Genome‐wide identification of the bZIP transcription factor family in sainfoin ( Onobrychis viciifolia Scop.) and expression profiles in response to aluminum stress

Abstract Background Sainfoin ( Onobrychis viciifolia Scop.) is a perennial forage legume valued for its forage quality and stress tolerance. In southern China, acidic soils limit leguminous forage diversity; therefore, improving sainfoin tolerance to acidic soils and aluminum (Al) toxicity could broaden its agricultural use. Basic leucine zipper (bZIP) transcription factors are conserved regulators of stress signaling, but their roles in sainfoin Al responses remain unclear. Methods We identified OvbZIP genes genome‐wide and characterized their phylogeny, conserved motifs, gene structures, promoter cis‐elements, duplication/synteny patterns, and Ka/Ks ratios. Al‐responsive candidates were prioritized using RNA‐seq and weighted gene co‐expression network analysis (WGCNA), followed by quantitative reverse transcription polymerase chain reaction, subcellular localization, and yeast assays. Results A total of 236 OvbZIP genes were identified and classified into 10 subgroups. They showed subgroup‐specific motif conservation and exon‐intron divergence. Most OvbZIP genes were retained through inter‐subgenome homeologous duplication and evolved under purifying selection. Expression profiling revealed subgroup‐biased Al responsiveness, and WGCNA identified OvbZIP211 , OvbZIP195 , and OvbZIP228 as key candidates. All three genes were Al‐responsive, with stronger induction in the Al‐tolerant genotype, and with enhanced yeast growth under Al stress. Conclusions These findings provide a genomic framework for understanding OvbZIP evolution and identify candidate regulators of Al tolerance in sainfoin.