Atypical bHLH transcription factor that acts as negative regulator of a variety of shade avoidance syndrome (SAS) responses, including seedling elongation and photosynthetic pigment accumulation. Acts as direct transcriptional repressor of two auxin-responsive genes, SAUR15 and SAUR68. May function in integrating shade and hormone transcriptional networks in response to light and auxin changes. Homodimer. Plants overexpressing PAR1 are dwarf with compact rosettes and inflorescences, epinastic leaves, shorter flowering stems and siliques and a general dark-green phenotype. Plants silencing PAR1 exhibit a slight increase in the length of hypocotyls, cotyledons and primary leaves (PubMed:17948056). Belongs to the bHLH protein family.