Together, our results reveal mechanistic details of BR signaling mediated by a transcriptional activation module BIC1/BZR1/PIF4 and thus provide new insights into the molecular mechanisms underlying the integration of BR and light signaling in plants. In addition, we show that the interaction between BIC1 and BZR1 is evolutionally conserved in the model monocot plant Triticum aestivum (bread wheat). Chromatin immunoprecipitation assays demonstrate that BIC1 and BZR1/PIF4 interdependently associate with the promoters of common target genes. Simultaneously, BIC1 interacts with the transcription factor PIF4 to synergistically and interdependently activate expression of downstream genes including PIF4 itself, and to promote plant growth. We show that BIC1 positively regulates BR signaling and acts as a transcriptional coactivator for BZR1-dependent activation of BR-responsive genes. In this study, we used an efficient protein interaction screening system to identify blue-light inhibitor of cryptochromes 1 (BIC1) as a new BZR1-interacting protein in Arabidopsis thaliana. The BRASSINAZOLE-RESISTANT 1 (BZR1) transcription factor family plays an essential role in plant brassinosteroid (BR) signaling, but the signaling mechanism through which BZR1 and its homologs cooperate with certain coactivators to facilitate transcription of target genes remains incompletely understood.
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