Background: Seedlings, which germinate in the soil, generally undergo rapid hypocotyl elongation to seek light for growth and development. Brassinosteroids (BRs), one of the main classes of hormones that promote plant growth, play an important regulatory role in Arabidopsis thaliana seedling growth, including rapid hypocotyl elongation. Once exposed to the light, light signals inhibit the BR signaling pathway in seedlings, thereby controlling hypocotyl growth. However, the mechanism underlying how light inhibits BR pathway-mediated growth is still unclear.
Question: Arabidopsis nuclear factor YC proteins (NF-YCs) function as essential negative regulators of light-controlled hypocotyl elongation. We wanted to know if NF-YCs participate in the light-induced inhibition of the BR pathway.
Findings: We found that NF-YCs function as essential repressors of the BR pathway involved in photomorphogenesis. nf-ycQ, a loss-of-function mutant of four NF-YC homologs, shows long hypocotyls and is insensitive to BR. In the light, NF-YCs directly inhibit BR biosynthesis by regulating the transcription of the BR synthase gene BR6ox2. In addition, NF-YCs interact with the BIN2 protein, a critical repressor of BR signaling, and facilitate its stabilization, thus inhibiting the BR signaling pathway. Interestingly, NF-YCs regulate neither BR biosynthesis nor BR signaling in the dark.
Next steps: Given that the process from seed germination to seedling emergence involves the environmental transition from dark to light and that NF-YCs function differently under these two conditions, exploring the molecular mechanism of how light and dark affect NF-YC activity will be our next major focus of study.
Wenbin Zhang, Yang Tang, Yilong Hu, Yuhua Yang, Jiajia Cai, Hailun Liu, Chunyu Zhang, Xu Liu, and Xingliang Hou (2021). Arabidopsis NF-YCs play dual roles in repressing brassinosteroid biosynthesis and signaling during light-regulated hypocotyl elongation. Plant Cell. DOI: https://doi.org/10.1093/plcell/koab112