The Type III Effector NopM from Bradyrhizobium elkanii USDA61 Induces a Hypersensitive Response in Lotus japonicus Root Nodules.

Leguminous plants establish root nodule symbiosis, which is initiated by the recognition of rhizobial nodulation factors by plant receptor kinases. However, other factors, such as Type III effector proteins, also affect host specificity. We herein investigated the role of nodulation outer protein M (NopM), a Type III effector of Bradyrhizobium elkanii USDA61, in symbiosis with Lotus japonicus MG-20 and Lotus burttii. NopM, annotated as an E3 ubiquitin ligase, triggers an early senescence-like response, inducing brown nodules that hinder effective symbiosis. NopM shares structural features with E3 ubiquitin ligases derived from both pathogenic and symbiotic bacteria, including a leucine-rich-repeat and E3 ubiquitin ligase domain. The deletion of these domains or substitution of the cysteine residue, predicted to be the active site of the ubiquitin ligase domain, suppressed the formation of brown nodules. These results suggest that NopM interacts with target proteins through its leucine-rich-repeat domain and mediates ubiquitination via its ligase domain, thereby contributing to the induction of brown nodules. A transcriptome ana-lysis further suggested that the early senescence-like response closely resembled the plant hypersensitive response, with the up-regulation of defense-related genes. Therefore, L. japonicus may recognize NopM in infected nodule cells, leading to an immune response that disrupts symbiosis. The present study provides insights into the mole-cular mechanisms by which rhizobial effectors modulate symbiotic interactions in infected nodule cells, highlighting the ability of L. japonicus to activate immune responses even in nodule cells where rhizobia have been accepted.