX, by other
effectors and/or the PAMP INF, while several caused
S. Ali and G. Bakkeren
cell death themselves and are hence possible avirulence
factors (Wang et al., 2011).
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Hyaloperonospora arabidopsidis
H. arabidopsidis, formerly known as (Hyalo)peronospora
parasitica, is an obligate oomycete pathogen that causes
downy mildew disease on the model plant Arabidopsis.
From this pathogen, two avirulence genes have been
cloned and characterized: (Arabidopsis thaliana recognized 1), Atr1 and Atr13 (Table 1). The product of
the Atr1 gene is recognized by Arabidopsis R proteins from two ecotypes, Niederzenz (RPP1Nd ) and
Wassilewskija (RPP1WsB ) and hence the effector allele
is called Atr1NdWsB . Transient expression of Atr1NdWsB in
Arabidopsis leaves by particle bombardment triggered cell
death. Extensive searches for allelic variants found that
RPP1Nd recognized the product of a single allele of Atr1,
while RPP1WsB recognized the products of four different
diverged alleles and provided resistance to a wide range
of isolates (Rehmany et al., 2005). Atr1NdWsB encodes
a 311 aa protein with a predicted secretion signal and
a conserved RXLR motif. Atr1NdWsB is a highly polymorphic protein and six different alleles that differ in
about one-third of all residues were identified in eight
isolates; intense diversifying selection and recombination
played an important role in the evolution of this locus
(Rehmany et al., 2005; Kamoun, 2006). Recent molecular studies have shown that binding of RPP1 occurs at
the LRR domain and is a prerequisite for HR induction,
while the actual induction is facilitated by the TIR domain
(Krasileva et al., 2010).
ATR13 is recognized by resistance protein RPP13 and
triggers a HR when transiently expressed in Arabidopsis
cells using particle bombardment. The ATR13 gene
encodes a 187 aa effector protein for which no related
sequences have been found in public databases (Allen
et al., 2004). ATR13 revealed apparent domain structures:
in addition to the N-terminal SP and an RXLR motif,
it has a heptad leucine-isoleucine repeat motif that is
required for RPP13 recognition, followed by an imperfect direct repeat of 4 × 11 aa which lies between aa
residues 93 and 136. All pathogen Atr13 effector alleles as well as identified alleles of the interacting host
RPP13 resistance gene reveal a high level of aa polymorphisms among their protein products and apparently have
been under intense diversifying selection suggesting a coevolutionary arms race at these loci (Allen et al., 2004).
Both ATR1 and ATR13 suppress basal defence responses
of host plants when delivered by the P. syringae T3SS,
revealing a virulence function (Sohn et al., 2007).
440
Concluding remarks
Over the last two decades, the study of different fungal,
oomycete and bacterial pathogen effectors and their function during the interaction with host plants has led to
remarkable insights into the molecular basis underpinning plant diseases. These effectors facilitate a pathogen’s
entry into the host by suppressing defence responses at
multiple levels. Examples include interference with recognition of PAMPs by PRRs and/or downstream signalling
thereby evading basal immunity that otherwise would
be sufficient to stop infection. Effectors also have roles
in establishing feeding interactions and/or nutrient leakage from the host to the benefit of the pathogens. These
effectors function at the interface of pathogen and host
plant or inside plant cells. Inadvertently, when recognized
by components of the host surveillance system, they can
trigger defence responses, revealing avirulence or elicitor functions. The sequencing of complete genomes of
many bacterial, fungal, and oomycete plant pathogens and
subsequent computational analyses have revealed a myriad of effectors, many of which are potential avirulence
genes that can trigger ETI. These then have the potential of revealing novel resistance potential, in particular
resistance genes in varied germplasm if suitable assays
can be designed. This has become an important goal of
many research laboratories and has already led to the discovery of novel resistance genes (van der Hoorn et al.,
2000; Torto et al., 2003; Stergiopoulos et al., 2010). The
identification of complete sets of pathogen effectors, their
functions, and their host targets will advance the knowledge of molecular plant–pathogen interactions and spark
the design of novel disease control strategies.
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