Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 409

(Feng et al. 2003 ). CSN is commonly present in plants. Several molecular studies
have shown the presence of six loci, COP9 , FUS/COP11 , FUS5 , FUS4/COP8 ,
FUS11 , and FUS12 , encoding subunits of the COP9 signalosome, which are CSN8,
CSN1, CSN7, CSN4, CSN3, and CSN2, respectively in plants (Feng et al. 2003 ). CSN
is associated with multiple SCF-type E3 ubiquitin ligases (Wang et al. 2003 ). CSN
shows deneddylation activity toward the neddylated cullin subunit of SCF
complexes, which is important for SCF ubiquitin ligase activity (Yang et al. 2002 ).

10.2 Ubiquitin-Proteasome in Jasmonate Signaling System

10.2.1 JAZ Proteins Act as Repressors

of JA Signaling Pathway

Jasmonate (JA) signaling system plays important role in plant innate immunity
(Lozano-Durán et al. 2011 ; Qi et al. 2011 ; Zhang et al. 2012 ). In the absence of elici-
tor signals, the JA signaling pathway is generally repressed by a family of jasmonate
ZIM (for ZING FINGER PROTEIN EXPRESSED IN INFLORESCENCE
MERISTEM) domain (JAZ) proteins (Chini et al. 2007 ; Thines et al. 2007 ), which
recruit the corepressor TOPLESS through the linker NOVEL INTERACTOR OF JAZ
(Pauwels et al. 2010 ). JAZ family of proteins and related JAI3 (for JASMONATE-
INSENSITIVE 3) protein have been identifi ed as key suppressors of jasmonate sig-
naling (Chini et al. 2007 ; Thines et al. 2007 ). The JAI3 protein contains a ZIM domain
and hence it was renamed as JASMONATE ZIM DOMAIN3 (JAZ3; Chini et al.
2007 ). Within 30 min of JA treatment, several genes encoding individual members of
the JAZ protein family showed strong induction in Arabidopsis (Mandaokar et al.
2006 ). JAZ1 and JAZ3/JAI3 are the repressors of the JA signaling pathway (Chini
et al. 2007 ; Thines et al. 2007 ). Both JAZ1 and JAZ3/JAI3 each interact with JIN1
( J ASMONATE IN SENSITIVE1, also known as MYC2 [ MY ELO C YTOMATOSIS 2] )
(Chini et al. 2007 ). JIN1 / MYC2 encodes a basic helix-loop-helix-type transcription
factor involved in the transcriptional regulation of JA-responsive gene expression
(Lorenzo et al. 2004 ). It is suggested that, in the absence of a JA signal, JAZ1 and
JAZ3 repress JIN1/MYC2. The Arabidopsis bHLH transcription factors MYC3 and
NYC4 are targets of JAZ repressors and act additively with MYC2 in the activation of
jasmonate response (Fernández-Calvo et al. 2011 ).

10.2.2 JA Signaling Pathway Is Activated by the Removal

of the JAZ Repressor Proteins by Ubiquitination

Once activated by stress signals, JA is rapidly synthesized and further converted into
numerous conjugates, including the highly bioactive (+)-7-iso-jasmonoyl-L-isoleucine
(JA-Ile) (Thines et al. 2007 ; Fonseca et al. 2009 ). Synthesized JA and its bioactive