BZR1 and BES1: From Brassinosteroid Signaling Termini to Multifunctional Integrators of Plant Growth and Environmental Adaptation

BZR1 (Brassinazole-resistant 1) and BES1 (bri1-EMS suppressor1) are the principal transcription factors that convert brassinosteroid (BR) perception into genome-wide transcriptional programs governing plant growth, development, and stress responses. Once regarded as end-effectors of a linear receptor-to-nucleus relay, they are now recognized as central hubs that integrate diverse endogenous and environmental cues into context-specific transcriptional outputs. We synthesize recent progress across four connected levels. Structurally, a modular architecture enables simultaneous target recognition, signal input, and output control. Mechanistically, the canonical BIN2-mediated phosphorylation switch is expanded by upstream regulators that couple membrane dynamics, carbon status, protein homeostasis, and nuclear import to BZR1/BES1, while ubiquitination, SUMOylation, redox modification, and acetylation form an interconnected post-translational network. During transcriptional execution, dephosphorylated BZR1/BES1 partition targets into activated and repressed sets through cis-element preference, corepressor recruitment, chromatin modifiers, and partnerships with factors such as PIF4 and ARF6. Physiologically, BZR1/BES1 coordinate anutrient-responsive metabolism, yield-related traits, and abiotic-stress tolerance, with marked functional divergence among crop orthologs. We highlight open questions in signal-integration specificity and discuss how BZR1/BES1 modules could be engineered for climate-resilient crops.