PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 97

treated with the two different MAMPs, at least during the early phase of defense
signaling system (Suzuki et al. 2007 ). Pep1 induced transcriptional induction of
MPK3 , WRKY29 , WRKY33 , and WRKY53 in Arabidopsis (Yamaguchi et al. 2010 ).
These have been reported to be induced by a fungal PAMP, chitin (Wan et al. 2004 ,
2008a , b ), and bacterial PAMPs fl g22 and elf18 (Zipfel et al. 2004 , 2006 ). Collectively
these results suggest that different PAMPs/HAMPs may induce almost same type of
early signaling related genes. Several studies have revealed that the PAMPs/HAMPs
activate conserved early basal defense responses (Garcia-Brugger et al. 2006 ; Jones
and Dangl 2006 ; Qutob et al. 2006 ; Thilmony et al. 2006 ; Ferrari et al. 2007 ).
The very early responses induced within seconds or minutes by the PAMPs include
protein phosphorylation followed by Ca 2+ infl ux, plasma membrane depolarization,
anion and K + effl ux, cytosol acidifi cation, activation of MAP kinases, transient pro-
duction of ROS and NO production (Boller and He 2009 ; Boller and Felix 2009 ). All
these early events are induced by most of the PAMPs (Asai et al. 2002 ; Zhang et al.
2002a , b , c ; Hu et al. 2004; Lecourieux et al. 2005 ; Garcia-Brugger et al. 2006 ;
Moscatiello et al. 2006 ; Denoux et al. 2008 ). Most PAMPs and HAMPs (but not all)
induce calcium ion infl ux (Lecourieux et al. 2002 , 2006 ; Garcia-Brugger et al. 2006 ;
Denoux et al. 2008 ; Erbs et al. 2008 ; Trouvelot et al. 2008 ; Aslam et al. 2009 ; Qi et al.
2010 ). MAPK signaling is activated by multiple PAMPs, including fl agellin, EF-Tu,
peptidoglycan, lipopolysaccharide, and bacterial HrpZ harpin, and fungal chitin in
Arabidopsis (Wu et al. 2011 ). Production of ROS by various PAMPs and HAMPs has
been widely reported (Huffaker et al. 2006 ; Denoux et al. 2008 ).
The PRRs FLS2 and EFR are induced also by bacterial LPS, fungal chitin, and the
oomycete-derived NPP1 (Zipfel et al. 2006 ). Overall, these different PAMPs seem to
trigger changes in a common set of genes, indicating that plants do not distinguish
bacteria, fungi, and oomycetes on the basis of the signaling signature of their PAMPs.
Rather, presence of one type of PAMP seems to serve as an indicator of injury or
danger in general, resulting in plant innate immune systems (Zipfel et al. 2006 ).

2.22 Magnitude and Timing of Expression of Early Signaling

Systems May Vary Depending on Specifi c PAMPs

Although various PAMPs/HAMPs may induce same set of early signaling events,
such as the same Ca 2+ infl ux, activation of the same MAPK cascades, and similar
production and accumulation of ROS and NO, the induction of these events may
vary in magnitude and timing depending on the specifi c PAMPs/HAMPs. Lecourieux
et al. ( 2005 ) showed that the PAMPs fl g22,

-1,3-glucan, four different elicitins, and
harpin, and the HAMP OGs induced changes in Ca 2+ concentration in tobacco cells,
but these changes in Ca 2+ concentration varied in magnitude and timing, depending
on the PAMP/HAMP. The proteinaceous PAMPs (fl g22, elicitins, harpin) induced a
pronounced and sustainable [Ca 2+ ] nuc elevation, relative to the small effects of the
PAMP

β

-1,3-glucan and the HAMP oligogalacturonides in induction of calcium
signatures (Lecourieux et al. 2005 ). Aslam et al. ( 2009 ) showed that each PAMP
may elicit different calcium signature. A comparison of calcium infl ux patterns

β