PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 93

have been found to be induced by ET (Reddy et al. 2000). Ca 2+ may play an
important role in ET signal transduction (Raz and Fluhr 1992; Kwak and Lee
1997; Gallardo et al. 1999; Reddy et al. 2000).
Several PAMPs are known to induce production of ethylene in plant cells. The
oomycete PAMP cryptogein induced production of ethylene in tobacco cells (Milat
et al. 1991). The bacterial PAMP fl g22 up-regulated ACS genes encoding ACC
synthase, which is the key enzyme involved in biosynthesis of ethylene (Denoux
et al. 2008 ). Flg22 induces ET production through activation of ACS6, an ET bio-
synthetic enzyme (Liu and Zhang 2004 ). The PAMP Nep1 induced the expression of
transcripts encoding ACC synthase and ethylene-responsive element binding fac-
tors (ERF), which are involved in ethylene signaling (Bae et al. 2006 ). The maize
HAMP Zm Pep1-treated maize leaves emitted a fi ve-fold increase in ethylene (ET).
Expression of the gene encoding ACC oxidase also responded to ZmPep1 treatment
(Huffaker et al. 2011 ).
Flg22 treatment up-regulated the expression of the ET-responsive transcription
factor ERF1 (Ethylene-Responsive element-binding Factor 1) in Arabidopsis (Clay
et al. 2009 ). ERF1 is a downstream component of ethylene signaling system
(Berrocal-Lobo and Molina 2004 ). ET signaling is required for the full induction of
ERF1 in response to fl g22 (Clay et al. 2009 ). ERF5, another ethylene-responsive
element-binding factor, is induced by the fungal PAMP chitin. It induces defense
against Pseudomonas syringae pv. tomato in Arabidopsis (Son et al. 2012 ).
The transcription factor MYB51 is also induced by ET and MYB51 acts down-
stream of ET signaling for the callose response (Clay et al. 2009 ). MYB51 induced
expression of all known indole glucosinolate (IGS) biosynthetic enzymes. MYB51 is
involved in the transcriptional activation of IGS biosynthetic gene ASA1. ASA1
expression is ET-inducible (Clay et al. 2009 ). ASA1 gene expression was also induced
by Flg22 treatment (Clay et al. 2009 ). The HAMP oligogalacturonide induced the ET
biosynthesis enzymes ACC synthases (ACS7 and ACS8) (Denoux et al. 2008 ).
ET is required for the oxidative burst contributing to plant immunity (Mersmann
et al. 2010 ). ROS production is triggered by fl g22 in Arabidopsis . The ROS produc-
tion was diminished in ethylene-insensitive mutants (Mersmann et al. 2010 ).
Ethylene signaling also regulates accumulation of the FLS2 receptor (Mersmann
et al. 2010 ). FLS2 accumulation was reduced in etr1 and ein2 , indicating a require-
ment of ethylene signaling for FLS2 expression (Mersmann et al. 2010 ).

2.20.11 Abscisic Acid Signaling System

Abscisic acid (ABA) signaling system is another important component in plant
immune system activated by PAMPs (Adie et al. 2007 ). PAMPs trigger increases in
ABA concentrations, inducing disease resistance (Whenham et al. 1986) or suscep-
tibility (Koga et al. 2004 ; Schmidt et al. 2008 ). Xanthoxin is the fi rst cytoplasmic
precursor for the biosynthesis of ABA. It is converted to abscisic aldehyde by the
action of a dehydrogenase/reductase encoded by ABA2 . Abscisic aldehyde is