parts) by utilizing specific mechanisms (Huang et al. 2011; Reinhold-Hurek and
Hurek 2011).
The endophytes prior to penetration form a biofilm on the root surface, in which a
single microbial cell adheres to the surface, multiplies to form multiple
microcolonies, which are linked and embedded in a matrix of extracellular
exopolysaccharides (Fig. 10.2). These biofilms provide protection from external
stress, decrease microbial competition, and promote the host-plant growth (Ramey
et al. 2004). The formation of biofilms further involves chemical signals among
bacteria by quorum sensing, which allows microbial communities to respond fervently as a synchronized single unit to inhibit hostile organisms, enhance nutrient
uptake, adapt to altering environmental conditions, and controls bacterial size and
population. These diffusible signals can be N-acyl-homoserine lactones (AHLs),
2-heptyl-3-hydroxy-4-quinoline and autoinducer-2 in Proteobacteria, gammabutyrolactones in Streptomyces, cis-11-methyl-2-dodecanoic acid in Xanthomonas,
and oligopeptides in Gram-positive bacteria (Danhorn and Fuqua 2007). Further,
swarming, a common property of motile PGPR such as strains of genera, Bacillus
and Pseudomonas occurs, in which the bacterial cells translocate on a surface-linked
with neighboring bacteria by extensive flagella in a coordinated manner that
facilitates in root-colonization (Tremblay et al. 2007; Oura et al. 2015). Besides,
Trichoderma spp. form certain structures analogous to the appressorium of plantpathogenic fungi for colonizing the root hairs (Mukherjee et al. 2013). In Bacillus
subtilis for root colonization, there is a stimulation and expression of genes involved
in biofilm matrix production, in response to the polysaccharides exuded from host
plant cell walls that function as signaling molecules (Beauregard et al. 2013). This
biofilm matrix is considered as the mutualistic interface in which bacterial cells
integrate both self-derived and host solutes and chemical signals for the coordination
of plant growth promotion, nutrition, and ISR (Fig. 10.2).
The further entry of PGPR in the root takes place by root hairs, root apex, or
cracks in the newly emerged lateral roots, which is facilitated by cell wall-degrading
cellulase and pectinase exo-enzymes (Reinhold-Hurek and Hurek 2011). After
colonization, Pseudomonas, Bacillus, and Trichoderma strains can initiate an
auxin-dependent plant growth-promoting activity including increased root hair
length, abundant lateral root formation, and enhanced plant biomass production,
though these can function independently from the triggering of ISR (Zhang et al.
2007; Contreras-Cornejo et al. 2009; Zamioudis et al. 2013).
10.6.3 Suppression of Plant PTI or ETI
Colonization of roots further necessitates local suppression of PTI or ETI to protect
the PGPR against MAMP or effector triggered production of antimicrobial
compounds (Figs. 10.2 and 10.3). The ISR-inducing beneficial microbes must
suppress and evade plant immune responses to establish a prolonged mutualism
with the host (Wang et al. 2012; Zamioudis and Pieterse 2012). For suppressing the
ETI responses and promoting fungal biotrophy, the AMF Rhizophagus intraradices
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
307
Hurek 2011).
The endophytes prior to penetration form a biofilm on the root surface, in which a
single microbial cell adheres to the surface, multiplies to form multiple
microcolonies, which are linked and embedded in a matrix of extracellular
exopolysaccharides (Fig. 10.2). These biofilms provide protection from external
stress, decrease microbial competition, and promote the host-plant growth (Ramey
et al. 2004). The formation of biofilms further involves chemical signals among
bacteria by quorum sensing, which allows microbial communities to respond fervently as a synchronized single unit to inhibit hostile organisms, enhance nutrient
uptake, adapt to altering environmental conditions, and controls bacterial size and
population. These diffusible signals can be N-acyl-homoserine lactones (AHLs),
2-heptyl-3-hydroxy-4-quinoline and autoinducer-2 in Proteobacteria, gammabutyrolactones in Streptomyces, cis-11-methyl-2-dodecanoic acid in Xanthomonas,
and oligopeptides in Gram-positive bacteria (Danhorn and Fuqua 2007). Further,
swarming, a common property of motile PGPR such as strains of genera, Bacillus
and Pseudomonas occurs, in which the bacterial cells translocate on a surface-linked
with neighboring bacteria by extensive flagella in a coordinated manner that
facilitates in root-colonization (Tremblay et al. 2007; Oura et al. 2015). Besides,
Trichoderma spp. form certain structures analogous to the appressorium of plantpathogenic fungi for colonizing the root hairs (Mukherjee et al. 2013). In Bacillus
subtilis for root colonization, there is a stimulation and expression of genes involved
in biofilm matrix production, in response to the polysaccharides exuded from host
plant cell walls that function as signaling molecules (Beauregard et al. 2013). This
biofilm matrix is considered as the mutualistic interface in which bacterial cells
integrate both self-derived and host solutes and chemical signals for the coordination
of plant growth promotion, nutrition, and ISR (Fig. 10.2).
The further entry of PGPR in the root takes place by root hairs, root apex, or
cracks in the newly emerged lateral roots, which is facilitated by cell wall-degrading
cellulase and pectinase exo-enzymes (Reinhold-Hurek and Hurek 2011). After
colonization, Pseudomonas, Bacillus, and Trichoderma strains can initiate an
auxin-dependent plant growth-promoting activity including increased root hair
length, abundant lateral root formation, and enhanced plant biomass production,
though these can function independently from the triggering of ISR (Zhang et al.
2007; Contreras-Cornejo et al. 2009; Zamioudis et al. 2013).
10.6.3 Suppression of Plant PTI or ETI
Colonization of roots further necessitates local suppression of PTI or ETI to protect
the PGPR against MAMP or effector triggered production of antimicrobial
compounds (Figs. 10.2 and 10.3). The ISR-inducing beneficial microbes must
suppress and evade plant immune responses to establish a prolonged mutualism
with the host (Wang et al. 2012; Zamioudis and Pieterse 2012). For suppressing the
ETI responses and promoting fungal biotrophy, the AMF Rhizophagus intraradices
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
307