Elucidation of the symbiotic incompatibility mechanisms between Vigna radiata and Bradyrhizobium vignae ORS3257 mediated by nodulation outer protein P2

<i>Bradyrhizobium vignae</i> ORS3257 is an efficient symbiotic strain for <i>Vigna unguiculata</i> and <i>V. mungo</i> but fails with <i>V. radiata</i> due to an effector-triggered immunity response mediated by the nodulation outer protein P2 (NopP2). To understand this incompatibility, we identified NopP2 interacting proteins in <i>V. radiata</i> cv. KPS1, including enolase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), monodehydroascorbate reductase (MDHAR), and serine hydroxymethyltransferase (SHMT) as targets. Protein-protein interaction assays confirmed that NopP2 binds to these enzymes, and further analysis revealed their co-localization on the plasma membrane. Comparative transcriptomic analysis revealed NopP2 stimulates genes related to plant defense response (<i>PR1</i>, <i>PR5</i>, <i>MYB13</i>, and <i>TAO1</i>), hydrogen peroxide (<i>SOD</i>, <i>POX10</i>, and <i>POX16</i>), and cell wall lignification (<i>LAC</i>). NopP2 did not alter the expression of genes encoding the target enzymes but interfered with MDHAR activity, leading to high H₂O₂ accumulation in roots. These findings suggest that NopP2 contributes to symbiotic incompatibility in <i>V. radiata</i> by inducing a multifaceted defense response and initiating cell wall lignification early in infection.