Atypical and probable non DNA-binding bHLH transcription factor that acts as a positive regulator of grain size. Binds the transcription repressor APG and forms a heterodimer of antagonistic bHLH transcription factors that regulates grain length and weight by controlling cell elongation in lemma and palea. May be involved in the control of lamina inclination through brassinosteroid signaling pathway. Interacts with APG. Plants over-expressing ILI6 show enhanced bending of the lamina joints and produce grains increased in size, caused by elongated cells in lemma. Belongs to the bHLH protein family.