Genome-Wide Identification of the BraBLH Gene Family and Its Regulatory Networks in Chinese Cabbage Leafy Head Formation

Leafy head formation is a critical agronomic trait that determines the yield and quality of Chinese cabbage (Brassica rapa), involving complex morphological transitions. While BEL1-like homeodomain (BLH) transcription factors regulate development, their specific roles in heading remain elusive. To systematically identify the BraBLH gene family and elucidate its expression networks and regulatory functions during leafy head formation, a total of 36 BraBLH genes were identified. The phylogenetic analysis revealed that Brassica rapa had a specific clade (Group II). A high-resolution spatiotemporal expression framework for the BraBLH family was constructed. Subsequently, comparative transcriptomics of heading, non-heading, and GA3-treated heading plants highlighted multiple differentially expressed BraBLH genes, suggesting that this gene family may contribute to the regulation of leafy head formation in Chinese cabbage. Subsequently, a high-resolution spatiotemporal expression framework for the BraBLH family was constructed. Within this framework, BraBLH5 emerged as a pivotal node integrating GA signaling, peaking during the rosette stage to initiate heading. In contrast, a distinct negative module (BraBLH9/26) was significantly upregulated in the non-heading line. Spatial profiling further revealed functional compartmentalization within the family, where specific members maintained shoot apical meristem identity, whereas others drove inner leaf expansion and incurvature. Furthermore, subcellular localization analysis confirmed that BraBLH5 was localized to the nucleus. Ultimately, this systems-level study elucidates a transcriptome-driven framework for heading, highlighting a GA-mediated BraBLH5 mechanism and providing valuable candidates for Brassica functional genomics.