N. benthamiana as a surrogate host for studying the pathogenicity of A. citrulli, the causal agent of bacterial fruit blotch of cucurbits

Acidovorax citrulli is the causal agent of bacterial fruit blotch (BFB) of cucurbits. Despite its significant agricultural importance, the mechanisms associated with A. citrulli pathogenicity have not been extensively investigated. In this study, we used Nicotiana benthamiana and Nicotiana tabacum as surrogate hosts for studying A. citrulli virulence and nonhost resistance triggered by bacterial type III-secreted effectors. Two A. citrulli strains, M6 and AAC00-1, that represent the two major groups among A. citrulli populations, induced disease symptoms on N. benthamiana, but triggered hypersensitive response (HR) on N. tabacum plants. Transient expression of 19 T3S effectors in N. benthamiana and N. tabacum leaves revealed that three effectors, Aave1548, Aave2708, and Aave2166, could trigger water-soaking-like cell death in N. benthamiana and HR in N. tabacum. Knockout effector gene Aave1548 in strains M6 and AAC00-1 reduced A. citrulli virulence on N. benthamiana and melon (Cucumis melo L.). Transient expression Aave1548 and Aave2166 effectors triggered nonhost HR in N. tabacum, and this reaction depends on the functionality of the immune signaling component, NtSGT1