Identification of USDA61 Type III Effectors Determining Symbiosis with .

USDA61 possesses a functional type III secretion system (T3SS) that controls host-specific symbioses with legumes. Here, we demonstrated that T3SS is essential for the nodulation of several southern Asiatic cultivars. Strikingly, inactivation of either Nod factor synthesis or T3SS in abolished nodulation of the plants. Among the effectors, NopL was identified as a key determinant for T3SS-dependent symbiosis. Mutations of other effector genes, such as , , and , also impacted symbiotic effectiveness, depending on host genotypes. The deletion mutant formed no nodules on but infection thread formation was still maintained, thereby suggesting its pivotal role in nodule organogenesis. Phylogenetic analyses revealed that NopL was exclusively conserved among and () species and showed a different phylogenetic lineage from T3SS. These findings suggest that evolved a unique symbiotic signaling cascade that requires both NFs and T3Es (NopL).