FLS2 recognizes a bacterial flagellin peptide fragment (flg22) and triggers defense response (Gomez-Gomez and Boller, 2002). Recent studies by Robatzek et al. (2006) demonstrated that FLS2 is ubiquitously expressed and plasma-membrane localized in the absence of a ligand, but the addition of the ligand, flg22, rapidly induces internalization of FLS2 into endosomes (Fig. 1.2b). Therefore unlike BRI1, which exhibits constitutive endocycles, FLS2 is subjected to a ligand-induced endocytosis (Robatzek et al., 2006). The internalized FLS2 is most likely targeted for destruction to quench the innate immune response (Robatzek et al., 2006).
Two PAMP receptors, FLS2 and EFR, recognize unrelated ligands, flg22 and elf18, respectively, via direct binding (Chinchilla et al., 2006; Zipfel et al., 2006). Nevertheless, both FLS2 and EFR trigger a common set of target gene expression potentially via a downstream map kinase cascade (Zipfel et al., 2006). «-
Figure 1.2 (a) Model of BRI1 activation. (Left) In the absence of BR, both BRI1 and BAK1 form homodimers. The BRI1 homodimer is inactive due to autoinhibition of its C-terminus as well as association with the negative regulator, BKI1. Both BRI1 and BAK1 are constitutively recycled into endomembranes. (Right) The BR binding to BRI1 leads to its activation (and dissociation of BKI1) via phosphorylation, and formation of receptor heterodimer (heterotetramer) with BAK1. The receptor heterodimers are internalized into endosomes, and they signals to inhibit BIN2 kinase via unknown mechanism. In the absence of BR signaling, BIN2 inhibits BES1/BZR1 activity via phosphorylation. Dephosphorylation of BES1/BZR1 transcription factors (mediated via BSU1 phosphatase) activates the target gene expression (b). Model of FLS2 activation. (Left) In the absence of PAMP (flg22), both FLS2 and BAK1 likely form homodimers. (Right) The flg22 binding to FLS2 leads to its activation via phosphorylation and formation of receptor heterodimer (heterotetramer) with BAK1. The active receptor complex is internalized rapidly via ligand-dependent manner. The signal may be transduced via mitogen-activated protein kinase (MAP kinase) cascades and eventually trigger innate immune response likely via WRKY-family transcription factors.
How can two distinct ligand-receptor pairs induce common downstream targets? Through a genome-wide transcriptome analysis of PAMP-induced genes and biochemical purification, Chinchilla et al. (2007) and Hesse et al. (2007) made an intriguing discovery that BAK1, a known heterodimeric partner of BRI1, in addition acts as a heterodimeric partner of FLS2 for defense signaling (Chinchilla et al, 2007; Heese et al, 2007) (Fig. 1.2b). Similar to fls2, bak1 mutant seedlings are insensitive to flg22, indicating that BAK1 is required for flagellin perception in Arabidopsis. Application of flg22 rapidly induced FLS2-BAK1 receptor complex formation. These results point to the mechanism by which the recognition of a specific PAMP by FLS2 leads to association with BAK1 and active receptor complex formation, which in turn signals to downstream components. Similar to BRI1, FLS2 may exist as an inactive homodimer at the plasma membrane in the absence of its ligand. Consistently, the absence of BAK1 has no effects on flg22 binding to FLS2 (Chinchilla et al, 2007).
BAK1's role as a transducer of PAMP response goes beyond FLS2, given that BAK1 is required in restricting other bacterial and oomycetes infections (Heese et al, 2007). Furthermore, BAK/SERK-family LRR-RLKs control multiple developmental processes, such as somatic embryogenesis and mi-crosporocyte formation. Thus, BAK/SERK-family RLKs may form an active receptor heterodimers with numbers of different LRR-RLKs.
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