BAK1 is a leucine-rich repeat receptor-like kinase that functions as a coreceptor with the brassinosteroid (BR) receptor BRI1 and the flagellin receptor FLS2, and as a negative regulator of programmed cell death. BAK1 has been shown to autophosphorylate on numerous serine/threonine sites in vitro as well as to transphosphorylate associated receptor kinases both in vitro and in planta. In the present study we identify Tyr-610 in the carboxyl-terminal domain of BAK1 as a major site of autophosphorylation that is brassinolide-induced in vivo and requires a kinase-active BAK1. Expression of BAK1(Y610F)-Flag in transgenic plants lacking the endogenous bak1 and its functional paralogue, bkk1, produced plants that were viable but extremely small and generally resembled BR signaling mutants, whereas an acidic substitution for Tyr-610 to mimic phosphorylation restored normal growth. Several lines of evidence support the notion that BR signaling is impaired in the BAK1(Y610F)-Flag plants, and are consistent with the recently proposed sequential transphosphorylation model for BRI1/BAK1 interaction and activation. In contrast, the FLS2-mediated inhibition of seedling growth by the flg22 elicitor occurred normally in the Y610F-directed mutant. However, expression of many defense genes was dramatically reduced in BAK1(Y610F) plants and the nonpathogenic hrpA mutant of Pseudomonas syringae was able to grow rapidly in the mutant. These results indicate that phosphorylation of Tyr-610 is required for some but not all functions of BAK1, and adds significantly to the emerging notion that tyrosine phosphorylation could play an important role in plant receptor kinase signaling.