The Ralstonia solanacearum effector RipV1 acts as a novel E3 ubiquitin ligase to suppress plant PAMP-triggered immunity responses and promote susceptibility in potato

Bacterial wilt caused by Ralstonia solanacearum is a destructive plant disease, particularly in potato (Solanum tuberosum). R. solanacearum deploys a diverse and potent arsenal of type III effectors to inhibit the plant immune system. However, the understanding of individual effectors promoting susceptibility in host plants and interfering with plant immunity responses is still limited. Here, we demonstrated that the type III effector RipV1 functioned as a novel E3 ubiquitin ligase (NEL) effector and exhibited E3 ubiquitin ligase activity in vitro. Transient expression of RipV1 suppressed plant pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) responses in Nicotiana benthamiana, such as the expression of PTI-related genes and the reactive oxygen species (ROS) burst. Prolonged expression of RipV1 induced cell death in N. benthamiana leaves. Notably, mutating the conserved cysteine residue of RipV1 abolished its E3 ligase activity and its ability to suppress plant PTI responses. This study also revealed the indispensability of RipV1 for R. solanacearum's full virulence in potato. Transgenic potato plants overexpressing ripV1 but not the catalytic mutant ripV1-C444A displayed enhanced susceptibility to R. solanacearum. RipV1 was observed to localize specifically to the plant plasma membrane, with its N-terminus being pivotal in determining this localization. These findings showcase that RipV1 acts as a NEL effector and contributes to R. solanacearum virulence by suppressing plant PTI responses through its E3 activity.