A single-nucleus atlas of ligular region uncovers brassinosteroid-driven cell fate transitions controlling leaf angle in maize.

The ligular region is a critical determinant of leaf angle (LA) and plant architecture in maize, yet its developmental mechanisms remain largely unknown. In this study, we generate a comprehensive single-nucleus RNA-seq atlas of the maize ligular region, spanning from leaf primordium through the preligule band (PLB) to the mature stage. By integrating cellular and transcriptomic analyses, we reveal that brassinosteroid (BR) inhibits cell division in the adaxial division zone while promoting elongation in adaxial hypodermal cells. Consequently, BR treatment leads to ectopic emergence of the auricle and ligule. At early stages, GDSL esterases/lipase genes (GELP2/3/6) act as key regulators of emergence, as their disruption causes both ectopic initiation and impaired outgrowth of the ligule and auricle. At later stages, adaxial hypodermal cells serve as pivotal sites for BR-mediated cell elongation and LA expansion, where the transcription factors LIGULELESS1 (LG1) and BRASSINAZOLE-RESISTANT1 (BZR1) cooperatively drive cell elongation to increase LA. Strikingly, comparative snRNA-seq analyses reveal that variation in adaxial cell elongation and subsequent hypodermal lignification explains LA diversity across inbred lines. Together, our findings elucidate the molecular mechanism of BR-driven cell fate transitions in the ligular region and offer potential targets for breeding high-yield maize varieties with ideal plant architecture.