Brassinosteroids at the Crossroads of Crop Architecture and Nitrogen Efficiency: Engineering Smart Ideotypes for the Next Green Revolution

Brassinosteroids (BRs), a class of essential plant steroid hormones, have emerged as central regulators in optimizing crop architecture, yield potential, and nutrient use efficiency (NUE). Through crosstalk with gibberellin (GA), auxin (IAA), strigolactone (SL), and nitrogen (N) signaling pathways, BRs coordinate cell elongation, tillering, and nutrient assimilation to optimize growth-resource balance. Allelic variations affecting BR biosynthesis or perception often generate compact, erect-leaf plant types suited for dense planting and enhanced lodging resistance-key traits for high-yield ideotypes. This review outlines BR signaling networks and crosstalk with GA, IAA, SL, and N pathways in cereals. Two principal regulatory hubs are emphasized: the Zinc Finger protein (ZnF)-BRASSINOSTEROID INSENSITIVE1 KINASE INHIBITOR1 (BKI1)-BRASSINOSTEROID INSENSITIVE1 (BRI1) receptor module, which fine-tunes BR perception and determines architectural traits, and the GLYCOGEN SYNTHASE KINASE 2 (GSK2)-BRASSINAZOLE-RESISTANT1 (BZR1)-DWARF AND LOW-TILLERING (DLT)-SMALL ORGAN SIZE1 (SMOS1)-GROWTH-REGULATING FACTOR4 (GRF4)-DELLA regulatory module, which integrates BR signaling with GA responsiveness and nitrogen metabolism. Moreover, deletion of the "r-e-z" haploblock, encompassing Rht-B1b, EamA-B, and ZnF-B, elicits a semi-dwarf phenotype with 6.48%-15.25% yield increases. These interconnected networks establish a molecular framework for engineering BR-driven cereal ideotypes. Future breeding could improve resource efficiency by fine-tuning BR activity in shoots for compact growth and promoting it in roots for enhanced nutrient uptake. Integrating genomics and precision gene editing will enable fine-tuning of BR signaling intensity and its crosstalk with other hormonal and nutrient pathways. By prioritizing growth optimization over mere growth maximization, BR-based strategies offer a sustainable path toward high-yield, nitrogen-efficient cereal production.