Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 417

(González-Lamothe et al. 2006 ). CMPG1 gene was also rapidly induced after elici-
tation with the oomycete elicitor Pep-13 in parsley (Kirsch et al. 2001 ). Arabidopsis
PUB20 expression is induced after treatment with the oomycete pathogen elicitor
Pmg and bacterial elicitor fl g22 (Heise et al. 2002 ; Navarro et al. 2004 ). These
results suggest the involvement of U-box E3 ligases in defense signaling system.
Another U-box protein was detected in Arabidopsis and it was called PUB17
(PLANT U-BOX17) and it is a homolog of tobacco ARCE276 (Yang et al. 2006 ).
Arabidopsis PUB17 knockout plants lost the R genes RPM1 - and RPS4 -mediated
resistance against Pseudomonas syringae pv. tomato . Transiently expressing PUB17 in
Cf-9 tobacco silenced for ACRE276 restored defense-related hypersensitive response
(Yang et al. 2006 ). These observations suggest the U-box E3 ubiquitin ligase PUB17 is
also involved in defense signaling system. Another U-box protein involved in ubiqui-
tin- proteasome proteolytic pathway is ARC1 in Brassica napus (Yang et al. 2006 ).
SGT1 (for Suppressor of the G2 allele of SKP1) is a plant disease resistance
response protein required for the function of multiple R genes (Austin et al. 2002 ;
Azevedo et al. 2002 ; Peart et al. 2002 ; Tör et al. 2002 ; Fu et al. 2009 ; Kud et al.
2013 ). It is a co-chaperone in the HSP90-SGT1-RAR1 molecular chaperone com-
plex, a core modulator in plant immunity (Seo et al. 2008 ). SGT1 associates with
SKP1, a component of the SCF-type E3 complexes (Liu et al. 2002 ). It is required
for the function of an SCF complex (Kitagawa et al. 1999 ; Peart et al. 2002 ). It
suggests that SGT1 plays key role in ubiquitin-proteasome-mediated proteolytic
pathway. SGT1 plays an important regulatory role in early R -gene-mediated plant
defense responses (Austin et al. 2002 ; Azevedo et al. 2002 ; Bieri et al. 2004 ; Wang
et al. 2006 ; Seo et al. 2008 ). SGT1 functions in R protein accumulation in disease
resistance (Azevedo et al. 2006 ). Silencing of SGT1 in Nicotiana benthamiana
results in reduced steady-state levels of R proteins (Azevedo et al. 2006 ). It suggests
that the major function of SGT1 may be in conferring stability to R proteins,
probably by degrading the proteins involved in reducing the stability of R proteins.
It has been shown that SGT1 contributes to the Prf -mediated defense responses by
stabilizing Prf protein via its co-chaperone activity (Kud et al. 2013 ).

10.6 Small Ubiquitin-Like Modifi er (SUMO)

in Plant Immunity

10.6.1 Role of SUMOylation in SA Biosynthesis

SUMO is a post-translational modifi cation that can be reversibly conjugated to target
proteins, similar to its well-known cousin Ubiquitin (Miura and Hasegawa 2010 ). In
contrast to ubiquitinylation, reversible SUMO conjugation only requires the actions
of the SUMO activating enzymes SAE1/SAE2, the SUMO conjugating enzyme
SCE1 and SUMO proteases. SUMO conjugation is promoted by SUMO E3 ligases,
such as SIZ1 or HPY2 (NSE1) (Ishida et al. 2009 ; Miura and Hasegawa 2010 ).
SUMOylation plays an important role in SA signaling system. It alters the levels
of SA in the immune response pathway. Several regulatory proteins are involved in