BRI1-associated receptor kinase 1 (BAK1), is a plant protein. BAK1 is a leucine-rich repeat receptor-like kinase has been reported to regulate the brassinosteroid receptor BRI1. It is involved in signalling by FLS2 and EFR. Plants carrying bak1 mutations show normal flagellin binding but abnormal early and late flagellin-triggered responses, indicating that BAK1 acts as a positive regulator in signalling. The bak1-mutant plants also show a reduction in early, but not late, EF-Tu-triggered responses. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. In vivo FLS2 and BAK1 form a complex, in a specific ligand-dependent manner, within the first minutes of stimulation with flagellin. Thus, BAK1 is not only associated with developmental regulation through