Atypical and probable non DNA-binding bHLH transcription factor that integrates multiple signaling pathways to regulate cell elongation and plant development. Binds IBH1, forming a pair of antagonistic bHLH transcription factors that function downstream of BZR1 to mediate brassinosteroid regulation of cell elongation. Regulates light responses by binding and inhibiting the activity of the bHLH transcription factor HFR1, a critical regulator of light signaling and shade avoidance. May have a regulatory role in various aspects of gibberellin-dependent growth and development. Interacts with IBH1 and HFR1. Expressed in roots, leaves, stems and flowers. By gibberellin and epibrassinolide. Gain-of-function mutants (T-DNA tagging) show long hypocotyls, pale green and slightly narrow leaves, elongated petioles and early flowering. They are not sensitive to the gibberellin inhibitor paclobutrazol during seed germination (PubMed:16527868, PubMed:20009022, PubMed:23221598).