Atypical and probable non DNA-binding bHLH transcription factor required for MONOPTEROS-dependent root initiation in embryo. Promotes the correct definition of the hypophysis cell division plane. Transcriptionally controlled by MONOPTEROS. Moves from its site of synthesis in pro-embryos cells into the hypophysis. Regulates brassinosteroid (BR) signaling by sequestering negative BR signaling components. May function as positive regulator of gibberellin signaling. May play a role in the regulation of light signaling and possibly auxin signaling. Homodimer (Probable). Interacts with BHLH 147, BHLH148, BHLH149, BHLH150 and IBH1. Interacts with SIEL (PubMed:21924907). Expressed in root and shoot meristems, and young siliques. Low levels detected in all aerial tissues. At the globular stage, expressed in cells adjacent to the hypophysis and at later embryonic stages, restricted to the future root stem cells. Not induced by exogenous gibberellin. Plants over-expressing PRE3 show long hypocotyls, pale green and slightly narrow leaves, elongated petioles and early flowering. They are not sensitive to the gibberellin inhibitor paclobutrazol during seed germination (PubMed:16527868, PubMed:22339648).