Deciphering synergy of beneficial bacteria and quercetin dihydrate for enhanced resilience in Chinese Kale against combined cadmium and salinity stress

• Cadmium and salinity stress significantly decreased plant growth in Chinese kale plants. • The combined application Bacillus strains and quercetin dihydrate synergistically improve the plant growth under normal and stressed conditions. • Synergistic action of quercetin dihydrate and beneficial bacteria enhanced cadmium and salinity tolerance in Chinese kale by activating both biochemical defenses and metabolic adjustments • In molecular docking analysis, quercetin demonstrated a stable interaction with stress-responsive proteins. • Pathway-level modifications promoting stress tolerance were identified by metabolomic analysis. Degradation of agricultural lands due to the accumulation of trace elements and salinity is anticipated to emerge as a critical issue impeding sustainable agricultural production and food security. This study aimed to mitigate combined salinity and cadmium metal stress in Chinese kale ( Brassica oleracea var. alboglabra ) using Quercetin dihydrate (QDH) and Bacillus strains ( Bacillus cereus Z-01 and Bacillus simplex Z-09). The potting mix was artificially spiked with Cd metal (90 mg/kg soil) and subjected to salinity stress (100 mM). QDH (50 µM) was applied as foliar spray, whereas bacteria were provided as a soil drench in different combinations. The onset of Cd and salinity stress significantly decreased ( P <0.05) shoot length (32.5%), root length (44.8%), shoot dry biomass (55.17%), and root dry biomass (50.1%) compared with control. The combined application of strain Z-09 and QDH performed best, restoring the plant growth parameters to non-stressed levels. Z-09 + QDH significantly increased shoot length (36.21%), root length (37.47%), shoot dry biomass (58.2%), and root dry biomass (25.3%) compared with the stress control plants. Evaluation of photosynthetic pigment levels and enzyme activities showed that Z-09 + QDH application under combined stress increased chlorophyll a (50.8 %), chlorophyll b (60.3 %), and total chlorophyll (54.2 %) contents and antioxidant enzyme activities in Kale plants under stress conditions. In-silico studies were performed to further support the role of QDH in stress mitigation, which confirmed strong interactions of QDH with key stress-responsive proteins (MAPK, DREB, BHLH, and SOS1),. To elucidate the metabolic contribution, non-targeted metabolomic analysis was performed which showed that metabolites with varying abundance belonged to TCA cycle, carbon metabolism, amino acids metabolism, and phenylpropanoid pathways, along with notable accumulation of osmolytes, flavonoids, and organic acids that contributed to stress mitigation. Our findings indicated a synergistic action of quercetin dihydrate and beneficial bacteria to enhance cadmium and salinity tolerance in Chinese kale by activating both biochemical defenses and metabolic adjustments.