A Phytosulfokine signaling module activates the Nod factor receptor to control soybean nodulation

The rhizobia-legume symbiosis, essential for nitrogen fixation, is initiated through Nod Factor (NF) perception by the NF receptors (NFRs). However, the mechanisms regulating NFR activity remain poorly defined. Here, we identify a NODULE INCEPTION 1 (GmNIN1)- Subtilisin-like Protease 1.2 (GmSBT1.2) - Phytosulfokine 4 (GmPSK4) - Phytosulfokine Receptor 1 (GmPSKR1) signaling module that directly activates the GmNFR1ɑ to promote nodulation in soybean. We demonstrate that GmNIN1a specifically activates GmSBT1.2b/1.2d, which are important for nodulation. GmSBT1.2b cleaves the proprotein of the phytosulfokine peptide GmPSK4a into mature form. Knockout of GmPSK4a and its homolog GmPSK4b severely impairs nodulation. Genetic analyses confirm the functionally interdependency of GmSBT1.2b/1.2d and GmPSK4a/4b. GmPSKR1a/1b/1c, identified as GmPSK4 receptors, are beneficial to nodulation. Notably, GmPSKR1a physically interacts with GmNFR1ɑ and enhances its kinase activity. Thus, the activation of GmNFR1ɑ by GmPSKR1a is important to its role in initiating NF signaling. Our findings uncover a previously unknown regulatory mechanism that controls the core NF signaling, providing fundamental insights into the governance of symbiotic nitrogen fixation.