PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 95

2.20.12 PAMP-Induced Expression of Transcription Factors

Transcription factors are the master regulators of expression of genes involved in
many cellular processes and they regulate the gene transcription processes by mod-
ulating the rate of transcription initiation of target genes (Du et al. 2009). They
play important role in activating plant immune responses either positively (Liu
et al. 2007 ; Qiu et al. 2007 ; Moreau et al. 2012 ) or negatively (Li et al. 2007;
Nurmberg et al. 2007; Kim et al. 2008b ; Sun et al. 2010 ; Moreau et al. 2012 ). In
Arabidopsis , there are 72 expressed WRKY genes, and many of them are impli-
cated in the regulation of the plant immune responses positively or negatively via
modulation of the JA/SA signaling pathways (Eulgem and Somssich 2007 ). WRKY
proteins are transcription factors that recognize the W-box elements that exist in
the promoters of many PR genes (Ulker and Somssich 2004 ). WRKY transcription
factors bind to W-box DNA elements (C/TTGACC/T) that are found in the promot-
ers of many defense-related genes, including PR-1 and NPR1 (Maleck et al. 2000;
Yu et al. 2001 ; Eulgem and Somssich 2007 ). Multiple W-box DNA elements were
predicted in the promoter region of PEPR1 and PROPEP1-5 genes in Arabidopsis
and therefore, the WRKY transcription factors may play an important role in the
amplifi cation of the HAMP Pep peptide signal (Yamaguchi et al. 2010 ). WRKY
transcription factors also regulate the expression of their own genes and/or other
WRKY genes (Eulgem and Somssich 2007 ).
After Flg22 treatment WRKY70 was consistently induced around 20-fold
in A . thaliana . WRKY70 induces expression of SA-responsive PR genes (Li
et al. 2004). WRKY70 has been shown to function downstream of ROS and SA
(Knoth et al. 2007 ). The WRKY70 transcription factor acts as a node of conver-
gence for jasmonate-mediated and salicylate-mediated signals in plant defense
(Li et al. 2004). It modulates the selection of signaling pathways in plant defense
(Li et al. 2006).
Flg22 induces activation of several other transcription factors in Arabidopsis .
It activates WRKY33 and WRKY40 transcription factors (Denoux et al. 2008 ).
These transcription factors function as activators of JA signaling system and repressors
of SA signaling system (Zheng et al. 2006 ). WRKY33 positively regulates
JA-mediated responses, while it retards the SA-mediated PR genes expression
(Zheng et al. 2006 ). The transcription factors WRKY22 and WRKY29 are known
to function downstream of the FLS2-mediated immune response in Arabidopsis .
Overexpression of the AtWRKY29 constitutively activates the plant defense
response against bacterial invasion (Asai et al. 2002 ). WRKY29, WRKY33, and
WRKY53 are induced by the HAMP Pep1 and they are positive regulators of
defense responses (Asai et al. 2002 ; Zheng et al. 2006 ; Murray et al. 2007 ).
Nine WRKY genes were induced by the PAMP Nep1 treatment, while WRKY65
was repressed in A . thaliana (Bae et al. 2006 ). Keates et al. ( 2003 ) detected the
induction of WRKY18 within 15 min after treatment with the PAMP Nep1. These
results suggest that PAMPs/HAMPs may trigger the expression of several transcrip-
tion factors and trigger the plant innate immune responses.