Atypical and probable non DNA-binding bHLH transcription factor that acts as transcriptional repressor that negatively regulates cell and organ elongation in response to gibberellin (GA) and brassinosteroid (BR) signaling. Is able to form heterodimer with BHLH49, thus inhibiting DNA binding of BHLH49, which is a transcriptional activator that regulates the expression of a subset of genes involved in cell expansion by binding to the G-box motif. Binds and inhibits HBI1, a positive regulator of cell elongation that directly binds to the promoters and activated the two expansin genes EXPA1 and EXPA8, encoding cell wall loosening enzymes. The ability of IBH1 to inhibit BHLH49 and HBI1 is counteracted by binding to the antagonist bHLH transcription factor PRE1, restoring the transcriptional activity of BHLH49 and HBI1 and resulting in induction of cell elongation. Functions redundantly with IBL1/BHLH159 in a regulation node known as the incoherent feed-forward loop (FFL) (PubMed:24505057). Interacts with PRE1, PRE3, PRE4, PRE5, HBI1, BHLH49, BHLH63, BHLH74, BHLH76 and BHLH77. Highly expressed in roots and at lower levels in rosette leaves, stems and cauline leaves. Repressed by epibrassinolide. No visible phenotype. Plants over-expressing IBH1 are dwarf with round-shaped, dark-green leaves, short petioles, siliques and roots, and are insensitive to brassinosteroid (PubMed:20009022, PubMed:23161888, PubMed:23221598). Plants silencing IBH1 exhibit increased length of hypocotyls and leaves (PubMed:23161888). Belongs to the bHLH protein family.