Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 419

The SIZ1 gene, which encodes an Arabidopsis SUMO E3 ligase, regulates SAR.
Mutant siz1 plants exhibit constitutive SAR characterized by elevated accumulation
of salicylic acid and increased resistance to Pseudomonas syringae pv. tomato .
Transfer of the NahG gene to siz1 plants results in reversal of these phenotypes back
to wild-type. Analyses of the double mutants, npr1 siz1 , pad4 siz1 , ndr1 siz1
revealed that SIZ1 controls SA signaling (Lee et al. 2007 ). The results suggest that
SUMOylation suppresses SA signaling at the level of transcription. It is suggested
that SUMO conjugation of the SA-dependent transcription factors may transform
the transcription factors into repressors. Pathogen invasion/PAMP application,
which activates phosphorylation, may revert back the transcription repressors into
transcription activators (van den Burg and Takken 2010 ). Sumoylation transforms
an activator into a repressor and phosphorylation induced by MAP kinases may
convert the repressor into activator. The interaction between sumoylation and phosphor-
ylation may determine the immune response. Plants with disturbed SUMOylation
levels exhibit constitutive expression of early and late defense genes, increased
accumulation of SA, and increased disease resistance (Lee et al. 2007 ; van den Burg
and Takken 2009 ).

10.7 Pathogens May Subvert Ubiquitin-Proteasome

System to Cause Disease

Ubiquitin-proteasome system has been shown to be involved in plant immune sys-
tem. Many E3 ligase class RING fi nger proteins have been found to be induced in
plants by pathogen attack and play an important role in plant defense (Zeng et al.
2006 ). The fungal PAMP chitin up-regulated the ATL2 and ATL6 genes encoding
RING proteins in Arabidopsis (Salinas-Mondragón et al. 1999 ). The bacterial
PAMP fl g22 upregulated the genes encoding ten putative RING fi nger E3 ligases in
Arabidopsis (Navarro et al. 2004 ). The fungal PAMP N-acetyloligosaccharide has
been shown to induce the expression of EL5 RING E3 ligase (Takai et al. 2002 ).
Overexpression of the pepper E3 ubiquitin ligase RING1 gene confers enhanced
resistance against Pseudomonas syringae pv. tomato and Hyaloperonospora arabi-
dopsidis in Arabidopsis , which was accompanied by rapid production of SA and
various PR genes (Lee et al. 2011 ). U-box E3 ligases have also been implicated in
plant defense response (Zeng et al. 2004 ; González-Lamothe et al. 2006 ; Yang et al.
2006 ). Some E3 ubiquitin ligases have been shown to negatively regulate plant
defense responses (Hong et al. 2004 ; Trujillo et al. 2008 ; Zhang et al. 2012 ).
Recently it has been reported that some pathogens interfere with the ubiquitin-
proteasome system and subvert the defense responses induced by the proteolysis to
cause disease. Park et al. ( 2012 ) showed that the rice blast pathogen Magnaporthe
oryzae produces an effector called AvrPiz-t. The effector targets the RING E3 ubiq-
uitin ligase APIP6 to suppress PAMP-triggered immunity in rice. AvrPiz-1 accumu-
lates in the specialized structure called the biotrophic interfacial complex and is