Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 416

such a regulatory factor would be proteins that displace negative transcriptional regu-
lators of PR genes whose expression may be associated with son1 -mediated defense
responses. The target of SON1 may be NPR1 itself (Kim and Delaney 2002 ).

10.5 Ubiquitin-Proteasome in R -Gene Mediated

Early Signaling System

Several R genes encode receptor proteins (Kawchuck et al. 2001 ) and on perception
of the external stimuli by the receptor, a well-orchestrated signaling system is
activated. Ubiquitin-proteasome-mediated proteolytic pathway plays an important
role in the rice R protein XA21-mediated signaling system (Wang et al. 2006 ; Yang
et al. 2006 ). TheXA21 protein carries both a leucine-rich repeat motif and a serine-
threonine kinase-like domain, suggesting a role in cell surface recognition of a
pathogen ligand and subsequent activation of an intracellular defense response
(Song et al. 1995 ). Lee et al. ( 2009 ) reported that XA21 is a pattern recognition
receptor (PRR) and it recognizes a 194-amino acid protein designated Ax21 as a
pathogen-associated molecular pattern (PAMP).
A RING domain-containing protein named as XB3 (for XA21 binding protein 3)
interacts with the rice bacterial blight resistance gene Xa21-encoded protein (Wang
et al. 2006 ). The RING domain ubiquitinates XB3 protein, indicating that XB3 is an E3
ubiquitin ligase. XB3 is specifi cally transphosphorylated by the kinase domain of
XA21 (Wang et al. 2006 ). The activated XB3 may ubiquitinate a third protein and tar-
get its degradation. The degraded protein may be a negative regulator of the defense
signaling (Wang et al. 2006 ). It is suggested that XB3 protein may be involved in con-
ferring stability to the R protein XA21; otherwise, the XA21 protein would have been
degraded by a ubiquitin-proteasome-mediated proteolytic pathway. Thus the ubiquitin
ligase activity of XB3 protein is involved in the R gene-mediated signaling system.
Cf-9 is a disease resistance gene in tomato conferring resistance against
Cladosporiun fulvum . Transgenic tobacco plants expressing this gene were devel-
oped. Upon an elicitor treatment, the gene ACRE276 (Avr9/Cf-9 Rapidly Elicited
gene 276) was upregulated and the gene encoded an E3 ubiquitin ligase requiring
an intact U-box domain (Yang et al. 2006 ). ACRE276 RNA interference (RNAi)
silencing in tobacco resulted in loss of defense response induced by Cf resistance
genes. ACRE276 RNAi plants also lost defense responses induced by a viral elicitor
(Yang et al. 2006 ). Another ACRE gene, ACRE74, was also found to be induced
upon elicitor treatment in tobacco (González-Lamothe et al. 2006 ). ACRE74
encodes a U-box E3 ligase homolog, highly related to parsley CMPG1 (Kirsch et al.
2001 ) and Arabidopsis thaliana PLANT U-BOX20 (PUB20) and PUB21 proteins
(Azevedo et al. 2001 ), and was called NtCMPG1. The NtCMPG1 was shown to be
involved in induction of HR (hypersensitive response) in Cf9 tobacco after AVR9
elicitor infi ltration (González-Lamothe et al. 2006 ). A homolog of CMPG1 was
detected in tomato and it conferred resistance to the fungal pathogen Cladosporium
fulvum . It was shown to be involved in the Pto/AvrPto-mediated bacterial disease
resistance and Inf1-mediated oomycete resistance responses in tomato