Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 10, 2007
Publication Date: April 8, 2008
Citation: Qu, X., Wanner, L.A., Christ, B.J. 2008. Using the TxtAB Operon to Quantify Pathogenic Streptomyces in Potato Tubers and Soil. Phytopathology. 98:405-412.
Interpretive Summary: Bacteria causing the tuber disease potato common scab can be transmitted through soil and on seed potato tubers. Sensitive, specific and quantitative assays are needed to determine the relative significance of soil-borne and seed-borne disease-causing bacteria. We have designed a sensitive quantitative PCR (polymerase chain reaction) assay for the common scab-causing bacteria, and demonstrated its utility, specificity and sensitivity in both soil and potato tuber samples. This assay will be useful in determining whether there is a threshold number of pathogenic bacteria for incidence of the disease, and could become a useful component of seed potato and soil testing and evaluation.
The phytotoxin thaxtomin, produced by plant pathogenic Streptomyces species, is a pathogenicity determinant for common scab. In this study a SYBR Green quantitative real-time PCR assay using primers targeted on the txtAB operon of Streptomyces was developed to quantify pathogenic bacterial populations in potatoes and soil. The real-time PCR assay was specific for pathogenic Streptomyces strains in tests with DNA from pathogenic and nonpathogenic isolates of Streptomyces. The detection limit of the assay was 10 fg of the target DNA. Cycle threshold (CT) values were linearly correlated with the concentration of the target DNA (correlation coefficient R2 = 0.99) and were not affected by the presence of plant DNA extracts, indicating the usefulness of the assay for quantitative analyses of the pathogenic bacteria in plant tissues. The amount of pathogenic Streptomyces DNA present in asymptomatic and symptomatic tubers assessed using the assay ranged from 101 to 106 pg per gram tuber tissue. A standard curve was established to quantify pathogenic Streptomyces in soil. Numbers of pathogenic Streptomyces colony forming units ranged from 103 to 106 per gram soil from potato fields where common scab was found. This real-time PCR assay using primers designed from the txtAB operon allows rapid, accurate and cost effective quantification of pathogenic Streptomyces strains in potato tubers and in soil.