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Bacterial fruit blotch(BFB),caused by Acidovorax avenae subsp. citrulli, is a serious bacterial disease of cucurbitaceae. It mainly harms the leaves and the fruit of melon and watermelon, causes a large number of lesion and fruit to decay and makes a serious threat to the production of melon. In recent years, this disease is becoming more and more serious in china, and has caused great damage on growth of economy. Therefore, we need to find the effective control measures as soon as possible, and the study on the pathogenesis of bacterial fruit blotch has great guiding significance to its prevention and treatment.The progress of pathogen infect host plants is a complicated physiological and biochemical process. typeⅢsecretion system which is made up of hrp genes can transfer effect factors and avirulent to plant cell. This is very important to the pathogenicity on host and hypersensitive response(HR) on non-host. At present, the research of bacterial fruit blotch focuses on pathogen identification, pathogen detection methods and techniques, spread mechanism, control methods and so on. The pathogenic molecular mechanism of Acidovorax avenae subsp. citrulli is still poorly understood, there have no report about hrp gene.The 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 Bacterial Fruit Blotch of Melon. Disruption mutants of strain xj112 were successfully generated by a single cross-over event. Electron microscopy showed, 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 and the pathogenicity in melon leaves; while the hrpG mutant was significantly reduced. furthermore, growth analysis results showed that, colonization of hpaA, hrcT, hrcC and hrpG mutant was significantly decreased, respectively. Accordingly, the function of mutants 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 affected pathogenicity on host plants and disease resistance on non-host and resistant plants.
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