"Black rot" is caused by the pathogenic bacterium Xanthomonas campestris pathovar campestris (or Xcc). Under favorable conditions (high humidity and temperature), Xcc infects vegetable crops by spreading through the plants' vascular tissues, turning the veins in their leaves yellow and black, and causing V-shaped lesions along the margins of the leaves. All vegetables in the crucifer family, including broccoli, Brussels sprouts, cabbage, cauliflower, kale, mustard, radish, rutabaga, and turnip, are potential hosts for Xcc. The model plant Arabidopsis thaliana is also susceptible to Xcc infection. Surprisingly, however, some wild cruciferous weed species do not manifest the characteristic symptoms of "black rot" disease when infected.
To date, there is no effective treatment for Xcc infection, so in hopes of developing a treatment, scientists at four Chinese institutions (the Institute of Microbiology at the Chinese Academy of Sciences, the Chinese National Human Genome Center at Shanghai, Guangxi University, and the Chinese National Human Genome Center at Beijing) have focused their efforts on characterizing the genes responsible for Xcc pathogenicity. In their study published today, the investigators describe the identification of 75 different genes responsible for Xcc virulence. These genes appear to belong to 13 different functional categories or related metabolic pathways. The researchers hope that the molecular characterization of these pathogenicity-related genes will lead to the development of a treatment for "black rot" disease.
Employing whole-genome comparative genomic approaches, the authors sequenced the complete genome of an Xcc strain that was isolated from an infected cauliflower plant in England during the 1950's. They then compared this sequence to a previously published sequence from a cabbage-derived Xcc strain. Although the gene content of the two strains was very similar, the authors identified several genes located on strain-specific chromosomal elements that were unique to each strain. In addition, there were dramatic differences in the genomic arrangement of the two strains; the scientists identified significant rearrangements between the genomes, including major translocations, inversions, insertions, and deletions.
In order to functionally characterize Xcc and identify genes implicated in its pathogenicity, the researchers then screened an Xcc transposon insertional mutant library in its host plant (cabbage). They screened a total of 16,512 Xcc mutants on individual cabbage plants and, of these, 172 proved to be non-pathogenic. Upon further characterization of the 172 non-pathogenic mutants, the researchers came up with a non-redundant list of 75 genes or non-coding regions that are involved in Xcc pathogenicity.
Interestingly, the researchers identified three genes that were implicated in pathogenicity but that were not present in the previously described Xcc genomic sequence. To test the biological implications of this observation, they inoculated five different vegetable species with the three mutants corresponding to these strain-specific genes, and they observed significant differences in the response of each host species to infection. The authors point out that these findings highlight the role of genome dynamics in the evolution of pathogenicity in Xcc in response to different host species.