Concanamycins Are Key Contributors to the Virulence of the Potato Common Scab Pathogen Streptomyces scabiei

The soil-dwelling bacterium Streptomyces scabiei is distributed worldwide and is the best-characterised causative agent of common scab disease, which impacts potato crops and causes significant economic losses to growers. The principal pathogenicity factor responsible for common scab development is the phytotoxin thaxtomin A, which functions as a cellulose biosynthesis inhibitor in plants. S. scabiei also produces polyketide compounds belonging to the concanamycin family, which serve as inhibitors of eukaryotic vacuolar-type ATPases and have been shown to exhibit phytotoxic activity against different plant species. It has been proposed that concanamycins contribute to the virulence of S. scabiei, but direct evidence of this has been lacking. Using constructed strains of S. scabiei that are either unable to produce concanamycins or produce elevated levels of the metabolites, we showed that concanamycins enhance the severity of disease symptoms induced by S. scabiei on radish seedlings and potato tuber tissue. We demonstrated that concanamycin production is controlled by two regulatory genes that are situated within the concanamycin biosynthetic gene cluster, and that production of concanamycins and thaxtomin A by S. scabiei is modulated by different nutritional signals. The concanamycin biosynthetic gene cluster is conserved in other common scab-causing Streptomyces spp., suggesting that these metabolites may function as important virulence determinants in multiple phytopathogenic species. Overall, this study expands our understanding of the molecular factors that enable plant host colonisation and common scab disease development by S. scabiei.