Quercetin as a selective quorum sensing inhibitor: in silico and in vitro analyses against Pseudomonas syringae with limited impact on plant growth-promoting bacteria.

Quorum sensing (QS) is a bacterial communication mechanism that coordinates gene expression and regulates population-dependent behaviors such as biofilm formation, virulence, and antibiotic resistance. Pseudomonas syringae, a major plant pathogen, causes bacterial blight in legumes, significantly reducing crop productivity. In contrast, plant growth-promoting bacteria (PGPR) produce secondary metabolites that enhance plant growth and protect against pathogens. This study aimed to identify plant-derived compounds capable of selectively disrupting the QS of P. syringae without adversely affecting beneficial PGPR strains, using a combined in silico and in vitro approach. The QS-specific mode of action was functionally confirmed using a Chromobacterium violaceum biosensor assay, which revealed potent, dose-dependent inhibition of violacein pigment. Molecular docking and MM-GBSA (Molecular Mechanics Generalized Born Surface Area) analyses identified quercetin as the most potent compound, exhibiting strong binding affinity (docking score: - 10.363 kcal/mol; binding energy: - 55.89 kcal/mol) to QS-related proteins in P. syringae. Beneficial bacteria, including Klebsiella pneumoniae and Sinorhizobium meliloti, showed lower affinities to quercetin. Chromobacterium violaceum, a well-known QS reporter strain used as a control, exhibited the lowest sub-minimum inhibitory concentration (47.22 μg/mL). Experimentally, quercetin inhibited bacterial growth by 90-95% at 1024 μg/mL. The sub-MIC values for P. syringae, K. pneumoniae, and S. meliloti were 156.7, 242.6, and 535.7 μg/mL, respectively. At 64 μg/mL, quercetin suppressed biofilm formation by 92% in P. syringae, 95-98% in K. pneumoniae and S. meliloti, and 78% in C. violaceum. These findings highlight quercetin's selective disruption of QS and biofilm formation in P. syringae, underscoring its potential as a targeted biocontrol agent for legume crop protection.