Salt stress responses of Brassica napus roots and functional characterization of BnSAUR12 in salt tolerance.

Salt stress severely limits crop productivity by affecting plant growth and development. Rapeseed (Brassica napus L.) shows considerable adaptability to saline environments; however, the molecular basis of root responses to salt stress remains largely unclear. In this study, we investigated the physiological, hormonal, and transcriptional responses of rapeseed roots under salt stress. Salt treatment altered root system architecture by inhibiting hypocotyl elongation and lateral root development while promoting primary root elongation. Targeted hormone metabolomic analysis revealed extensive changes in endogenous hormone profiles, with auxin-related metabolites showing the most pronounced changes. Transcriptome analysis further demonstrated that auxin-responsive pathways were strongly activated under salt stress, indicating an important role of auxin signaling in root adaptation. To explore the genes involved in this process, we performed a genome-wide characterization of the Small Auxin-Up RNA (SAUR) gene family in B. napus. A total of 225 BnSAUR genes were identified and analyzed based on their phylogenetic relationships, gene structures, conserved motifs, chromosomal distribution, evolutionary relationships, cis-regulatory elements, and potential microRNA regulation. Integrative analysis of transcriptome sequencing, published RNA sequencing, and assay for transposase-accessible chromatin sequencing data identified multiple salt-responsive BnSAUR genes, and their expression patterns were further confirmed by real-time quantitative polymerase chain reaction. Among them, BnSAUR12 was selected for functional characterization, and its loss reduced salt tolerance during seed germination in rapeseed. Together, our study expands our understanding of the molecular basis of salt tolerance in rapeseed and provides potential candidate genes for improving stress resilience in oil crops.