Abstract: [Objective] The poplar bacterial canker caused by Lonsdalea quercina subsp. populi is a disease that is a seriously harm to poplar industry. In order to clarify the role of the hrcJ gene playing in the pathogenesis of L. quercina subsp. populi, it's necessary to reveal the function of each gene in type Ⅲ secretion system of the pathogen and provide a theoretical basis for elucidating the pathogenic mechnism of pathogens.[Method]The hrcJ inserted mutant was constructed by homologous recombination. A suicide vector, pEX18Km-hrcJ,that carries hrcJ gene of the pathogen strain N-5-1 was constructed. The suicide vector was introduced into the wild-type strain N-5-1 by using amphiphilic bonding method. The hrcJ gene insertion mutants were screened on LB plates containing the corresponding concentration of kanamycin and rifampicin. Then the hrcJ inserted mutant was verified by PCR and Southern blot. One-year-old seedling of Populus ×euramericana cv.'74/76' were inoculated in an incubator, with strains of the wild-type, the hrcJ gene mutant, and the complemented mutant HBhrcJ respectively, for pathogenecity test.[Result]The PCR amplified fragment sizes are consistent with those expectations and Southern hybridization signal stripe size was the same as expected, showing that the suicide vector pEX18Km-hrcJ has been properly integrated into the target gene. That is the hrcJ gene has achieved insertion mutation due to the homologous exchange. Pathogenicity test on one-year-old poplar branches indicated that the hrcJ gene mutant was significantly less virulent than wild-type, while the complemented mutant HBhrcJ restored the virulence to the wild-type level. Growth analysis showed that growth capacity of hrcJ gene mutant had no significant change compared with that of wild-type, and the ability of biofilm formation evaluated by OD₅₇₀ was similar among strains N-5-1, ΔhrcJ and HBhrcJ. Motility test showed that the motility of hrcJ gene mutant was decreased by 21 percent compared with that of wild-type.[Conclusion] The suicide vector pEX18Km can be successfully used to construct pathogen gene insertion mutation in Lonsdalea quercina subsp. populi, and an alternative method is provided for pathogenic gene research of the pathogen. This result suggests that the hrcJ gene is required for pathogenicity of L. quercina on host plant poplar and hypersensitive reaction on non host plant tobacco. The hrcJ gene mutant of L. quercina did not affect growth of the pathogen and the formation of biofilm, but the obviously decreased motility.

Key words: Lonsdalea quercina subsp. populi, hrcJ, type-Ⅲ secretion system, pathogenicity