Heavy metal tolerance and metal ion homeostasis in soybean (Glycine max L.) driven by the synergistic effect of Pseudomonas qingdaonensis strain BD1 and Illite

Heavy metal (HM) toxicity is a major constraint for plants, soils, and the environment. Thus, eco-friendly and cost-effective strategies are needed to mitigate HM stress. The bacterial strain Pseudomonas qingdaonensis BD1 and the clay mineral Illite have been identified as promising agents for alleviating lead (Pb), arsenic (As), and cadmium (Cd) stress in soybean. However, their synergistic effects on soybean under combined metal stress (Pb+As+Cd) remain underexplored. In this study, soybean plants grown under controlled conditions were treated with BD1 isolates and 3 % Illite to counter the phytotoxic effects of Pb, As, and Cd (1.5 mM each). HM stress impaired soybean growth by increasing oxidative damage and disrupting photosynthetic functioning, whereas the co-administration of BD1 +Illite restored morpho-physiological performance, including improved chlorophyll (Chl a and Chl b) content, enhanced maximum quantum efficiency of PSII (Fv/Fm), and increased net photosynthetic rate (Pn), indicating effective protection of the photosynthetic apparatus under stress conditions. These physiological improvements were accompanied by enhanced key enzymatic antioxidant activities, including SOD (218.3 %), CAT (84.5 %), POD (57.5 %), and APX (60.9 %), as well as the non-enzymatic antioxidant GSH (108.2 %). These improvements led to reduced malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) levels. BD1 +Illite also increased the accumulation of sugars and free amino acids, improved the uptake of Ca, K, and Si, and simultaneously reduced Cd, As, and Pb accumulation. Moreover, the treatment modulated phytohormone levels by decreasing abscisic acid (40.3 %) and salicylic acid (13.8 %) while increasing jasmonic acid (44.3 %). BD1 +Illite downregulated GmNCED3 and GmPAL1, but upregulated GmCYP707A2, GmLAX1, and GmCDPK5. Metal ion homeostasis and detoxification-related genes (GmNRAMP5A, GmMT1, GmMT2, GmPCS1, and GmWRKY142) were also differentially expressed, indicating coordinated responses that enhance HM tolerance in soybean. Overall, these findings highlight plant–microbe interaction-based, eco-friendly, and cost-effective strategies to reduce HM toxicity in soybean and other legumes.