BZR1 coordinates multiple pathways to promote axillary bud outgrowth in model M. truncatula and forage yield in alfalfa

Brassinosteroids (BRs) regulate plant architecture through the transcription factor BZR1, yet the molecular mechanisms underlying its role in axillary bud (AB) outgrowth and branching remain poorly characterized in M. truncatula. Here, we investigated the function of MtBZR1 using a null mutant (mtbzr1) through integrated phenotypic, transcriptomic, and hormonal analyses. Phenotypic comparisons revealed that the mtbzr1 mutant exhibited a 22.9 % reduction in primary branches, shorter ABs, and significantly lower biomass accumulation than the wild type (WT). Treatment with 24-epibrassinolide (EBR) promoted AB elongation in isolated stem segments of WT, whereas this response was severely attenuated in mtbzr1, confirming the essential role of MtBZR1 in BR-mediated AB outgrowth. Transcriptomic profiling identified 27,624 differentially expressed genes (DEGs) in the ABs of mtbzr1 compared to WT, with pronounced suppression of photosynthesis-related genes and cell cycle regulators. Hormone pathway analysis revealed transcriptional repression of auxin biosynthesis genes, coupled with elevated expression of strigolactone (SL) and abscisic acid (ABA) biosynthetic/signaling components. Quantification of endogenous hormones confirmed reduced indole-3-acetic acid (IAA) and elevated ABA levels in the ABs of mtbzr1. Overexpression of the homologous gene MsBZR1 (74.0 % protein identity to MtBZR1) in alfalfa (Medicago sativa L.) resulted in a 43.1–59.8 % increase in branching, consequently improving biomass, highlighting its yield promotion potential in forage crop. Collectively, these findings delineate the MtBZR1-mediated regulatory network controlling AB development in M. truncatula and provide strategic genetic targets for forage productivity enhancement.