Dual specificity kinase acting on both serine/threonine- and tyrosine-containing substrates. Positively regulates the BR-dependent plant growth pathway and negatively regulates the BR-independent cell-death pathway. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Interacts with the EF-Tu receptor EFR and FLS2 in a specific ligand-induced manner (PubMed:21693696). Interacts with TMK4/BARK1 (PubMed:23921992). Interacts with ERECTA in a EPF2-induced manner. Interacts with ERL1 in a EPF1-induced manner. Interacts with TMM (PubMed:26320950). Autophosphorylated on Thr and Tyr residues. No visible phenotype; due to the redundancy with BAK1. Bak1 and bkk1 double mutants are seedling lethal. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. Intron retention.