PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 59

2.12.4 BAK1 Functions as an Adapter or Signaling

Partner for Regulation of PRRs

BAK1 is a central regulator of innate immunity in plants (Heese et al. 2007 ). PAMP-
PRR binding leads to the formation of a molecular platform that involves the recruit-
ment of adaptor proteins to trigger downstream signaling (Heese et al. 2007 ). BAK1
probably acts as a co-activator of the receptor complex to enhance various signaling
pathways (Wang et al. 2008a ). Upon PAMP perception, PRR rapidly associates with
BAK1, thereby initiating downstream signaling. BAK1 acts downstream of several
PRRs (Nicaise et al. 2009 ). BAK1 acts not only as an adapter of multiple PRRs includ-
ing the PRRs FLS2, EFR, PEPR1 and PEPR2, but also as an adapter of other trans-
membrane proteins BIR1 and SOBIR1. Unlike the direct binding of the PAMP to PRR
(Chinchilla et al. 2007b ; Kinoshita et al. 2005 ), BAK1 more likely functions as an
adapter or signaling partner for the regulation of PRRs. Furthermore, BAK1 is required
for the immune responses triggered by multiple PAMPs including fl agellin, the bacte-
rial elongation factor EF-Tu, peptidoglycans, lipopolysaccharides, cold-shock protein,
HrpZ (harpin), and the oomycete elicitor INF1 in Arabidopsis and tobacco (Chinchilla
et al. 2007a ; Heese et al. 2007 ; Shan et al. 2008 ). A BAK1 ortholog has been detected
in rice (Li et al. 2009a ) and it may be involved in the PAMP Ax21-mediated immunity
(Segonzac and Zipfel 2011 ). Thus, BAK1 appears to associate with multiple PRRs to
integrate specifi c PAMP perception into convergent downstream signaling. It is still not
known how the PAMP signal is transmitted from the BAK1-associated receptor com-
plexes at the plasma membrane to intracellular events (Lu et al. 2010 ).

2.12.5 Rapid Heteromerization and Phosphorylation of PRRs

and Their Associated Kinase BAK1

PRRs form tight complexes with the receptor kinase BAK1 instantaneously after
ligand binding. FLS2-BAK1 heteromerization occurs almost instantaneously after
perception of the PAMP fl g22. Flg22 can induce formation of a stable FLS2-BAK1
complex in microsomal membrane preparations in vitro (Schulze et al. 2010 ).
However, the kinase inhibitor K-252a does not prevent complex formation. The
results suggest that kinase activity of BAK1 is essential for FLS2 signaling, but not
for fl g22 induced association of FLS2 and BAK1 (Schulze et al. 2010 ).
Schulze et al. ( 2010 ) detected de novo phosphorylation of both FLS2 and BAK1
within 15 s of stimulation with fl g22. Similarly, brassinolide induces BAK1 phos-
phorylation within seconds. Bacterial EF-Tu and AtPep1 induce rapid formation of
heterocomplexes consisting of de novo phosphorylated BAK1 and proteins repre-
senting the ligand-specifi c binding receptors EF-Tu receptor and Pep1 receptor,
respectively. It is suggested that several LRR-RKs form tight complexes with BAK1
almost instantaneously after ligand binding and the subsequent phosphorylation
events are key initial steps in signal transduction (Schulze et al. 2010 ).