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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 goveing the regulation of lignin biosynthesis remain to be elucidated.In this study,we identified a lignin biosynthesisassociated 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 exteal 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.