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

Abstract

BACKGROUNDCitrus 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.RESULTSIn 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.CONCLUSIONEAE‐1 and Acts produced by Streptomyces sp. GLL‐9 have high potential as novel antimicrobial agents against plant pathogens. © 2023 Society of Chemical Industry.

Antibacterial Mechanism, Control Efficiency, and Nontarget Toxicity Evaluation of Actinomycin X2 against Xanthomonas citri Subsp. citri

“…Similar to the present study, it was reported that Act-X 2 directly bonded to the protein kinase PKNB to inhibit the proliferation of multidrug-resistant tuberculosis bacteria (TB) . In addition, Act-X 2 was found to be a broad-spectrum antibacterial agent due to its action targets being various, involving both proteins and nucleic acids, , which was consistent with the present observations.…”

“…GLL-9 according to our previous report. 24 The chemical structure, high-performance liquid chromatography (HPLC) purity, 1 H, 13 C NMR, and high-resolution mass spectrometry (HRMS) spectra of Act-X 2 are shown in the Supporting Information. Nutrient broth (NB) and nutrient agar (NA) were purchased from Hope Biotechnology Co., Ltd. (Qingdao, China).…”

“…Act-X 2 was isolated from the fermentation products of endophyte Streptomyces sp. GLL-9 according to our previous report . The chemical structure, high-performance liquid chromatography (HPLC) purity, 1 H, 13 C NMR, and high-resolution mass spectrometry (HRMS) spectra of Act-X 2 are shown in the Supporting Information.…”

“…GLL-9 from the branches of Gannan navel oranges, which was capable of producing Act-X 2 with a high yield in rice medium (about 6.03 mg/g of cultural medium), and the in vitro minimum inhibitory concentration (MIC) of Act-X 2 on Xcc was 31.25 μg/mL. 24 However, the antibacterial mechanism, in vivo control efficiency, and the nontarget toxicity of Act-X 2 remain unknown. Thus, in this study, the antibacterial mechanism of Act-X 2 against Xcc was investigated by evaluating its impact on bacterial growth, reactive oxygen species (ROS) levels, biofilm formation, intracellular protein content, and cell membrane integrity.…”

“…In a recent investigation, we identified a strain, Streptomyces sp. GLL-9 from the branches of Gannan navel oranges, which was capable of producing Act-X 2 with a high yield in rice medium (about 6.03 mg/g of cultural medium), and the in vitro minimum inhibitory concentration (MIC) of Act-X 2 on Xcc was 31.25 μg/mL . However, the antibacterial mechanism, in vivo control efficiency, and the nontarget toxicity of Act-X 2 remain unknown.…”