The Ralstonia solanacearum Effector RipP1 Interacts with Nicotiana benthamiana FRL4a to Suppress Ethylene Signaling and Modulate Bacterial Wilt Susceptibility

RipP1 is a well-characterized avirulence effector that induces a hypersensitive response (HR) in three tobacco species. However, the molecular mechanisms by which host proteins recognize RipP1 to activate a defense response and modulate host–pathogen interactions remain largely unknown. In this study, we screened a Nicotiana benthamiana cDNA library via yeast two-hybrid assay and identified FRIGIDA-like protein 4a (FRL4a) as a host protein interacting with RipP1. Secondary structure analysis of FRL4a and construction of serial mutants revealed that the ClyA-like domain of FRL4a is the key region mediating its interaction with RipP1. Using virus-induced gene silencing (VIGS) and quantitative real-time PCR (qPCR) analysis, we found that the ability of RipP1 to induce HR was significantly attenuated in FRL4a-silenced plants, and RipP1 no longer suppressed the ethylene signaling pathway. Pathogenicity tests by inoculating R. solanacearum on N. benthamiana with different FRL4a expression levels showed enhanced bacterial wilt resistance in FRL4a-silenced plants but increased susceptibility in FRL4a-overexpressing plants. Collectively, these findings demonstrate that RipP1 suppresses the ethylene pathway through its interaction with FRL4a, and FRL4a acts as a negative regulator of tobacco resistance to bacterial wilt.