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A substrate for MPK4 has been identifi ed and it was designated MKS1 (for MAP
Kinase 4 Substrate 1) (Andreasson et al. 2005 ). MPK4 interacts with the nuclear
protein MKS1 that in turn interacts with two WRKY transcription factors,
WRKY25 and WRKY33 (Andreasson et al. 2005 ). The molecular phenotypes of
plants over- or under-expressing MKS1 indicate that it mediates some effects of
MPK4 on SA-mediated resistance responses. The results suggest that the MKS1 is
required for SA-dependent resistance in Arabidopsis (Andreasson et al. 2005 ). The
transcription factors WRKY25 and WRKY33 may function as downstream com-
ponents of the MPK4-mediated signaling pathway and contribute to repression of
SA-dependent disease resistance response (Andreasson et al. 2005 ).
An Arabidopsis MAPKKK, EDR1 ( E nhanced D isease R esistance1), negatively
regulates SA-mediated defense responses (Frye et al. 2001 ). Inactivation of EDR1
interacting receptors by fungal pathogen - derived elicitor signals activated
SA-inducible defense responses. The Arabidopsis mutant edr1 showed enhanced
disease resistance response. These results suggest that the mitogen-activated protein
kinase is involved in SA-mediated resistance response (Frye et al. 2001 ).
7.15
MAP Kinase Cascades Activate JA Signaling System
Some MAP kinase cascades are involved in jasmonic acid (JA) signaling system.
The Arabidopsis MKK3-MPK6 cascade is involved mainly in JA signaling sys-
tem. This cascade negatively regulates ATMYC2 function (Takahashi et al.
2007a ). Arabidopsis JIN1 ( J ASMONATE IN SENSITIVE1 , also known as MYC2
[ MY ELO C YTOMATOSIS 2 ] ) encodes a basic helix-loop-helix (bHLH) - type
transcription factor. MYC2 involved in the transcriptional regulation of
JA-responsive gene expression (Lorenzo et al. 2004 ; Chini et al. 2007 ). ATMYC2
plays a predominant role in JA pathway (Boter et al. 2004 ). ATMYC2 was shown
to function as a downstream factor of the MKK3-MPK6 cascade in JA signaling
(Takahashi et al. 2007a ).
JAZ (for JA SMONATE Z IM [ Z inc-fi nger protein expressed in Infl orescence
Meristem]-domain) family of transcriptional repressors has been identifi ed as an
important component in a receptor complex involved in the JA perception process
(Sheard et al. 2010 ). JAZ proteins have been identifi ed as key regulators of jasmonate
signaling (Chini et al. 2007 ; Thines et al. 2007 ). JAZ proteins negatively regulate the
key transcriptional activator of jasmonate responses, MYC2 (Chini et al. 2007 ).
JAZ proteins interact with MYC2 involved in the transcriptional regulation of
JA-responsive gene expression (Lorenzo et al. 2004 ; Chini et al. 2007 ). It is sug-
gested that, in the absence of a JA signal, JAZ proteins repress MYC2. Upon sensing
of the JA signals, JAZ repressors are recruited to the SCF E3 complex for ubiquitina-
tion and subsequent degradation by the proteasome. The removal of these repressors
then paves the way for MYC2 to regulate JA-dependent gene expression.
MYC2 is a master regulator of the JA signaling pathway. MYC2 is required for
induced systemic resistance (ISR) triggered by benefi cial soil microbes (Kazan and
 
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