Streptomyces scabiei and its toxin thaxtomin A induce scopoletin biosynthesis in tobacco and Arabidopsis thaliana.

Streptomyces scabiei is the predominant causal agent of common scab of potato in North America. The virulence of common scab-causing streptomycetes relies on their capacity to synthesize thaxtomins. In this study, the effects of S. scabiei infection and of thaxtomin A, the main toxin produced by S. scabiei, were tested for the elicitation of plant defense molecules in the model plants tobacco (Nicotiana tabacum) and Arabidopsis thaliana. Tobacco leaves infected with spores of S. scabiei strain EF-35 or infiltrated with purified thaxtomin A produced a blue fluorescent compound that was not detected in leaves infiltrated with spores of a S. scabiei mutant deficient in thaxtomin A biosynthesis. Thin layer chromatography and high performance liquid chromatography identified this fluorescent compound as scopoletin, a plant defense phytoalexin. Arabidopsis seedlings grown in liquid medium also excreted scopoletin as a reaction to S. scabiei and thaxtomin A. The effects of the presence of scopoletin on S. scabiei were also investigated. The phytoalexin scopoletin caused a slight reduction of bacterial growth and a severe decrease of thaxtomin A production. Scopoletin was shown to inhibit thaxtomin A production by repression of a gene involved in the toxin biosynthesis.