Calcium- and calmodulin-dependent protein kinase necessary and sufficient for dedifferentiation of root cortical cells into nodule initials. Not required for calcium spiking (PubMed:16810257). Acts as central regulator of the nodule organogenesis program. Required for root hair curling and infection thread (IT) formation upon rhizobial infection, and arbuscule formation during arbuscular mycorrhiza (AM) fungal infection. Phosphorylates the downstream target IPD3, a protein required for root infection by symbiotic rhizobia and AM fungi (PubMed:19074278). Phosphorylates the downstream target CIP73, a protein required for root nodule organogenesis (PubMed:21209278). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by calcium/calmodulin binding after calcium-induced autophosphorylation. Interacts with IPD3 (PubMed:19074278, PubMed:21209278). Interacts with CIP73 (PubMed:21209278). Mainly expressed in roots and nodules. Detected in leaves, stems and cotyledons. Very low up-regulation 8 days after inoculation with bacteria. Autophosphorylation stimulated by calcium. Occurs probably by an intermolecular mechanism. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. CaMK subfamily.