Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 353

demonstrated that silencing of MPK6 compromised disease resistance response
(Menke et al. 2004 ). The Arabidopsis CTR1 (Constitutive triple Response 1) is a
Raf-like kinase considered as a MAPKKK and it controls MPK3/6 activation via
MKK9 (MAPK kinase9). It negatively regulates ET signaling (Guo and Ecker 2004 ;
Yoo et al. 2008 ). The Arabidopsis MAP kinase MPK4 has been shown to be a
requirement for induction of a subset of ET-regulated genes (Brodersen et al. 2006 ).
The ethylene-dependent defense signaling pathway begins with the induction of
ethylene biosynthesis. 1-Amino-cyclopropane-1-carboxylic acid (ACC) synthase
(ACS) is the key enzyme involved in ethylene biosynthesis. S-adenosyl-methionine
(SAM) is converted to ACC by ACC synthase (Wang et al. 2002 ). ACS is PAMP
elicitor-inducible enzyme involved in induction of biosynthesis of ethylene
(Li et al. 2012 ). Another enzyme involved in biosynthesis of ethylene is ACC oxi-
dase. The enzyme oxidatively cleaves the ACC resulting in generation of ethylene
(Wang et al. 2002 ).
Several ACS genes have been identifi ed and transcriptional activation of these
genes contributes to the increase in ACS isozymes in plants (Vogel et al. 1998 ; Chae
et al. 2003 ; Skottke et al. 2011 ). Phosphorylation of the ACC synthases appears to
be essential for the activation of these enzymes. In the absence of phosphorylation
by mitogen-activated protein kinase, the newly synthesized ACC synthases are rap-
idly degraded through ubiquitin-proteasome pathway, resulting in no net increase in
the ACS proteins (Liu and Zhang 2004 ; Joo et al. 2008 ). ACC synthase isozymes
have been shown to be substrates for E3 ligases (Dreher and Callis 2007 ).
The Arabidopsis ACS isoforms ACS2 and ACS6 have been shown to be sub-
strates of MPK3 and MPK6 (Liu and Zhang 2004 ; Han et al. 2010 ). The ACS
isoforms were shown to be phosphorylated and stabilized by MPK3 and MPK6
functioning in the MAPK cascade consisting of MEKK1-MKK4/MKK5-MPK3/
MPK6 (Han et al. 2010 ; Li et al. 2012 ). Phosphorylation of the two ACC syn-
thases, ACS2 and ACS6 by MPK3 and MPK6 prevents rapid degradation of
ACS2/ACS6 by the 26S proteasome pathway, resulting in an increase in cellular
ACS activity (Han et al. 2010 ). The phosphorylation resulted in ACS stability
and accumulation of ACS isozymes, which led to increased synthesis of ACC
(Liu and Zhang 2004 ). Another enzyme involved in biosynthesis of ethylene,
ACC oxidase, oxidatively cleaves the accumulated ACC resulting in generation
of ethylene (Wang et al. 2002 ). In addition to direct phosphorylation modifi ca-
tion and stabilization of ACS proteins, MPK3 and MPK6 also regulate the
expression of ACS genes through another MPK3/MPK6 substrate, the WRKY
transcription factor WRKY 33 (Fig. 7.6 ; Li et al. 2012 ). WRKY33 binds to the
W-boxes in the ACS2 / ACS6 promoters in vivo and is directly involved in MPK3/
MPK6-induced ACS2 / ACS6 gene expression. Regulations of ACS isoforms at
both transcriptional and post-translational levels by MPK3 and MPK6 seem to
contribute to the high-level ethylene production in plants challenged by invading
pathogens (Li et al. 2012 ). These studies suggest that MPK3 and MPK6 not only
function in the phosphorylation-induced stabilization of ACS2/ACS5 proteins,
but also signal the ACS2 and ACS6 gene activation after Botrytis cinerea in
Arabidopsis (Li et al. 2012 ).