Plant biology

Dual-Personality Inhibitor

Science Signaling  29 Nov 2011:
Vol. 4, Issue 201, pp. ec332
DOI: 10.1126/scisignal.4201ec332

Brassinosteroids are plant steroid hormones that promote growth and differentiation by activating the membrane-localized receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1), which phosphorylates various substrates, including one of its inhibitors, BRI1 KINASE INHIBITOR 1 (BKI1). Wang et al. found that Arabidopsis plants overexpressing a fluorescently tagged version of the C terminus of BKI1 (BKI1-C-YFP) were larger than control plants, a phenotype suggestive of enhanced brassinosteroid signaling. Changes in gene expression in response to the brassinosteroid epi-brassinolide were accentuated in BKI1-C-YFP lines. Treatment of seedlings with epi-brassinolide elicited phosphorylation of BKI1, and mass spectrometric analysis of BKI1 phosphorylated in vitro by BRI1 identified four potential phosphorylation sites. Transgenic plants expressing a BKI1 mutant with alanine substitutions at Ser270 and Ser274 (BKI1S270/274A) were smaller than control plants, and biochemical analysis suggested that these two serine residues were the main sites of BRI1-mediated phosphorylation. BKI1 interacted with the phosphoserine and phosphothreonine-interacting proteins 14-3-3κ and 14-3-3λ in various assays. The interaction between BKI1 and the 14-3-3 proteins required intact serine residues at positions 270 and 274 (which are located in the C terminus of BKI1) and was increased by epi-brassinolide. Overexpression of 14-3-3κ resulted in slightly smaller plants and rescued the dwarf phenotype of BKI1, but not BKI1S270/274A, plants. 14-3-3κ interacts with BES1 (BRI1-EMS suppressor 1), a transcription factor that becomes dephosphorylated during brassinosteroid signaling and translocates to the nucleus. The association between 14-3-3κ and BES1 was reduced by epi-brassinolide, and BKI1-C expression increased the nuclear localization of dephosphorylated BES1. Phosphorylated BKI1, but not unphosphorylated BKI1 or BKI1S270/274A, reduced the interaction of 14-3-3κ with BES1. Thus, BKI1 can both inhibit and promote brassinosteroid signaling, and the C terminus of BKI1 promotes brassinosteroid signaling by titrating 14-3-3 proteins from BES1.

H. Wang, C. Yang, C. Zhang, N. Wang, D. Lu, J. Wang, S. Zhang, Z.-X. Wang, H. Ma, X. Wang, Dual role of BKI1 and 14-3-3 s in brassinosteroid signaling to link receptor with transcription factors. Dev. Cell 21, 825–834 (2011). [PubMed]