Formation and Dissociation of the BSS1 Protein Complex Regulates Plant Development via Brassinosteroid Signaling

• Published in: The Plant cell 2015-02, Vol.27 (2), p.375-390
• Main Authors: Shimada, Setsuko, Komatsu, Tomoyuki, Yamagami, Ayumi, Nakazawa, Miki, Matsui, Minami, Kawaide, Hiroshi, Natsume, Masahiro, Osada, Hiroyuki, Asami, Tadao, Nakano, Takeshi
• Format: Article
• Language: English
• Subjects: Arabidopsis - drug effects; Arabidopsis - genetics; Arabidopsis - metabolism; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; Brassinosteroids; Brassinosteroids - metabolism; Brassinosteroids - pharmacology; Cell Nucleus - drug effects; Cell Nucleus - metabolism; Cytosol; Cytosol - drug effects; Cytosol - metabolism; Darkness; Fluorescence; Gene expression regulation; Gene Expression Regulation, Plant - drug effects; Green Fluorescent Proteins - metabolism; Hypocotyls; Models, Biological; Multiprotein Complexes - metabolism; Mutation - genetics; Phenotype; Phenotypes; Plant cells; Plant Development - drug effects; Plant growth regulators; Plants; Protein Binding - drug effects; RNA, Messenger - genetics; RNA, Messenger - metabolism; Seedlings; Signal Transduction - drug effects; Signal Transduction - genetics; Triazoles - pharmacology
• ISSN: 1040-4651; 1532-298X
• DOI: 10.1105/tpc.114.131508

Brassinosteroids (BRs) play important roles in plant development and the response to environmental cues. BIL1/BZR1 is a master transcription factor in BR signaling, but the mechanisms that lead to the finely tuned targeting of BIL1/BZR1 by BRs are unknown. Here, we identified BRZ-SENSITIVE-SHORT HYPOCOTYL1 (BSS1) as a negative regulator of BR signaling in a chemical-biological analysis involving brassinazole (Brz), a specific BR biosynthesis inhibitor. The bss1-1D mutant, which overexpresses BSS1, exhibited a Brz-hypersensitive phenotype in hypocotyl elongation. BSS1 encodes a BTB-POZ domain protein with ankyrin repeats, known as BLADE ON PETIOLE1 (BOP1), which is an important regulator of leaf morphogenesis. The bss1-1D mutant exhibited an increased accumulation of phosphorylated BIL1/BZR1 and a negative regulation of BR-responsive genes. The number of fluorescent BSS1/BOP1-GFP puncta increased in response to Brz treatment, and the puncta were diffused by BR treatment in the root and hypocotyl. We show that BSS1/BOP1 directly interacts with BIL1/BZR1 or BES1. The large protein complex formed between BSS1/BOP1 and BIL1/BZR1 was only detected in the cytosol. The nuclear BIL1/BZR1 increased in the BSS1/BOP1-deficient background and decreased in the BSS1/BOP1-overexpressing background. Our study suggests that the BSS1/BOP1 protein complex inhibits the transport of BIL1/BZR1 to the nucleus from the cytosol and negatively regulates BR signaling.