Actinomycins produced by endophyte Streptomyces sp. GLL‐9 from navel orange plant exhibit high antimicrobial effect against Xanthomonas citri susp. citri and Penicillium italicum

Citrus canker and citrus blue mold are two severe diseases in citrus plants, which are mainly caused by Xanthomonas citri susp. citri (Xcc) and Penicillium italicum, respectively. The currently widely used pesticides for these two diseases are harmful to human health and the environment. Therefore, searching for novel antimicrobial agents, especially from natural resources, is getting increasing interest.In this study, the crude extract of Streptomyces sp. GLL-9, an endophyte from a navel orange tree, was found to exhibit excellent antimicrobial effects against Xcc and P. italicum. Bioassay-guided isolation led to the discovery of three actinomycins (Acts), actinomycin X2 (Act-X2 ), actinomycin D (ActD), and actinomycin XOβ (Act-XOβ ). The MIC (minimum inhibitory concentration) values of Act-X2 , ActD, and Act-XOβ were 31.25, 62.50, and 62.50 μg mL-1 against Xcc, respectively, while 62.50 (Act-X2 ) and 125.00 μg mL-1 (ActD) against P. italicum, being better or comparable to the positive controls. The highest yield of Acts was obtained by solid-state fermentation with rice containing 1% L-tryptophan as a culture medium, being 6.03, 3.07, and 1.02 mg g-1 , for Act-X2 , ActD, and Act-XOβ , respectively. The ethyl acetate extract of Streptomyces sp. GLL-9 cultivated under the optimal fermentation conditions (EAE-1) can efficiently control these two citrus diseases by excessively producing reactive oxygen species (ROS) in both pathogens, damaging the cell membranes of P. italicum, and inhibiting the growth of Xcc. In addition, Act-X2 , ActD, and EAE-1 displayed broad-spectrum antifungal activity.EAE-1 and Acts produced by Streptomyces sp. GLL-9 have high potential as novel antimicrobial agents against plant pathogens. © 2023 Society of Chemical Industry.