The Ralstonia solanacearum effector RipI induces a defence reaction by interacting with the bHLH93 transcription factor in Nicotiana benthamiana

Ralstonia solanacearum releases a set of effectors into plant cells that modify the host defence reaction. The role of the effector protein RipI during infection has not been elucidated. In this study, we demonstrated that transient overexpression of RipI induces the hypersensitive response (HR), up-regulating the HR marker gene hin1, in Nicotiana benthamiana. Deletion of R. solanacearum ripI led to increased virulence in tomato (Solanum lycopersicum) plants. Through yeast two-hybrid and pull-down assays, we identified an interaction between the N. benthamiana transcription factor bHLH93 and RipI, both of which could be localized in the nucleus of Arabidopsis protoplasts. Silencing of bHLH93 markedly attenuated the RipI-induced HR and induced expression of the PDF1.2 defence gene. These data demonstrate that the R. solanacearum effector RipI induces a host defence reaction by interacting with the bHLH93 transcription factor.