| Summary: | <i>Bradyrhizobium elkanii</i> USDA61 possesses a functional type III secretion system (T3SS) that controls host-specific symbioses with legumes. Here, we demonstrated that <i>B. elkanii</i> T3SS is essential for the nodulation of several southern Asiatic <i>Vigna mungo</i> cultivars. Strikingly, inactivation of either Nod factor synthesis or T3SS in <i>B. elkanii</i> abolished nodulation of the <i>V. mungo</i> plants. Among the effectors, NopL was identified as a key determinant for T3SS-dependent symbiosis. Mutations of other effector genes, such as <i>innB</i>, <i>nopP2</i>, and <i>bel2-5</i>, also impacted symbiotic effectiveness, depending on host genotypes. The <i>nopL</i> deletion mutant formed no nodules on <i>V. mungo,</i> but infection thread formation was still maintained, thereby suggesting its pivotal role in nodule organogenesis. Phylogenetic analyses revealed that NopL was exclusively conserved among <i>Bradyrhizobium</i> and <i>Sinorhizobium</i> (<i>Ensifer</i>) species and showed a different phylogenetic lineage from T3SS. These findings suggest that <i>V. mungo</i> evolved a unique symbiotic signaling cascade that requires both NFs and T3Es (NopL).
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