ORIGINAL PAPER Streptomyces scabiei and its toxin thaxtomin A induce scopoletin biosynthesis in tobacco and Arabidopsis thaliana Sylvain Lerat • Amadou H. Babana • Mohamed El Oirdi • Abdelbassed El Hadrami • Fouad Daayf • Nathalie Beaudoin • Kamal Bouarab • Carole Beaulieu Received: 14 August 2009 / Revised: 24 September 2009 / Accepted: 6 October 2009 / Published online: 27 October 2009 Ó Springer-Verlag 2009 Abstract 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 phy- toalexin. 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 scopo- letin 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. Keywords Common scab  Model plants  Scopoletin  Streptomyces scabiei  Thaxtomin A Introduction Common scab is an important bacterial disease of potato (Solanum tuberosum) and several root-vegetables (Goyer and Beaulieu 1997; Lerat et al. 2009a). Based on economic losses, common scab is ranked amongst the most important disease affecting potatoes in Canada (Hill and Lazarovits, 2005). Streptomyces scabiei (or scabies), a gram positive filamentous soil bacterium, is considered to be the main causal agent in North America. This pathogen does not only induce common scab, it is also known to reduce growth and to induce root necrosis on most monocotyle- dons and dicotyledons (Leiner et al. 1996). S. scabiei pathogenicity is correlated with the ability to produce toxins called thaxtomins. Thaxtomins are 4-nitroindol-3-yl- containing 2,5 dioxopiperazines (King et al. 1992) and thaxtomin A is the most abundant toxin produced by S. scabiei (King et al. 1989). Thaxtomin A appears to be essential for symptom development. Purified thaxtomin A reproduces symptoms of common scab when applied on aseptic potato tubers (Lawrence et al. 1990). Likewise, NTG mutagenesis on the S. scabiei pathogenic strain EF-35 showed that mutants producing no or reduced amounts of thaxtomin A were non-pathogenic or showed reduced virulence (Goyer et al. Communicated by D. Zaitlin. S. Lerat  M. El Oirdi  N. Beaudoin  K. Bouarab (&)  C. Beaulieu (&) Centre SE ` VE, De ´partement de Biologie, Universite ´ de Sherbrooke, 2500 boulevard de l’Universite ´, Sherbrooke, QC J1K 2R1, Canada e-mail: Kamal.Bouarab@USherbrooke.ca C. Beaulieu e-mail: Carole.Beaulieu@USherbrooke.ca A. H. Babana De ´partement de Biologie, Universite ´ de Bamako, Bamako BPE3206, Mali A. El Hadrami  F. Daayf Department of Plant Science, 222 Agriculture Building, University of Manitoba, Winnipeg, MB R3T 2N2, Canada 123 Plant Cell Rep (2009) 28:1895–1903 DOI 10.1007/s00299-009-0792-1