Soil-Gradient–Derived Bacterial Synthetic Communities Enhance Drought Tolerance in <i>Quercus pubescens </i>and <i>Sorbus domestica</i> Seedlings
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Climate change-induced drought threatens forest restoration by limiting seedling establishment. To address this, we designed synthetic microbial communities (SynComs) to enhance drought tolerance in Quercus pubescens and Sorbus domestica. Bacteria were isolated from distinct soil compartments and depths, based on drought legacy effects. Functional characterization revealed compartment-specific traits, including exopolysaccharide production, auxin and abscisic acid biosynthesis, siderophore production, and osmotic tolerance. We assembled four-strain SynComs guided by ecological origin and functional complementarity. Biofilm assays showed that weak individual producers could significantly enhance community performance, indicating emergent cooperativity. SynComs and single strains were tested on Arabidopsis thaliana, Q. pubescens, and S. domestica, selected for contrasting root strategies within the root economics space. SynComs reduced drought symptoms by 47% in Q. pubescens (SynCom B) and 71% in S. domestica (SynCom F), outperforming individual strains. Exopolysaccharide-producing SynCom B aligned with Q. pubescens conservative roots, while hormone-producing SynCom F matched S. domestica acquisitive strategy. Predictive modeling identified bacterial identity and symptom timing as key predictors of drought tolerance, with stem diameter increment as a physiological indicator. Overall, SynCom effectiveness reflected compatibility between microbial traits, host strategies, and drought context—supporting their use in climate-adaptive forest restoration.