PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 57

2.12 Transmembrane Proteins Interacting with PRRs

in PAMP-PRR Signaling Complex

2.12.1 Signaling Adapters/Amplifi ers in PAMP-PRR

Signaling Complex

Plant pattern recognizing receptors (PRRs) interact with additional transmem-
brane proteins that act as signaling adapters or amplifi ers to achieve full func-
tionality (Zipfel 2009 ). These transmembrane proteins include BAK1 (for B RI1
(BRASSINOSTEROID INSENSITIVE1)- A SSOCIATED K INASE1), BIK1
( B OTRYTIS- I NDUCED K INASE 1 ), and BIR1 ( B RANCHING I NHIBITING
R ECEPTOR 1 ) (Postel et al. 2010 ; Zhang and Zhou 2010 ; Zhang et al. 2010a ).
BAK1 is also called SERK3 (for Somatic EMBRYO RECEPTOR KINASE3) and
it belongs to the LRR-receptor-like kinase (RLK). BAK1 belongs to the LRR type
II SERK subfamily (Chinchilla et al. 2007b ; Heese et al. 2007 ; Zipfel 2008 ).
BAK1 is a component of diverse processes, including brassinosteroid signaling,
light responses, cell death, and plant innate immunity (Chinchilla et al. 2009 ).
BAK1 was originally identifi ed as a BRI1-associated receptor kinase mediating
brassinosteroid signaling (Li et al. 2002 ; Nam and Li 2002 ). Brassinosteroids
(BR), a class of plant hormone with essential roles in plant growth and develop-
ment, are perceived by LRR-RK BRI1, which is structurally similar to the PRR
FLS2 (Belkhadir et al. 2006 ). BAK1 is a positive regulator of PAMP-triggered
plant immunity and it acts as an adaptor of multiple LRR-RKs that act in defense
signaling, including the PRRs FLS2, EFR, PEPR1 and PEPR2 (Chinchilla et al.
2007a , b ; Ryan et al. 2007 ; Gao et al. 2009a ; Postel et al. 2010 ; Schulze et al.
2010 ). It also acts as an adaptor of the receptor kinases BIR1 and SOBIR1, which
seem to act as part of a presumed PRR complex(es) and/or at a downstream step
in the signaling cascade (Saijo 2010 ).
BAK1 forms a complex with the RLK BIR1 to negatively regulate defense
responses (Gao et al. 2009a ). BIK1 is a receptor-like cytoplasmic kinase
(RLCK). The PAMP fl g22 interacts with BAK1 to phosphorylate BIK1, which
seems to act as positive regulator of the PAMP signaling pathway (Lu et al.
2010 ; Zhang et al. 2010a ). BIK1 is an essential component in PAMP signal
transduction, which links the PAMP receptor complex to downstream intracel-
lular signaling (Lu et al. 2010 ).
Zhang et al. ( 2010a ) identifi ed a number of PBS1-like (PBL) RLCKs (cytoplas-
mic receptor-like kinases), including BIK1 and several other PBLs, as components
in PAMP- signaling pathways. PBL proteins belong to the subfamily VII of cyto-
plasmic receptor-like protein kinases (Zhang et al. 2010a ). PBL1, PBL2, and PBS1
appear to additively contribute to PAMP- signaling (Zhang et al. 2010a ). AtPHOS32,
AtPHOS34, and AtPHOS43 are the other signaling components in PAMP-triggered
immunity (Peck et al. 2001 ; Merkouropoulos et al. 2008 ). Another LRR-RLK,
ERECTA has been shown to interact with PRRs (Godiard et al. 2003 ; Llorente et al.