La production de thaxtomine A par le Streptomyces scabies EF-35 in vitro et dans un sol sablonneux
Common potato scab is a disease that results in a lower quality of potato tubers. Streptomyces scabies, a soil-borne actinomycete, is considered to be the principal causal agent of common scab. Pathogenicity of Streptomyces scabies appears to be positively correlated with thaxtomins production. Thaxtomins are a group of phytotoxins, first isolated by King's research group in 1989 from scab lesions on potato tubers. Thaxtomin A is the main thaxtomin produced by Streptomyces scabies. This pathogen was found to produce thaxtomin A in culture media containing potato suberin. Suberin is a specific plant cell wall polymer composed of polyaliphatic and polyphenolic domains. The objective of this study is to identify a suberin constituent that is responsible for the induction of thaxtomin A production in Streptomyces scabies. In Chapter 1, we first compared thaxtomin A production in culture media supplemented with different concentrations of suberin. This allowed us to determine the optimal concentration of suberin for thaxtomin A production. We found that the addition of 0.2% suberin in Streptomyces scabies minimal medium allowed a maximal production of thaxtomin A. We also developed a protocol to extract thaxtomin A from sandy soil. The method was especially efficient when thaxtomin A exceeded 100 [micro]g/10 g. Using this method, we were able to extract thaxtomin A from a sandy soil amended with suberin and inoculated with Streptomyces scabies spores or mycelium. In Chapter II, we determined thaxtomin A production in Streptomyces scabies culture medium containing the chemical degradation products of suberin. The aliphatic portion of suberin was obtained by the BF[indice inférieur 3]-methanol depolymerization method, whereas the phenolic portion was recovered by alkaline nitrobenzene oxidation (NBO). There was no thaxtomin A production in medium supplemented with the aliphatic portion of suberin. It was not possible to determine the effect of the phenolic portion on thaxtomin A production because this chemical fraction inhibited Streptomyces scabies growth.
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