text: Because BIK1 is a possible regulator of the FLS2-BAK1 complex, it is speculated that in vitro, BAK1 phosphorylates BIK1, which then phosphorylates both FLS2 and BAK1.  However, in vivo, BIK1 is not phosphorylated until about 5-10 minutes after the addition of FS2, and the peak phosphorylation occurs just after the phosphorylation of the FS2-BAK1 complex. It is speculated that BIK1 activation might be enhanced through transphosphorylation by BAK1 rather than by FLS2 because FLS2 more likely serves as a scaffold protein for the arrangement of the BAK1-FLS2 complex. This hypothesis will require more testing in vivo. Research has shown that BIK1 and BAK1 are signaling partners for the flagellin receptor FLS2 and that the three together initiate defense response. However, BIK1 and BAK1 phosphorylate different residues of the FLS2 receptor with the exception of only a select few. This suggests that both BAK1 and BIK1 play unique roles in defense response by a series of phosphorylation reactions with one another and the flagellin receptor FLS2.