Paralogous <scp>LRR</scp> receptor kinases confer symbiosis specificity between arbuscular mycorrhizal and root nodule symbioses in <i>Lotus japonicus</i>

Arbuscular mycorrhizal (AM) and root nodule (RN) symbiosis play essential roles in plant nutrient acquisition and share a common symbiotic signal transduction pathway, yet they produce distinct developmental outcomes. Here, we identify arbuscular mycorrhiza-induced kinase 2 (AMK2), a leucine-rich repeat receptor-like kinase (LRR-RLK) in Lotus japonicus, as a key regulator of AM symbiosis. AMK2 is a paralog of the rhizobial infection receptor Rhizobial Infection Receptor-like Kinase 1 (RinRK1), highlighting an evolutionary link between receptors controlling different symbiotic programs. AMK2 expression is strongly induced following AM fungal (AMF) inoculation and is directly activated by the AM-specific transcription factor CBX1 through conserved CTTC cis-regulatory motifs. The AMK2 protein localizes specifically to arbuscule-containing cells, and amk2 mutants exhibit severely reduced arbuscule formation. Domain-swapping experiments between RinRK1 and AMK2 demonstrate that symbiosis specificity is determined by their intracellular kinase domains, whereas their extracellular domains are functionally interchangeable. Together, our findings show that two evolutionarily related LRR-RLK receptors have been differentially recruited to regulate AM and RN symbioses, providing mechanistic insights into how shared signaling components have diversified to control distinct mutualistic interactions between plants and microbes.