The Rhizobial effector NopT targets Nod factor receptors to regulate symbiosis inLotus japonicus

It is well-studied that type-III effectors are essential required by gram-negative pathogens to directly target different cellular pathways from hosts, thus enhancing bacterial infection. However, in the unique context of legume-rhizobium symbiosis, the role of rhizobial effectors in regulating plant symbiotic pathways remains a subject of hypothesized understanding. Here, we show that NopT, a YopT-type cysteine protease fromSinorhizobium frediiNGR234 that could directly target symbiotic signaling pathway by associating with two plants’ Nod factor receptors (NFR1 and NFR5). NopT inhibits cell death triggered by co-overexpression of NFR1/NFR5 inNicotiana benthamiana. NopT interacts with both NFR1 and NFR5. NopT proteolytically cleaves NFR5 in bothin vitroandin vivoassays, but is later inactivatedviaphosphorylation by NFR1. NopT plays an essential role in mediating rhizobial infection inL. japonicus. The truncated NopT, lacking 50 N–terminal amino acids post-autocleavage, retains the ability to cleave NFR5 but no longer interacts with NFR1. Intriguingly, certainSinorhizobiumspecies exclusivley harbor this truncated NopT in their genomes. These findings unveil an intricate regulatory interplay between rhizobia and legumes, wherein rhizobial NopT actively cleaves NFR5, and this cleavage process is subsequently suppressed following NopT phosphorylation by NFR1. This discovery presents a compelling narrative of a nuanced regulatory interplay between rhizobia and legumes, highlighting rhizobial effector as a direct mediator targeting symbiotic pathway from plants.