Phosphorylation of LTF1, an MYB Transcription Factor in Populus, Acts as a Sensory Switch Regulating Lignin Biosynthesis in Wood Cells

Lignin is specifically deposited in plant secondary cell walls, and initiation of lignin biosynthesis is regulated by a variety of developmental and environmental signals. However, the mechanisms governing the regulation of lignin biosynthesis remain to be elucidated. In this study, we identified a lignin biosynthesis-associated transcription factor (LTF) from Populus, LTF1, which binds the promoter of a key lignin biosynthetic gene encoding 4-coumarate-CoA ligase (4CL). We showed that LTF1 in its unphosphorylated state functions as a regulator restraining lignin biosynthesis. When LTF1 becomes phosphorylated by PdMPK6 in response to external stimuli such as wounding, it undergoes degradation through a proteasome pathway, resulting in activation of lignification. Expression of a phosphorylation-null mutant version of LTF1 led to stable protein accumulation and persistent attenuation of lignification in wood cells. Taken together, our study reveals a mechanism whereby LTF1 phosphorylation acts as a sensory switch to regulate lignin biosynthesis in response to environmental stimuli. The discovery of novel modulators and mechanisms modifying lignin biosynthesis has important implications for improving the utilization of cell-wall biomass.