A Ralstonia solanacearum effector RipAD perturbs aquaporin NbPIP oligomeric status to suppress plant immunity

Abstract

Bacterial wilt caused by Ralstonia solanacearum is a devastating disease that affects a wide range of crops, including tobacco. The pathogen promotes infection by secreting type III effector (T3E) proteins into host plants to facilitate pathogenicity; however, the molecular mechanisms for many of these effectors remain elusive. Here, we characterize RipAD, a conserved T3E that significantly contributes to R. solanacearum virulence in Nicotiana tabacum and Nicotiana benthamiana. We demonstrate that RipAD dampens early immune responses, including pathogen-associated molecular pattern-triggered reactive oxygen species (ROS) bursts and MAPK activation. This immunosuppressive function depends on the dual localization of RipAD to the plasma membrane and the nucleus. Mechanistically, RipAD physically associates with the N-terminal domains of the plasma membrane intrinsic proteins NbPIP2;3 and NbPIP2;4. We show that these aquaporins act as positive regulators of ROS accumulation and plant immunity. Crucially, RipAD binding is associated with reduced homo- and hetero-oligomerization of NbPIP2;3 and NbPIP2;4, thereby compromising their stability and function. Furthermore, silencing NbPIP2;3 and NbPIP2;4 increases tobacco susceptibility and rescues the virulence defect of a RipAD deletion mutant. These findings reveal a novel virulence strategy wherein a bacterial effector targets the structural integrity of host aquaporin complexes to block ROS signaling and suppress immunity.