DMI3 autophosphorylation at the C-terminus of its calmodulin-binding domain dismantles CaM-DMI3-IPD3 to initiate root nodulation

DMI3 (Doesn't Make Infections 3), a calcium/calmodulin-dependent protein kinase (CCaMK), decodes rhizobia-induced Ca2+ signals through phosphorylation of its interacting partner, IPD3, thereby initiating root nodule symbiosis (RNS). However, the precise mechanism by which DMI3 activates this pathway remains unclear. Here, we show that phosphorylation of a triplet motif (S343S344T345) located at the C-terminus of the calmodulin-binding domain acts as a molecular switch. We identified T345 as a phosphorylation site that exhibits regulatory functions similar to those of S343 and S344. Phosphomimic mutations at any single residue of the three sites blocked DMI3-dependent nodulation, indicating that the kinase must initially remain non-phosphorylated at these positions. Conversely, simultaneous phosphorylation of at least two residues was required for symbiotic activity, revealing a transition from a phosphorylation-free state to a hyperphosphorylated (simultaneous phosphorylation of at least two residues) status at these sites. Phosphomimic mutations within the triplet enhanced DMI3 autophosphorylation (auto-P) but severely impaired its interaction with Ca2+/CaM and IPD3. In addition, the physical interaction between DMI3 and IPD3 suppressed IPD3-triggered MtNIN transcription and nodulation, indicating that DMI3 also acts as a negative regulator by sequestering activated IPD3. Thus, we propose a model in which repeated auto-P at the triplet contributes to RNS activation by dissociating the CaM-DMI3-IPD3 complex, thereby releasing phosphorylated and activated IPD3 to initiate the downstream transcriptional cascade. Our findings uncover a dual role for DMI3-as both a kinase and a scaffold-and clarify how auto-P within the triplet motif licenses IPD3 to trigger RNS.