SRG16 alleviates cadmium-induced heavy metal stress in soybean (Glycine max (L.) Merr.)

Cadmium (Cd) is among the most common toxic heavy metal pollutants in nature. Cd exposure causes physical damage and toxicity in many organisms. In this study, we observed root growth from soybean (Glycine max (L.) Merr.) plants that were exposed to 15 mg L^-1 Cd showed significant growth inhibition, accompanied by reduced root biomass and plant height. Through transcriptome analysis and functional verification, we identified soybean Stress Responsive Gene 16 (GmSRG16) as a negative regulator of Cd tolerance. GmSRG16 knockout significantly increased soybean plants' heavy metal tolerance, a phenotype that also resulted from the overexpression of its downstream gene, Cinnamyl alcohol dehydrogenase 9-related (GmCAD9L), a key gene associated with the phenylpropanoid metabolic pathway. Consistently, overexpression of GmCAD9L enhanced Cd tolerance, indicating that GmCAD9L is a candidate downstream component of the GmSRG16-mediated Cd response pathway. Multi-omics analyses further showed enhanced phenylalanine/phenylpropanoid metabolism in Gmsrg16 plants, leading to increased accumulation of coniferin, a downstream lignin precursor derived from phenylalanine metabolism. Exogenous application of 2 μM coniferin significantly alleviated Cd toxicity and reduced Cd uptake in soybean. Furthermore, in vitro complexation assays and isothermal titration calorimetry (ITC) demonstrated that coniferin directly binds Cd²⁺ to form soluble complexes, thereby reducing Cd-induced oxidative damage and maintaining antioxidant enzyme activities. Collectively, our results identify the GmSRG16 - GmCAD9L module as an important regulator of Cd tolerance and reveal coniferin-mediated Cd chelation as a potential mechanism for mitigating Cd toxicity in soybean.