Abstract The hypersensitive response and pathogenicity(hrp) genes exist in Gram-negative plant pathogenic bacteria and are responsible for the pathogenicity of bacteria．They can induce hypersensitive response(HR) on non-host and resistant plants. hpaA, hrcT, hrcC and hrpG genes were cloned from the hrp gene cluster of Acidovorax avenae subsp. citrulli. Disruption mutants of strain xjl12 were successfully generated by a single cross-over event. Electron microscopy observation showed that the cells of hpaA and hrpG mutants lacked detectable flagella and obviously changed in cell morphology, while hrcT and hrcC mutants' flagella and cell morphology did not change. Surprisingly, hpaA, hrcT and hrcC mutants lost HR-induced capacity in tobacco(Nicotiana tabacam) and the pathogenicity in melon(Hami cantaloupe) leaves; while the hrpG mutant was significantly reduced. Furthermore, growth analysis results showed that colonization of hpaA, hrcT, hrcC and hrpG mutants were significantly decreased, respectively. Accordingly, the mutant phenotypes nearly recovered after complement. Overall, these results indicate that the hrp genes of bacterial fruit blotch of melon as a key component of type Ⅲ secretion system affect pathogenicity on host plants and disease resistance on non-host and resistant plants.
