Unlocking the Potential of Catechyl Lignin: Molecular Regulation of Biosynthesis, Structural Organization, And Valorization

Catechyl lignin (C-lignin), a type of lignin found in vanilla seed coats and some members of the Cactaceae and Euphorbiaceae families, has gained significant attention. C-lignin possesses a homogeneous linear structure that is characterized by the presence of a catechol group comprising a benzene ring with two hydroxyl groups. C-lignin also has substructures according to specific linkage patterns between its constituent monolignols, i.e., coniferyl (C), hydroxyphenyl (H), guaiacyl (G), and sinapyl (S) monomers. The linear structure of C-lignin makes it an ideal source for the development of carbon fiber-based composites, whereas its nonetherified structure and low molecular weight favor its microbial conversion to various useful products. However, to fully realize the potential of C-lignin, it is important to obtain its significant quantities through gene regulation. Technology must be developed to address these challenges and achieve the goals successfully. Genetic engineering techniques have been developed to increase C-lignin accumulation in specific plants for valorization. The extraction of C-lignin from biomass materials involves various effective methods to depolymerize it, producing aromatic compounds like propyl and propenyl catechol. This makes C-lignin a promising material for depolymerization and unlocking its valuable use, thanks to its homogeneous catechyl units. This review explores the biochemical and molecular regulation of C-lignin biosynthesis. It discusses the role of various enzymes, genes, and regulators in the accumulation of C-lignin in different plant species. The review also delves into the techniques, catalysts, and solvents employed in the extraction of C-lignin. Additionally, it discusses the depolymerization of C-lignin to produce various aromatic compounds, as well as its application in developing carbon fibers and polymeric composites. Finally, the review highlights the challenges and potential opportunities associated with utilizing the homogeneous and linear structure of C-lignin for future applications.