PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 78

not have any signifi cant role in biogenesis and accumulation of FLS2 (Li et al.
2009b ). Like CRT3 and UGGT, ERD2b is also not required for FLS2 function
(Li et al. 2009b ). ERD2b is Golgi-localized and it is required for CRT3 protein
accumulation (Li et al. 2009b ). However, the erdj3b-1 mutant was impaired in fl g22
responses. ERDj3B is an ER-localized member of the HSP40 co-chaperone family
and it may be involved in fl g22 biogenesis (Nekrasov et al. 2009 ). It has also been
reported that in the ER-QC mutants, EFR levels are greatly reduced while FLS2
levels remain unaffected, suggesting that ER-QC may not be involved in FLS2 sig-
naling (Saijo 2010 ).

2.17.7 Role of BiP3 in ERQC of the Rice PRR XA21

XA21 is glycosylated and is primarily localized to the ER and also to the plasma
membrane (Park et al. 2010a ). BiP3, the ER-localized chaperone HSP70, regulates
XA21 processing and stability. BiP may serve as a PRR chaperone, and it may be
involved in processing and degradation of XA21 (Park et al. 2010a ). The rice BiP3
gene encodes a 666 amino acid protein with an approximately 45 kDa domain at the
N-terminus that is predicted to carry ATPase catalytic activity and a domain of
approximately 25 kDa at the C-terminus having a predicted substrate-binding
domain (Park et al. 2010a ). In BiP3 -overexpressing rice plants, XA21-mediated
immunity is down-regulated and XA21 stability is signifi cantly decreased. The
results indicate that BiP3 regulates XA21 protein stability and processing and this
regulation is critical for resistance to the bacterial pathogen Xanthomonas oryzae
pv. oryzae (Park et al. 2010a ).
BiPs are known to be involved in targeting unfolded glycoproteins for ER-associated
degradation (ERAD) activity (Kleizen and Braakman 2004 ). If glycoproteins are
not able to acquire their native performance within an appropriate time, misfolded
or unassembled proteins are retained due to the ER-QC system. If unfolded and/or
misfolded proteins may over accumulate in ER after PRR signaling and the cells
may either initiate ER-associated cell death or attenuate the signal transduction
pathway (Park et al. 2010a ). BiP is involved in degradation of these proteins and
these proteins are ultimately destroyed by ERAD (Kleizen and Braakman 2004 ;
Meusser et al. 2005 ).
The rice PRR XA21 is highly glycosylated and this N-glycosylation is impor-
tant for correct protein folding and ERAD (Kleizen and Braakman 2004 ). It has
been shown that BiP3 accumulation drives glycosylated XA21 to the ERAD sys-
tem, inhibiting its further processing (Park et al. 2010a ). Fine control of membrane-
resident PRR activity is essentially achieved by a combination of proper ER
folding, degradation and traffi cking of PRRs. Strict elimination of the misfolded
proteins may occur by the action of BiP, which would avoid precocious immune
activation (Saijo 2010 ).