Brassica campestris ssp . chinensis HY5 regulates hypocotyl elongation by binding the promoter of BcMYBH

Abstract Light is a necessary environmental factor for plant growth and development. HY5 (ELONGATED HYPOCOTYL 5), regarded as a crucial transcriptional factor for regulating hypocotyl, was clearly in Arabidopsis thaliana , but its regulatory mechanism in non-heading Chinese cabbage (NHCC) has not been understood. In this study, we used BcHY5 as a bait to screen two BcB-box (BBX) proteins, BcBBX21, and BcBBX25, and applied multiple genetic and biochemical approaches to identify the roles of BcHY5 and two B-BBX proteins. Overexpression of the two BBX proteins in Arabidopsis was used to investigate their functions. BcHY5 can bind the G-box of pro BcMYBH and inhibit BcMYBH transcription. BcHY5 can form a complex with BcBBX21, which promotes BcHY5’s regulation of BcMYBH and increases the transcription inhibition of BcMYBH , resulting in the shorter hypocotyl. BcBBX25 can interact with BcHY5 to release the inhibition of BcMYBH by BcHY5 , allowing the hypocotyl to return to normal. The C-terminal exchange confirmed that the interaction between BcBBXs and BcHY5 was determined by the two B-box domains of the N-terminal and the opposite effect of BcBBXs on BcHY5 was caused by C-terminal difference. The current results demonstrate the fine-tuned regulation of hypocotyl length via transcriptional regulation of BcMYBH by BcHY5-associated proteins and provide insights into Cruciferous hypocotyl length.