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

Discovery of Antibacterial Compounds against Xanthomonas citri subsp. citri from a Marine Fungus Aspergillus terreus SCSIO 41202 and the Mode of Action

Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is a severe citrus disease. Currently, copper-containing pesticides are widely used to manage this disease, posing high risks to the environment and human health. This study reports the discovery of naturally occurring anti-Xcc compounds from a deep-sea fungus, Aspergillus terreus SCSIO 41202, and the possible mode of action. The ethyl acetate extract of A. terreus was subjected to bioassay-guided isolation, resulting in the discovery of eight anti-Xcc compounds (1-8) with minimum inhibitory concentrations (MICs) ranging from 0.078 to 0.625 mg/mL. The chemical structures of these eight metabolites were determined by integrative analysis of various spectroscopic data. Among these compounds, Asperporonin A (1) and Asperporonin B (2) were identified as novel compounds with a very unusual structural skeleton. The electronic circular dichroism was used to determine the absolute configurations of 1 and 2 through quantum chemical calculation. A bioconversion pathway involving pinacol rearrangement was proposed to produce the unusual compounds (1-2). Compound 6 exhibited an excellent anti-Xcc effect with a MIC value of 0.078 mg/mL, which was significantly more potent than the positive control CuSO4 (MIC = 0.3125 mg/mL). Compound 6 inhibited cell growth by disrupting biofilm formation, destroying the cell membrane, and inducing the accumulation of reactive oxygen species. In vivo tests indicated that compound 6 is highly effective in controlling citrus canker disease. These results indicate that compounds 1-8, especially 6, have the potential as lead compounds for the development of new, environmentally friendly, and efficient anti-Xcc pesticides.

Burkholderia ambifaria H8 as an effective biocontrol strain against maize stalk rot via producing volatile dimethyl disulfide

BACKGROUND

Maize stalk rot (MSR) caused by Fusarium graminearum is the primary factor contributing to the reduction in maize yield and quality. However, this soil-borne disease presents a significant challenge for sustainable control through field management and chemical agents. The screening of novel biocontrol agents can aid in developing innovative and successful strategies for MSR control.

RESULTS

A total of 407 strains of bacteria were isolated from the rhizosphere soil of a resistant maize inbred line. One strain exhibited significant antagonistic activity in plate and pot experiments, and was identified as Burkholderia ambifaria H8. The strain could significantly inhibit the mycelial growth and spore germination of F. graminearum, induce resistance to stalk rot, and promote plant growth. The volatile compounds produced by strain H8 and its secondary metabolites in the sterile fermentation broth exhibited antagonistic activity. The primary volatile compound produced by strain H8 was identified as dimethyl disulfide (DMDS) using gas chromatography tandem mass spectrometry. Through in vitro antagonistic activity assays and microscopic observation, it was confirmed that DMDS was capable of inhibiting mycelial growth and disrupting the mycelial structure of F. graminearum, suggesting it may be the major active compound for strain H8. The transcriptome data of F. graminearum further indicated that strain H8 and its volatile compounds could alter pathogenic fungi metabolism, influence the related metabolic pathways, and potentially induce cell apoptosis within F. graminearum.

CONCLUSION

Our results showed that B. ambifaria H8 was capable of producing the volatile substance dimethyl disulfide, which influenced the synthesis and permeability of cell membranes in pathogens. Thus, B. ambifaria H8 was found to be a promising biological control agent against MSR. © 2024 Society of Chemical Industry.

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

The present study investigated the antibacterial mechanism, control efficiency, and nontarget toxicity of actinomycin X2 (Act-X2) against Xanthomonas citri subsp. citri (Xcc) for the first time. Act-X2 almost completely inhibited the proliferation of Xcc in the growth curve assay at a concentration of 0.25 MIC (minimum inhibitory concentration, MIC = 31.25 μg/mL). This inhibitory effect was achieved by increasing the production of reactive oxygen species (ROS), blocking the formation of biofilms, obstructing the synthesis of intracellular proteins, and decreasing the enzymatic activities of malate dehydrogenase (MDH) and succinate dehydrogenase (SDH) of Xcc. Molecular docking and quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) analysis results indicated that Act-X2 steadily bonded to the RNA polymerase, ribosome, malate dehydrogenase, and succinate dehydrogenase to inhibit their activities, thus drastically reducing the expression levels of related genes. Act-X2 showed far more effectiveness than the commercially available pesticide Cu2(OH)3Cl in the prevention and therapy of citrus canker disease. Furthermore, the nontarget toxicity evaluation demonstrated that Act-X2 was not phytotoxic to citrus trees and exhibited minimal toxicity to earthworms in both contact and soil toxic assays. This study suggests that Act-X2 has the potential as an effective and environmentally friendly antibacterial agent.

Control of citrus blue and green molds by Actinomycin X2 and its possible antifungal mechanism

Citrus blue and green molds caused by Penicillium digitatum, P. italicum, and P. polonicum, are the major postharvest diseases of citrus fruit. In the present study, Actinomycin X2 (Act-X2), a naturally occurring antibiotic produced by Streptomyces species, was found to show excellent antifungal effect against these three pathogens with a minimum inhibitory concentration (MIC) value of 62.5 μg/mL for them all, which was better than the positive control thiophanate-methyl. Act-X2 significantly reduced the percentage of spore germination, and highly inhibited the mycelial growth of P. italicum, P. digitatum, and P. polonicum with EC50 values being 34.34, 13.76, and 37.48 μg/mL, respectively. In addition, Act-X2 greatly decreased the intracellular protein content while increasing the reactive oxygen species (ROS) level and superoxide anion (O2-) content in the mycelia of pathogens. In vivo test indicated that Act-X2 strongly inhibited the infection of navel oranges by these three Penicillium species, with an inhibition percentage of >50% for them all at the concentration of 10 MIC. Transcriptome analysis suggested that Act-X2 might highly influence the ribosomal functions of P. polonicum, which was supported as well by the molecular docking analysis of Act-X2 with some key functional proteins and RNAs of the ribosome. Furthermore, Act-X2 significantly reduced the decay percentage and improved the firmness, color, and sugar-acid ratio of navel oranges spray-inoculated with P. polonicum during the postharvest storage at 4 °C for 60 d.