Necessary for male gametophyte development, as well as ovule specification and function (PubMed:16367950). Required for the development of high-ordered vascular strands within the leaf and a correlated control of leaf shape, size and symmetry (PubMed:16367950). LRR-rich receptor-like kinase (LRR-RLK) involved in the perception of CLE45 peptide ligand which mediates root growth inhibition by repressing protophloem differentiation; this mechanism requires CRN (PubMed:23569225, PubMed:28607033, PubMed:25049386). 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, PubMed:25049386). Necessary for CLE45 peptide-triggered accumulation of MAKR5 in developing sieve elements (PubMed:27354416). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with CLE45, especially in roots (PubMed:28607033). Binds to the dimer CLV2/CRN (PubMed:28607033). Expressed in seedlings, roots, leaves, stems, inflorescences, flowers and siliques (PubMed:16367950). In roots, confined to protophloem and sieve element precursor cells (PubMed:23569225, PubMed:25049386). Expressed in roots developing protophloem, up to the end of the transition zone. Accumulates upon CLE45 peptide application (PubMed:23569225). Accumulation is promoted by CRN, especially at later stages of protophloem development (PubMed:28607033). When associated with BAM1 and BAM2 disruptions, loss of stem cells at the shoot and flower meristems. The disruption of BAM3 restores root protophloem and mersitem phenotypes observed in brx mutants (PubMed:23569225). The bam3 mutant resists to root growth inhibition mediated by CLE45 peptide ligand (PubMed:23569225). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.