UV mutagenesis‐enhanced Streptomyces nogalater produces a novel streptothricin‐like compound with strong activity against cabbage soft rot

Abstract BACKGROUND A total of 1000 soil‐derived actinomycetes were isolated and screened for antibacterial activity against Pectobacterium carotovorum subsp . carotovorum (cabbage soft‐rot disease). The most potent isolate, KRA705‐18 , was selected and UV mutagenesis was performed to improve its bioactivity. The active compound was purified, and its molecular formula and structure were identified through multiple spectroscopic analysis. RESULTS An irradiation time of 90 s was found to be optimal for mutant selection, yielding a 1.09% survival rate. After two successive mutagenesis cycles, the mutant strain ( Streptomyces sp . KRA0514‐6 ) exhibited the strongest and most stable antibacterial activity, consistently achieving over 90% inhibition within 24 h. The culture broth of the mutant achieved complete (100%) cabbage soft rot control at a 250‐fold dilution, comparable to oxolinic acid (200 ppm) under glasshouse conditions. Field trials further showed stable activity at the same dilution rates, with control efficacies of more than 70% at 21 days after treatment. Phylogenetic analysis based on 16S rRNA gene sequencing identified the mutant strain as Streptomyces sp. KRA0514‐6 , closely related to Streptomyces nogalater JCM4799T. Bioassay‐guided fractionation and purification of culture filtrate using C18 open‐column chromatography, Sephadex LH‐20, preparative high‐performance liquid chromatography and hydrophilic interaction liquid chromatography led to the isolation of the active substance HL‐3. In vivo glasshouse assays confirmed that purified HL‐3 provided 100% cabbage soft rot disease control at concentrations >1 ppm. HR‐ESI‐MS analysis determined the molecular weight of HL‐3 to be 459 g mol −1 , leading to the identification of its molecular formula as C 17 H 29 N 7 O 8 . Finally, by nuclear magnetic resonance spectroscopic analysis, HL‐3 was identified as novel streptothricin‐like compound containing N ‐methylglycine instead of β ‐lysine. CONCLUSION Overall, UV mutagenesis effectively enhanced the antibacterial potential of the wild strain, and the purified compound, HL‐3 , exhibited potent activity against cabbage soft rot disease, which can be a promising biocontrol agent for sustainable plant disease management strategies. The results of this study provide new insights into streptothricin structure–activity relationships and suggest the existence of unknown biosynthetic mechanisms of S. nogalater . © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.