Molecular mechanisms and regulation of lignin synthesis, transmembrane transport, and cell wall polymerization in plants

As a key component of plant secondary cell walls, lignin biosynthesis is a complex process encompassing intracellular monomer synthesis, transmembrane transport, and in situ polymerization within the cell wall. The content and structure of lignin directly influence plant mechanical strength, stress resistance, and biomass utilization efficiency. In recent years, research in this field has expanded from the functional characterization of individual enzyme genes to the systematic dissection of multi-level regulatory networks. At the biosynthetic level, transcription factors and epigenetic modifications coordinately regulate the spatiotemporal-specific expression of lignin biosynthetic genes. At the transport level, monolignols can traverse the plasma membrane through multiple routes, including passive diffusion, active efflux mediated by ATP-binding cassette (ABC) transporters, and vesicle-mediated secondary active transport, among which the proton gradient-driven vesicular transport provides a key mechanism for the directional export of glycosylated monomers. At the polymerization level, laccases and peroxidases, serving as the core oxidative enzymes, jointly determine the spatial distribution and structural properties of the lignin polymer through substrate specificity, precise localization within cell wall microdomains, and copper homeostasis-dependent activity regulation, while silicon modulates the polymerization process via multidimensional mechanisms such as altering polymerization kinetics and participating in chemical bond formation. This review systematically summarizes the molecular mechanisms underlying the entire chain of lignin biosynthesis, from synthesis and transport to polymerization. Furthermore, it proposes future research directions in response to current controversies and challenges, aiming to provide a theoretical reference for plant cell wall biology research and forest tree genetic improvement.