Cellobiose Transport: The Key to Trigger Virulence in the Plant Pathogenic Bacterium Streptomyces Scabies

Undergraduate #63
Discipline: Biological Sciences
Subcategory: Cell and Molecular Biology

Joren Jeico Salazar - California State University, Bakersfield
Co-Author(s): Isolde M. Francis, California State University Bakersfield, CA

The genus Streptomyces comprises several hundred bacterial species that are mostly saprophytes and best known for the production of important secondary metabolites such as antibiotics. Besides these harmless species, some are phytopathogens causing raised or pitted scab lesions on root and tuber crops like potato, radish, beet, peanut, and sweet potato. Their primary virulence determinants are part of a family of plant toxins called thaxtomins. Thaxtomin A, the main component made by S. scabies, S. acidiscabies and S. turgidiscabies, affects the plant cell wall by interfering with the activity of cellulose synthase leading to stunted plant growth, cell hypertrophy and tissue necrosis. The disaccharide cellobiose, a subunit of the plant cell wall polymer cellulose, has been shown to trigger thaxtomin production. Recently, the cellobiose transporter of these pathogens was identified and shown to be essential for the onset of virulence. The genes encoding this transporter are highly conserved among streptomycetes as they use cellobiose as a nutrient. However, in S. scabies, another gene cluster was found with high homology to the first transporter. Interestingly, this gene cluster seems to be absent in the other pathogens, S. acidiscabies and S. turgidiscabies.

In order to evaluate its role in virulence, specific mutants in S. scabies were created using the Redirect PCR targeting strategy by deleting the gene encoding the solute binding protein that confers substrate specificity (Δscab2421) and the transcriptional regulator of the gene cluster (Δscab2431). Although Δscab2431 is currently being evaluated, the virulence phenotype of Δscab2421 showed that this transporter is indeed involved in virulence. Upon inoculation of radish seedlings, Δscab2421 was less virulent than the wild type strain. This is most likely due to reduced toxin production as seen when growing the strains in oat bran broth, a complex plant-mimicking medium known to induce thaxtomin production, whereas toxin production by Δscab2421 was unaffected when grown in minimal medium containing cellobiose. These results indicate that under more complex conditions, as would be in plant-microbe interactions, the loss of this transporter cannot be compensated for by the actual cellobiose transporter that was identified earlier. Further research will focus on the elucidation of the specificity of this transporter and identifying its counterpart in the other pathogenic Streptomyces species.

Funder Acknowledgement(s): This study was supported, in part, by an RCU grant from CSU Bakersfield awarded to Isolde M. Francis.

Faculty Advisor: Isolde M. Francis,