P-432 - Understanding NopP variation diversity and function in regulating Sinorhizobium-soybean symbiosis affinity

Rhizobium-legume symbiosis is a highly host-specific interaction governed by intricate molecular dialogues and immune evasion mechanisms. This study investigates the genetic basis underlying the expanded host range of Ensifer sesbaniae strain E2, which uniquely forms nitrogen-fixing nodules on soybean, a non-native host for its typical strain. Comparative genomics revealed that E2 acquired a symbiotic plasmid from Sinorhizobium fredii, enabling soybean nodulation. Pangenome profiling identified plasmid-derived Sym genes are critical for symbiosis. Remarkably, E2 exhibited broad-spectrum nodulation across diverse soybean genotypes, circumventing soybean nucleotide-binding leucine-rich repeat (NLR) Rfg1/Rj2-mediated restriction. Functional analysis of rhizobial effector NopP revealed two key amino acid substitutions (K63E, D184G) in E2 compared to incompatible S. fredii strains. K63E and D184G variant attenuated effector-triggered immunity (ETI), as shown by reduced cell death in tobacco leaves. NopP knockout (E2ΔnopP) severely impaired nodulation, underscoring its role in symbiosis establishment.These findings demonstrate that horizontal gene transfer of symbiotic plasmids, coupled with effector evolution, enables rhizobia to bypass host immune surveillance and adapt to novel legumes. This work provides insights into engineering rhizobial strains with expanded host ranges, advancing efforts to optimize symbiotic nitrogen fixation for sustainable agriculture.