Fine Mapping of BrTCP1 As a Key Regulator of Branching in Flowering Chinese Cabbage (Brassica rapa subsp. chinensis)

Branching is a critical agronomic trait in flowering Chinese cabbage (Brassica rapa subsp. chinensis), influencing plant architecture and yield. In this study, there was a highly significant difference between CX010 (single primary rosette branches) and BCT18 (multiple primary rosette branches), and a population was constructed based on them. Phenotypic analysis revealed significant differences in primary rosette branch numbers, with BCT18 showing up to 15 branches and CX010 displaying only one main stem branch. Genetic analysis of 840 F2 individuals indicated that branching was controlled by quantitative trait loci (QTL) with a normal distribution of branch numbers. Using bulked segregant analysis coupled with sequencing (BSA-seq), we identified a candidate interval of approximately 2.96 Mb on chromosome A07 linked to branching. Fine mapping narrowed this to a 172 kb region containing 29 genes, with BraA07g032600.3C (BrTCP1) as the most likely candidate. cDNA cloning and sequence analysis of the BrTCP1 gene revealed several variations in BCT18 compared to CX010, including a 6-bp insertion, 10 SNPs, and two single-nucleotide deletions. Expression analysis indicated that BrTCP1 was highly expressed in the rosette stems of CX010, suggesting its role in inhibiting branch formation. The Arabidopsis homologous TCP1 mutant exhibited a multi-branching phenotype. In Arabidopsis, TCP1 is homologous to BRC1, which is the regulatory hub for branching in various plants. These findings reveal BrTCP1 might be a key regulator of branching in flowering Chinese cabbage, providing insights into the molecular mechanisms underlying this trait and offering a framework for genetic improvement in Brassica crops.