Direct coupling of nod factor signaling to vesicular trafficking initiates legume nodulation

Legume–rhizobium symbiosis requires coordinated receptor signaling and membrane trafficking to initiate infection thread formation in root hairs. Here, we identify the soybean Qa-SNARE GmSYNTAXIN111a (GmSYP111a), a close paralog of the cytokinesis-associated protein KNOLLE, as a critical regulator of symbiotic infection. Kinase-client assays and in vivo immunoprecipitation–mass spectrometry showed that GmSYP111a is phosphorylated by the receptor kinase GmSymRKβ at Ser-8 and Ser-128. BiFC, co-immunoprecipitation, and kinase assays validated the interaction and phosphorylation. Nod factor perception promoted clathrin-mediated endocytosis of the GmSymRKβ–GmSYP111a complex and its relocalization to intracellular vesicles. Structural modeling and interaction assays suggest that dual phosphorylation exposes an endocytic motif that recruits the TPLATE adaptor; accordingly, non-phosphorylatable mutations impaired internalization, whereas phosphomimetic substitutions induced endocytosis without rhizobial stimulation. GmSYP111a also interacted with VAMP72, linking endocytic recruitment to vesicle fusion. Genetic analyses in soybean and Lotus japonicus established a conserved requirement for GmSYP111a in nodule initiation. These findings define a phosphorylation-dependent SNARE switch that couples Nod factor signaling to membrane trafficking during legume nodulation. Soybean plants initiate symbiosis with nitrogen-fixing bacteria by remodeling cell membranes. This study shows that bacterial signals activate a receptor that switches on a membrane fusion protein, enabling infection thread and nodule formation.