Abstract：Bacterial fruit blotch of melons is an important disease on cucurbit plants which is caused by Acidovorax citrulli. Since now it has been reported that this disease can cause serious damages of local melons in United States, Australia, Brazil, China and many other countries. So far, to prevent bacterial fruit blotch of melons, we should find out the mechanism of Acidovorax citrulli. In this experiment, we constructed several type Ⅳ pili related gene-inserted mutants, and obtained two mutants with dramatically reduced virulence by seed-transmission assays which was identified for pilO and pilQ genes, then we tested some related phenotypes about them. By transmission electron microscopy, we found that wild type and pilO mutant could form type Ⅳ pili, whereas pilQ mutant lost type Ⅳ pili. Results of optical microscope observation showed that pilO and pilQ mutants, compared with wild type, both lost twitching motility completely, meanwhile their swimming ability and biofilm formation also reduced significantly. For complementary strains, virulence, twitching motility, swimming ability and biofilm formation recovered partly. Compared with wild-type, the growth of mutants showed no significant difference in MMX medium, and they could also stimulate hypersensitive response in non-host. PilO mutant lost some abilities mediated by type Ⅳ pili but still could form type Ⅳ pili showed that, pilO gene was not directly involved in synthesis of type Ⅳ pili, but it was functional gene of type Ⅳ pili in A. citrulli. PilQ mutant not only lost the abilities mediated by type Ⅳ pili but also couldn’t form type Ⅳ pili showed that, pilQ gene was directly involved in synthesis of type Ⅳ pili and was the synthetic and functional gene of type Ⅳ pili in A. citrulli. From this we knew that type Ⅳ pili of A. citrulli was related to twitching motility, virulence, swimming ability and biofilm formation.
