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 (By similarity). Interacts with APG. Belongs to the bHLH protein family.