PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 109

important transmembrane protein, which interacts with PRRs and acts downstream
of PRRs. (Chinchilla et al. 2007a , b ; Heese et al. 2007 ). BAK1 is a signal amplifi er
(Nicaise et al. 2009 ). BAK1has been shown to be required for full function of
PAMP-PRR signaling complex to activate plant immune responses (Schulze et al.
2010 ). BAK1 is required for proper functionality of several PRRs including FLS2,
EFR, CERK1, PEPR1, and PEPR2 (Postel et al. 2010 ; Zhang and Zhou 2010 ).
Some effectors block the action of BAK1. The effectors AvrPto and AvrPtoB
bind to BAK1 and thereby blocking its interaction with the PRR FLS2 resulting in
suppression of immunity (Shan et al. 2008 ). AvrPtoB binds BAK1 during infection
and impede BAK1-dependent plant defense responses (Shan et al. 2008 ). AvrPtoB
directly targets the kinase domains of BAK1 (Shan et al. 2008 ). Suppression of
BAK1 by AvrPtoB may occur by inhibition of kinase activity and may have broad
effects through the multiple BAK1-dependent PAMP-signaling pathways (Shan
et al. 2008 ). AvrPtoB targets BAK1 to disrupt the complex (Hann et al. 2010 ).
However, Xiang et al. ( 2011 ) provided evidences to show BAK1 is not targeted by
the P . syringae effector AvrPto in plants.

2.31.5 Effectors May Target the Receptor-Like Cytoplasmic

Kinases BIK1 and PBL1

Downstream of PAMP-PRR-BAK1 signaling complex, several receptor-like
cytoplasmic kinases (RLCK) play important role in regulation of the signaling
pathways. The important RLCKs involved in the signaling complex in Arabidopsis
include BIK1 (Botrytis-induced kinase 1), PBS1 (AvrPphB susceptible 1), and
PBS1-like (PBL) proteins. The BIK1 plays an important role in mediating early
fl agellin signaling from the FLS2/BAK1 receptor complex (Lu et al. 2010 ).
BIK1 forms a complex with unstimulated FLS2 in plants, and the PAMP fl g22
induces a rapid phosphorylation of BIK1 in both an FLS2- and BAK-dependent
manner (Lu et al. 2010 ; Zhang et al. 2010a ; Wu et al. 2011 ). BIK1 links the
PAMP-PRR signaling complex to downstream intracellular signaling (Lu et al.
2010 ). PBS1-like (PBL) cytoplasmic receptor-like kinases (RLCKs) act addi-
tively with BIK1 in plant innate immune system (Zhang et al. 2010a ). PBS
receptor-like kinases are required for signaling from multiple PAMPs and act
downstream of FLS2, EFR, and CERK1 to trigger immune responses. It has
been shown that BIK1 and PBS proteins integrate immune signaling from mul-
tiple PRRs (Zhang et al. 2010a ).
Zhang et al. ( 2010a ) presented evidence that several related Arabidopsis
cytoplasmic receptor kinases, exemplifi ed by BIK1 and PBS1 are cleaved by
AvrPphB, an effector from the pathogenic bacterium P . syringae (Zhang et al.
2010a ). In addition to cleavage of BIK1 and PBS1, AvrPphB targets about 10 PBL
kinases, including PBL1 and PBL2 (Zhang et al. 2010a ). The effector AvrPphB is a
cysteine protease that cleaves PBS1 kinase (Ade et al. 2007 ). Plants lacking BIK1