Anti-Fireblight Potential of Fabclavines, Synthesized by the Type Strains of Xenorhabdus szentirmaii and Xenorhabdus budapestensis

The options for controlling agricultural plant-pathogenic bacterial diseases with antibiotics are rather limited. Applications of naturally derived antimicrobial peptides may offer an alternative. Objectives. The cell-free conditioned media of Xenorhabdus budapestensis and X. szentirmaii bacteria inactivate Erwinia amylovora, the causative agent of fire blight in Rosaceae, both in vitro and in planta. The working hypothesis was that the active ingredient was fabclavine. To test this hypothesis, inducible fabclavine-producing strains were constructed in both Xenorhabdus species. The anti-Erwinia potential of non-induced and induced fabclavine-producing cultures was compared in vitro and in planta. Methods. Promoter exchange mutants known to exclusively produce the fabclavine antibiotic were generated and their fabclavine production was confirmed by LC-HRMS, and in vitro bioassays. Finally, apple blossoms in an incubator were subjected to antimicrobial and control treatments before experimental inoculation of the buds with Erwinia amylovora strain Ea1. Results. Cell-free medium from both induced fabclavine-producing Xenorhabdus species exerted strong antagonistic effects on Ea1 cells in vitro, whereas the non-induced ones showed substantially reduced antibacterial activity. Similarly, both induced culture media prevented fire blight symptoms in experimentally inoculated flowers in a dose-dependent manner. The non-induced X. szentirmaii mutant strain was completely inactive in planta, whereas the X. budapestensis strain exhibited some protective activity. Conclusions. The results support the hypothesis that the fermentation product of the inducible Xenorhabdus mutants producing only fabclavines can be used for protection against E. amylovora. Some aspects of the application perspectives are discussed in comparison with conventional streptomycin and kasugamycin treatments.