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Involvement of the Plant Polymer Suberin and the Disaccharide Cellobiose in Triggering Thaxtomin A Biosynthesis, a Phytotoxin Produced by the Pathogenic Agent Streptomyces scabies

Streptomyces scabies is a gram-positive soil bacterium recognized as the main causal agent of common scab. Pathogenicity in Streptomyces spp. depends on their capacity to synthesize phytotoxins called thaxtomins. Genes involved in biosynthesis of these secondary metabolites are known to be induced by cellobiose, a plant disaccharide. However, growth of S. scabies in a minimal medium containing cellobiose as a carbon source is very poor and only generates traces of thaxtomins. The effect of suberin, a lipid plant polymer, on thaxtomin A biosynthesis and the expression of genes involved in its biosynthetic pathway was analyzed. S. scabies was grown in a starch-containing minimal medium supplemented with cellobiose (0.5%), suberin (0.1%), or both. The presence of both cellobiose and suberin doubled bacterial growth and triggered thaxtomin A production, which correlated with the upregulation (up to 342-fold) of genes involved in thaxtomins synthesis. The addition of either suberin or cellobiose alone did not affect these parameters. Suberin appeared to stimulate the onset of secondary metabolism, which is a prerequisite to the production of molecules such as thaxtomin A, while cellobiose induced the biosynthesis of this secondary metabolite.