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upstream of SA signaling and involved in SA production in Arabidopsis . Mutations
in eds1 (for enhanced disease susceptibility1 ), or pad4 (for phytoalexin-defi cient4 )
lead to reduced SA levels in infected leaves (Zhou et al. 1998 ; Gupta et al. 2000 ;
Feys et al. 2001 ). EDS1 is required for SA production and it controls SA production
to amplify defense signals (Rustérucci et al. 2001 ; Eulgem et al. 2004 ; Song et al.
2004 ). PAD4 is a key regulator acting at upstream of SA (Lippok et al. 2007 ).
Arabidopsis plants carrying pad4 mutations have a defect in accumulation of SA
upon pathogen infection (Zhou et al. 1998 ). PAD4 is required for amplifi cation of
weak signals to a level suffi cient for activation of SA signaling (Jirage et al. 1999 ).
The PAD4 protein sequence displays similarity to triacyl glycerol lipases and other
esterases (Jirage et al. 1999 ). It is suggested that EDS1 and PAD4 transduce ROS-
derived signals leading to SA production (Rustérucci et al. 2001 ; Wiermer et al.
2005 ). EDS1 and PAD4 may have a fundamental role in transducing redox signals.
EDS1 forms several molecularly and spatially distinct complexes with PAD4
(Wiermer et al. 2005 ; Xing and Chen 2006 ). Another gene EDS5 encodes a protein,
which transports precursors for SA biosynthesis. EDS5 exhibits homology to
multidrug and toxin extrusion (MATE) transporter proteins from animals (Nawrath
et al. 2002 ). EDS5 expression requires PAD4, placing EDS5 downstream of PAD4
(Nawrath et al. 2002 ). SIZ1 gene, which encodes an Arabidopsis SUMO E3 ligase,
regulates SA-mediated plant immunity. SIZ1 interacts epistatically with PAD4 and
EDS1 and inhibits the SA biosynthesis pathway (Lee et al. 2006 ).
10.6.2
Role of SUMOylation in SA-Mediated Systemic
Acquired Resistance
SUMO conjugation suppresses defense signaling in unstressed healthy plants
without any exposure to PAMP signals or pathogen invasion. Upon pathogen/PAMP
perception plant innate immune receptors activate various signaling pathways
that trigger host defenses. SUMO (Small ubiquitin-like modifi er) conjugation is
essential to suppress defense signaling in non-infected plants. SUMO conjugation
can transform transcription activators into repressors, thereby preventing defense
induction in the absence of a pathogen signal (van der Burg and Takken 2010 ).
SAR is activated upon recognition of pathogens and activation of SAR requires
SA, which induces SA-responsive gene expression. The SA-induced changes in
gene expression have been found to have a link to chromatin remodeling, such as
histone modifi cations and histone replacement. The recruitment of chromatin-
modifying complexes to SA-responsive loci controls their basal and SA-induced
expression (March-Diaz et al. 2008 ; van den Burg and Takken 2009 , 2010 ;
Jaskiewicz et al. 2011 ). Basal repression of these loci may require the post- translational
modifi er SUMO. SUMO conjugation has been reported to control the activity,
assembly and disassembly of chromatin-modifying complexes to transcription
complexes (van den Burg and Takken 2009 ). SUMO conjugation determines recruit-
ment and activity of chromatin-modifying enzymes, and thereby indirectly controls
gene expression (van der Burg and Takken 2010 ).
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