Abstract
Living organisms can be primed for potentiated responses to recurring stresses based on prior experience. However, the molecular basis of immune priming remains elusive in plants that lack adaptive immunity. Here, we report that bacterial challenges can prepare plants for fungal attacks by inducing juxtamembrane phosphorylation of CERK1, the co-receptor indispensable for signaling in response to the fungal elicitor chitin. This phosphorylation is mediated by BAK1, a co-receptor for signaling in response to multiple elicitors. BAK1 interacts with CERK1, and loss of BAK1 reduces priming phosphorylation of CERK1. Juxtamembrane phosphomimetic mutations of CERK1 confer accelerated chitin responses and fortified fungal resistance without triggering constitutive immunity, whereas juxtamembrane phosphodeficient mutations diminish bacteria-induced protection against fungal infection. These findings reveal that crosstalk between cell-surface immune co-receptors can prime defense and demonstrate that juxtamembrane phosphorylation of plant receptor-like kinases can occur independent of kinase activation to place the protein into a prime state.