During nodulation, plays a central role in nodule organogenesis (PubMed:22874912). Involved in the perception of Nod factors, the first step of recognition of rhizobia prior to nodulation (PubMed:12753588). Necessary in epidermal cells to induce cortical cell divisions leading to nodule primordia formation (PubMed:22874912). Required during root nodule symbiosis with Sinorhizobium meliloti by triggering infection threads and release of bacteria into the cytoplasm of cells in the infection zone of developing nodules, especially in cells derived from the meristem (PubMed:25351493, PubMed:16844829, PubMed:22087221). Promotes plant fitness (e.g. fruit weight and leaf number) (PubMed:23173081). Involved in resistance to oomycetes (e.g. Aphanomyces euteiches) and to fungi (e.g. Colletotrichum trifolii). Forms heteromeric complexes with LYK3 at the cell periphery in nodules (PubMed:25351493). Interacts with ROP10 (PubMed:25794934). 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. Mostly expressed in roots and nodules and, to a lower extent, in stems (PubMed:16844829, PubMed:22874912). 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). In non-inoculated roots, present in root hair cells of lateral roots and fades out in older regions of roots (PubMed:16844829). In young roots, localized at high levels in both epidermis and cortical layers (PubMed:22874912). Associates with primordium formation and with infection throughout the nodulation process. First day after inoculation by S.meliloti, restricted to discrete areas of root epidermis. During the second day, observed in the inner cortex, in regions where cell divisions lead to the formation of nodule primordia and in the middle cortex of regions where root hairs undergo root hair curling (PubMed:16844829). After three to four days, detected in outer cortical cells directly underlying infected root hairs and through which infection threads occur. Weak levels remain in the epidermis (PubMed:16844829, PubMed:22874912). After five days, highly expressed in the central nodule tissue of young, emerging nodules characterized by a highly ramified infection thread network (PubMed:16844829). In mature nodules, restricted to the infection zone, and, at low levels, in interzone II/III, but excluded from the nitrogen-fixing zone (PubMed:16844829, PubMed:22874912). Localized on infection threads before rhizobia are released (PubMed:25351493, PubMed:16844829). LysM domains are involved in Nod factors perception, especially in chitooligosaccharide-binding. Highly N-glycosylated. Nod(-) phenotype characterized by absence of response to Nod factors (e.g. no rapid calcium flux, no calcium spiking, absence of early nodulin gene expression, and no root hair deformation) (PubMed:12753588). In the nfp-1 mutant, there is impaired nodulation response to S.meliloti, with no root hair curling or infection thread formation and no Nod factor-induced root hair deformation (PubMed:16844829). There are smaller nodules with an aberrant infection zone that contains more cell layers with smaller cells in the central tissue. Infection threads enter the cells, but the release of the bacteria is hampered, and the few released bacteria fail to develop into mature symbiosomes (PubMed:25351493). Increased susceptibility to the root oomycete A.euteiches and to the fungus C.trifolii (PubMed:23432463). Belongs to the protein kinase superfamily. Tyr protein kinase family.