Identification Ofbradyrhizobium Elkaniiusda61 Type Iii Effectors Determining Symbiosis Withvigna Mungo

Bradyrhizobium elkaniiUSDA61 possesses a functional type III secretion system (T3SS) that controls host-specific symbioses with legumes. Here, we demonstrated thatB. elkaniiT3SS is essential for the nodulation of several southern AsiaticVigna mungocultivars. Strikingly, inactivation of either Nod factor synthesis or T3SS inB. elkaniiabolished nodulation of theV. mungoplants. Among the effectors, NopL was identified as a key determinant for T3SS-dependent symbiosis. Mutations of other effector genes, such asinnB,nopP2, andbel2-5, also impacted symbiotic effectiveness, depending on host genotypes. ThenopLdeletion mutant formed no nodules onV. mungo,but infection thread formation was still maintained, thereby suggesting its pivotal role in nodule organogenesis. Phylogenetic analyses revealed that NopL was exclusively conserved amongBradyrhizobiumandSinorhizobium(Ensifer) species and showed a different phylogenetic lineage from T3SS. These findings suggest thatV. mungoevolved a unique symbiotic signaling cascade that requires both NFs and T3Es (NopL).