Characterization of antagonistic rice phyllosphere bacteria against Pyricularia oryzae and GC-MS profiling of bioactive metabolites.

Rice phyllosphere bacteria have potential as biocontrol candidates against foliar pathogens, but their antagonistic potential against Pyricularia oryzae remains poorly characterized. This study aimed to identify and evaluate rice phyllosphere bacteria exhibiting antagonistic activity against P. oryzae, followed by ultrastructural observation and metabolite profiling of the selected isolate using scanning electron microscopy (SEM) and gas chromatography-mass spectrometry (GC-MS). A total of 75 bacterial isolates were obtained from the phyllosphere of healthy rice plants. Of these, 52 isolates were classified as non-pathogenic and non-hemolytic, with 12 isolates exhibiting antagonistic activity in the dual culture assay. Among them, OPW17, OPW19, and SR10 showed the highest radial growth inhibition, reaching 57.2%, 51.1%, and 50.0%, respectively, on day 9, and were selected for further characterization. Volatile-mediated inhibition assays also showed significant reductions in fungal colony growth. Molecular identification based on 16 S rRNA analysis identified OPW17 as Paenibacillus polymyxa, OPW19 as Bacillus cereus, and SR10 as Serratia marcescens. SEM observations revealed deformation and damage to fungal hyphae of P. oryzae after dual culture with bacterial antagonists. GC-MS profiling of OPW17 metabolite extract tentatively identified several compounds, including n-hexadecanoic acid and cis-13-octadecenoic acid. These findings support the potential of the selected rice phyllosphere bacterial isolate as biocontrol candidates against P. oryzae. Further studies are needed to characterize the volatile fraction, validate the activities of detected metabolites, and evaluate the selected isolates under greenhouse and field conditions.