RinRK1's extracellular domain acts as a host-specific gatekeeper for rhizobial infection in Medicago truncatula

Legume nodulation by nitrogen-fixing rhizobia displays strict host specificity, primarily determined by rhizobial nodulation factors (NFs). While the kinase domain of NF receptors in Lotus japonicus suffices for nodule organogenesis, their extracellular domains govern rhizobial infection. In Sinorhizobium meliloti, specific NF modifications (nodL-dependent acetylation and nodF-dependent N-linked C16:2 acyl chains) are critical for infecting certain Medicago truncatula ecotypes, with LYK2bis essential for nodL-mediated infection. However, how NF receptors discern NF modifications to enable rhizobial infection remains unknown. Here, we demonstrate that M. truncatula RinRK1 (MtRinRK1) interacts with NF receptors and is indispensable for root hair infection of ecotype R108 by S. meliloti nodF nodL mutant producing modified NFs. Notably, a screen of 50 M. truncatula natural variants revealed only R108 forms functional nodules with S. meliloti nodF nodL, and we pinpointed three residues in MtRinRK1 extracellular domains essential for this infection. Despite homology with L. japonicus RinRK1 (LjRinRK1), these proteins are functionally non-interchangeable, with their extracellular domains acting as host-specific determinants. Our findings establish RinRK1 as a core regulator of NF-dependent host specificity in rhizobia infection.