Identification of Genetic and Chemical Factors Affecting Type III Secretion System Expression in pv. Biovar 3 Using a Luciferase Reporter Construct.

The type III secretion system (T3SS) is a key factor in the pathogenesis of pv. biovar 3 (3), the causal agent of a global kiwifruit bacterial canker pandemic. To monitor the T3SS expression levels in 3, we constructed a luciferase reporter plasmid-expressing HrpA-NLuc fusion protein. The expression of HrpA-NLuc was induced in -inducing conditions whereas the level of luciferase activity correlated with the expression of / genes in 3 confirmed the reliability of the reporter construct. Based on the readout of the NLuc reporter construct, three small molecule compounds 4-methoxy-cinnamic acid, sulforaphane, and ferulic acid were determined as T3SS inhibitors in 3, whereas sodium acetate was determined to be a T3SS inducer. Moreover, the aqueous extract of fruit inhibited the accumulation of HrpA-NLuc in 3 in medium and in planta. Additionally, the T3SS inhibitors suppress 3 virulence, whereas the T3SS inducer promotes 3 virulence on kiwifruit. Thus, our findings may provide clues to why the fruit is not infected by 3, and the 3 T3SS inhibitors have potential as alternatives to current nonspecific antimicrobials for disease management.