Location: Floral and Nursery Plants ResearchTitle: Prophage Rs551 and its repressor gene orf14 reduce virulence and increase competitive fitness of its Ralstonia solanacearum carrier strain UW551 Author
Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2017
Publication Date: 12/22/2017
Citation: Ebrahim, A., Stulberg, M.J., Huang, Q. 2017. Prophage Rs551 and its repressor gene orf14 reduce virulence and increase competitive fitness of its Ralstonia solanacearum carrier strain UW551. Frontiers in Microbiology. 8:2480. https://doi.org/10.3389/fmicb.2017.02480.
DOI: https://doi.org/10.3389/fmicb.2017.02480 Interpretive Summary: Ralstonia solanacearum is a bacterial pathogen that causes millions of dollars of crop losses in a wide range of plant species worldwide. One strain in particular, the r3b2 subgroup, is such a threat to U.S. agriculture that is has been designated a select agent in the U.S. and is a quarantine pathogen in Europe and Canada. ARS scientists in Beltsville, MD discovered a virus associated with the bacterium that offers the bacterium a competitive advantage. The virus appears to play a role in regulating the virulence of its carrier bacterial strain and in helping the bacterium to persist in the environment, which in turn prolongs the symbiotic relationship between the virus and the bacterium. This study helps define the relationship between the virus and the bacterium so that effective controls for this important plant pathogen can be developed.
Technical Abstract: We previously characterized a filamentous lysogenic bacteriophage, 'Rs551, isolated directly from the race 3 biovar 2 phylotype IIB sequevar 1 strain UW551 of R. solanacearum grown under normal culture conditions. The genome of 'Rs551 was identified with 100% identity in the deposited genomes of 11 race 3 biovar 2 phylotype IIB sequevar 1 strains of R. solanacearum, indicating evolutionary and biological importance, and ORF14 of 'Rs551 was annotated as a putative type-2 repressor. In this study, we determined the effect of the prophage and its ORF14 on the virulence and competitive fitness of its lysogenic host strain UW551 by deleting the orf14 gene only, and nine of the prophage’s 14 genes including orf14 and eight out of nine structural genes, respectively, from the genome of UW551. The two mutants were increased in extracellular polysaccharide production, twitching motility, expression of targeted virulence and virulence regulatory genes (pilT, egl, pehC, hrPB, and phcA), and virulence, suggesting that the virulence of UW551 was negatively regulated by 'Rs551, at least partially through ORF14. Interestingly, we found that the 'Rs551-carrying wt strain UW551 significantly outcompeted the 'Rs551-susceptible strain RUN302 of R. solanacearum in tomato plants co-inoculated with the two strains. When each of the two mutant strains was co-inoculated with RUN302, however, the mutants were significantly out-competed by RUN302 for the same colonization site. Our results suggest that ecologically, 'Rs551 may play an important role by regulating the virulence of and offering a competitive fitness to its carrier bacterial strain for persistence of the bacterium in the environment, which in turn prolongs the symbiotic relationship between the phage 'Rs551 and the R. solanacearum strain UW551.