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and/or PBS1 were compromised in their PTI response to several PAMPs suggesting
that at least these two targets of AvrPphB are important in PTI and are virulence
targets of
P
.
syringae
(Zhang et al.
2010a
; Lu et al.
2010
).
2.31.6
Effectors May Inhibit Autophosphorylation of PRRs
Most of the PRRs identifi ed are receptor kinases and the PAMPs induce autophos-
phorylation of these PRRs (Gómez-Gómez et al.
2001
; Wang et al.
2001
; Robatzek
et al.
2006
; Kanzaki et al.
2008
; Xiang et al.
2008
; Chen et al.
2010d
; Kim et al.
2010
; Petutschnig et al.
2010
). PAMP-induced autophosphorylation of PRRs is
required for activation of the PRRs (Chen et al.
2010d
; Park et al.
2010a
,
b
). The
effector AvrPto produced by
P
.
syringae
pv.
tomato
interacts
in vivo
with the PRRs
FLS2 and EFR and inhibits their autophosphorylation in the dose-dependent man-
ner (Xiang et al.
2008
). The effector thus prevents activation of the function of the
PRRs. AvrPto inhibits all responses induced by several PAMPs (He et al.
2006
;
Hann and Rathjen
2007
; Xiao et al. 2007).
The
P
.
syringae
effector HopF2 has been found to be a potent suppressor of early
immune gene transcription signaling activated by multiple PAMPs, including bacte-
rial fl agellin, ef-Tu, peptidoglycan, lipopolysaccharide and HrpZ1 harpin, and fun-
gal chitin (Wu et al.
2011
). HopF2 is targeted to the plant plasma membrane through
a putative myristoylation site and the membrane association appears to be required
for its PAMP-suppression function (Wu et al.
2011
). The plasma membrane-
associated cytoplasmic kinase BIK1is phosphorylated within 1 min upon fl agellin
perception. Expression of HopF2 in plants potently diminished the fl agellin-induced
phosphorylation of BIK1 (Wu et al.
2011
). These results suggest that HopF2 likely
intercepts PAMP signaling at the plasma membrane immediately of signal percep-
tion by PRR by inhibiting the phosphorylation of the protein kinase BIK1 (Wu et al.
2011
). These studies suggest that the effectors may interfere with the function of
PRRs by inhibiting autophosphorylation of the PRRs.
2.32
PAMP-Induced Small RNA-Mediated RNA Silencing
2.32.1
RNA Silencing Is an Immune System in Plants
Small RNAs (sRNAs) are non-protein-coding RNAs of 20 to 30-nucleotide length
(Ghildiyal and Zamore 2009; Havecker et al. 2010; Kulcheski et al. 2011; al. 2011).
These small RNAs have been identifi ed as regulatory RNAs that modulate gene
expression at both the transcriptional and posttranscriptional levels (Zhao et al.
2012). They act as sequence-specifi c repressors of target gene expression, either at
the transcriptional level through DNA and/or histone methylation or at the posttran-
scriptional level through transcriptional cleavage or translational inhibition
(Ramachandran and Chen 2008a, b).
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