Phytopathogenic bacteria are often facultative parasites, and
an inoculum threshold is generally required for infection
(10
4 cells of P. syringae per g of bean leaf). Some protozoa
including Phytomonas are phytopathogens. These
trypanosomes are responsible for heart-rot disease of coconut palms, and they can be transmitted back and forth from
host plants to insects. Finally, some viruses such as poxvirus
cause significant damages, but most of the time, they need
vectors to infect their host plants. For instance, parasitic
nematodes (e.g., Xiphinema spp.) serve as vectors of viruses
such as the grapevine fanleaf virus (GFLV). Moreover, the
wound caused by stylet penetration of the parasitic
nematodes may also facilitate the plant infection by fungal
(Verticillium), oomycete (Phytophthora), or bacterial
(Clavibacter michiganense) pathogens. Besides nematodes,
the main virus vectors are Hemiptera insects, but some
viruses can also be transmitted to the plant by fungi
(Olpidium spp.) or protists (Polymyxa betae) or between
plants through parasitic plants such as dodder (Agrios 1997).
Infection Processes
Infection of the plant is done in several steps. The contact of
the plant with spores (constituting the primary inoculum) of
fungal pathogens involves, as presented above, several environmental factors including wind, water, insects, and microfauna that vectorize the pathogens, but also the biochemical
and mechanical infectious potential of these pathogens. Production of fungal mucilage consisting of polysaccharides and/
or glycoproteins ensures the adhesion of spores and germ tubes
to the plant tissues. Enzymatic hydrolysis of the cuticle surface
(U. necator on grapevine) or production of small proteins such
as hydrophobins Mgp1 (Magnaporthe grisea on rice)
enhances the adhesion of the propagules to the plant (Ebbole
2007). In most cases, there is no specific site of infection on the
surface of the plant. Fungi penetrate the aerial part of the plant
by piercing the cuticle, growing through the anthesis,
exploiting wounds (opportunistic fungi), or growing between
the stomata guard cells. The fungal germ tube generally forms
an appressorium that is simply a swelling of the tip of the
hypha (B. cinerea) or results from cell differentiation (case of
rusts) to puncture through plant cell wall using high physical
pressure, cell wall degrading enzymes, or both. In the case of
pathogenic bacteria too, penetration occurs most often through
natural openings, such as nectaries of flowers (E. amylovora),
stomata or hydathodes (X. campestris), or wounds (P.
syringae). Their lytic enzymes play an important role in virulence (Kazemi-Pour et al. 2004). Quorum sensing is sometimes involved in the infectious process (Smadja et al. 2004).
Different types of trophic relationships can then be established. Some fungi such as Venturia inaequalis, responsible
for apple scab, establish under the leaf cuticle and use epidermal walls as trophic base thanks to pectinases they produce.
Others, such as powdery mildews, exploit the cytoplasm of
epidermal cells thanks to a sucker they develop from the
surface of the leaf. In rust or mildew agents, colonization of
a first cell is followed by the development of secondary hyphae
that gradually colonize other cells. Similar strategies are
observed among soil-borne fungi and oomycetes when
penetrating the plant root and crown. For instance, no specific
site of infection was found for F. oxysporum attacking tomato
roots (Olivain et al. 2006), and a set of extracellular enzymes is
produced by the pathogens to degrade cellulose, hemicelluloses, and pectins of the plant epidermis and parenchyma
leading to necroses, generalized rot, or tracheomycoses.
Interplay with Plant Defense Systems
As we saw previously, plants interact with many
microorganisms, some of which being neutral or beneficial,
while others are deleterious. It is important for plants to
recognize and discriminate between them and to be able to
respond accordingly. Conversely, it is important for beneficial and pathogenic microbes to modulate the host immune
system to establish an intimate relationship or to prevent
defense reactions. The plant, for instance, can respond to
the presence of pathogens by the production of volatile
compounds that will either promote or block microbial
development, as for B. cinerea on strawberry (AbandaNkpwatt et al. 2006). It now appears that plants rely on the
innate immunity of each cell and on systemic signals
emanating from the infection site. This immune system
allows them to recognize and respond specifically to invading pathogens (Jones and Dangl 2006).
Upon infection, compounds such as lipopolysaccharides,
flagellin, peptidoglycans, and chitin are released from the
pathogens. These microbial elicitors called microbeassociated molecular patterns (MAMPs), also referred to as
pathogen-associated molecular patterns (PAMPs), are
recognized at the surface of the host cell by receptor proteins
called pattern recognition receptors (PRRs). MAMPs are basic
components of all classes of pathogens. They may be
constituents of the wall of the pathogen (exogenous elicitors)
that are recognized by receptors. They may also correspond to
the constituents of the plant cell wall that are released after
degradation by microbial enzymes and recognized by
receptors. A number of these common MAMPs and their
role in the elicitation process have been recently described
(Pel and Pieterse 2013). Recognition by the plant of these nonself compounds is a first step toward an effective immune
response. PRRs generally consist of an extracellular leucinerich repeat (LRR) domain and an intracellular kinase domain.
Stimulation of PRRs leads to PAMP-triggered immunity
(PTI), which provides a first line of defense against most
nonspecific pathogens (Jones and Dangl 2006).
The second line of defense involves recognition by intracellular receptors of pathogen virulence molecules called
effectors. The pathogens deliver effectors into the host cell
by type III secretion (for bacteria) or using haustoria or other
intracellular structures by an unknown mechanism (for fungi
11 Microorganisms and Biotic Interactions
413
an inoculum threshold is generally required for infection
(10
4 cells of P. syringae per g of bean leaf). Some protozoa
including Phytomonas are phytopathogens. These
trypanosomes are responsible for heart-rot disease of coconut palms, and they can be transmitted back and forth from
host plants to insects. Finally, some viruses such as poxvirus
cause significant damages, but most of the time, they need
vectors to infect their host plants. For instance, parasitic
nematodes (e.g., Xiphinema spp.) serve as vectors of viruses
such as the grapevine fanleaf virus (GFLV). Moreover, the
wound caused by stylet penetration of the parasitic
nematodes may also facilitate the plant infection by fungal
(Verticillium), oomycete (Phytophthora), or bacterial
(Clavibacter michiganense) pathogens. Besides nematodes,
the main virus vectors are Hemiptera insects, but some
viruses can also be transmitted to the plant by fungi
(Olpidium spp.) or protists (Polymyxa betae) or between
plants through parasitic plants such as dodder (Agrios 1997).
Infection Processes
Infection of the plant is done in several steps. The contact of
the plant with spores (constituting the primary inoculum) of
fungal pathogens involves, as presented above, several environmental factors including wind, water, insects, and microfauna that vectorize the pathogens, but also the biochemical
and mechanical infectious potential of these pathogens. Production of fungal mucilage consisting of polysaccharides and/
or glycoproteins ensures the adhesion of spores and germ tubes
to the plant tissues. Enzymatic hydrolysis of the cuticle surface
(U. necator on grapevine) or production of small proteins such
as hydrophobins Mgp1 (Magnaporthe grisea on rice)
enhances the adhesion of the propagules to the plant (Ebbole
2007). In most cases, there is no specific site of infection on the
surface of the plant. Fungi penetrate the aerial part of the plant
by piercing the cuticle, growing through the anthesis,
exploiting wounds (opportunistic fungi), or growing between
the stomata guard cells. The fungal germ tube generally forms
an appressorium that is simply a swelling of the tip of the
hypha (B. cinerea) or results from cell differentiation (case of
rusts) to puncture through plant cell wall using high physical
pressure, cell wall degrading enzymes, or both. In the case of
pathogenic bacteria too, penetration occurs most often through
natural openings, such as nectaries of flowers (E. amylovora),
stomata or hydathodes (X. campestris), or wounds (P.
syringae). Their lytic enzymes play an important role in virulence (Kazemi-Pour et al. 2004). Quorum sensing is sometimes involved in the infectious process (Smadja et al. 2004).
Different types of trophic relationships can then be established. Some fungi such as Venturia inaequalis, responsible
for apple scab, establish under the leaf cuticle and use epidermal walls as trophic base thanks to pectinases they produce.
Others, such as powdery mildews, exploit the cytoplasm of
epidermal cells thanks to a sucker they develop from the
surface of the leaf. In rust or mildew agents, colonization of
a first cell is followed by the development of secondary hyphae
that gradually colonize other cells. Similar strategies are
observed among soil-borne fungi and oomycetes when
penetrating the plant root and crown. For instance, no specific
site of infection was found for F. oxysporum attacking tomato
roots (Olivain et al. 2006), and a set of extracellular enzymes is
produced by the pathogens to degrade cellulose, hemicelluloses, and pectins of the plant epidermis and parenchyma
leading to necroses, generalized rot, or tracheomycoses.
Interplay with Plant Defense Systems
As we saw previously, plants interact with many
microorganisms, some of which being neutral or beneficial,
while others are deleterious. It is important for plants to
recognize and discriminate between them and to be able to
respond accordingly. Conversely, it is important for beneficial and pathogenic microbes to modulate the host immune
system to establish an intimate relationship or to prevent
defense reactions. The plant, for instance, can respond to
the presence of pathogens by the production of volatile
compounds that will either promote or block microbial
development, as for B. cinerea on strawberry (AbandaNkpwatt et al. 2006). It now appears that plants rely on the
innate immunity of each cell and on systemic signals
emanating from the infection site. This immune system
allows them to recognize and respond specifically to invading pathogens (Jones and Dangl 2006).
Upon infection, compounds such as lipopolysaccharides,
flagellin, peptidoglycans, and chitin are released from the
pathogens. These microbial elicitors called microbeassociated molecular patterns (MAMPs), also referred to as
pathogen-associated molecular patterns (PAMPs), are
recognized at the surface of the host cell by receptor proteins
called pattern recognition receptors (PRRs). MAMPs are basic
components of all classes of pathogens. They may be
constituents of the wall of the pathogen (exogenous elicitors)
that are recognized by receptors. They may also correspond to
the constituents of the plant cell wall that are released after
degradation by microbial enzymes and recognized by
receptors. A number of these common MAMPs and their
role in the elicitation process have been recently described
(Pel and Pieterse 2013). Recognition by the plant of these nonself compounds is a first step toward an effective immune
response. PRRs generally consist of an extracellular leucinerich repeat (LRR) domain and an intracellular kinase domain.
Stimulation of PRRs leads to PAMP-triggered immunity
(PTI), which provides a first line of defense against most
nonspecific pathogens (Jones and Dangl 2006).
The second line of defense involves recognition by intracellular receptors of pathogen virulence molecules called
effectors. The pathogens deliver effectors into the host cell
by type III secretion (for bacteria) or using haustoria or other
intracellular structures by an unknown mechanism (for fungi
11 Microorganisms and Biotic Interactions
413