PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 92

2.20.10 Ethylene Signaling System

Ethylene (ET) signaling system is another important component in plant's innate
immune system (Cao et al. 2006 ; Xu et al. 2007; Lin et al. 2008 ; Gaige et al. 2010 ;
Al-Daoud and Cameron 2011; Sun et al. 2010 ; Zhu et al. 2011 ). Methionine (L-Met)
is the precursor of ethylene. It is converted to S -adenosylmethionine ( S -AdoMet) by
the action of S -AdoMet synthetase (SAM synthetase) (Wang et al. 2002). Nitric
oxide (NO) induces SAM synthetase, which catalyzes the conversion of ATP and
methionine into S -adenosyl-Met (Zago et al. 2006 ). S-AdoMet is converted to
1-aminocyclopropane −1-carboxylic acid (ACC) by ACC synthase (ACS) (Chae
et al. 2003; Peng et al. 2005). Accumulation of ACS isozymes leads to increased
synthesis of ACC (Liu and Zhang 2004 ) and the ACC is oxidized by ACC oxidase
(ACO) to form ET (Wang et al. 2002; Vidhyasekaran 2007a ). Both Ca 2+ and NO
signaling systems are involved in ethylene biosynthesis. The Ca 2+ infl ux activates
ACC oxidase (Gallardo et al. 1999), whereas NO induces ACC synthase, resulting
in accumulation of ethylene (Lamotte et al. 2004 ).
Downstream components of ET signal transduction system in Arabidopsis
include activation of ET receptors. In Arabidopsis , ethylene is perceived by a family
of fi ve membrane-bound receptors (ETR1, ERS1, ETR2, EIN4, and ERS2), which
transmit the signal to downstream effectors (O'Malley et al. 2005; Wang et al. 2006 ;
Qu et al. 2007; Grefen et al. 2008). Ethylene receptor ETR1 has been shown to
mediate ROS signaling in Arabidopsis (Desikan et al. 2005). ETR1 functions as an
ROS sensor. ROS up-regulates four ethylene-responsive element-binding proteins
(EREBPs), the ethylene-responsive transcription factor (ERF1), and a CEO1-like
protein, which is a potential cofactor of EREBP transcription factors in tobacco
(Vandenabeele et al. 2003). NO signaling activates EIN3 , which is involved in acti-
vation of transcription of ethylene-responsive genes (Chang and Stadler 2001).
The HAMP oligogalacturonides induced several plant cell membrane-bound
ethylene receptors such as ETR1, EIN2, ERF1 and ERF4 (Denoux et al. 2008 ).
Flg22 induced expression of the ethylene receptors ETR1 and EIN4 (Denoux et al.
2008 ). In Arabidopsis , ethylene is perceived by membrane-bound receptors such as
ETR1 and EIN4, which transmit the signal to downstream effectors (Qu et al.
2007; Grefen et al. 2008).
A MAPK cascade, MAPKKK (CTR1) - MKK9 - MPK3/MPK6, has been
shown to be an important downstream component in ET signaling system induced
by PAMPs (Yoo et al. 2008 ). The MAPKKK CTR1 is a negative regulator of
defense responses. Both the inhibition of CTR1 and activation of MKK9 are
induced by ethylene signaling (Yoo et al. 2008 ). ET activates JA biosynthesis
(O'Donnell et al. 1996) and also activates JA-inducible defense gene expression
(Gu et al. 2002; Brown et al. 2003; Tournier et al. 2003; McGrath et al. 2005 ).
The PAMP-induced ET also induces Ca 2+ infl ux, which acts at downstream of ET
(Kwak and Lee 1997). Ca 2+ from intracellular pools, but not Ca 2+ from the apo-
plast, may interact with ET signal transduction (Petruzzelli et al. 2003). The
CaM binding proteins (EICBPs) isolated from tomato, parsley, and Arabidopsis