title: Tryptophan regulates thaxtomin A and indole-3-acetic acid production in Streptomyces scabiei and modifies its interactions with radish seedlings.
abstract: The virulence of Streptomyces scabiei, the causal agent of common scab, depends mainly on the production of the toxin thaxtomin A. S. scabiei also produces indole-3-acetic acid (IAA) but the role of this hormone in the interaction between pathogenic streptomycetes and plants has not yet been elucidated. Tryptophan is a biosynthetic precursor of both IAA and thaxtomin A. In this study, the effect of tryptophan on thaxtomin A and IAA production as well as its effect on the transcription of the corresponding biosynthetic genes in S. scabiei has been analyzed. In vitro IAA production depended on the availability of tryptophan. However, addition of this amino acid to the culture medium inhibited the biosynthesis of thaxtomin A. Expression of thaxtomin A biosynthetic genes nos and txtA were strongly repressed in the presence of tryptophan; however, modulation of the expression was not observed for the IAA biosynthetic genes iaaM and iaaH. The effects of an exogenous tryptophan supply on S. scabiei virulence were assessed on radish seedlings. Addition of tryptophan reduced symptoms on inoculated radish roots compared with seedlings grown in the absence of the bacterium, by way of inhibition of thaxtomin A production and increase of IAA biosynthesis.
authors.affiliation: Departement de Biologie, Universite de Sherbrooke, Sherbrooke, Quebec, Canada.
authors.forename: Geneviève S
authors.initials: GS
authors.lastname: Legault
authors.forename: Sylvain
authors.initials: S
authors.lastname: Lerat
authors.forename: Philippe
authors.initials: P
authors.lastname: Nicolas
authors.forename: Carole
authors.initials: C
authors.lastname: Beaulieu
mesh_terms: D015870:Gene Expression; D015964:Gene Expression Regulation, Bacterial; D017343:Genes, Plant / Q000235:genetics; D005856:Germ-Free Life; D054884:Host-Pathogen Interactions; D007210:Indoleacetic Acids / Q000378:metabolism*; D007211:Indoles / Q000378:metabolism*; D010879:Piperazines / Q000378:metabolism*; D010935:Plant Diseases / Q000382:microbiology*; D010937:Plant Growth Regulators / Q000378:metabolism; D031224:Raphanus / Q000235:genetics / Q000382:microbiology* / Q000502:physiology; D036226:Seedlings / Q000235:genetics / Q000378:metabolism; D013302:Streptomyces / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism / Q000472:pathogenicity; D013997:Time Factors; D014364:Tryptophan / Q000494:pharmacology*; D014774:Virulence
chemical_list: D007210:Indoleacetic Acids; D007211:Indoles; D010879:Piperazines; D010937:Plant Growth Regulators; C406691:thaxtomine A; C030737:indoleacetic acid; D014364:Tryptophan