Rhizobial effector NopM ubiquitinates Nod factor receptor NFR5 and promotes rhizobial infection in Lotus japonicus

Bacterial pathogens and nitrogen-fixing rhizobia employ type III protein secretion system (T3SS) effectors as potent tools to manipulate plant signaling pathways, thereby facilitating infection and host colonization. However, the molecular mechanisms by which rhizobial effectors affect legume infection remain largely elusive. In this study, we investigated the symbiotic role of T3SS effectors in the interaction between Sinorhizobium fredii NGR234 and Lotus japonicus. Mutants deficient in the T3SS genes Tts1 or NopA showed enhanced rhizobial infection of L. japonicus roots. Further mutant analysis showed that the NopT effector negatively affects infection, while the NopM effector in the absence of NopT promotes infection. Notably, NopM interacts with the Nod factor receptors LjNFR1 and LjNFR5. NopM ubiquitinates LjNFR5 on ten lysine residues as identified using mass spectrometry. Expression of NopM in L. japonicus resulted in an approximately twofold increase in LjNFR5 protein levels and enhanced rhizobial infection. Our findings indicate that NopM directly interferes with the symbiotic signaling pathway through interaction and ubiquitination of Nod factor receptors, which likely benefits rhizobial infection of L. japonicus. Our research contributes to the intricate interplay between the Nod factor signaling pathway and rhizobial T3SS effectors delivered into host cells. Significance StatementThis study unravels intricate mechanisms underlying the symbiotic interaction between Sinorhizobium fredii NGR234 and Lotus japonicus. By focusing on the role of the rhizobial effector NopM, this research sheds light on how bacteria manipulate host symbiotic signaling to promote infection and colonization. NopM interacts with NFRs and ubiquitinates LjNFR5, suggesting a novel strategy employed by rhizobia to enhance symbiotic signaling.