The Plant Cell,
1711-1722, October 1996
1996 American Society
Department of Biochemistry, Max-Planck-lnstitut für Züchtungsforschung, Carl-von-Linne-Weg 10, D-50829 Cologne,
Fungi constitute a highly versatile group of eukaryotic carbon-
heterotrophic organisms that have successfully occupied most
natural habitats. The vast majority of fungi are strict sapro-
phytes; <10% of the
known fungal sp-ecies are able
to colonize plants, and an even smaller fraction of these are
to a plant pathogen? Second, what mechanisms control the
degree of virulence on the host once pathogenicity has been
capable of causing disease. Among the causal agents of in-
fectious diseases of crop plants, however, phytopathogenic
fungi play the dominant role not only by causing devastating
epidemics, but also through the less spectacular although per-
sistent and significant annual crop yield losses that have made
fungal pathogens of plants a serious economic factor, attract-
ing the attention of farmers, plant breeders, and scientists alike.
species of flowering plants are attacked
by pathogenic fungi. However, a single plant species can be
host to only a few fungal species, and similarly, most fungi
usually have a limited host range. The evolution of fungal
phytopathogens toward a high degree of specialization for in-
dividual plant species may be reflected in the different levels
specialization observed in extant plant-funga1 interactions
(Scheffer, 1991). The first level may be Seen in opportunistic
parasites, which enter plants through wounds or require other-
wise weakened plants for colonization. These fungal species
are usually characterized by a broad host range but a rela-
tively low virulence, that is, they cause only mild disease
symptoms. The next level comprises true pathogens that rely
on living plants to grow byt that under certain circumstances
can survive outside of their hosts. Many of the more serious
plant pathogens are found at this level; most are highly viru-
lent on only a limited number of host species. Finally, the
highest level of complexity is achieved by obligate pathogens,
for which the living host plant is an absolute prerequisite to
fulfill their complete life cycle.
Therefore, in a simplified view
the evolution toward
To colonize plants, fungal microorganisms have evolved strate-
gies to invade plant tissue, to optimize growth in the plant, and
to propagate. Bacteria and viruses, as well as some oppor-
tunistic fungal parasites, often depend on natural openings
or wounds for invasion. In contrast, many true phytopathogenic
fungi have evolved mechanisms to actively traverse the plant's
outer structural barriers, the cuticle and the epidermal Cell wall.
To gain entrance, fungi generally secrete a cocktail of hydro-
lytic enzymes, including cutinases, cellulases, pectinases, and
proteases. Because these enzymes are also required for the
saprophytic lifestyle, they are unlikely to represent the tools
specifically developed by fungi to implement pathogenesis,
and each individual hydrolytic enzyme may not be absolutely
necessary for penetration. This does not, however, preclude
the adaptation of their structure or their biosynthetic regula-
tion to the specific needs of a pathogen on a particular host
Experiments addressing the role of cutinase illustrate this
point. The cuticle covers the aerial parts of living plants and
needs to be pierced before other pathogenetic mechanisms
can become effective. Therefore,' enzymatic degradation of cu-
tin, the structural polymer of the plant cuticle, has been
postulated as crucial for fungal pathogenicity, and cutinase
is presumed to be a key player in the penetration process. In-
deed, severa1 lines of evidence demonstrate the pivotal role
phytopathogenicity, an ancestral fungus needed first to gain
attributesenabling it to live on numerous plant species before
refining those traits and/or developing additional devices to
increase its virulence on individual plant species, thus gain-
ing an edge on competing pathogens. The strategies pursued
by fungal pathogens in this process vary in different types of
interactions with their hosts (Keen, 1986). In this review, there-
focus on two intriguing questions. First, what are the
mechanisms that facilitate the transition of asaprophytic fungus
of cutinase during the infection process (reviewed in Kolattukudy,
1985). For example, inhibition of the enzyme by using differ-
ent chemical inhibitors or cutinase-specific antibodies was
shown to prevent infection by Necfria haemafococca
of pea stem segments with intact cuticle
but not of those with mechanically breached cuticle. Further
support for the role of cutinase came from the observation that
a wound pathogen affecting papayafruits, Mycosphaerelle sp,
that does not produce cutinase, was able to penetrate intact
1712 The Plant Cell
fruits only when transformed with the cutinase gene from
haemafococca (Dickman et al., 1989). However, the importance
of cutinase for plant penetration has been questioned (Schafer,
1994) and may vary between fungi, with other mechanisms
compensating for the lack of this enzyme. In addition, cutinase
may also be involved in prepenetration processes, for exam-
ple, by altering the adhesive properties of the cuticle and thus
facilitating fungal attachment to plant surfaces (Nicholson and
Epstein, 1991) or by releasing signal molecules required for
early fungal development on the plant (Kolattukudy et al., 1995).
Alternatively, or in combination with hydrolytic enzymes,
some fungi have developed a more complex and sophisticated
mechanism to penetrate the cuticle of host plants. In general,
phytopathogenic fungi form specialized penetration organs,
called appressoria, at the tip of their germ tubes; these or-
gans are firmly attached to the plant surface by extracellular
adhesives. As it develops, the porosity of the appressorium
wall of mechanically penetrating fungi is markedly reduced
by melanin incorporation, allowing high turgor pressure (>8
megapascals; Howard et al., 1991) to build up inside. This pres-
sure is focused effectively on a small area at the base of the
appressorium that is kept free of wall material and melanin.
From this penetration pore, an infection peg develops and
pierces through the cuticle and cell wall, possibly assisted by
hydrolytic enzymes (reviewed in Mendgen and Deising, 1993).
Studies of the rice blast fungus Magnaporthe grisea have
illustrated the importance of melanin for infection peg penetra-
tion; melanin-deficient mutants are unable
infect intact plants,
but some mutants retain pathogenicity on leaves with abraded
(i.e., wounded) epidermis (Chumley and Valent, 1990; Kubo
and Furusawa, 1991). Furthermore, melanized appressoria of
grisea were capable of pushing penetration pegs through
plastic membranes (Howard et al., 1991). These results sug-
gest that melanin is an essential factor for mechanically
Other fungal species, including some rusts, have not evolved
a direct penetration mechanism and instead bypass the plant
cuticle and outer cell wall by entering through the stomata.
These fungi have developed a poorly understood mechanism
to locate these openings on the plant surface (Hoch et al., 1987;
Correa and Hoch, 1995). Thus, penetration is likely to be con-
trolled by a combination of different factors.
addition to fungal
compounds, these factors may include plant surface structures
as well as activators or inhibitors of fungal spore germination
and germ tube formation.
IMPAIRMENT OF PLANT FUNCTIONS
After penetration, the next step in a fungal strategy to colo-
nize a plant species is often the secretion of toxins or plant
hormonelike compounds that manipulate the plant’s physiol-
ogy to the benefit of the pathogen. This interference can consist
simply of killing plant cells for the purpose of nutrient uptake
or a more subtle redirecting of the cellular machinery (Keen,
is achieved through the production of phytotoxins
with varying degrees of specificity toward different plants. Some
toxins are host selective (see Walton, 1996, in this issue),
whereas others are active in a wide range of plant species.
Phytotoxins have been identified in a broad spectrum of
pathogens, but their actual role in pathogenesis remains poorly
understood in most cases. However, in some plant-fungus in-
teractions, genetic and biochemical studies revealed that toxins
are the determinants of specificity. In these cases, resistance
or susceptibility to the fungus always correlates with insensi-
tivity or sensitivity to the toxins. Consequently, these
host-selective toxins, which are produced mainly by species
of the fungal genera Alternaria and Cochliobolus, have attracted
much attention (see Walton, 1996, in this issue). In contrast,
host-nonselective toxins are active on both host and nonhost
species. Although this nonselectivity contradicts a role in host-
range determination, these toxins may nevertheless have a
crucial function during fungal pathogenesis on a particular
host. Alternatively, they may be interpreted as remnants of ear-
lier stages of fungal evolution toward phytopathogenicity whose
activity may be obstructed in most plants by detoxification or
other mechanisms. Two examples illustrate the potential func-
tion of host-nonselective toxins.
Which processes are affected by fungal host-nonselective
toxins? The mode of action of only a small portion of these
toxins has been elucidated. However, severa1 fungal toxins tar-
get the plant plasma membrane-localized H+-ATPase. This
enzyme plays a central role in many cellular functions. For ex-
ample, in mediating ATP-dependent H+ extrusion, the enzyme
helps to establish an inwardly directed proton electrochemi-
cal gradient that
required for a number of “uphill” splute
transport processes. H+-ATPase activity is also involved in
various turgor-related processes and in the regulation of the
intracellular pH (Briskin and Hanson, 1992; Michelet and
Boutry, 1995; see also Gianinazzi-Pearson, 1996, in this issue).
This ATPase is activated by the host-nonselective toxin
fusicoccin, the major phytotoxic metabolite of the peach and
almond pathogen Fusicoccum amygdali that appears to be
generally active in higher plants (Marrè, 1979; Marrè and
Ballarin-Denti, 1985). The ensuing increased uptake of K+
(and other cations), CI-, and water by the stomatal guard cells
causes the irreversible opening of the stomata and the disease-
typical wilting of leaves (Ballio, 1991). The plasma mem-
brane-localized fusicoccin receptor is a member of the 14-3-3
superfamily of eukaryotic proteins (Korthout and De Boer, 1994;
Oecking et al., 1994) that is present in all higher plants (Meyer
et al., 1993). Members of the family of 14-3-3 proteins serve
a multitude of distinct functions, often through regulating the
phosphorylation state of proteins (Aitken et al., 1992). This sug-
gests that fusicoccin may affect a signaling pathway leading
to H+-ATPase stimulation through modulation of protein ki-
nase/phosphatase activity. However, the ubiquitous distribution
of the fusicoccin receptor contradicts the limited number of
host species of
amygdali. Despite the potential of fusicoc-
Funga1 lnfection of Plants 1713
cin, an additional asyet-unknown factor(s) must be responsible
for drastically restricting the fungal host range.
A second fundamental plant process, energy transfer dur-
ing light-driven photophosphorylation in chloroplasts, is
inhibited by a different host-nonselective toxin, tentoxin. In sen-
sitive plants, this cyclic tetrapeptide, which is produced by the
broad host range pathogen Alternaria alternata, causes seed-
ling chlorosis and the arrest of sensitive plant growth. Different
from fusicoccin, however, sensitivity to tentoxin varies between
plant species (Durbin and Uchytil, 1977). The toxin target ap-
pears to be the
subunit complex of the chloroplast coupling
factor 1. This is suggested by comparison of the sequences
subunit-encoding atpB genes from six closely related
toxin-sensitive and 4nsensitive Nicotiana species. The pres-
of a glutamate residue at position 83 correlated with
tentoxin insensitivity, whereas an aspartate residue correlated
with sensitivity (Avni et al., 1992). In addition, tentoxin binding
required the presence of the
subunit (Hu et al., 1993). De-
spite the identification of a highly specific target for tentoxin
action, it has not yet been demonstrated unequivocally that
sensitivity of a plant species to the toxin is the only factor
responsible for its susceptibility to the pathogen.
In summary, although the mode of action of some host-
nonselective fungal toxins has been analyzed in great detail,
their contribution to overall pathogenicity of the fungi that pro-
duce them remains to be defined. They clearly deserve more
attention because their role in pathogenesis may be more sig-
nificant than generally presumed, but also because they
to study physiological processes in
plants (Ballio, 1991).
ELlClTORS AND SUPPRESSORS: FUNGAL HOST
A fungus capable of actively penetrating plants and of produc-
ing a toxin that affects a fundamental biochemical process has
the potential to be a universal phytopathogen. Yet such a patho-
gen does not exist. Instead, all fungal plant pathogens grow
preferentially or exclusively on a limited number of hosts. There-
fore, additional factors must exist by which the host range of
a pathogen is restricted.
Soon after coming into contact with a plant, fungal patho-
gens are likely to be detected by the plant and confronted with
an active defense system. Clearly then, a successful plant de-
fense response must be based on an effective surveillance
system that enables an early recognition of the threat and, as
a consequence, the activation of defense-specific processes
that act to prevent further fungal development. Successful
pathogens, in turn, need to neutralize the plant resistance
on. The result of such coevolutionary dynamics
is seen in the contemporary highly specialized plant-fungus
The molecular bases for recognition of potential pathogens
by plants outside of gene-for-gene systems (see below) are
poorly understood. Plants may recognize an aggressor through
non-self factors that are present on the fungal surface (e.g.,
chitin and glucan fragments) or are secreted by the pathogen
(e.g., proteins) and/or through self determinants such as plant
cell wall fragments (e.g., oligogalacturonates) that are released
by an invading pathogen through the activity of hydrolytic en-
zymes. After recognition of the pathogen, a multitude of plant
resistance-associated reactions is initiated: ion fluxes across
the plant plasma membrane, the generation of highly reactive
oxygen species (the oxidative burst), the phosphorylation of
specific proteins, the activation of enzymes involved in strength-
ening of the cell wall, the transcriptional activation of numerous
defense genes, the induction of phytoalexins, localized cell
death at the infection sites (the hypersensitive response [HR]),
and the induction of systemic acquired resistance in dista1 plant
organs (Baron and Zambryski, 1995; Kombrink and Somssich,
1995, and references therein; see also Bent, 1996; Crute and
Pink, 1996; Dangl et al., 1996; Hammond-Kosack and Jones,
1996; Ryals et al., 1996, in this issue).
Although the actual role of particular defense reactions in
restricting further fungal progression in specific interactions
has only been incompletely unveiled, plant resistance or sus-
ceptibility is presumed
be determined after a sequential
exchange of signals between pathogen and host. From a num-
ber of fungi, molecules have been isolated that trigger mOSt
or at least some of these plant defense reactions. These com-
pounds are called elicitors, and several recent review articles
have covered various aspects related to their function (Cate
and Hahn, 1994; Ebel and Cosio, 1994; Ebel and Scheel, 1996;
Although some of the more general elicitors such as oligo-
N-acetylglucosamines and oligogalacturonates are active in
several plants, others appear to be species specific. The most
extensive data are available for two different elicitors from
sojae. A glycoprotein derived from culture filtrates
of this fungus induces many defense reactions in suspension-
cultured cells of the nonhost species parsley but not in the
host species soybean. In contrast, a specific heptaglucan elic-
itor from mycelial walls of the same fungus is active in soybean
and other leguminous species but not in parsley (Parker et
An interesting group of small proteinaceous elicitors, termed
elicitins, are secreted by species of
diseases on various plants (Ricci et al., 1989; Yu, 1995). Be-
cause elicitins were also found to be produced by another
Oomycete, Pythium vexam, they may be ubiquitous in this fun-
gal class (Huet et al., 1995). The purified proteins induce
necrosis and other defense reactions at the site of application
but also distally after their translocation, thus mimicking the
effects of fungal infection. In addition, they trigger SAR in
tobacco and other solanaceous species. On tobacco, the vir-
parasitica is inversely correlated with elicitin
secretion (Bonnet et al., 1994; Kamoun et al., 1994), implying
1714 The Plant Cell
that elicitins may be genus-specific determinants of resistance
in this (and other solanaceous) species. In contrast, elicitins
may be cultivar specific in some Brassica species because
they have been shown to function as elicitors of necrosis only
in a few cultivars of Raphanus sativus and Brassica campes-
(Kamoun et al., 19934 1994).
Molecular analyses using a cloned elicitin-encoding gene,
parA7, from a tomato isolate of
parasifica (Kamoun et al.,
1993a) revealed that elicitin genes occur as a multigene fam-
ily in this fungus. Interestingly, isolates that do not produce
elicitins and are virulent on tobacco were found to have re-
tained a set of elicitin genes. This indicates that the virulence
these isolates may be the result of mutations within loci that
regulate the transcription
the entire elicitin gene family.
The production of elicitors and the ensuing recognition by
the plant are counterproductive for fungal pathogenesis. There-
fore, fungi must develop mechanisms to elude recognition by
the host or to interfere with plant defense mechanisms. One
strategy could include the secretion of suppressors of the de-
fense response,(Bushnell and Rowell, 1981). In the most
common model, elicitor activity is explained by binding to a
specific cell surface-localized plant receptor that initiates a
defense-related signal transduction cascade. By comparison,
suppressors may interfere directly with elicitor binding, sig-
na1 transduction, gene activation,
the activity of defense
factors from the plant.
Evidence for the actual existence of suppressors came from
the observation that successful infection by virulent fungal
races frequently renders plant tissues more susceptible to nor-
mally avirulent fungi, indicating that plant tissue can be
conditioned toward susceptibility (Heath, 1982). To date, sup-
pressors have been described for only a few phytopathogenic
fungi (Shiraishi et al., 1991b), those from the pea pathogen
Mycosphaerella pinodes being the best characterized.
The structurally related glycopeptides supprescin A and
were purified from germination fluid of
et al., 1992). Treatment
pea leaves with a mixture of both
supprescins increased the infection frequency of several other-
wise nonpathogenic fungi (Shiraishi et al., 1978, 1991b).
Furthermore, suppressor specificity coincided with the host
pinodes. The fungus infects different leguminous
species to varying degrees. When species that are highly sus-
pinodes were treated with 'supprescins, they
became highly susceptible to the nonpathogen A. alternata.
Conversely, after supprescin treatment,
levels stayed lower in species that are less susceptible to
pinodes. This observation indicates that the supprescins may
be the determinants of host species specificity for
(Oku et al., 1980; Shiraishi et al., 1991b).
How do these suppressors allow the fungus to escape the
host resistance mechanism? A polysaccharide elicitor also
present in fungal germination fluid induces the accumulation
of the pea phytoalexin pisatin as well as of two biosynthetic
enzymes, phenylalanine ammonia-lyase and chalcone syn-
thase. In the presence of supprescins, this response is delayed
(Yamada et al., 1989). Furthermore, the elicitor-activated tran-
scription of the phenylalanine ammonia-lyase gene was found
to be rapidly deactivated upon suppressor treatment of pea
tissue (Wada et al., 1995). In vitro studies demonstrated that
inhibits the pea plasma membrane H+-ATPase
in a noncompetitive manner (Kato et al., 1993). However, inhi-
bition also occurred in isolated membranes from four nonhost
species. By contrast, after treating leaves with the suppres-
sors, cytochemical observations employing the lead
precipitation technique in combination with electron micros-
copy revealed that the ATPase was inhibited only in the host
plant, pea (Shiraishi et al., 1991a). Taken together, these data
indicate that the suppressors may not function simply by in-
hibiting elicitor binding to a receptor in pea membranes but
rather by affecting the signaling pathway that leads to the ac-
tivation of the resistance response.
In addition to suppressor formation, several alternative fun-
gal strategies to counter or to avoid the plant defense response
can be envisaged. For example,
a recognized elicitor is es-
sentia1 for pathogenesis, the pathogen could develop
mechanisms to increase its ability to tolerate the plant defense
the elicitor component is not crucial, deletion of
the respective gene may lead to increased virulence (see
AVIRULENCE GENE PRODUCTS
Genetic analyses of races of fungal pathogens and cultivars
of host species demonstrated that pathogen recognition is of-
ten determined by the interaction of plant resistance genes
with single avirulence genes of the pathogen (Flor, 1955, 1971;
also see reviews in this issue by Alfano and Collmer, 1996;
Bent, 1996; Crute and Pink, 1996). This gene-for-gene hypoth-
esis may be interpreted in biochemical terms as the interaction
of a race-specific pathogen elicitor with either a cultivar-specific
plant receptor or alternatively with a cultivar-specific signal
transduction compound (Keen, 1982, 1990). In other words,
resistant plant cultivars are capable of utilizing specific fea-
tures of pathogen races to trigger their defense response.
During the past few years, results obtained with a number
of pathosystems, mainly involving bacterial pathogens, have
corroborated the gene-for-gene complementarity at the mo-
lecular level. The small genome size of bacteria and the
availability of efficient molecular biology techniques enabled
the cloning of many avirulence genes from bacterial patho-
gens by genetic complementation. A shotgun strategy is,
however, not practical for fungi because of their larger genomes
and the fact that transformation protocols do not exist for many
phytopathogenic fungal species, in particular for most obligate
biotrophs. In addition, even if fungi can be transformed, trans-
formation frequencies are usually low. The alternative, a
map-based cloning strategy, is not applicable to the Imper-
fect Fungi, which lack a sexual stage and hence cannot be
Fungal lnfection of Plants 1715
Species Avirulence Gene
vr4 135 (105) Elicitor
63 (28) Elicitor
82 (60) Elicitor Toxin Cultivar d,
Cultivar d, i,
m, point mutation;
deletion; i, insertion.
crossed. Many serious plant pathogens belong to this group
of fungi; the identification of their avirulence genes depends
on the purification of race-specific elicitors of plant defense
responses. To date, avirulence genes have only been cloned
from a few fungal species. These include the tomato leaf mold
pathogen Cladosporium fulvum and the barley leaf scald patho-
gen Rhynchosporium secalis, which are both lmperfect Fungi.
In addition, a map-based cloning strategy was recently used
successfully to clone avirulence genes from the Ascomycete
grisea (Table 1).
In apoplastic fluids from C. fulvum-infected susceptible
race-specific elicitors, AVR4 and AVR9, were
identified (Scholtens-loma and De Wit, 1988; Joosten et al.,
1994) that trigger the HR in tomato cultivars carrying the
resistance genes 0-4 and Cf-9 (see Bent, 1996; Hammond-
Kosack and Jones, 1996, in this issue). Characterization of the
cloned Avr4 and Avr9 genes (Van Kan et al., 1991; Van den
Ackerveken et al., 1992; Joosten et al., 1994) revealed that the
gene products are cysteine-rich preproproteins that are
processed by fungal and/or plant proteases
yield active pro-
teins of 105 (AVR4) and 28 (AVR9) amino acids, respectively
(Van den Ackerveken et al., 1993; Joosten et al., 1994). An-
other race-specific elicitor, NIP1, was isolated from culture
secalis (Figure 1). The 82-amino acid product
gene is processed
yield a 60-amino acid mature
protein that is also cysteine rich (Rohe et al., 1995). In barley
plants with the resistance gene
this elicitor triggers severa1
defense reactions (Hahn et al., 1993). However, the HR is not
involved in the resistance of barley to
and Knogge, 1990).
Genetic complementation and gene disruption were used
to analyze the role of these elicitors during pathogenesis. Af-
ter transfer of the genes into virulent fungal races, transformants
were isolated that are avirulent on plants carrying the respec-
tive resistance genes (van den Ackerveken et al., 1992; Joosten
et al., 1994; Rohe et al., 1995). In addition, replacement by
nonfunctional genes in avirulent races through homologous
recombination (Marmeisse et al., 1993; W. Knogge, unpub-
lished data) yielded virulent fungi. Similarly, in
fulvum isolate, a frame-shift mutation was detected in Avr4, lead-
a truncated gene product (Joosten et al., 1994). These
results demonstrate that the genes Avr4, Av& and
sufficient and necessary to condition avirulence in combina-
tion with the corresponding plant resistance genes and thus
by definition are avirulence genes.
The intriguing questions now are whether the avirulence
gene products bind to specific plant receptors and whether
these receptors are encoded by the corresponding resistance
genes. Severa1 tomato resistance genes including Cf-9 have
been cloned and found to encode proteins with putative secre-
tory signal sequences, single transmembrane domains, and
short cytoplasmic tails, indicating their membrane-anchored
extracellular localization. In addition, a role of the gene prod-
ucts in recognition is suggested by the occurrence of
leucine-rich repeats in the putative extracellular domain (for
a more thorough discussion, see Bent, 1996, in this issue).
Studies with AVR9 revealed high-affinity binding sites on
plasma membranes isolated from Cf-9 plants. However, bind-
ing was also detected to membranes from Cf-O plants lacking
C. fulvum and from other solanaceous species,
all carrying Cf-9 homologous genes (Jones et al., 1994;
Kooman-Gersmann et al., 1996). Therefore, the question of
whether the Cf proteins interact directly with the fungal aviru-
lence gene products is still open for debate (see
Hammond-Kosack and Jones, 1996, in this issue).
Which mechanisms allow races of these fungal pathogens
to grow on host plants carrying resistance genes? The Avr9
gene is unique to races of C. fulvum that are avirulent on Cf-9
tomato plants but is absent from all virulent races (Van Kan
et ai., 1991). In contrast, all fungal races contain theAvr4 gene
(Joosten et al., 1994). However, whereas this gene
in all races avirulent on Cf-4 tomato plants, the virulent races
carry alleles with single nucleotide alterations that frequently
affect cysteine residues or, in one case, with a frame-shift mu-
tation. AI1 races transcribe the Avr4 gene, but the products of
the alleles from virulent races were not detectable
apoplastic compartment of susceptible host cultivars, indicat-
ing that the mutated AVR4 proteins are either unstable or not
secreted (Joosten et ai., 1994; De Wit, 1995; see also
Hammond-Kosack and Jones, 1996, in this issue).
virulence factors, including
also additional toxic proteins
susceptible host cultivars lacking
resistance gene Rrs1, these factors mediate
death through stimulation
plant plasma membrane
through additional unknown mechanisms (NIP2).
Rrs1 gene product
machinery that enables
host defense genes upon specific interaction with
one of the
secalis races that
Rrs1 barley carry
nipl alleles, both
which encode elicitor-active pro-
three amino acid positions.
virulent races lack
gene products that
elicitor inactive. These
fourth amino acid alteration
avirulence genes from
secalis were isolated after
contrast, cultivar-specific avirulence genes were
gene that prevents infection
cultivar Yashiro-mochi resides near
the tip of a
acid protein that shares
Funga1 lnfection of Plants 1717
tral Zn2+-proteases (Valent and Chumley, 1994; De Wit, 1995).
Mutant analysis revealed that gain of virulence can result from
DNA deletion as well as from DNA insertion at the chro-
mosomal tip. In addition, in some virulent isolates, point
mutations were identified in the putative protease motif (De
Wit, 1995). Although direct evidence for protease activity of
AVR2-YAMO is still missing, this avirulence gene product may
be functionally different from the
lence proteins. The latter are presumably the actual ligands
in the resistance-causing signal perception process, whereas
the AVRP-YAMO gene product may function by releasing an
active elicitor from a plant or fungal precursor molecule.
gene family from M.
exemplifies the con-
cept that resistance at the plant species level can also be
controlled by single fungal genes that function in a way very
similar to cultivar-specific avirulence genes (Heath, 1991).
These genes encode glycine-rich, hydrophilic proteins with
putative secretory signal sequences and prohibit pathogen-
icity on weeping lovegrass (fragrosris curvula). The host range
of M. grisea includes
grass species. The fungus exists
in a number of genetically distinct, asexually reproducing popu-
lations, only one of which favors rice (Valent and Chumley,
1991). The single species-specific avirulence genes PWLl and
were isolated from a finger millet (Eleusine coracana)
and a rice isolate (Kang et al., 1995; Sweigard et al., 1995),
homologs have been detected in many fungal strains
isolated from different host species. However, no correlation
has been found between the presence of PWL gene sequences
in fungal strains and their inability
infect weeping lovegrass,
indicating that not all members of this gene family function
as avirulence signals. This was substantiated by the finding
that two apparently allelic
homologs, PWL3 from a finger
millet pathogen and PWL4 from a weeping lovegrass isolate,
did not affect the ability of transformed
grisea strains to in-
fect weeping lovegrass. The inactivity of PWL4 appears to be
caused by improper expression of the gene, because
comes functional when put under the control of the PWLl
promoters. 6y contrast, PWL3 remained inactive in these
experimen:s, indicating that the gene product is nonfunctional
(Kang et al., 1995). In addition, as with the avirulence genes
secalis, pathogenicity can also be re-
stored by deletion of particular
genes or be retained in
grisea strains by single base pair changes
(Sweigard et al., 1995).
FUNGAL AVIRULENCE GENES
deletion of avirulence genes is advantageous for fungi to
overcome recognition by resistant host plants, why is this
strategy not always followed by pathogens? Clearly, the role
of these genes as avirulence determinants is coincidental and
a function of the plant defense mechanism. But what are the
genuine gene functions? In the case of the PWL genes, se-
quence comparison and mutation frequencies higher than
those in the rest of the
grisea genome strongly indicate that
this gene family is highly dynamic and rapidly evolving (Kang
et al., 1995). In addition, although spontaneous
tions occurred in fungal strains without affecting the fitness
under laboratory conditions, only one field isolate was found
to lack PWLP-related DNA (Sweigard et al., 1995). Therefore,
one can speculate that the PWL genes have a function that
may be required for fungal fitness in the field.
fulvum Avr4 gene is induced
plants and expressed
during pathogenesis in compatible interactions. In addition,
the occurrence of Avr4 alleles in virulent races suggests a func-
tion in virulence. However, a frame-shift mutation detected in
the Avr4 allele from a natural isolate had no visible effect on
fungal development in the plant, indicating that the gene is
dispensable (Joosten et al., 1994). The AvrQ gene is activated
after fungal hyphae have passed the stomata; it is highly ex-
pressed in hyphae growing in the vicinity of vascular tissue.
In vitro, AvrS expression requires nitrogen-limiting conditions
(Van den Ackerveken et al:, 1994), possibly reflecting the situ-
ation found in the apoplast of tomato leaves. The AvrS promoter
contains severa1 copies of a sequence motif that was identi-
fied as the recognition site of regulatory proteins in Neurospora
crassa (Caddick, 1992) and Aspergillus nidulans (Fu and
Marzluf, 1990; Marzluf et al., 1992). Deletion of a number of
these elements from the AvrQ promoter abolishes the induc-
ibility of the gene under low nitrogen conditions, suggesting
that they are functional in the transcriptional regulation of the
fulvum gene (De Wit, 1995). Nevertheless, under labora-
tory conditions, the AvrQ gene appears to be dispensable for
fungal development in the plant, as indicated by the lack of
the gene in races virulent on Cf-9 tomato and by disruption
mutants. As with the
genes, a possible role of the gene
for fungal development under field conditions has been dis-
cussed (De Wit, 1995).
In contrast to these avirulence genes, an actual function is
suggested for the
secalis. This gene ap-
pears to encode a factor that,
addition to its role in
determining avirulence, is essential for the expression of viru-
lence (Figure 1). Evidence comes from a fungal
transformant that displays a level of virulence that is reduced
compared with the parenta1 nipl' race on rrsl barley. This
phenotype is, however, similar to that of wild-type nipl- races,
regardless of the presence or absence of the
gene in the
host (W. Knogge, unpublished data). A contribution of NlPl
to fungal virulence is further substantiated by the finding that
it is a necrosis-jnducing protein in all barley cultivars
tested, although at higher concentrations than those required
for elicitor activity. Moreover,
is toxic to other mono- and sev-
era1 dicotyledonous plants (Wevelsiep et al., 1991; W. Knogge,
unpublished data). At least in part, this host-nonselective toxic
be based on an indirect stimulation of the
plasma membrane H+-ATPase (Wevelsiep et al., 1993). This
observation now raises the question whether the dual func-
tions of NIP1 are mediated through the same plant receptor
or whether the elicitor receptor (the one triggering resistance)
is distinct from the toxin receptor (the one conditioning disease).
1718 The Plant Cell
Receptor Models for NlPl Function.
The avirulence (elicitor) function of NlPl is depicted on the ft, an
the virulence (toxin) function
IIPI is depicted on the right in each panel.
Avirulence activity of NlPl is mediated through the product of barley resistance gene
whereas toxic activity results from the interaction
with a different plasma membrane receptor. The product of the recessive
allele in susceptible plants may not allow efficient binding of NIPI.
Alternatively, signal transduction upon binding may be impaired.
Both avirulence function and toxicity of NlPl are mediated through the same receptor that is encoded by the
locus. Again, differences
between the products of the
alleles in the efficiency
NlPl binding or in signal transduction may determine whether the plant
defense response is triggered.
(C) A single NlPl receptor mediates the toxic activity. However, it is not encoded by the
locus and requires an interaction with the product
gene to initiate the signal transduction pathway leading
the plant defense response. The
gene product may interact
with the NIPI receptor
trigger the defense response. Alternatively, transmission
the signal may be inefficient. In this model, the resistance
gene encodes a signal transduction component that may or may not be localized in the plasma membrane.
required to determine whether
the elicitor receptor
the resistance gene
elicitor receptors from plants. For the heptaglucan
fragment of the
protein elicitor from
membranes from soybean
1994, 1995). lsolation
these elicitor receptors should be
future (Honée and Nürnberger, 1995) and
further insight into the defense-related
tems of plants. Furthermore, isolation and characterization of
the receptor(s) for AVR9, NIP1, and additional avirulence
the question whether elicitor
tor genes are structurally related to
encoded by plant
resistance genes (see Bent, 1996; Hammond-Kosack and
Many questions concerning fungal infection
unanswered. However, research in this field obtains its sig-
nificance from the fact that these microorganisms are major
many crop species. An understanding
afford insights into the evolution
into the highly dynamic
lution with plants. In addition, the various factors fungi
developed to manipulate the physiology
their hosts to
optimize the parasitic lifestyle represent valuable tools to
study the affected plant processes. This
by the impact
other phytotoxins on unravel-
ing the roles of the plasma membrane H+-ATPase in plant
Furthermore, the compounds of fungal origin that are uti-
their hosts to initiate the
machinery offer the
potential for fine-tuned analyses
signaling pathways in plant
cells under microbial attack. These analyses, in turn, impinge
on cellular functions in healthy plants. Because disease re-
sistance is a response
plant tissues, not only of single cells,
one such function is cell-to-cell communication. A major ap-
strategies to counter
fungal pathogens on crop
aims to place fungal avirulence genes under the control
fined pathogen-responsive plant promoters (De Wit, 1992).
such constructs with
the complementary plant resistance genes should provide
transgenic plants with both components of the switch required
turn on the resistance response. Therefore, experimenta-
the coming years
likely not only to deliver answers
to the questions raised
the present article but
the results in molecular breeding approaches to improve re-
plants to disease.
Drs. Thorsten Nürnberger and lmre E. Somssich are gratefully ac-
knowledged for critical discussions
the manuscript. The work in
my laboratory was supported by grant
0136101A from the German
Research and Technology and by grants Kn 3293 from
the Deutsche Forschungsgemeinschaft.
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