Identification Of Gene Involved In The Plant Signal Sensing System Of Xanthomonas Campestris Pv. Campestris

Xanthomonas campestris pv. campestris (Xcc) is the casual agent of black rot of crucifers. The hrp gene cluster is essential for virulence of Xcc. Expression of hrp genes is repressed in rich medium, and is induced in minimal medium and inplanta. This indicated the existence of a signal sensing system in the bacterium. But the plant signal sensing system had not been identified in Xcc so far. In this work, by searching the genome sequence of Xcc 8004, we found 6 genes whose predicted proteins have significant amino acid sequence similarity to components of the plant signal sensing system (PrhA, PrhJ, PrhI and PrhR) of Ralstonia solanacearum [Brito et al 2002]. To investigate the functions of these genes in Xcc, the genes XC0519 and XC0520 were inactivated by transposonal and gene-specific plasmid integration mutagenesis method. The virulence of the resulting mutants was detected in Chinese radish by leaf clipping method. Ten days post-inoculation, lesion length with a mean of 11.5 mm was observed on leaves inoculated with the wild type strain, while mean lesion lengths of 4.9 mm and 8.4 mm were observed with XC0519 and XC0520 mutants, respectively. These results demonstrated that XC0519 and XC0520 were required for full virulence of Xcc. To determined if XC0519 and XC0520 play any role in hrp gene expression, the GUS reporter system of hrpF and hrpBl were constructed. Expression of hrpF and hrpBl in the XC0519 mutation background significantly reduced compare to that in the wild-type XC0519 background. These results suggested that XC0519 play roles in plant signal(s) sensing and hrp genes regulation.