AUTHOR=Wang Qiao , Dai Xinren , Pang Hongying , Cheng Yanxia , Huang Xiong , Li Hui , Yan Xiaojing , Lu Fachuang , Wei Hairong , Sederoff Ronald R. , Li Quanzi 

TITLE=BEL1-like Homeodomain Protein BLH6a Is a Negative Regulator of CAld5H2 in Sinapyl Alcohol Monolignol Biosynthesis in Poplar

JOURNAL=Frontiers in Plant Science

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.695223

DOI=10.3389/fpls.2021.695223

ISSN=1664-462X

ABSTRACT=<p>Lignin is one of the major components of xylem cell walls in tree stems. The lignin in the wood of most flowering plants (dicotyledonous angiosperms) is typically polymerized from three monolignol precursors, coniferyl alcohol, sinapyl alcohol, and <italic>p</italic>-coumaroyl alcohol, resulting in guaiacyl (G), syringyl (S), and hydroxyphenyl (H) subunits, respectively. In this study, we focus on the transcriptional regulation of a coniferaldehyde 5-hydroxylase (<italic>CAld5H2</italic>) gene, which encodes a key enzyme for sinapyl alcohol biosynthesis. We carried out a yeast one-hybrid (Y1H) screen to identify candidate upstream transcription factors (TFs) regulating <italic>CAld5H2</italic>. We obtained 12 upstream TFs as potential regulators of <italic>CAld5H2</italic>. One of these TF genes, BLH6a, encodes a BEL1-like homeodomain (BLH) protein and negatively regulated the <italic>CAld5H2</italic> promoter activity. The direct regulation of <italic>CAld5H2</italic> promoter by BLH6a was supported by chromatin immunoprecipitation–quantitative polymerase chain reaction (ChIP–qPCR) and dominant repression of BLH6a in transgenic plants. Luciferase complementation imaging analyses showed extensive protein<bold>–</bold>protein interactions among these 12 TFs. We propose that BLH6a is a negative regulator of <italic>CAld5H2</italic>, which acts through combinatorial regulation of multiple TFs for sinapyl alcohol (S monolignol) biosynthesis in poplar.</p>