The study investigates the role of BAK1 (BRI1-associated receptor kinase 1) in plant defense signaling initiated by flagellin and elongation factor Tu (EF-Tu). BAK1, a leucine-rich repeat receptor-like kinase, is involved in signaling by FLS2 and EFR, two pattern-recognition receptors for bacterial PAMPs. Plants with bak1 mutations show normal flagellin binding but abnormal early and late responses to flagellin, indicating that BAK1 acts as a positive regulator in signaling. bak1 mutants also show reduced early but not late responses to EF-Tu. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. The authors provide evidence that FLS2 and BAK1 form a complex in vivo, specifically in response to flagellin, within the first minutes of stimulation. This complex formation suggests that BAK1 has a functional role in PRR-dependent signaling, which initiates innate immunity. The study also shows that BAK1 interacts with FLS2 in a stimulus-dependent manner, and that this interaction is specific to flagellin. These findings highlight the importance of BAK1 in plant defense signaling and its potential role as an adaptor or co-receptor for various receptors.The study investigates the role of BAK1 (BRI1-associated receptor kinase 1) in plant defense signaling initiated by flagellin and elongation factor Tu (EF-Tu). BAK1, a leucine-rich repeat receptor-like kinase, is involved in signaling by FLS2 and EFR, two pattern-recognition receptors for bacterial PAMPs. Plants with bak1 mutations show normal flagellin binding but abnormal early and late responses to flagellin, indicating that BAK1 acts as a positive regulator in signaling. bak1 mutants also show reduced early but not late responses to EF-Tu. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. The authors provide evidence that FLS2 and BAK1 form a complex in vivo, specifically in response to flagellin, within the first minutes of stimulation. This complex formation suggests that BAK1 has a functional role in PRR-dependent signaling, which initiates innate immunity. The study also shows that BAK1 interacts with FLS2 in a stimulus-dependent manner, and that this interaction is specific to flagellin. These findings highlight the importance of BAK1 in plant defense signaling and its potential role as an adaptor or co-receptor for various receptors.