PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 90

contribute to a signal amplifi cation loop involving ROS, SA, and NPR1 that is
required to potentiate innate immune system (Mishina and Zeier 2006 ). Another
protein, GH3.5, has been shown to be involved in SA accumulation in A. thaliana
and it possesses adenylation activity on SA (Zhang et al. 2007c ). The protein
CDR1 has also been shown to take part in SA signaling (Xia et al. 2004). The
CDR1 may be an aspartic protease. It induces accumulation of SA and also induces
oxidative burst, suggesting that ROS-mediated SA accumulation is mediated by
CDR1 (Xia et al. 2004). Several signaling systems, including Ca 2+ - signaling net-
work system (Garcia-Brugger et al. 2006 ), G-proteins (Fujiwara et al. 2006 ),
MAPK signaling systems (Zhang et al. 2007a ), ROS signaling system (Torres et al.
2006 ; Ahn et al. 2007 ), and NO signaling system (Durner et al. 1998) act upstream
of SA synthesis.
Several SA-binding (SAB) proteins have been shown to be involved in accu-
mulation of SA in plants. The fi rst SAB protein identifi ed is the cytosolic (per-
oxisomal) tobacco catalase that reversibly binds SA (Chen et al. 1993). SA
inhibits H 2 O 2 -degrading activity of catalase and the SA-mediated inhibition of
catalase may generate H 2 O 2 , which may activate the ROS signaling system
(Chen et al. 1993). A second specifi c high-affi nity SA-binding protein, SABP2,
has been identifi ed as a methyl salicylate esterase whose function is to convert
biologically inactive methyl salicylate to active SA (Kumar and Klessig 2008;
Vlot et al. 2008; Manosalva et al. 2010 ; Liu et al. 2011a , b ). The third SA-binding
protein, SAB3, identifi ed in tobacco is the chloroplast carbonic anhydrase
(Slaymaker et al. 2002). It shows antioxidant activity and SA may inhibit the
antioxidant activity by binding with SABP3. The inhibition of antioxidant
enzymes may enhance ROS levels (Slaymaker et al. 2002). Azelaic acid is a
long-distance priming signal (Parker 2009). It primes plants to accumulate SA
upon infection by pathogens (Jung et al. 2009 ).
SA triggers ROS and NO signaling systems (Blee et al. 2004 ; Zottini et al. 2007;
Kobeasy et al. 2011 ). It also activates MAPK signaling cascade (Uppalapati et al.
2004 ; Brodersen et al. 2006). SA elevates NPR1 (for non-expresser of PR gene1 ),
which is a master regulator of SA-mediated defense responses (Chern et al. 2008 ).
SA induces increased expression of several WRKY and ERF transcription factors in
Arabidopsis (Knoth et al. 2007 ; Mao et al. 2007 ; Miao and Zentgraf 2007 ; Zheng
et al. 2006 , 2007 ; Zhang et al. 2007d ; Grennan 2008 ; Moreau et al. 2012 ). These
transcription factors are necessary for the inducible expression of several defense
genes (Yu et al. 2001 ; Grennan 2008 ).
Flg22 activates the transcription factor WRKY7, which is a negative regulator of
SA-mediated responses in Arabidopsis (Denoux et al. 2008 ). Probably the tran-
scription factor would have suppressed the SA- induced defense response. The
transgenic Arabidopsis plants over expressing WRKY7 showed reduced expression
of defense-related genes, including PR1 (Kim et al. 2006 ).
Although most of the PAMPs activate SA signaling system, there are also reports
that SA signaling system may not be necessary to activate the plant immune systems
against a particular pathogen. Ferrari et al. ( 2007 ) showed that resistance to Botrytis
cinerea induced in Arabidopsis by fl g22 was independent of SA signaling.