Putative receptor for S.meliloti Nod factor signals essential for the establishment of the nitrogen-fixing, root nodule symbiosis with S.meliloti (PubMed:20971894, PubMed:12947035, PubMed:17586690, PubMed:25351493). Involved in the control of root hair curling after S.meliloti infection, probably by modulating the reorganization of the microtubular cytoskeleton in epidermal and cortical cells (PubMed:17586690, PubMed:11290290). Regulates a subset of Nod factor-induced genes (PubMed:17586690). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Forms homodimers and homooligomers (By similarity). Forms heteromeric complexes with NFP at the cell periphery in nodules (PubMed:25351493). Interacts with PUB1 (PubMed:20971894). Removed from the plasma membrane upon the release of rhizobia into the host cytoplasm. Vacuolar localization is observed in cells undergoing breakdown of the receptors. Expressed in the epidermal and root hair cells of the developing root hair zone during nonsymbiotic growth. Accumulates in roots and nodules during symbiotic growth with rhizobia (PubMed:12947035, PubMed:20971894). Localized at the cell periphery in a narrow zone of about two cell layers (e.g. L1/L2 zone) at the nodule apex upon infection by rhizobia, from the meristem to the infection zone (at protein level) (PubMed:25351493). Following inoculation with S.meliloti, accumulates strongly in the nodule primordia. In young nodules, expressed in a broad apical region and in the vasculature. In mature nodules, predominantly observed in the apical part of the nodule encompassing the pre-infection and infection zones (PubMed:20971894). Localized on infection threads before rhizobia are released (PubMed:25351493). Autophosphorylated. In the lyk3-4 mutant, increased nodulation in plants infected by S.meliloti, mostly resulting in tubular infection threads, and, to a lower extent, in sac-like structures (PubMed:20971894, PubMed:12947035). The weak hcl-4 mutant is unable to form curled root hairs, a step preceding infection thread formation upon infection by S.meliloti and leading to nodulation. In the hcl-2 mutant, S.meliloti induces extensive root hair deformation and continuous curling, but is unable to induce the formation of tight root hair curls and the subsequent infection threads (PubMed:17586690). In the hcl-2 mutant, there is reduced S. meliloti Nod factor-mediated induction of cortical cell division foci. Impaired asymmetric microtubule network formation in curled root hairs, and failure of activated cortical cells to become polarised and to exhibit the microtubular cytoplasmic bridges characteristic of the pre-infection threads induced by rhizobia (PubMed:11290290). Increased infection threads in nodules due to a slower release of bacteria into the cytoplasm (PubMed:25351493). Normal susceptibility to the root oomycete A.euteiches and to the fungus C.trifolii (PubMed:23432463). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.