Acts as a regulator of cell proliferation and elongation in the root and shoot (PubMed:25049386, PubMed:28652362). Regulates roots architecture and primary root protophloem differentiation (PubMed:25049386, PubMed:28652362). BRX, BAM3, and CLE45 act together to regulate the transition of protophloem cells from proliferation to differentiation, thus impinging on postembryonic growth capacity of the root meristem (PubMed:23569225). Probable transcription regulator. Regulated by the auxin response factor ARF5. Polarly localized in vascular cells and subject to endocytic recycling. Required for CPD expression and for correct nuclear auxin response. Mediates cross-talk between the auxin and brassinosteroid pathways. BRX is a target for auxin-induced, proteasome-mediated degradation. Homodimer and heterodimer with BRXL1. Interacts with NGA1 and ARF5. The translocation to the nucleus depends on the auxin concentration. Polar localization at the plasma membrane, rootward oriented, in developing root protophloem cells (PubMed:28652362). Expressed in the developing protophloem up to the elongation zone in root meristem of young seedlings, in the columella and the phloem vasculature throughout the root and in the phloem vasculature in the shoot (PubMed:23569225). Detected in the shoot meristem and in primordia. Low expression in stomata. Confined to sieve element precursor cells and to protophloem (PubMed:25049386). Ubiquitously expressed in early embryos and becomes restricted to the vasculature in the mature embryo and at later stages of development (PubMed:19465596). Expressed in roots developing protophloem, up to the end of the transition zone, as well as in the root tip meristem (PubMed:23569225). Up-regulated by auxin and down-regulated by brassinolide. The DZC domain is necessary and sufficient for dimerization. A C-terminal fragment containing both DZC domains is able to rescue the short root phenotype. The N-terminal part (1-57) promotes BRX membrane association and is required for efficient degradation in the nucleus. Impaired primary root protophloem differentiation. Short primary root and increased number of lateral roots (PubMed:25049386, PubMed:28652362). Absent sieve element precursor division, occurrence of gap cells lacking actin filament, associated with a reduction in the number of early dividing protophloem cells, in root meristem size and root growth; these phenotypes are partially rescued in plants also missing MAKR5 (PubMed:25049386, PubMed:28652362, PubMed:27354416). Patchy expression of SUC2 (PubMed:25049386). Reduced cotyledon and leaf size. Enhanced response to abscisic acid-mediated inhibition of root growth and insensitivity to cytokinin induced inhibition of lateral root initiation. The double mutant brx ops double mutant has strongly reduced root length and meristem size, with missing protophloem strands (PubMed:28652362). The disruption of BAM3 restores root protophloem and mersitem phenotypes observed in brx mutants (PubMed:23569225). Ectopic overexpression of BAM3 but slightly reduced CLE45 levels in root meristems (PubMed:23569225). The truncated variant in cv. Uk-1 results in short root phenotype. Belongs to the BRX family.