A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence

Abstract

Plants sense potential microbial invaders by using patternrecognition receptors to recognize pathogen-associated molecular patterns (PAMPs) 1 . In Arabidopsis thaliana, the leucine-rich repeat receptor kinases flagellin-sensitive 2 (FLS2) (ref.2) and elongation factor Tu receptor (EFR) (ref.3) act as pattern-recognition receptors for the bacterial PAMPs flagellin 4 and elongation factor Tu (EF-Tu) (ref. 5) and contribute to resistance against bacterial pathogens. Little is known about the molecular mechanisms that link receptor activation to intracellular signal transduction. Here we show that BAK1 (BRI1-associated receptor kinase 1), a leucinerich repeat receptor-like kinase that has been reported to regulate the brassinosteroid receptor BRI1 (refs 6,7), is involved in signalling by FLS2 and EFR. Plants carrying bak1 mutations show normal flagellin binding but abnormal early and late flagellintriggered responses, indicating that BAK1 acts as a positive regulator in signalling. The bak1-mutant plants also show a reduction in early, but not late, EF-Tu-triggered responses. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. We provide evidence that FLS2 and BAK1 form a complex in vivo, in a specific ligand-dependent manner, within the first minutes of stimulation with flagellin. Thus, BAK1 is not only associated with developmental regulation through the plant hormone receptor BRI1 (refs 6,7), but also has a functional role in PRR-dependent signalling, which initiates innate immunity.PAMPs have key roles as activators of the innate immune response in animals 8 and, analogously, as 'general elicitors' of defence responses in plants [1][2][3][4][5][9][10][11] . We have previously characterized FLS2 and EFR as the pattern-recognition receptors (PRRs) for flagellin (represented by a 22-amino-acid peptide, flg22) and for EF-Tu (represented by the peptides elf18 and elf26, which correspond to its amino terminus), respectively 2-5,10 . Flagellin and EF-Tu rapidly induce a common set of Arabidopsis genes for leucine-rich repeat receptor-like kinases (LRR-RLKs), including FLS2 and EFR themselves 3,11 . This led to the assumption that some of these PAMPinduced LRR-RLKs might encode additional components of PAMP perception or signalling. Using a reverse genetic approach, we tested a collection of insertional mutants in these LRR-RLKs (previously used for identification of the EFR gene 3 ) for responsiveness to flg22 and found that two mutants with insertions in the LRR-RLK gene At4g33430 have reduced sensitivity to flg22 in seedling growth assays (Fig. 1a,b). In more than 10 repetitions of seedling growth assays with these mutants, we always observed a clear reduction (but never a complete loss) of sensitivity to flg22 and flg22-related peptides. By contrast, the mutants seemed to be as sensitive as the wild type to treatment with elf18 in more than five seedling growth assays (Fig. 1b and data not shown).

Crystal Structure of the Complex between Pseudomonas Effector AvrPtoB and the Tomato Pto Kinase Reveals Both a Shared and a Unique Interface Compared with AvrPto-Pto

“…Moreover, both the FLS2 kinase domain and Pto bind AvrPto in a competitive manner, supporting structural similarity between FLS2-AvrPto and Pto-AvrPto interaction (Xiang et al, 2008). Interestingly, AvrPto also appears to bind BAK1 (Shan et al, 2008), a protein required for FLS2 and EFR1 function (Chinchilla et al, 2007;Heese et al, 2007).…”

Arabidopsis thaliana Pattern Recognition Receptors for Bacterial Elongation Factor Tu and Flagellin Can Be Combined to Form Functional Chimeric Receptors

“…EFR and FLS2, although detecting different ligands, share all functional aspects of receptor activation and signal output. Activation of both receptors occurs according to the address-message concept, presumably involving ligand-induced, conformational changes that allow heteromerization with co-receptors such as BAK1 (18,21,39,40). Thus, for substitutions with parts of FLS2 that fully retain elf binding, one would rather expect functionality of receptor activation to be retained as well.…”

Functional importance of BAK1 tyrosine phosphorylation in vivo

“…That phosphorylation of BAK1 at the Tyr-610 sites plays a positive role in BR which is also known as BAK1, can multitask 24 as coreceptor with BRI1 in BR signaling, 2,25 BIR1 in control of cell death, 26 FLS2 in flg22 signaling, 27 EFR in elf18 signaling, 28 and PEPR1 and PEPR2 in Pep1 signaling. 29,30 In its role as coreceptor, BAK1 is thought to bind to the receptor kinase in a ligand-dependent manner, and to then autophosphorylate and also transphosphorylate sites on the receptor kinase.…”