The HD-ZIP I transcription factor BcLMI1 regulates bolting time in <i>Brassica campestris</i> by repressing <i>BcTCP4</i>

Abstract During the domestication of Brassica campestris (syn. B. rapa), significant variations in bolting time have emerged, critically impacting crop yield and quality. To elucidate the genetic basis of bolting time regulation in B. campestris, we integrated selective-sweep analysis on 365 resequenced accessions with genome-wide association studies (GWAS). This identified genes including BcLMI1-LF (LATE MERISTEM IDENTITY 1), an HD-ZIP I subfamily gene in the least fractionated subgenome of B. campestris. Further ‌Gene Ontology (GO) enrichment analysis indicated BcLMI1-LF was a key candidate regulator of bolting.‌ Haplotype analysis demonstrated the association of Hap3 of BcLMI1-LF with the late bolting phenotype. Functional validation confirmed that BcLMI1-LF-Hap3 overexpression significantly delays bolting. Mechanistically, Y1H, DLRA, and ChIP-qPCR assays established that BcLMI1-LF-Hap3 directly represses BcTCP4-LF (TEOSINTE BRANCHED 1/CYCLOIDEA/PCF 4), a conserved bolting regulator.‌ ‌This study advances understanding of bolting regulation in B. campestris and provides molecular targets for breeding late-bolting crops.