Root nodule symbiosis in legumes begins when the LysM receptor kinases NFR1 and NFR5 perceive rhizobial Nod factors at the root surface. A recent study showed that a 23-residue juxtamembrane motif of NFR1, named Symbiosis Determinant 1 (SD1), and two of its residues are necessary and sufficient to convert chitin-immunity receptor kinases such as Lotus CERK6 and barley RLK4 into receptors that activate symbiotic signaling in Lotus japonicus, with an additional independent contribution from a 45-residue C-terminal region of the NFR1 kinase domain. What remains unknown is how these intracellular surfaces actually transduce the signal: which proteins bind SD1 and the C-terminal region upon Nod factor perception, and how these ligand-regulated interactions coordinate rhizobial infection with nodule organogenesis. The problem is to establish this molecular mechanism.

Background

Legumes, and the single non-legume genus Parasponia, form root nodule symbioses with nitrogen-fixing rhizobia: bacteria are accommodated intracellularly in root organs (nodules) where they fix atmospheric nitrogen for the plant. The interaction is initiated when root epidermal cells perceive lipo-chitooligosaccharide signaling molecules ('Nod factors') secreted by rhizobia. Perception is mediated by LysM (lysin motif) receptor kinases at the plasma membrane: in the model legume Lotus japonicus the Nod factor receptor NFR1, an active protein kinase, partners with NFR5, a catalytically inactive pseudokinase, to initiate the common symbiosis signaling pathway — via the SYMRK receptor-like kinase, nuclear calcium oscillations decoded by CCaMK (a calcium/calmodulin-dependent protein kinase), and the NIN transcription factor — which activates two programs that must be coordinated in space and time: infection (rhizobia enter root hairs through infection threads, cell-wall-lined tunnels guiding bacteria into the root) and organogenesis (nodule primordia form from cortical cell divisions). Nearly identical LysM receptors also mediate immunity: the Lotus chitin receptor CERK6 recognizes chitin fragments of fungal cell walls and triggers a burst of reactive oxygen species (ROS), a hallmark of pattern-triggered immunity. How two structurally conserved receptor kinases activate mutually exclusive programs — defense versus mutualism — in the same cell is a central question in plant biology. Earlier work established that the extracellular LysM domains determine ligand specificity (Bozsoki et al., Science 2020), while the intracellular domains determine pathway choice, and that a juxtamembrane motif on the NFR5 side promotes NFR1–NFR5 interactions and is required for symbiotic signaling, also from the barley NFR5-type receptor RLK10 (Hansen et al., PNAS 2024). Tsitsikli et al. (Nature 2025) then showed that a conserved 23-residue motif in the C-terminal part of the juxtamembrane region of NFR1, adjacent to the kinase domain — termed Symbiosis Determinant 1 (SD1) — is critical for symbiotic proficiency: two SD1 residues (M306 and A308 in NFR1; introducing methionine and alanine at the corresponding CERK6 positions T304 and D306) are indispensable hallmarks of NFR1-type receptors and sufficient to convert the kinase outputs of Lotus CERK6 and of barley RLK4, a non-legume chitin receptor, to symbiotic signaling in Lotus (nodule formation and rhizobial infection); the minimal two-residue engineering in CERK6 signals less efficiently than native NFR1 (a third change, T318K, partially restores efficiency, and NFR1-levels of nodulation are reached when the entire SD1 is present), while a full-SD1 construct in barley RLK4 also efficiently initiates nodule formation and rhizobial infection in Lotus nfr1 roots. A separate 45-residue C-terminal region of the NFR1 kinase domain contributes to symbiotic signaling independently of SD1, and neither SD1 nor the C-terminal region, when placed into CERK6, abolishes chitin-triggered ROS, whereas engineering SD1 into the intracellular region of barley RLK4 abolishes its capacity to activate ROS production — symbiosis can be added to a legume chitin receptor without removing immunity, but not without a trade-off in the cereal receptor tested. Related recent work shows that chitin-triggered symbiosis and immunity can be separated at the receptor-complex level in Medicago (the LYK8 receptor; Zhang et al., Current Biology 2024), and that the protein EPP1 is the cytoplasmic mediator recruited to the activated SYMRK receptor, acting downstream of NFR1/NFR5 and upstream of CCaMK (Nørgaard et al., bioRxiv 2025) — but tethering EPP1 to NFR1 does not trigger nodulation, so the connection from the NFR1 intracellular surfaces to the cytoplasm remains undefined. SD1 is maintained across legume NFR1-type receptors and appears to be a specific signature of legume Nod factor receptors: Parasponia, which independently evolved rhizobial nodulation by recruiting a LysM-type mycorrhizal receptor (Op den Camp et al., Science 2011), lacks the motif yet still nodulates, and the source study speculates that the evolution and maintenance of SD1 in the legume clade may have contributed to a very efficient symbiosis in papilionoid legumes. The source study presents these results as a blueprint for engineering native receptors to support root nodule symbiosis in non-legumes; whether SD1-engineered native receptors confer full symbiotic function in a non-legume crop's own signaling context remains an outlook rather than a demonstrated result.

Problem Statement

Establish how the intracellular determinants of the NFR1 Nod factor receptor — the Symbiosis Determinant 1 (SD1) motif in the juxtamembrane region and the 45-residue C-terminal region of the kinase domain — transduce symbiotic signaling. Specifically: (i) identify the protein(s) that physically bind the SD1 surface and the NFR1 kinase C-terminal region upon Nod factor perception (candidates include the NFR5 co-receptor or as-yet-unknown cytoplasmic components); and (ii) define the molecular mechanism by which these ligand-regulated protein–protein interactions coordinate the two symbiotic programs — rhizobial infection via infection threads and root nodule organogenesis via cortical NIN activation and nodule primordium formation — in space and time, including how SD1 regulates the signaling output of the NFR1–NFR5 heteromeric complex and how the surface-exposed residues of the kinase C-terminal region contribute, for example by creating more stable protein–protein interaction interfaces or by enabling dynamic interactions with multiple partners. A complete answer addresses both determinants named in the objective: the source study reports that the necessity for SD1 can be overcome by NFR1 residues present in the C-terminal region of the kinase and vice versa, so a mechanism established for only one of the two surfaces does not resolve the problem; partial resolutions of individual components (for example, partner identification alone) constitute genuine but incomplete progress.

References

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