Rhizobial effector NopM mediates the ubiquitination of the Nod factor receptor NFR5 and promotes rhizobial symbiosis in Lotus japonicus

Bacterial pathogens and most nitrogen-fixing rhizobia deploy type III effectors (T3Es) as potent tools to manipulate plant signaling pathways, thereby facilitating infection and colonization. However, how rhizobial T3Es regulate legume symbiosis remains poorly understood. Here, we show that NopM, a T3E from Sinorhizobium fredii NGR234, contributes to infection and nodulation in Lotus japonicus Gifu. Loss of nopM in an NGR234ΔnopT mutant reduces infection and nodulation, whereas expression of NopM under the control of the L. japonicus NIN promoter enhances these processes. NopM associates with the Nod factor (NF) receptors NFR1 and NFR5 and physically interacts with their cytosolic domains in vitro. Notably, NopM selectively mediates the ubiquitination of NFR5. Expression of NopM in hairy roots of NFR5-HA transgenic plants correlates with increased NFR5 protein abundance compared with an inactive NopM variant. Taken together, our results indicate that NopM promotes symbiosis by increasing NFR5 abundance, thereby expanding our understanding of rhizobial T3E functions and the co-evolution of legume-rhizobium symbiosis.