Origin, Conservation and Functional Diversification of the BES1/BZR1 Gene Family During Early Land Plant Terrestrialization in Bryophytes

BRI1-EMS-SUPPRESSOR 1 (BES1)/BRASSINAZOLE-RESISTANT 1 (BZR1) transcription factors serve as core regulators of brassinosteroid (BR) signal in seed plants, where they control diverse developmental processes, including cell elongation, vascular development, and environmental responses; however, their evolutionary trajectory and functional diversification in early-diverging land plants remain poorly characterized. In this study, we systematically characterized the BES1/BZR1 gene family across 12 representative bryophyte species covering hornworts, liverworts and mosses, with Arabidopsis thaliana included as a vascular plant outgroup. In total, 19 non-redundant BES1/BZR1 homologs were identified within bryophyte genomes. Phylogenetic reconstruction, synteny analysis and Ka/Ks selection pressure analyses collectively revealed that this gene family is evolutionarily conserved throughout bryophytes, with moss-specific lineage expansion; most paralogous gene pairs have experienced strong purifying selection during evolution. Further analyses of gene structural organization, conserved protein motifs and cis-acting promoter elements uncovered universally conserved core domains alongside lineage-specific structural and regulatory variations. Subcellular localization assays demonstrated that the majority of tested bryophyte BES1/BZR1 proteins primarily accumulate in the nucleus, and autoluminescent reporter assays verified that multiple homologs modulate E-box-driven transcriptional activity. Transcriptional expression profiling indicated that BES1/BZR1 genes from Marchantia polymorpha and Sphagnum fallax are transcriptionally responsive to exogenous BR treatment, while several paralogs in S. fallax additionally exhibit altered expression under drought stress. Collectively, our results demonstrate that the BES1/BZR1 family originated at an early stage of land plant evolution, followed by lineage-specific gene expansion and divergent transcriptional regulation in bryophytes. This work advances our understanding of ancestral BR signaling and stress response modules in the early terrestrial plant lineages.