Atypical bHLH transcription factor that acts as positive regulator of cell elongation downstream of multiple external and endogenous signals by direct binding to the promoters and activation of the two expansin genes EXPA1 and EXPA8, encoding cell wall loosening enzymes. Transcriptional activity is inhibited when binding to the bHLH transcription factor IBH1. Homodimer (Probable). Interacts with IBH1. Highly expressed in hypocotyls and cotyledons. Expressed in leaves, stems, and flowers. Plants over-expressing HBI1 show increased petiole length and early flowering, and plants suppressing HBI1 display dwarf, short-petiole and late-flowering phenotype. Although HBI1 contains a degenerate basic motif it can bind DNA in vitro. May be due to a competing acceptor splice site.