Functions redundandly with IBH1/BHLH158 in a regulation node known as the incoherent feed-forward loop (FFL). Acts as transcriptional repressor that negatively regulates cell and organ elongation in response to gibberellin (GA) and brassinosteroid (BR) signaling. No visible phenotype. Plants over-expressing IBL1 are dwarf with round-shaped, dark-green leaves and short petioles. Belongs to the bHLH protein family.