PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 66

Ethylene perception and signaling are crucial for FLS2 gene transcription
(Boutrot et al. 2010 ). FLS2 promoter revealed the presence of nine potential EIN3/
EILs binding sites (Boutrot et al. 2010 ), suggesting that EIN3 may bind to the pro-
moter of the FLS2 gene to infl uence its transcription. EIN3 binds to two positions in
FLS2 promoter in Arabidopsis seedlings treated with ethylene. The results suggest
that endogenous ethylene controls FLS2 expression transcriptionally through direct
binding of the transcription factor EIN3 and potentially, EIL1 to the FLS2 promoter
(Boutrot et al. 2010 ).
Flg22 induces MAP kinases, which phosphorylate the ethylene biosynthetic
enzymes ACC synthases 2 and 6 as well as EIN3, leading to its stabilization (Liu
and Zhang 2004 ; Yoo et al. 2008 ). It is suggested that in the absence of fl agellin,
endogenous ethylene may ensure a constitutive level of FLS2 expression. On fl g22
binding, FLS2 may activate MPK6 that, in turn may phosphorylate ACS2/6 and
further leads to EIN3 stabilization, resulting in increased ethylene production and
signaling (Boutrot et al. 2010 ). These studies suggest that endogenous ethylene may
play an important role in PAMP-triggered expression of PRR genes in plants.

2.14 PAMPs Induce Phosphorylation of PRRs

2.14.1 PAMP-Induced Autophosphorylation of PRRs

Most of the PRRs identifi ed are receptor kinases and these protein kinase PRRs
are known to be activated by PAMPs (Segonzac and Zipfel 2011 ). Before activa-
tion, the protein kinases are frequently autophosphorylated (Schlessinger 2000 ;
Gómez-Gómez et al. 2001 ; Wang et al. 2005 ; Kanzaki et al. 2008 ). The PAMP
fl g22 induces autophosphosphorylation of the PRR FLS2 and the PRR receptor
kinase is phosphorylated by its own serine/threonine kinase (Gómez-Gómez
et al. 2001 ; Wang et al. 2001 ). Mutation of the threonine residue 867 in FLS2
hampers the autophosphorylation response, suggesting that autophosphoryla-
tion of the FLS2 occurs at threonine residue 867 (Robatzek et al. 2006 ).
Autophosphorylation of the PRR EFR has also been reported, suggesting that
EFR carries active kinase domain (Xiang et al. 2008 ).
The rice PRR, XA21, recognizes the PAMP, Ax21 (Activator of XA21-mediated
immunity), which is highly conserved in all sequenced genomes of Xanthomonas
and in Xylella (Lee et al. 2006a , 2009 ). It has been shown that the intracellular non-
RD cytoplasmic kinase domain of XA21 contains intrinsic kinase activity (Liu
et al. 2002). Chen et al. ( 2010b ) showed that XA21 juxtamembrane (JM) domain
is required for kinase autophosphorylation. Threonine 705 in the XA21 JM domain
is essential for XA21 autophosphorylation in vitro and XA21-mediated innate
immunity in vivo . The replacement of Thr 705 by an alanine or glutamic acid abol-
ishes XA21 autophosphorylation (Chen et al. 2010d , e ). Threonine residues analo-
gous to Thr 705 of XA21 are present in the JM domains of most RD and non-RD
plant receptor-like kinases (Chen et al. 2010d ).