Bacterial blight (BB) of rice, caused by Xanthomonas oryzae pv. oryzae (Xoo), is the most devastating bacterial disease in rice. A virulence-attenuated mutant strain HNU89K9 of X. oryzae pv. oryzae (KACC10331), with a transposon insertion in the pilQ gene was used for this study. The pilQ was involved in the gene cluster pilMNOPQ of the Xoo genome. Growth rate of the pilQ mutant was similar to that of wild-type. At level of amino acids, PilQ of Xoo showed that a high sequence identities more than 94% and 70% to Xanthomonas species and to Xyllela fastidiosa, respectively but a low sequence homology less than 30% to other bacterial species. The twitching motility forming a marginal fringe on PSA media was observed on colony of the wild-type strain KACC10331, but not in mutant HNU89K9. Wild-type Xoo cells formed a biofilm on the surface of the PVC plastic test tube, while the mutant strain HNU89K9 did not form a biofilm. The results suggest that the pilQ gene of X. oryzae pv. oryzae plays a critical role in pathogenicity, twitching motility, and biofilm formation.

A transposon mutant library was constructed from the bacterial blight pathogen Xanthomonas oryzae pv . oryzae ( Xoo ) KACC10331 by Tn5 transposon mutagenesis. The susceptible rice cultivar Milyang 23 was inoculated with a total of 24 540 mutants resistant to kanamycin and 67 avirulent or reduced-pathogenicity mutant strains were selected for study. Southern hybridization verified that 84 mutant strains had single-copy insertions and their single-transposon insertion sites were identified by sequencing analysis combined with thermal asymmetric interlaced (TAIL)-PCR. The single-transposon-tagged sequences of 21 mutant strains belonged to pathogenicity-related genes previously reported in Xanthomonas species, while the other 46 single-transposon-tagged sequences included diverse functional genes encoding, five cell-wall-degrading enzymes, three fimbrial and flagella assembly regulators, five regulatory proteins, 15 metabolic regulators and 18 hypothetical proteins, which were identified as novel pathogenicity genes of Xoo .