The Effector RipAW Enhances Ralstonia solanacearum Invasion in Arabidopsis via CBP60g/SARD1-Dependent and -Independent Pathways

Abstract

CaM-binding Protein 60-like G (CBP60g) and Systemic Acquired Resistance Deficient 1 (SARD1) are key immune signalling regulators that redundantly promote salicylic acid (SA) biosynthesis and plant immunity. Pathogen effectors often target these immune nodes to suppress plant defence. However, the role of bacterial effectors in disabling CBP60g and SARD1 to increase plant susceptibility remains unclear. In this study, we show that RipAW, an E3 ligase effector from Ralstonia solanacearum, induces root architecture changes and enhances plant susceptibility to R. solanacearum in Est::RipAW transgenic plants. The constitutively expressed RipAW (C177S), lacking E3 ligase activity, did not affect root architecture or plant susceptibility, indicating that RipAW's E3 ligase activity is crucial for these phenotypes. Transcriptional profiling of Est::RipAW plants revealed strong up-regulation of CBP60g and SARD1, while the SA signalling pathway remained in a basal state. Transient expression of RipAW and CBP60g in Nicotiana benthamiana showed that RipAW associates with CBP60g and affects its stability. Genetic analysis revealed that loss-of-function mutations in CBP60g and SARD1 increased plant susceptibility to R. solanacearum, but did not enhance RipAW-mediated pathogen growth. Furthermore, growth of the R. solanacearum ΔRipAW null mutant strain was reduced in wild-type plants but restored in cbp60g/sard1 mutant plants, confirming that the promotion of RipAW on bacterial growth is dependent on CBP60g and SARD1. Surprisingly, CBP60g and SARD1 were not involved in R. solanacearum-induced and RipAW-triggered root architecture changes. Overall, our findings demonstrate that RipAW increases plant susceptibility to R. solanacearum via both CBP60g/SARD1-dependent and -independent pathways.