Defining the Secondary Metabolites in the <i>Pseudomonas protegens</i> PBL3 Secretome with Antagonistic Activity Against <i>Burkholderia glumae</i>

Rice production worldwide is threatened by the disease bacterial panicle blight (BPB) caused by <i>Burkholderia glumae</i>. Despite the threat, resources to control this disease, such as completely resistant cultivars or effective chemical methods, are still lacking. However, the need to control this disease has paved the way to explore biologically based approaches harnessing the antimicrobial activities of environmental bacteria. Previously, the bacterium <i>Pseudomonas protegens</i> PBL3 was identified as a potential biological control agent against <i>B. glumae</i> due to its antimicrobial activity against <i>B. glumae.</i> Such antimicrobial activity in vitro and in planta was associated with the <i>P. protegens</i> PBL3 bacteria-free secreted fraction (secretome), although the specific molecules responsible for this activity have remained elusive. In this work, we advance the characterization of the <i>P. protegens</i> PBL3 secretome by evaluating the antimicrobial activity in vitro of selected secondary metabolites predicted by the <i>P. protegens</i> PBL3 genomic sequence against <i>B. glumae</i>. In addition, using reversed-phase liquid chromatography tandem mass spectrometry of the <i>P. protegens</i> PBL3 secretome enabled us to successfully detect and quantify pyoluteorin, 2,4-diacetylphloroglucinol, and pyochelin. Among those, pyoluteorin and 2,4-diacetylphloroglucinol reduced the growth of <i>B. glumae</i> in vitro, along with reducing the symptoms of BPB and bacterial growth in planta, suggesting that these compounds could be effective as biopesticides to mitigate BPB.