Flavonoids naringenin chalcone, naringenin, dihydrotricin, and tricin are lignin monomers in papyrus

8 figuras.- refefencias.- Versión aceptada del artículo.- Supplementary data kiab469_Suppl. https://academic.oup.com/plphys/advance-article/doi/10.1093/plphys/kiab469/6400262#supplementary-data

Recent studies demonstrate that several polyphenolic compounds produced from beyond the canonical monolignol biosynthetic pathways can behave as lignin monomers, participating in radical coupling reactions and being incorporated into lignin polymers. Here, we show various classes of flavonoids, the chalconoid naringenin chalcone, the flavanones naringenin and dihydrotricin, and the flavone tricin, incorporate into the lignin polymer of papyrus (Cyperus papyrus L.) rind. These flavonoids were released from the rind lignin by DFRC, a chemical degradative method that cleaves the β-ether linkages, indicating that at least a fraction of each was integrated into the lignin as β-ether linked structures. Due to the particular structure of tricin and dihydrotricin, whose C-3ʹ and C-5ʹ positions at their B-rings are occupied by methoxy groups, these compounds can only be incorporated into the lignin through 4ʹ–O–β bonds. However, naringenin chalcone and naringenin have no substituents at these positions and can therefore form additional carbon-carbon linkages, including 3ʹ– or 5ʹ–β linkages that form phenylcoumaran structures not susceptible to cleavage by DFRC. Furthermore, NMR analysis indicated that naringenin chalcone can also form additional linkages through its conjugated double bond. The discovery expands the range of flavonoids incorporated into natural lignins, further broadens the traditional definition of lignin, and enhances the premise that any phenolic compound present at the cell wall during lignification could be oxidized and potentially integrated into the lignin structure, depending only on its chemical compatibility. This study indicates that papyrus lignin has a unique structure as it is the only lignin known to date that integrates such a diversity of phenolic compounds from different classes of flavonoids. This discovery will open up new ways to engineer and design lignins with specific properties and for enhanced value.

This work was funded by the Spanish State Research Agency and the European Regional 51 Development Fund (project AGL2017-83036-R), the Consejería de Transformación 52 Económica, Industria, Conocimiento y Universidades, Junta de Andalucía (project P20-53 00017), the DOE Great Lakes Bioenergy Research Center (DOE BER Office of Science DE54 SC0018409), and the Austrian Biorefinery Center Tulln (ABCT). MJR thanks the Spanish 55 Ministry of Science, Innovation and Universities for a FPI fellowship (PRE2018-083267).

Peer reviewed