Ahn et al. 2007; De Vleesschauwer et al. 2008; Hossain et al. 2008; Korolev et al.
2008; Van der Ent et al. 2009b; Weller et al. 2012).
In plant-root, initiation of ISR requires certain essential signaling components.
For instance, in response to ISR-inducing PGPR Pseudomonas fluorescens
WCS417r and PGPF Trichoderma, there is a significant expression of the MYB
transcription factor gene MYB72 of the R2R3-type that encodes MYB72, a rootspecific transcription factor which functions with other signaling components and is
necessary for the instigation of ISR in the epiblema and cortex of Arabidopsis roots
(Van der Ent et al. 2008; Segarra et al. 2009). Typically, during iron-deficiency
conditions, roots extrude protons by H
+
-ATPase to make Fe(III)/Fe
3+ more soluble
by acidification of the soil environment. This is then reduced to Fe(II)/Fe
2+ by ferric
chelate reductase-FRO2 and is transported into root cells by the iron transporter,
IRT1. In Arabidopsis roots colonized by ISR-inducing rhizobacterial Pseudomonas
strains, there is a coordinated up-regulation of MYB72 with FRO2, IRT1, and other
iron deficiency-regulated genes (Pieterse et al. 2014). Alternatively, MYB72
functions in the production and/or secretion of root semiochemicals that further
stimulate ISR by PGPR. Thus, this indicates a link between iron homeostasis and
ISR initiation, as MYB72 is induced under iron-limited or distorted iron uptake
conditions (Palmer et al. 2013).
The transcriptional factor NPR1 that has essential role as coregulator of
SA-dependent PR genes in SAR is also known to be required for JA/ET-dependent
rhizobacteria-mediated ISR by many PGPR and PGPF without PR gene activation
(Fig. 10.3). For the SA signaling, NPR1 is associated within the nucleus, while for
JA/ET signaling in ISR it is deliberated to have a cytosolic function (Spoel et al.
2003; Dong 2004; Pajerowska-Mukhtar et al. 2013). In Arabidopsis, ISR is
associated with an enhanced expression of the JA/ET-responsive defense genes
VSP, PDF1.2 and HEL against the insect herbivore Spodoptera exigua (Van Wees
et al. 1999). In carnation, ISR is accompanied with increased accumulation of
phytoalexins at the site of pathogen infection (Van Peer et al. 1991) (Fig. 10.3).
Moreover, the PGPM mediated ISR is commonly based on priming, in which
whole plant is sensitized by an enhanced activation of a combination of cellular
defenses including altered gene expression and production of structural barriers
(Conrath et al. 2006; Liu et al. 2007). There can be increased frequency of callose
depositions at the infection site to effectively block pathogen access that is observed
in the ISR of Pseudomonas fluorescens WCS417r in Arabidopsis against downy
mildew pathogen Hyaloperonospora arabidopsidis (Fig. 10.3) (Van der Ent et al.
2009a). However, callose deposition is considered to be regulated by plant hormone
ABA, as biotrophic pathogen H. arabidopsidis is insensitive to JA/ET-dependent
defenses. Additional priming can be observed by Bacillus subtilis FB17 mediated
ISR in Arabidopsis, where there is an augmented closure of the stomata in response
to infection of its leaves by Pseudomonas syringae (Kumar et al. 2012). Thus, the
regulation by hormone ABA and the structural barriers of priming provide an
auxiliary stratum of protection that extends the effective range of ISR.
In Arabidopsis, during the ISR priming condition, transcription factor genes of
the AP2/ERF family transcription factors (TFs) are highly expressed (Memelink
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
309
2008; Van der Ent et al. 2009b; Weller et al. 2012).
In plant-root, initiation of ISR requires certain essential signaling components.
For instance, in response to ISR-inducing PGPR Pseudomonas fluorescens
WCS417r and PGPF Trichoderma, there is a significant expression of the MYB
transcription factor gene MYB72 of the R2R3-type that encodes MYB72, a rootspecific transcription factor which functions with other signaling components and is
necessary for the instigation of ISR in the epiblema and cortex of Arabidopsis roots
(Van der Ent et al. 2008; Segarra et al. 2009). Typically, during iron-deficiency
conditions, roots extrude protons by H
+
-ATPase to make Fe(III)/Fe
3+ more soluble
by acidification of the soil environment. This is then reduced to Fe(II)/Fe
2+ by ferric
chelate reductase-FRO2 and is transported into root cells by the iron transporter,
IRT1. In Arabidopsis roots colonized by ISR-inducing rhizobacterial Pseudomonas
strains, there is a coordinated up-regulation of MYB72 with FRO2, IRT1, and other
iron deficiency-regulated genes (Pieterse et al. 2014). Alternatively, MYB72
functions in the production and/or secretion of root semiochemicals that further
stimulate ISR by PGPR. Thus, this indicates a link between iron homeostasis and
ISR initiation, as MYB72 is induced under iron-limited or distorted iron uptake
conditions (Palmer et al. 2013).
The transcriptional factor NPR1 that has essential role as coregulator of
SA-dependent PR genes in SAR is also known to be required for JA/ET-dependent
rhizobacteria-mediated ISR by many PGPR and PGPF without PR gene activation
(Fig. 10.3). For the SA signaling, NPR1 is associated within the nucleus, while for
JA/ET signaling in ISR it is deliberated to have a cytosolic function (Spoel et al.
2003; Dong 2004; Pajerowska-Mukhtar et al. 2013). In Arabidopsis, ISR is
associated with an enhanced expression of the JA/ET-responsive defense genes
VSP, PDF1.2 and HEL against the insect herbivore Spodoptera exigua (Van Wees
et al. 1999). In carnation, ISR is accompanied with increased accumulation of
phytoalexins at the site of pathogen infection (Van Peer et al. 1991) (Fig. 10.3).
Moreover, the PGPM mediated ISR is commonly based on priming, in which
whole plant is sensitized by an enhanced activation of a combination of cellular
defenses including altered gene expression and production of structural barriers
(Conrath et al. 2006; Liu et al. 2007). There can be increased frequency of callose
depositions at the infection site to effectively block pathogen access that is observed
in the ISR of Pseudomonas fluorescens WCS417r in Arabidopsis against downy
mildew pathogen Hyaloperonospora arabidopsidis (Fig. 10.3) (Van der Ent et al.
2009a). However, callose deposition is considered to be regulated by plant hormone
ABA, as biotrophic pathogen H. arabidopsidis is insensitive to JA/ET-dependent
defenses. Additional priming can be observed by Bacillus subtilis FB17 mediated
ISR in Arabidopsis, where there is an augmented closure of the stomata in response
to infection of its leaves by Pseudomonas syringae (Kumar et al. 2012). Thus, the
regulation by hormone ABA and the structural barriers of priming provide an
auxiliary stratum of protection that extends the effective range of ISR.
In Arabidopsis, during the ISR priming condition, transcription factor genes of
the AP2/ERF family transcription factors (TFs) are highly expressed (Memelink
10 Microbe-Mediated Biotic Stress Signaling and Resistance Mechanisms in Plants
309