Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 414

10.4 Ubiquitin-Proteasome in SA Signaling System

10.4.1 Ubiquitin-Proteasome May Be Involved in Regulation

of SA Levels in the SA Signaling System

Protein degradation through ubiquitin-proteasome pathway has been shown to play
an important role in SA-regulated defense signaling. Ubiquitin proteasome may
positively or negatively regulate SA accumulation. SGT1 (for Suppressor of the G2
allele of SKP1) associates with SKP1, a component of the SCF-type E3 complexes
(Kitagawa et al. 1999 ; Liu et al. 2002 ; Peart et al. 2002 ). SGT1 plays key role in
ubiquitin-proteasome-mediated proteolytic pathway (Seo et al. 2008 ). SGT1 is
involved in basal defense response besides effector triggered immunity (ETI).
Silencing GmSGT1-2 impaired resistance to virulent bacterial pathogens and systemic
acquired resistance (SAR) in soybean (Fu et al. 2009 ). Overexpression of
OsSGT1 in rice signifi cantly induced basal resistance to both the rice bacterial
blight pathogen Xanthomonas oryzae pv. oryzae and the fungal pathogen
Magnaporthe oryzae (Wang et al. 2008 ). SGT1 genes have been shown to be
required for SA accumulation in Arabidopsis for induction of disease resistance
(Zhou et al. 2008 ). SGT1 triggers expression of various SA-regulated defense-
related genes including PR-1 , PR-2 , PR-5 , RPW8.1 , RPW8.2 , WRKY6 , WRKY29 ,
and EDS1 (Zhou et al. 2008 ). The results suggest that ubiquitin-proteasome
mediated proteolytic pathway positively regulates SA levels and triggers SA signal-
ing system triggering defense responses.
RPN1a, a 26S proteasome subunit has been shown to be required for SA -
mediated innate immunity in Arabidopsis (Yao et al. 2012 ). EDR2 (ENHANCED
DISEASE RESISTANCE2) negatively regulates SA-based defense response against
the powdery mildew pathogen Golovinomyces cichoracearum (Vorwerk et al.
2007 ). Loss-of-function mutations in EDR2 lead to enhanced resistance against the
pathogen (Yao et al. 2012 ). Mutations in the gene encoding RPN1a, a subunit of the
26S proteasome, suppressed edr2 -associated disease resistance. RPN1a also has
been shown to be required for edr1 and pmr4 (recessive R gene)-mediated powdery
mildew resistance. The rpn1a mutant displayed enhanced susceptibility against
both G . cichoracearum and the bacterial pathogen Pseudomonas syringae pv.
tomato . The rpn1a mutant showed defects in SA accumulation upon P . syringae
pv. tomato infection (Yao et al. 2012 ). The results suggest that the 26S proteosome
is involved in triggering SA accumulation, probably by removing/degrading an
inhibitor of SA biosynthesis.
The pepper E3 ubiquitin ligase RING1 gene, CaRING1 , has been shown to regu-
late SA accumulation in pepper plants (Lee et al. 2011 ). Overexpression of CaRING1
gene in Arabidopsis thaliana showed enhanced accumulation of SA, while in
pepper plants, virus-induced gene silencing of CaRING1 lowered SA levels. The
results suggest that CaRING1 modulates SA levels. The E3 ubiquitin ligase activity
of CaRING1 modulated SA signaling in innate immune system. Overexpression of
CaRING1 in A. thaliana conferred enhanced resistance to the bacterial pathogen