PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 58

2005 ). Identifi cation of extensive engagement of membrane-localized receptors and
regulators in signaling suggests that these membrane proteins represent a key aspect
of plant immunity.

2.12.2 BAK1 Is Required for Proper Functionality of PRRs

BAK1 appears to be a key component in plant innate immune system and suppres-
sion of BAK1 gene expression results in suppression of the plant innate immunity.
BAK1 -silenced plants were found to be more susceptible to adapted and non-adapted
Pseudomonas and to the oomycete Hyaloperonospora parasitica (Heese et al.
2007 ). Arabidopsis bak1 mutants are also extremely susceptible to necrotrophic
fungi Botrytis cinerea and Alternaria brassicicola (Kemmerling et al. 2007 ). These
results demonstrate the importance of BAK1 in immune signaling.
BAK1and/or its homologs have been shown to be required for full function of
PAMP-PRR signaling complex to activate plant immune responses (Schulze et al.
2010 ). Chinchilla et al. ( 2007b ) reported that plants mutated in BAK1 were strongly
affected in the PAMP elf18 responses. Arabidopsis bak1 mutants are largely
impaired in the PAMP fl g22-induced defense responses (Chinchilla et al. 2007b ;
Heese et al. 2007 ). Silencing of NbBAK1 in Nicotiana benthamiana results in
decreased responsiveness to the PAMPs CSP22 and INF1 (Heese et al. 2007 ).
BAK1 is required for proper functionality of several PRRs including FLS2, EFR,
CERK1, PEPR1, and PEPR2 (Postel et al. 2010 ; Zhang and Zhou 2010 ). BAK1 was
identifi ed as a positive regulator of the PRRs FLS2, EFR, and CSP22 (Chinchilla
et al. 2007a , b ; Heese et al. 2007 ; Zipfel 2008 ). It has been demonstrated that BAK1
forms a complex with FLS2 in a ligand-dependent fashion and is necessary for
FLS2 signaling (Chinchilla et al. 2007b ; Heese et al. 2007 ). BAK1 interacts with
PRRs and acts downstream of PRRs. BAK1 has been shown to be dispensable for
the PAMP fl g22 binding, but it interacts with the PRR FLS2 in a ligand-dependent
manner shortly after elicitation (Chinchilla et al. 2007b ; Heese et al. 2007 ).

2.12.3 BAK1 Acts Downstream of PRR Perception by PAMP

The BAK1 kinase activity is required for FLS2-mediated signaling but not fl g22
induced association of FLS2 and BAK1 (Schulze et al. 2010 ). It suggests that BAK1
acts downstream of FLS2 perception to regulate signaling (Chinchilla et al. 2007b ).
BAK1 is also required for the activation of another PRR CERK1 in chitin signaling
(Zhang and Zhou 2010 ).
Flg22 perception by FLS2 triggers an interaction between FLS2 and BAK1
(Chinchilla et al. 2007b ; Heese et al. 2007 ). The fl g22 induced FLS2-BAK1 asso-
ciation occurs within seconds and is accompanied by increased phosphorylation on
both FLS2 and BAK1 (Schulze et al. 2010 ).