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

Abstract

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