led to the definition of resistant plant genotypes and
adapted pathogenic fungal races (Takken and Rep, 2010)
(cf. Box 11.3). The gene-for-gene relationship is well known
and exists in many host–pathogen interactions, but in
F. oxysporum, such interactions occur within a formae
specialis (Rep et al. 2005). In rare cases, the formae speciales
are quite homogenous. Finally, the same F. oxysporum species also includes nonpathogenic strains (Edel et al. 2001).
Phytopathogen Control
Control of microbial pathogens of crops relies on a combination of various methods. Fungicides, which represent
about 20 % of the pesticides used in agriculture, are ineffective against soil-borne bio-aggressors, and for human health
and environmental quality, their use must be reduced. Prophylaxis should be preferred. This involves the regular
cleaning of farm machinery and irrigation systems, but also
the removal of dead plant matter from fields as well as the
management (burial) of crop residues to minimize potential
reservoirs for the plant pathogens. Similarly, many
pathogens can live on and in seeds. Therefore, seed
companies must ensure the quality of plant seeds. Innovative
farming practices should be promoted. This includes new
rotation schemes (longer and more diverse than the present
ones), the use of intermediate crops having sanitizing
impact, and appropriate tillage systems to preserve both
soil structure and biological activity (multitrophic
interactions), so that pathogen populations decline between
host plantings. These practices should be combined with the
use of resistant cultivars when possible as well as biological
control agents if any in given conditions. Based on what was
shown previously, these control methods (when used) can
only limit the extent of damage (Box 11.2).
11.3.5.3 Symbiosis
Certain bacteria and fungi form a mutualistic symbiosis with
plants (Cardon and Whitbeck 2007). Those between bacteria
and plants consist mainly of nitrogen-fixing symbioses involving the floating fern Azolla (with the cyanobacterium
Anabaena), phylogenetically diverse plants called actinorhizal
(with the actinobacterium Frankia), and Fabaceae (with
selected alphaproteobacteria, such as Rhizobium, and selected
betaproteobacteria) (Mathesius 2003; Huguet et al. 2005). The
Rhizobium–Fabaceae dialogue involves molecular signals
produced by the plant (including flavonoids) and leads to
bacterial synthesis of a lipo-chito-oligosaccharide termed
Nod factor, which is a signal inducing root nodulation
(Mathesius 2003) (Fig. 11.10a). Host specificity depends
largely on the chemical specificity of these two types of
signals. In the case of Frankia and of photosynthetic
Bradyrhizobium, the bacterial signal involved is different,
and indeed their genomes are devoid of canonical nod genes
(Giraud et al. 2007; Normand et al. 2007). The Fabaceae
nodule is formed by proliferation of cells from the cortex and
the pericycle. The bacteria usually enter the plant through root
hairs, which are deformed during the interaction, and find
themselves in an infection thread of plant origin. This thread
enables proliferation and delivery of bacteria to plant cells in
the nodule. Once endocytosed by nodule plant cells, the bacteria differentiate into bacteroids but remain separated from
the cytoplasm of the plant cell by the peribacteroid membrane.
In Medicago truncatula, this transformation into bacteroids is
triggered by nodule-specific cysteine-rich peptides resembling
antimicrobials from the plant innate immune response (Van de
Velde et al. 2010). The peribacteroid membrane and the bacteroid(s) it contains constitute the symbiosome. The nitrogen
fixed by the bacteroids will serve as nitrogen source for the
plant, which in return provides a source of carbon and energy
(25–35 ATP necessary for each N 2 reduced) in the form of
dicarboxylic acids (mainly malate). The nitrogenase is sensitive to oxygen but is protected by low oxygen diffusion within
Box 11.3: Resistance and Host Specificity
in Plant–Pathogen Relationships
Christian Steinberg
In fungi and bacteria, isolates of the same species
causing the same symptoms on a particular plant genotype, whether a cultivated (cultivar) or wild variety,
are a race.
The concepts of race and variety result from a
pathogen–plant coevolution. Indeed, the emergence
of a resistance gene in the host plant gives rise to a
new variety, which will no longer be infested by the
original pathogen (race 0), as is the initial variety. The
appearance by mutation of a gene for virulence in the
pathogen, allowing it to circumvent this resistance,
gives birth to race 1, capable of infecting both
varieties, while race 0 is limited to the first variety,
and so on.
The nature of the resistance of the host plant can be
divided into two types. In horizontal resistance, many
genes are involved (polygenic resistance). Vertical
resistance in a single gene gives the plant (cultivar or
variety) total resistance to a very specific biotype
(race) of the parasite, but the variety concerned is
sensitive to other biotypes of the pathogen. Any biotype able to overcome a vertical resistance factor is
said to be virulent.
11 Microorganisms and Biotic Interactions
415
adapted pathogenic fungal races (Takken and Rep, 2010)
(cf. Box 11.3). The gene-for-gene relationship is well known
and exists in many host–pathogen interactions, but in
F. oxysporum, such interactions occur within a formae
specialis (Rep et al. 2005). In rare cases, the formae speciales
are quite homogenous. Finally, the same F. oxysporum species also includes nonpathogenic strains (Edel et al. 2001).
Phytopathogen Control
Control of microbial pathogens of crops relies on a combination of various methods. Fungicides, which represent
about 20 % of the pesticides used in agriculture, are ineffective against soil-borne bio-aggressors, and for human health
and environmental quality, their use must be reduced. Prophylaxis should be preferred. This involves the regular
cleaning of farm machinery and irrigation systems, but also
the removal of dead plant matter from fields as well as the
management (burial) of crop residues to minimize potential
reservoirs for the plant pathogens. Similarly, many
pathogens can live on and in seeds. Therefore, seed
companies must ensure the quality of plant seeds. Innovative
farming practices should be promoted. This includes new
rotation schemes (longer and more diverse than the present
ones), the use of intermediate crops having sanitizing
impact, and appropriate tillage systems to preserve both
soil structure and biological activity (multitrophic
interactions), so that pathogen populations decline between
host plantings. These practices should be combined with the
use of resistant cultivars when possible as well as biological
control agents if any in given conditions. Based on what was
shown previously, these control methods (when used) can
only limit the extent of damage (Box 11.2).
11.3.5.3 Symbiosis
Certain bacteria and fungi form a mutualistic symbiosis with
plants (Cardon and Whitbeck 2007). Those between bacteria
and plants consist mainly of nitrogen-fixing symbioses involving the floating fern Azolla (with the cyanobacterium
Anabaena), phylogenetically diverse plants called actinorhizal
(with the actinobacterium Frankia), and Fabaceae (with
selected alphaproteobacteria, such as Rhizobium, and selected
betaproteobacteria) (Mathesius 2003; Huguet et al. 2005). The
Rhizobium–Fabaceae dialogue involves molecular signals
produced by the plant (including flavonoids) and leads to
bacterial synthesis of a lipo-chito-oligosaccharide termed
Nod factor, which is a signal inducing root nodulation
(Mathesius 2003) (Fig. 11.10a). Host specificity depends
largely on the chemical specificity of these two types of
signals. In the case of Frankia and of photosynthetic
Bradyrhizobium, the bacterial signal involved is different,
and indeed their genomes are devoid of canonical nod genes
(Giraud et al. 2007; Normand et al. 2007). The Fabaceae
nodule is formed by proliferation of cells from the cortex and
the pericycle. The bacteria usually enter the plant through root
hairs, which are deformed during the interaction, and find
themselves in an infection thread of plant origin. This thread
enables proliferation and delivery of bacteria to plant cells in
the nodule. Once endocytosed by nodule plant cells, the bacteria differentiate into bacteroids but remain separated from
the cytoplasm of the plant cell by the peribacteroid membrane.
In Medicago truncatula, this transformation into bacteroids is
triggered by nodule-specific cysteine-rich peptides resembling
antimicrobials from the plant innate immune response (Van de
Velde et al. 2010). The peribacteroid membrane and the bacteroid(s) it contains constitute the symbiosome. The nitrogen
fixed by the bacteroids will serve as nitrogen source for the
plant, which in return provides a source of carbon and energy
(25–35 ATP necessary for each N 2 reduced) in the form of
dicarboxylic acids (mainly malate). The nitrogenase is sensitive to oxygen but is protected by low oxygen diffusion within
Box 11.3: Resistance and Host Specificity
in Plant–Pathogen Relationships
Christian Steinberg
In fungi and bacteria, isolates of the same species
causing the same symptoms on a particular plant genotype, whether a cultivated (cultivar) or wild variety,
are a race.
The concepts of race and variety result from a
pathogen–plant coevolution. Indeed, the emergence
of a resistance gene in the host plant gives rise to a
new variety, which will no longer be infested by the
original pathogen (race 0), as is the initial variety. The
appearance by mutation of a gene for virulence in the
pathogen, allowing it to circumvent this resistance,
gives birth to race 1, capable of infecting both
varieties, while race 0 is limited to the first variety,
and so on.
The nature of the resistance of the host plant can be
divided into two types. In horizontal resistance, many
genes are involved (polygenic resistance). Vertical
resistance in a single gene gives the plant (cultivar or
variety) total resistance to a very specific biotype
(race) of the parasite, but the variety concerned is
sensitive to other biotypes of the pathogen. Any biotype able to overcome a vertical resistance factor is
said to be virulent.
11 Microorganisms and Biotic Interactions
415