BcAHL24-MF1 promotes photomorphogenesis in Brassica campestris via inhibiting over-elongation of hypocotyl under light conditions

Hypocotyl length is regarded to be a crucial seedling trait, influencing many subsequent plant development processes. However, little is known about this trait in Brassica campestris syn. Brasscia rapa. Here, we performed a comparative observation on the early hypocotyl development between two cultivars, ‘SZQ’ belonging to pak-choi (B. campestris ssp. chinensis var. communis) with longer hypocotyls, and ‘WTC’ belonging to Tacai (B. campestris L. ssp. chinensis var. rosularis) with shortter hypocotyls, and found that the difference in auxin biosynthesis might contribute to the varied hypocotyl phenotype between these two cultivars. By applying GWAS analysis using a total of 226 B. campestris accessions, we identified that the AT-Hook motif nuclear localized (AHL) gene BcAHL24-MF1 contributed to the natural variation in hypocotyl length. Functional variation of BcAHL24-MF1 was attributed to four haplotypes featuring four SNPs within the promoter region, of which Hap I accumulated more transcripts with shorter hypocotyls. Constitutive overexpression of BcAHL24-MF1 in B. campestris caused decreased hypocotyl length under light circumstances and even constant darkness, as BcAHL24-MF1 repressed the PIF-mediated transcriptional activation of auxin biosynthesis genes BcYUC6-MF2 and BcYUC8-LF. Our research uncovered the important role of BcAHL24-MF1 in regulating light-triggered inhibition of hypocotyl elongation, therefore presenting a valuable genetic target for crop breeding.