Abstract
The leucine-rich repeat receptor-like kinase BRASSINOSTEROID-INSENSITIVE1 (BRI1) is the main ligand-perceiving receptor for brassinosteroids (BRs) in Arabidopsis (Arabidopsis thaliana). Binding of BRs to the ectodomain of plasma membrane (PM)-located BRI1 receptors initiates an intracellular signal transduction cascade that influences various aspects of plant growth and development. Even though the major components of BR signaling have been revealed and the PM was identified as the main site of BRI1 signaling activity, the very first steps of signal transmission are still elusive. Recently, it was shown that the initiation of BR signal transduction requires the interaction of BRI1 with its SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) coreceptors. In addition, the resolved structure of the BRI1 ectodomain suggested that BRI1-ASSOCIATED KINASE1 [BAK1](SERK3) may constitute a component of the ligand-perceiving receptor complex. Therefore, we investigated the spatial correlation between BRI1 and BAK1(SERK3) in the natural habitat of both leucine-rich repeat receptor-like kinases using comparative colocalization analysis and fluorescence lifetime imaging microscopy. We show that activation of BR signaling by exogenous ligand application resulted in both elevated colocalization between BRI1 and BAK1(SERK3) and an about 50% increase of receptor heterooligomerization in the PM of live Arabidopsis root epidermal cells. However, large populations of BRI1 and BAK1(SERK3) colocalized independently of BRs. Moreover, we could visualize that approximately 7% of the BRI1 PM pool constitutively heterooligomerizes with BAK1(SERK3) in live root cells. We propose that only small populations of PM-located BRI1 and BAK1(SERK3) receptors participate in active BR signaling and that the initiation of downstream signal transduction involves preassembled BRI1-BAK1(SERK3) heterooligomers.
Highlights
The leucine-rich repeat receptor-like kinase BRASSINOSTEROID-INSENSITIVE1 (BRI1) is the main ligand-perceiving receptor for brassinosteroids (BRs) in Arabidopsis (Arabidopsis thaliana)
Our results show that only around 10% of BRI1 receptors form heterooligomers with BRI1-ASSOCIATED KINASE1 (BAK1)(SERK3) during active BR signaling in the plasma membrane (PM), the main site of BRI1 signaling activity (Irani et al, 2012), of root epidermal cells
The observation that only small populations of BRI1 and SERK3 form heterooligomers is in line with those obtained by semiquantitative coimmunoprecipitation experiments (Albrecht et al, 2012)
Summary
The leucine-rich repeat receptor-like kinase BRASSINOSTEROID-INSENSITIVE1 (BRI1) is the main ligand-perceiving receptor for brassinosteroids (BRs) in Arabidopsis (Arabidopsis thaliana). Unlike animal steroid signaling, which employs intracellular steroid receptors, in Arabidopsis (Arabidopsis thaliana), BR signaling is mainly mediated via the plasma membrane (PM)-located leucine-rich repeat (LRR) receptor-like kinase (RLK) BRASSINOSTEROIDINSENSITIVE1 (BRI1; Li and Chory, 1997). This receptor perceives BRs at the cell surface (He et al, 2000; Kinoshita et al, 2005) and initiates an intracellular signal transduction cascade, which controls various aspects of plant growth and development (Clouse, 2011; Kutschera and Wang, 2012; Wang et al, 2012). The coreceptor BAK1 (SERK3) may already participate in BR perception rather than being recruited to hormone-bound BRI1 receptors, which would imply that the two RLKs can form constitutive heterooligomers
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