Effect of carbohydrates on the production of thaxtomin A by Streptomyces acidiscabies

Several Streptomyces species cause plant diseases, including S. scabies, S. acidiscabies and S. turgidiscabies, which produce common scab of potato and similar diseases of root crops. These species produce thaxtomins, dipeptide phytotoxins that are responsible for disease symptoms. Thaxtomins are produced in vivo on diseased potato tissue and in vitro in oat-based culture media, but the regulation of thaxtomin biosynthesis is not understood. S. acidiscabies was grown in a variety of media to assess the impact of medium components on thaxtomin A (ThxA) production. ThxA biosynthesis was not correlated with bacterial biomass, nor was it stimulated by α-solanine or α-chaconine, the two most prevalent potato glycoalkaloids. ThxA production was stimulated by oat bran broth, even after exhaustive extraction, suggesting that specific carbohydrates may influence ThxA biosynthesis. Oat bran contains high levels of xylans and glucans, and both of these carbohydrates, as well as xylans from wheat and tamarind, stimulated ThxA production, but not to the same extent as oat bran. Starches and simple sugars did not induce ThxA production. The data indicate that complex carbohydrates may act as environmental signals to plant pathogenic Streptomyces, allowing production of thaxtomin and enabling bacteria to colonize its host.