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

Abstract

Bradyrhizobium vignae ORS3257 is an efficient symbiotic strain for Vigna unguiculata and V. mungo but fails with V. radiata 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 V. radiata 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 (PR1, PR5, MYB13, and TAO1), hydrogen peroxide (SOD, POX10, and POX16), and cell wall lignification (LAC). 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 V. radiata by inducing a multifaceted defense response and initiating cell wall lignification early in infection.