Diketopiperazines from antagonistic <i>Pseudomonas</i> spp. and their activity against the root rot-causing pathogens, <i>Aphanomyces euteiches</i> and <i>Fusarium avenaceum</i>

AIMS: Root rot poses a significant threat to pulse crop production in Canada, the USA, and Europe, with limited effective management options currently available. This study investigates the basis of the antagonistic activity of two rhizosphere-derived bacterial strains, Pseudomonas synxantha rhizo25 (PS-S25) and Pseudomonas sp. rhizo66 (PD-S66) against two of the most damaging root rot-causing pathogens, Aphanomyces euteiches and Fusarium avenaceum. METHODS AND RESULTS: Competitive growth inhibition assays demonstrated that both isolates significantly suppressed plant pathogen growth, prompting further investigation into their bioactive secondary metabolites. Using a bioassay-guided fractionation approach, we isolated and characterized diketopiperazines (DKPs), specifically cyclo(Phe-Pro), from both strains. Spectroscopic analyses revealed distinct stereoisomers: cyclo(L-Phe-D-Pro) from PS-S25 and cyclo(L-Phe-L-Pro) from PD-S66. Both compounds inhibited the growth of both A. euteiches and F. avenaceum. CONCLUSIONS: These findings represent the first report of DKPs with activity against A. euteiches, highlighting their potential as active ingredients in biocontrol or natural products for controlling root rot in pulses. This study provides foundational insights into the biochemical mechanisms underlying the biocontrol activity of these two bacterial strains and supports the development of sustainable microbial-based disease management strategies for pulse crops.