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

Brassinosteroids (BRs) regulate plant architecture largely through the transcription factor BZR1, yet the molecular mechanisms underlying its role in axillary bud (AB) outgrowth and branching remain poorly characterized in Medicago truncatula. In this study, we investigated the function of MtBZR1 using a null mutant (mtbzr1) through integrated phenotypic, transcriptomic, and hormonal analyses. Compared to wild-type (WT), mtbzr1 mutant exhibited a 22.88% reduction in primary branche number, shorter AB length, and significantly decreased biomass accumulation. Exogenous application of 24-epibrassinolide (EBR) promoted AB elongation in the 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 mtbzr1 ABs, with pronounced suppression of photosynthesis-related genes and cell cycle regulators. Hormone pathways 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 mtbzr1 ABs. Overexpression of the homologous gene MsBZR1 (73.97% protein identity to MtBZR1) in alfalfa (M. sativa) resulted in 43.11-59.78% increases in branching and biomass, highlighting its yield promotion potential in forage crop. Collectively, these findings delineate the MtBZR1-mediated regulatory network controlling lateral bud development in M. truncatula and provide strategic genetic targets for forage productivity enhancement.