Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 412

et al. 2002 ; Feng et al. 2003 ). Proteome studies have revealed the participation of a
ubiquitin-conjugating protein in the JA-signal transduction pathway (Hondo et al.
2007 ). A cell wall protein fraction (CWP) elicitor derived from the biocontrol agent
Pythium oligandrum induced expression of LeATL6 gene encoding ubiquitin-ligase
enzyme E3 and triggered the synthesis of PR-6 and TPI-1 defense-related proteins
in tomato via JA-dependent signaling system (Hondo et al. 2007 ). The role of the
ubiquitin ligase3 enzyme in the JA signaling system was demonstrated by overex-
pressing LeATL6 under the control of the Caulifl ower mosaic virus 35S promoter in
tomato plants. Overexpression of the gene induced the defense genes PR-6 and TPI-
1 in wild tomato but not in the jai-1 mutant in which the JA-mediated signaling
pathway was impaired (Hondo et al. 2007 ). It suggests that LeATL6 may be a part
of the JA- signal transduction system. LeATL6 expression was induced by elicitor
treatment in jai-1 mutant tomato cells; however, JA-dependent expression of the
basic PR-6 and TPI-1 genes was not induced in elicitor-treated jai-1 mutants. These
results indicated that ubiquitin ligase E3 (LeATL6) may act upstream of JA signaling
(Hondo et al. 2007 ).
The expression of LeATL6 , which encodes RING-H2 zinc fi nger ubiquitin ligase
E3 was highly induced in tomato roots treated with the elicitor from P . oligandrum
(Takahashi et al. 2010 ). The target protein of LeATL6 was identified as
S-adenosylmethionine decarboxylase (SAMDC) (Takahashi et al. 2010 ), which is
involved in biosynthesis of polyamines (Kresge et al. 2007 ). Polyamines are known
to act in the JA-signaling system (Chen et al. 2004 ). The elicitor suppressed the
activity of SAMDC in treated tomato roots. The interaction of SAMDC with
LeATL6 and the decreased SAMDC activity may be associated with JA-dependent
induced resistance in tomato treated with P . oligandrum (Takahashi et al. 2010 ).
Two RING-type ubiquitin ligases, RGLG3 and RGLG4, have been found to be
essential for JA-mediated responses in Arabidopsis . Both RGLG3 and RGLG4 pos-
sessed ubiquitin ligase activities (Zhang et al. 2012 ). Altered expression of RGLG3
and RGLG4 affected JA-inducible gene expression. The ubiquitin ligases have been
found to act as upstream modulators of JA signaling (Zhang et al. 2012 ).

10.3 Ubiquitin-Proteasome in Ethylene Signaling System

Ethylene signaling system is an important component in defense signaling (Iwai
et al. 2006 ; Binder et al. 2007 ; Ralph et al. 2007 ; Dreher and Callis 2007 ). Ubiquitin-
proteasome proteolytic pathway has been shown to be involved in both regulation of
ethylene biosynthesis and downstream activation of transcription factors, leading to
transcription of defense genes. ACC synthase (ACS) isozymes have been shown
to be substrates for E3 ligases (Dreher and Callis 2007 ). Arabidopsis ethylene over-
production mutants ( eto2 and eto3 ) are shown to bear mutations in the ACS genes,
ACS5 and ACS9 , respectively (Vogel et al. 1998 ; Chae et al. 2003 ).
Bostick et al. ( 2004 ) showed that silencing of two ubiquitin-related proteins, RUB1
and RUB2, which modify cullin RING ligases and regulate their activity,