portable SAR signals, though SA itself does not get translocated (Maldonado et al.
2002; Champigny et al. 2011). However, the several metabolites that are putatively
considered to be involved in long-distance SAR signaling are methyl ester of SA
(MeSA), diterpenoid dehydroabietinal (DA), a glycerol-3-phosphate
(G3P)-dependent factor, azelaic acid (AzA), and pipecolic acid (Pip). Then a
FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1) transduces or amplifies
these long-distance signals in the systemic tissues (Mishina and Zeier 2006).
In SAR, other known signaling molecules involved is defense priming, in which
mitogen-activated protein kinases (MAPKs) such as MPK3 and MPK6 get
accumulated after pathogen infection, resulting in potentiated PR-1 gene expression,
callose formation, and systemic immunity (Beckers et al. 2009). Moreover, SAR
priming involves chromatin modifications in the promoters of WRKY transcription
factor genes that cause regulation of SA-dependent defenses from pathogens
(Jaskiewicz et al. 2011) (Fig. 10.1).
10.6 ISR Signaling
In plants, PGPM induce a resistance against various phytopathogens after an initial
infection and for forthcoming attacks. This forms the PGPM escorted ISR, facilitated
by production of allelochemicals (such as siderophores and antibiotics that effectively inhibit pathogen growth), and competition for ecotype and nutrient
(Choudhary and Johri 2009; Jain et al. 2013). Plants can procure a state of ISR to
a broad spectrum of disease causing pathogens with trivial effects on yield and
growth, subsequent to an interface with different PGPB including Pseudomonas
fluorescens, Pseudomonas putida, Bacillus pumilus, Serratia marcescens,
Paenibacillus alvei, Acinetobacter lwoffii, Chryseobacterium balustinum, and
Azospirillum brasilense (Van Hulten et al. 2006; Van Loon 2007). Similarly, the
PGPF Piriformospora indica that colonizes the roots of many plants confers disease
resistance systemically in response to pathogen attack by stimulating the host to
synthesize phosphatidic acid and triggering the OXI1 pathway (Camehl et al. 2011).
10.6.1 Elicitors of ISR
ISR-inducing beneficial microbes produce various elicitors that are necessary for the
commencement of ISR. Elicitation of ISR shows similarities to certain non-specific
plant defense reactions to the common MAMPs such as cell surface components of
lipopolysaccharides (LPS) and flagella (Erbs and Newmann 2003). Moreover,
diverse ISR occasioning determinants including effectors and volatiles have been
identified for certain rhizobacterial strains. For instance, LPS, iron-regulated
metabolites pyoverdin and SA, antibiotics, such as 2,4-diacetylphloroglucinol
(DAPG) and pyocyanin, flagella, N-acyl homoserine lactones, and biosurfactants
are considered to be involved in PGPR induced ISR (De Vleesschauwer and Hofte
2009). Furthermore, volatile organic compounds such as 2R- and 3R-butanediol
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
305
2002; Champigny et al. 2011). However, the several metabolites that are putatively
considered to be involved in long-distance SAR signaling are methyl ester of SA
(MeSA), diterpenoid dehydroabietinal (DA), a glycerol-3-phosphate
(G3P)-dependent factor, azelaic acid (AzA), and pipecolic acid (Pip). Then a
FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1) transduces or amplifies
these long-distance signals in the systemic tissues (Mishina and Zeier 2006).
In SAR, other known signaling molecules involved is defense priming, in which
mitogen-activated protein kinases (MAPKs) such as MPK3 and MPK6 get
accumulated after pathogen infection, resulting in potentiated PR-1 gene expression,
callose formation, and systemic immunity (Beckers et al. 2009). Moreover, SAR
priming involves chromatin modifications in the promoters of WRKY transcription
factor genes that cause regulation of SA-dependent defenses from pathogens
(Jaskiewicz et al. 2011) (Fig. 10.1).
10.6 ISR Signaling
In plants, PGPM induce a resistance against various phytopathogens after an initial
infection and for forthcoming attacks. This forms the PGPM escorted ISR, facilitated
by production of allelochemicals (such as siderophores and antibiotics that effectively inhibit pathogen growth), and competition for ecotype and nutrient
(Choudhary and Johri 2009; Jain et al. 2013). Plants can procure a state of ISR to
a broad spectrum of disease causing pathogens with trivial effects on yield and
growth, subsequent to an interface with different PGPB including Pseudomonas
fluorescens, Pseudomonas putida, Bacillus pumilus, Serratia marcescens,
Paenibacillus alvei, Acinetobacter lwoffii, Chryseobacterium balustinum, and
Azospirillum brasilense (Van Hulten et al. 2006; Van Loon 2007). Similarly, the
PGPF Piriformospora indica that colonizes the roots of many plants confers disease
resistance systemically in response to pathogen attack by stimulating the host to
synthesize phosphatidic acid and triggering the OXI1 pathway (Camehl et al. 2011).
10.6.1 Elicitors of ISR
ISR-inducing beneficial microbes produce various elicitors that are necessary for the
commencement of ISR. Elicitation of ISR shows similarities to certain non-specific
plant defense reactions to the common MAMPs such as cell surface components of
lipopolysaccharides (LPS) and flagella (Erbs and Newmann 2003). Moreover,
diverse ISR occasioning determinants including effectors and volatiles have been
identified for certain rhizobacterial strains. For instance, LPS, iron-regulated
metabolites pyoverdin and SA, antibiotics, such as 2,4-diacetylphloroglucinol
(DAPG) and pyocyanin, flagella, N-acyl homoserine lactones, and biosurfactants
are considered to be involved in PGPR induced ISR (De Vleesschauwer and Hofte
2009). Furthermore, volatile organic compounds such as 2R- and 3R-butanediol
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
305