Brassinosteroids Orchestrate Multi-Layered Drought Adaptation in <i>Quercus glauca</i> Through Integrated Transcriptomic and Metabolomic Reprogramming

Quercus glauca (Q. glauca) is an ecologically and economically important evergreen broadleaf species in subtropical Asia, yet its productivity is increasingly threatened by drought stress. Brassinosteroids (BRs), a class of plant steroidal hormones, play crucial roles in stress adaptation. In this study, we used an integrated multi-omics approach to investigate how exogenous BR application enhances drought resistance in Q. glauca. Physiological analyses showed that BR reduced oxidative damage by lowering ROS and MDA levels, while increasing antioxidant enzyme activities (SOD, POD, CAT) and osmoprotectants (proline, soluble sugars). Anatomical observations indicated BR preserved mesophyll structure and stomatal aperture. Transcriptomic analysis revealed that BR not only restored gene expression to the pre-stress state but also induced a new transcriptional program distinct from both CK and drought that was enriched in MAPK signaling, hormone crosstalk, and carbohydrate metabolism. Metabolomics confirmed the accumulation of protective metabolites (flavonoids, sterols, osmolytes) and strategic reallocation away from energy-costly secondary metabolism. Weighted Gene Co-expression Network Analysis identified hub genes (AKR2, ERF020-like, At4g00960-like) linking BR-responsive expression patterns to drought-mitigating traits. Collectively, these results support a multi-phase model in which BR orchestrates detoxification, metabolic rewiring, structural repair, and sustained signal perception. This study provides novel insights into BR-mediated drought resilience in Q. glauca and identifies molecular targets for silvicultural stress management.