Xenorhabdus and Photorhabdus Metabolites for Fungal Biocontrol and Application in Soybean Seed Protection

<i>Photorhabdus</i> and <i>Xenorhabdus</i> bacteria, members of the Morganellaceae family, are sources of novel natural products for the biocontrol of fungal pathogens in soybean production. This study demonstrated the inhibitory effects of metabolites from four <i>Photorhabdus</i> and <i>Xenorhabdus</i> strains (including a local isolate, <i>X. szentirmaii</i> PAM 25), against four key phytopathogenic fungi. Bacterial metabolite efficacy and fungal susceptibility varied. <i>Xenorhabdus szentirmaii</i> DSM 16338, <i>X. szentirmaii</i> PAM 25, and <i>X. doucetiae</i> demonstrated significant inhibition (>90%) against <i>Sclerotinia sclerotiorum</i>, <i>Botrytis cinerea</i>, and <i>Macrophomina phaseolina</i>, exhibiting superior efficacy compared to <i>X. nematophila</i> and <i>Photorhabdus kayaii</i>. <i>Fusarium oxysporum</i> demonstrated greater resistance to the bacterial supernatants. We identified fabclavine, pyrollizixenamide, and szentirazine from <i>X. szentirmaii</i>, and xenocoumacins from <i>X. doucetiae</i> as the antifungal bioactive compounds in the respective easyPACid mutants. Furthermore, we assessed the efficacy of <i>X. szentirmaii</i> PAM 25 and its metabolites in protecting soybean seeds from <i>S. sclerotiorum</i> and investigated the shelf stability of the bacterial metabolites as the fungus suppressors. Cell-free supernatant maintained >80% inhibition of <i>S. sclerotiorum</i> after one year at 5-35 °C. Importantly, the cell-free supernatant, as well as the bacterial culture, effectively inhibited <i>S. sclerotiorum</i> in seed treatments, ensuring ≥80% seed germination, comparable to thiophanate-methyl + fluazinam fungicide. This study demonstrates that the direct seed application of <i>Xenorhabdus</i> and <i>Photorhabdus</i> bacteria offers a practical and innovative biological control method against soil-borne fungal pathogens.