Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 411

(Devoto et al. 2002 ; Vierstra 2003 ). It suggests that COI1 may form a functional
E3-type ubiquitin ligase in plants to regulate expression of jasmonate responsive
genes by targeted ubiquitination of a histone deacetylase (Devoto and Turner 2003 ;
Zhou et al. 2005 ).
Ubiquitin-proteasome-mediated proteolytic pathway may activate JA-inducible
transcription factors. The COI1 activated four JA-inducible transcription factors,
which include WRKY18, At1g74930, At3g53600, and AtMYC2 in Arabidopsis.
These transcription factors positively regulated the jasmonate-mediated signaling
system (Wang et al. 2008 ; Kazan and Manners 2013 ). Both JAZ1 and JAZ3/JAI3
each interact with JIN1 (JASMONATE INSENSITIVE1, 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. Upon sensing of the JA
signals, JAZ repressors are recruited to the SCF E3 complex for ubiquitination and
subsequent degradation by the proteasome. The removal of these repressors then
paves the way for JIN1/MYC2 to regulate JA-dependent gene expression.
JA-Ile, but not jasmonic acid itself or MeJA, promotes the interaction between
SCF COI1 -JAZ complexes (Thines et al. 2007 ). This fi nding suggests that jasmonic
acid may not be the signal directly responsible for the activation of the JA signaling
pathway, but possibly it undergoes further modifi cations to be converted to a bio-
logically active signal. JAR1 ( JASMONATE RESISTANT1 ) encodes a JA-amino
synthetase, which activates conjugation of JA to Ile (Staswick and Tiryaki 2004 ).
JA-Ile produced by JAR1 promotes the interaction between JAZ and SCF COI1 and
takes part in the removal of repressors of JA signaling pathway. SCF COI1 targets key
regulators of JA signaling pathway for ubiquitination and subsequent degradation
by the 26S proteasome (Kazan and Manners 2008 ; Melotto et al. 2008 ; Kawamura
et al. 2009 ).
The COP9 signalasome (CSN) has been shown to be necessary for the function
of SCF COI1 ubiquitin ligase. COP9 signalosome associates physically with SCF COI1
(Feng et al. 2003 ). Most of the COI1-dependent JA-responsive genes also required
CSN function, and CSN abundance was shown to be important for JA responses
(Feng et al. 2003 ). These observations suggest that both CSN and SCF COI1 work
together to control genome expression and promote JA responses.

10.2.3 Ubiquitin-Proteasome-Mediated Proteolysis

in JA Signaling System

Downstream events in the JA signaling pathway are controlled by repressors of
downstream transcriptional networks and the degradation of these repressors is the
early downstream event (Kazan and Manners 2008 ). Ubiquitin-proteasome- mediated
proteolysis has been shown to be involved in jasmonate signaling system (Xu