PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 61

In addition to phosphorylation, fl g22 induces a dissociation of BIK1 from
FLS2 (Zhang et al. 2010a ; Lu et al. 2010 ). The flg22-induced BAK1-FLS2
association is not affected by the ATP-binding site mutant form of BIK1, which
does not dissociate from FLS2. In contrast, the fl g22-induced phosphorylation of
BIK1 and BIK1-FLS2 dissociation requires BAK1. Together, these results
support the proposal that BIK1 acts downstream of FLS2 and BAK1. ATP-
binding site and phosphorylation site mutant forms of BIK1 dominantly
inhibit PAMP triggered immunity (PTI), indicating that the activated BIK1
kinase positively regulates PTI signaling. It is possible that the dissociation of
the phosphorylated BIK1 and PBL1 proteins from FLS2 allows the activation of
other components downstream of BIK1 and PBL1 (Zhang et al. 2010a ). BIK1 is
also required for the PAMPs elf18 and chitin-induced responses. BIK1 interacts
with the PRRs CERK1 and EFR in protoplasts (Zhang et al. 2010a ). BAK1 and/
or its homologs are required for the activation of the receptor kinase CERK1 in
chitin signaling (Zhang and Zhou 2010 ). The ligand-induced EFR-BAK1 inter-
action has been reported in the PAMP EF-Tu signaling (Schulze et al. 2010 ) and
BIK1 has been shown to be required for interaction with the EFR (Zhang et al.
2010a ). These studies suggest that upon PAMP binding, a complex forms between
PRR, BAK1, and BIK1. An Arabidopsis bik1 mutant is severely compromised in
defense responses induced by fl g22, elf18, and chitin, indicating that BIK1 plays
a critical role in the integration of signals from multiple PRRs (Lu et al. 2010 ).
The results demonstrate that BIK1 mediates PAMP-triggered immunity signal
transduction from multiple PAMP receptor complexes. The BIK1 is an essential
component in PAMP signal transduction, which links the PAMP receptor
complex to downstream intracellular signaling (Lu et al. 2010 ). BIK1 may play
a central role in signal integration from multiple surface-localized receptors
(Zhang et al. 2010a ).

2.12.7 PBL Proteins

Zhang et al. ( 2010a ) identifi ed a number of the Arabidopsis resistance gene PBS1-
like (PBL) cytoplasmic receptor-like kinases (RLCKs) as components in PAMP-
signaling pathways. PBL proteins belong to the subfamily VII of cytoplasmic
receptor-like protein kinases. One of the PBL proteins, BIK1, is required for sig-
naling elicited by fl g22, elf18, and chitin and is essential for PAMP-induced resis-
tance to P . syringae. Other members including PBL1, PBL2, and PBS1 also
contribute to PAMP-triggered defenses. BIK1 is localized to the plasma mem-
brane (Veronese et al. 2006 ). PBS1 and many other PBL proteins may also local-
ize to the plasma membrane, because these proteins possess putative myristoylation
and palmitoylation sites at the N-terminus (Zhang et al. 2010a ). PBL1, PBL2, and
PBS1 appear to additively contribute to PAMP- signaling as their corresponding
mutants showed slightly reduced PAMP-induced responses (Zhang et al. 2010a ).