Regulation of Cell Death and Innate Immunity by Two Receptor-like Kinases in Arabidopsis

Abstract

Upon recognition of bacterial flagellin, the plant receptor FLS2 heterodimerizes with brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) and activates plant defense responses. Because constitutive activation of defense responses is detrimental, plant resistance signaling pathways must be negatively controlled, although the mechanisms involved are unclear. We identified Arabidopsis BIR1 as a BAK1-interacting receptor-like kinase. Knocking out BIR1 leads to extensive cell death, activation of constitutive defense responses, and impairment in the activation of MPK4, a negative regulator of plant resistance (R) protein signaling, by flagellin. sobir1-1, a mutant obtained in a screen for suppressors of the bir1-1 phenotype, rescued cell death observed in bir1-1. SOBIR1 encodes another receptor-like kinase whose overexpression activates cell death and defense responses. Our data suggest that BIR1 negatively regulates multiple plant resistance signaling pathways, one of which is the SOBIR1-dependent pathway identified here.

Cautionary Notes on the Use of C-Terminal BAK1 Fusion Proteins for Functional Studies

“…affect (in a nonmutually exclusive manner) phosphorylation events within the PRR-BAK1 complex, association and/or dissociation with downstream components, and phosphorylation events with direct substrates. It is becoming increasingly clear that, in addition to their role in BR responses, BAK1 and other SERKs in Arabidopsis and other plant species are master regulators of plant innate immunity (Heese et al, 2007;Shan et al, 2008;Fradin et al, 2009Fradin et al, , 2011Santos et al, 2009;Bar et al, 2010;Krol et al, 2010;Chaparro-Garcia et al, 2011;Mantelin et al, 2011;Roux et al, 2011;Yang et al, 2011) as well as of cell death control Kemmerling et al, 2007;Gao et al, 2009;Wang et al, 2011), pollen development (Albrecht et al, 2005;Colcombet et al, 2005), and organ abscission (Lewis et al, 2010). One must not assume that the model based on analyses between BRI1 and BAK1 is universal, and, in light of our results, it is essential that researchers analyze in detail and quantify the impact of modifying the SERK proteins in transgenic approaches when studying SERKdependent signaling pathways.…”

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.…”

CAST AWAY, a Membrane-Associated Receptor-Like Kinase, Inhibits Organ Abscission in Arabidopsis

“…This approach has been successfully used to detect interactions between membrane-bound LRR-RLKs, such as the SERK family members (SERK1, SERK2, BAK1, and BAK1-LIKE1), and BAK1-INTERACTING RECEPTOR-LIKE KINASE1 (Russinova et al, 2004;Albrecht et al, 2005;Karlova et al, 2006;Gao et al, 2009). To facilitate this analysis, constructs to express full-length versions of CST, HAE, and EVR with C-terminal translational fusions to either the N-terminal half of YFP (YFPn) or the C-terminal half of YFP (YFPc) were generated.…”