Trans-cinnamic acid coordinates PAL repression and C4H induction to modulate lignification in bamboo

Lignin deposition in rapidly elongating bamboo culm requires precise metabolic coordination; however, regulatory mechanisms balancing phenylpropanoid precursor supply and demand remain unclear. Here, we identify a self-regulating PAL-C4H module in bamboo where the pathway intermediate trans-cinnamic acid (t-CA) acts bidirectionally. Transcriptomic and biochemical analyses revealed that t-CA simultaneously activates downstream lignin biosynthesis by upregulating cinnamate 4-hydroxylase (PeC4H1/2) gene expression and enzymatic activity, and suppresses upstream phenylalanine ammonia-lyase (PAL) through both transcriptional (PePAL10/11 repression) and posttranslational (PeKFB9-mediated ubiquitination) controls. This feedback loop ensures resource prioritization for secondary cell wall formation during culm maturation, as evidenced by increased lignin content in t-CA-treated seedlings. Evolutionary analyses further suggest bamboo-specific optimization of this regulatory paradigm to support bamboo's unique growth kinetics. Our findings redefine flux control mechanisms in bamboo lignification and provide actionable targets for precision breeding of bamboo as a substitute for plastic.