ArticlePDF Available
The Plant Cell,
Vol.
8,
1711-1722, October 1996
O
1996 American Society
of
Plant Physiologists
Funga1 Infection
of
Plants
Wolfgang Knogge
Department of Biochemistry, Max-Planck-lnstitut für Züchtungsforschung, Carl-von-Linne-Weg 10, D-50829 Cologne,
Germany
INTRODUCTION
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
*lOO,OOO
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
established?
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.
All
of the
.v300,000
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
of
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
of
the evolution toward
ACTIVE PENETRATION
OF
THE PLANT
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
plant.
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-
fore,
I
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
(Fusar-
ium
solani
f
sp
@si)
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
N.
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
to
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
M.
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
penetrating fungi.
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.
In
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
BY
FUNGAL TOXlNS
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,
1986); often
it
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
is
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
F:
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
ap
subunit complex of the chloroplast coupling
factor 1. This is suggested by comparison of the sequences
of the
fl
subunit-encoding atpB genes from six closely related
toxin-sensitive and 4nsensitive Nicotiana species. The pres-
ente
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
a
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
represent valuable
tools
to study physiological processes in
plants (Ballio, 1991).
ELlClTORS AND SUPPRESSORS: FUNGAL HOST
RANGE DETERMINANTS
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
strategy, and
so
on. The result of such coevolutionary dynamics
is seen in the contemporary highly specialized plant-fungus
interactions.
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
to
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;
Knogge, 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
Phyrophrhora
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
al., 1988).
An interesting group of small proteinaceous elicitors, termed
elicitins, are secreted by species of
Phytophthora
that cause
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-
ulence of
/?
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-
tris
(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
of
these isolates may be the result of mutations within loci that
regulate the transcription
of
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,
or
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
6
were purified from germination fluid of
M.
pinodes (Shiraishi
et al., 1992). Treatment
of
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
range of
M.
pinodes. The fungus infects different leguminous
species to varying degrees. When species that are highly sus-
ceptible to
M.
pinodes were treated with 'supprescins, they
became highly susceptible to the nonpathogen A. alternata.
Conversely, after supprescin treatment,
A.
alternata infection
levels stayed lower in species that are less susceptible to
M.
pinodes. This observation indicates that the supprescins may
be the determinants of host species specificity for
M.
pinodes
(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
supprescin
B
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,
if
a recognized elicitor is es-
sentia1 for pathogenesis, the pathogen could develop
mechanisms to increase its ability to tolerate the plant defense
reactions.
If
the elicitor component is not crucial, deletion of
the respective gene may lead to increased virulence (see
below).
TRlGGERlNG
OF
PLANT RESISTANCE
BY
FUNGAL
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
Table
1.
Cloned
Fungal
Avirulence Genes
Fungal
Species Avirulence Gene
Gene
Producta Activityb
Functionb
Specificity
Leve1
VirulenceC
C.
fulvum A
vr4 135 (105) Elicitor
?
Cultivar
m
C.
fulvum A
vr9
63 (28) Elicitor
?
Cultivar d
R.
secalis nip
1
82 (60) Elicitor Toxin Cultivar d,
m
M.
grisea
AVR2-YAMO
223 Protease?
?
Cultivar d, i,
m
M.
grisea
PWLl
147
? ?
Species d?
M.
arisea
PWL2
145
? ?
Species
d,
m
lntrinsic
Gain
of
~~
a
Number
of
amino
acids
in
the
primary
translation
products
and in
the
processed proteins
(in
parentheses).
?,
unknown.
m, point mutation;
d,
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
M.
grisea (Table 1).
In apoplastic fluids from C. fulvum-infected susceptible
tomato leaves,
two
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
to
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
filtrates of
R.
secalis (Figure 1). The 82-amino acid product
of the
nip7
gene is processed
to
yield a 60-amino acid mature
protein that is also cysteine rich (Rohe et al., 1995). In barley
plants with the resistance gene
Rrsl,
this elicitor triggers severa1
defense reactions (Hahn et al., 1993). However, the HR is not
involved in the resistance of barley to
R.
secalis (Lehnackers
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
a
virulent
C.
fulvum isolate, a frame-shift mutation was detected in Avr4, lead-
ing
to
a truncated gene product (Joosten et al., 1994). These
results demonstrate that the genes Avr4, Av& and
nip7
are
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
resistance
to
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
is
identical
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
in
the
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).
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
C.
fulvum and
R.
secalis aviru-
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.
The
PWL
gene family from M.
grisea
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
>50
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
PWLP
were isolated from a finger millet (Eleusine coracana)
and a rice isolate (Kang et al., 1995; Sweigard et al., 1995),
respectively.
PWL
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
to
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
PWL
homologs, PWL3 from a finger
millet pathogen and PWL4 from a weeping lovegrass isolate,
did not affect the ability of transformed
M.
grisea strains to in-
fect weeping lovegrass. The inactivity of PWL4 appears to be
caused by improper expression of the gene, because
it
be-
comes functional when put under the control of the PWLl
or
PWLP
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
from
C.
fulvum and
R.
secalis, pathogenicity can also be re-
stored by deletion of particular
PWL
genes or be retained in
PWL-expressing
M.
grisea strains by single base pair changes
(Sweigard et al., 1995).
INTRINSIC FUNCTIONS
OF
FUNGAL AVIRULENCE GENES
If
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
M.
grisea genome strongly indicate that
this gene family is highly dynamic and rapidly evolving (Kang
et al., 1995). In addition, although spontaneous
PWLP
dele-
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.
The
C.
fulvum Avr4 gene is induced
in
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
C.
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
PWL
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
nipl
gene from
R.
secalis. This gene ap-
pears to encode a factor that,
in
addition to its role in
determining avirulence, is essential for the expression of viru-
lence (Figure 1). Evidence comes from a fungal
nipl
disruption
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
Rrsl
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,
it
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
activity appears
to
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
A
(defense responsesl
B
H+
..
nn
I
Q
h
li!
r
(defense responsesl
C
HS
I/
+
7
I
defense responsesl
Figure
2.
Receptor Models for NlPl Function.
The avirulence (elicitor) function of NlPl is depicted on the ft, an
ADP+Pi ATP
disease
disease
H+
ADP+P~
ATP
the virulence (toxin) function
c
I
disease
I
IIPI is depicted on the right in each panel.
(A)
Avirulence activity of NlPl is mediated through the product of barley resistance gene
Rrsl,
whereas toxic activity results from the interaction
with a different plasma membrane receptor. The product of the recessive
rrs7
allele in susceptible plants may not allow efficient binding of NIPI.
Alternatively, signal transduction upon binding may be impaired.
(e)
Both avirulence function and toxicity of NlPl are mediated through the same receptor that is encoded by the
Rrsl
locus. Again, differences
between the products of the
Rrsl
and
rrsl
alleles in the efficiency
of
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
Rrsl
locus and requires an interaction with the product
of the
Rrsl
gene to initiate the signal transduction pathway leading
to
the plant defense response. The
rrsl
gene product may interact
to0
poorty
with the NIPI receptor
to
trigger the defense response. Alternatively, transmission
of
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.
Funga1 lnfection
of
Plants 1719
Further experimentation
is
also
required to determine whether
the elicitor receptor
is encoded
by
the resistance gene
Rrsl
(Figure 2).
Current research
in
severa1
laboratories
focuses
on
the
iso-
lation
of
elicitor receptors from plants. For the heptaglucan
elicitor
as
well
as for
a
13-amino
acid
fragment of the
glyco-
protein elicitor from
P
sojae,
specific
binding sites
were
detected
on plasma
membranes from soybean
and
parsley, respectively
(Cosio et
al.,
1992;
Nürnberger
et
al.,
1994, 1995). lsolation
and
cloning
of
these elicitor receptors should be
successful
in
the
near
future (Honée and Nürnberger, 1995) and
will
give
further insight into the defense-related
signal
perception sys-
tems of plants. Furthermore, isolation and characterization of
the receptor(s) for AVR9, NIP1, and additional avirulence
gene
products
will
help to
answer
the question whether elicitor
recep-
tor genes are structurally related to
and
encoded by plant
resistance genes (see Bent, 1996; Hammond-Kosack and
Jones,
1996,
in
this
issue).
CONCLUDING
REMARKS
Many questions concerning fungal infection
of
plants remain
unanswered. However, research in this field obtains its sig-
nificance from the fact that these microorganisms are major
pathogens
of
many crop species. An understanding
of
fungal
pathogenicity
will
not
only
afford insights into the evolution
of
fungi
but
also
into the highly dynamic
process
of their
coevo-
lution with plants. In addition, the various factors fungi
developed to manipulate the physiology
of
their hosts to
optimize the parasitic lifestyle represent valuable tools to
study the affected plant processes. This
is
clearly demonstrated
by the impact
of
fusicoccin
and
other phytotoxins on unravel-
ing the roles of the plasma membrane H+-ATPase in plant
cells.
Furthermore, the compounds of fungal origin that are uti-
lized
by
their hosts to initiate the
defense
machinery offer the
potential for fine-tuned analyses
of
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
of
plant tissues, not only of single cells,
one such function is cell-to-cell communication. A major ap-
plied goal
of
this research
is
to
develop
strategies to counter
the effect
of
fungal pathogens on crop
species.
One approach
aims to place fungal avirulence genes under the control
of
de-
fined pathogen-responsive plant promoters (De Wit, 1992).
Transformation-mediated combination
of
such constructs with
the complementary plant resistance genes should provide
transgenic plants with both components of the switch required
to
turn on the resistance response. Therefore, experimenta-
tion
in
the coming years
is
likely not only to deliver answers
to the questions raised
in
the present article but
also
to utilize
the results in molecular breeding approaches to improve re-
sistance
of
plants to disease.
ACKNOWLEDGMENTS
Drs. Thorsten Nürnberger and lmre E. Somssich are gratefully ac-
knowledged for critical discussions
of
the manuscript. The work in
my laboratory was supported by grant
No.
0136101A from the German
Ministry
for
Research and Technology and by grants Kn 3293 from
the Deutsche Forschungsgemeinschaft.
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... The association of fungi and plants is ancient and involves many different fungi. Fungi are an important group of plant pathogens most plant diseases are caused by fungi but fewer than 10% of all known fungi can colonize living plants (Knogge 1996) [10] . Most fungi are decomposers, utilizing the remains of plants and other organisms as their food source. ...
... The association of fungi and plants is ancient and involves many different fungi. Fungi are an important group of plant pathogens most plant diseases are caused by fungi but fewer than 10% of all known fungi can colonize living plants (Knogge 1996) [10] . Most fungi are decomposers, utilizing the remains of plants and other organisms as their food source. ...
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... Występują dwie drogi, przy użyciu których grzyb może infekować roślinę: pierwszą z nich jest penetracja warstwy kutikuli i epidermy, a drugą penetracja aparatów szparkowych przez zmodyfikowane strzępki (Carlile i inni, 2001). Penetracja kutikuli jest kluczowym czynnikiem, dzięki któremu może dojść do infekcji (Knogge, 1996) i jest to pierwszy etap infekcji. ...
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... As mentioned above, some fungi are facultative parasites and may attack plant roots from a saprophytic base in the soil through the mycelium, progressively causing the death of the host and thereafter living as saprophytes (Zhou and Hyde, 2001). Some fungi have developed other mechanisms to colonize plant hosts, such as via specialized penetration organs, called appressoria, or via penetrating through wounds or natural openings, such as stomata (Knogge, 1996). ...
... As different diseases lead to specific physiological changes of the plant, a differentiation between these diseases can be possible on the basis of reflectance patterns (Jones and Dangl, 2006). Perthotrophic fungi like C. beticola penetrate leaves through the stoma and produce toxins and hydrolytic enzymes, which lead to cell death and consequent nutrient uptake of the fungi (Knogge, 1996). As a consequence of this, large necrotic tissue develops over time. ...
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... But sometimes, pathogens bypass these systems by releasing chemicals that inhibit these receptors or circumvent the membrane system by using a vector host (Liu et al. 2020). Upon recognizing the pathogen, plants activate numerous defense related genes, produce reactive oxygen species (ROS), undergo phosphorylation of proteins and change their ionic flux to induce SAR (Knogge 1996). ...
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... The genus Fusarium is a class of filamentous fungi that includes endophytes, saprophytes, and pathogens (Knogge, 1996). It is most notable for a devastating phytopathogenic fungus that causes severe losses in many economically important crops (Pietro et al., 2003;Michielse and Rep, 2010;Sharma et al., 2018). ...
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... partial pruning), that also reported a low yield and quality of agarwood resin (Liu et al. 2013). Fungi infect plants by secreting cellulases, pectinases, and proteases that degrade plant defense structures, such as the cuticle and cell walls (Knogge 1996). In response to this, plant cells activate defense mechanisms and repair programs, including the production of aromatic resins (Sen et al. 2017). ...
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... This region is highly favorable for hazelnut due to the hot weather and high humidity. These conditions, however, are also favorable to several pathogens including fungal phytopathogens (Knogge, 1996). Among these severe pathogens, Erysiphe corylacearum U. Braun & S. Takam (Fam. ...
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... Hurricane was released as a moderately ascochyta-resistant cultivar but within a year, aggressive A. lentis isolates were collected from Hurricane crops (Blake et al., 2017 (Hahn et al., 1993;Knogge, 1996;Rohe et al., 1995). The avirulence gene from Cladosporium fulvum, Avr5, similarly acts as an avirulence effector in resistant host cultivars and as a virulence factor in tomato cultivars that lack the Cf-5 resistance gene (Mesarich et al., 2014). ...
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Full-text available
Ascochyta lentis is a fungal pathogen that causes ascochyta blight in the important grain legume species lentil, but little is known about the molecular mechanism of disease or host specificity. We employed a map‐based cloning approach using a biparental A. lentis population to clone the gene AlAvr1‐1 that encodes avirulence towards the lentil cultivar PBA Hurricane XT. The mapping population was produced by mating A. lentis isolate P94‐24, which is pathogenic on the cultivar Nipper and avirulent towards Hurricane, and the isolate AlKewell, which is pathogenic towards Hurricane but not Nipper. Using agroinfiltration, we found that AlAvr1‐1 from the isolate P94‐24 causes necrosis in Hurricane but not in Nipper. The homologous corresponding gene in AlKewell, AlAvr1‐2, encodes a protein with amino acid variation at 23 sites and four of these sites have been positively selected in the P94‐24 branch of the phylogeny. Loss of AlAvr1‐1 in a gene knockout experiment produced a P94‐24 mutant strain that is virulent on Hurricane. Deletion of AlAvr1‐2 in AlKewell led to reduced pathogenicity on Hurricane, suggesting that the gene may contribute to disease in Hurricane. Deletion of AlAvr1‐2 did not affect virulence for Nipper and AlAvr1‐2 is therefore not an avirulence gene for Nipper. We conclude that the hemibiotrophic pathogen A. lentis has an avirulence effector, AlAvr1‐1, that triggers a hypersensitive resistance response in Hurricane. This is the first avirulence gene to be characterized in a legume pathogen from the Pleosporales and may help progress research on other damaging Ascochyta pathogens. The avirulence effector protein AlAvr1‐1 (g2688‐P) produced by Ascochyta lentis isolate P94‐24 induces necrosis and confers resistance in the lentil cultivar PBA Hurricane XT.
Article
Macrophomina phaseolina is a soil-borne fungal pathogen that incites charcoal rot in more than 500 plant species including melon, Cucumis melo. Disease incidence and severity are affected by host genetic background, plant age, temperature, and water economy. The non-genetic variation in disease severity exhibited by infected melon plants poses a challenge to breeding for resistance. The objective of this investigation was to advance toward a fast and reliable screening methodology for identifying resistant melon germplasm, to facilitate breeding for resistance to M. phaseolina. To achieve this goal, plants of 25 melon accessions were inoculated with M. phaseolina using the toothpick method and the plants were defined as resistant or susceptible to the pathogen based on degree of disease severity. Young plants of the melon accessions were tested in a plastic greenhouse, a glass greenhouse, and a growth chamber to assess the effects of the environment, primarily temperature, on disease severity. Also tested were maturing field-grown plants as well as branches that were detached from them and inoculated in the laboratory. Differences in disease severity among accessions were most evident under the high temperatures of the glasshouse experiments conducted in mid-summer. Two accessions, Qishu Meshullash and PI 164323, were the most consistently resistant to M. phaseolina over the wide range of environmental conditions posed by the present experiments. When only the six most resistant and six most susceptible accessions were considered, moderate to high correlations (r = 0.62–0.96) in disease severity were observed between the field-grown plants and their detached branches. Results from a half-diallele crossing scheme involving six of the accessions indicated that resistance of melon to M. phaseolina has both, dominant and additive components.
Article
A pathogenic fungus of pea, Mycosphaerella pinodes, secretes a so-called "suppressor" in its pycnospore germination fluid. The suppressor blocks the defense responses and induces local susceptibility (accessibility) in pea plants to agents that are not pathogenic in pea. The suppressor nonspecifically inhibits the ATPase activity in plasma membranes prepared from pea, soybean, kidney bean, cowpea and barley plants. However, cytochemical studies by electron microscopy indicate that the suppressor specifically inhibits the ATPase in pea cell membranes, but not in those of four other plant species tested. That is, the specificity of the suppressor appears at the cell and/or tissue level, but is not evident in vitro. Furthermore, the inhibitory effect of the suppressor is temporary because the ATPase activity recovers 9 h after the treatment. A similar effect was observed after inoculation with M. pinodes but not with a nonpathogen of pea, M. ligulicola. The role of the suppressor in host-parasite specificity is discussed.
Chapter
The initial process of attachment of fungal propagules to a host plant is essential to the successful establishment of pathogenesis. Attachment may be involved in recognition of the host surface, serve as a base around which the infection court can be altered, and may include adhesion of the propagule. It was considered initially that attachment was purely a chance event resulting from physical entrapment of the propagule or germling. We know now that attachment involves an active process of secretion of adhesive materials by the fungus that in some cases are highly specific for the recognition of, and binding to, a particular host species. Adhesive production may occur at a specific stage of conidium or germling development, but may best be considered a general phenomenon for the establishment of the fungus prior to penetration.
Chapter
Fungi are the most important group of pathogens on plants from an economic point of view. This results in part from the fact that fungi are extremely varied in the ways they attack plants and actively move through host tissues, forcibly breaching barriers by enzymatic and/or physical mechanisms. In addition, fungal pathogens exhibit a complex array of tropic responses which may contribute to complex infection processes and are important in determining host range. Fungal interactions with plants are based on parasitism, a nutritional relationship, but the economic and aesthetic damage which they cause may result from the operation of other factors that constitute pathogenicity. While these two terms are frequently used interchangeably, they are distinct; indeed, it is possible to have pathogenesis without parasitism and vice versa. Although fungal diseases have been studied for over a century, we are only beginning to understand the molecular factors which determine whether a certain fungus is pathogenic on a higher plant. In this paper, I will review some of the progress that has been made, point out deficiencies in our knowledge and speculate on the future, especially on ways in which the powerful machinery of molecular genetics is being brought to bear on fungal pathogens. Because of the survey nature of the paper, references are exemplary only.
Chapter
Plants are endowed with diverse mechanisms that protect them from pathogenic microorganisms. Active defense, including formation of many chemical and physical barriers (e.g., phytoalexin, infection inhibitor, pathogenesis related proteins, lignin, callose, etc.), is considered the main part of the resistance mechanism, because negating of such defense reactions by prior treatment with several metabolic inhibitors or pre-inoculation with compatible fungi allows pathogenic fungi to invade non-host plants. Barriers induced in plant tissues after fungal invasion seem to block penetration, growth, and reproduction of the pathogen. Resistance-inducing substances called inducers or elicitors are released from spores into spore-germination fluid of both pathogenic and nonpathogenic fungi (Hayami et al., 1982; Shiraishi et al., 1978b). As far as the authors know, there is no pathogen that does not produce elicitors. Elicitors from pathogenic fungi are also able to induce resistance in their host.
Chapter
Plants are resistant to most potential pathogens in their environment; in fact, relatively few true host-pathogen pairs exist in which the plant is susceptible and the pathogen virulent. Within a susceptible plant species, specific cultivars carry genes which give them resistance to specific pathogen races harboring the corresponding avirulence genes (Chaps. 15, 16, Vol. V, Part B). Several models have been proposed to explain the biochemical basis for gene-for-gene complementarity in such plant-pathogen interactions. One model envisions a ligand-receptor-like interaction between the products of a fungal avirulence gene (elicitor) and the corresponding plant resistance gene (receptor) that triggers the initiation of a multicomponent defense response in the plant. The same response can also be activated by other types of elicitors that are not encoded by avirulence genes, including compounds released from fungal or plant cell walls during early phases of pathogen attack or substances secreted by the pathogen. Exogenous elicitors are derived from the pathogen, whereas endogenous elicitors are released from the plant cell wall during pathogen attack. Whatever the source of the elicitor, the mechanism of signal perception appears to rely on the presence of specific receptors on the plant cell surface (Fig. 1) which initiate signaling processes that activate plant defenses. Typical elements of the multicomponent defense response include the hypersensitive reaction (HR), the production of reactive oxygen species (oxidative burst), the activation of defense-related genes, structural changes of the cell wall, and the synthesis of phytoalexins.
Article
By definition, a susceptible plant is one that cannot successfully defend itself against pathogen attack. Such a situation is relatively rare, and anyone plant species is commonly susceptible to infection by only a few of the thousands of micro-organisms known to cause disease in higher plants. Nevertheless, susceptibility to only one pathogen is sufficient to cause unacceptable yield losses in a commercial crop, and it is this that plant pathologists are ultimately trying to prevent. Thus it is important to know why defense mechanisms which apparently confer resistance of a plant towards one plant pathogen seem unable to control infection by another.
Article
The interaction between tomato and C. fulvum has proven to be an excellent model system to dissect a host-pathogen interaction that complies with the gene-for-gene model at the molecular level. Cloned avirulence and resistance genes provide valuable tools to study the molecular genetic mechanisms determining race-specific resistance. The transposon tagging approach, which has been very effective in bacteria to identify and characterize different types of genes, can also be employed to isolate fungal avirulence genes, as active transposons have been described in numerous fungi, such as Fusarium oxysporum. The interaction between the biotrophic fungus Cladosporium fulvum and tomato (Lycopersicon esculentum), has been studied extensively as a model system for plant-fungus gene-for-gene relationships. The fungal pathogen Magnaporthe grisea causes blast of rice and different grass species. The molecular basis of host specificity in the rice blast disease has been studied intensively in recent years. Many Phytophthora species produce a family of small extracellular proteins of ca. 10 kDa named elicitins, which induce necrosis and other defense responses particularly on tobacco and some radish and turnip cultivars. Transposon tagging involves the inactivation of a resistance gene by insertion of a transposable element.