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.