Atypical and probable non DNA-binding bHLH transcription factor that acts as negative regulator of cell elongation and plant development. Binds the transcription factor ILI1 and forms a heterodimer of antagonistic bHLH transcription factors that function downstream of BZR1 to mediate brassinosteroid regulation of cell elongation and lamina inclination. Interacts with ILI1. Highly expressed in roots and at lower levels in leaf blades, leaf sheaths, lamina joint, stems and panicles. Repressed by epibrassinolide. Plants over-expressing IBH1 show erect leaves with reduced angles of inclination. Belongs to the bHLH protein family.