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

Abstract

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.

Plant biomechanics and resilience to environmental changes are controlled by specific lignin chemistries in each vascular cell type and morphotype

“…Distinct TE morphotypes also differently accumulate X CHO residues ( Yamamoto et al, 2020 ). Additionally, noncanonical residues are also differently incorporated in lignin depending on tissues and plant species, such as benzaldehydes ( Ralph et al, 2001 ; Kim et al, 2003 ), coumaroyl esters ( Lapierre et al, 2021 ), stilbenoids ( del Río et al, 2017 ; Rencoret et al, 2019 ), flavonoids ( Lan et al, 2015 ; Rencoret et al, 2022 ), and other residues presenting phenyl (P) rings ( Faix and Meier, 1989 ; Kawamoto, 2017 ). Yet, the role of this conserved cell and morphotype-specific lignin chemistry for TEs is still not understood.…”

The Sorghum (Sorghum bicolor) Brown Midrib 30 Gene Encodes a Chalcone Isomerase Required for Cell Wall Lignification

“…This finding is unexpected and appears to be inconsistent with loss of CHI (Sobic.001G035600) function in bmr30-2 , because naringenin is the product of the CHI enzyme. The incorporation of naringenin, a flavonoid pathway intermediate has been previously documented in a rice flavone synthase (FNSII) mutant ( Lam et al, 2017 ) and most recently in a poplar CHS transgenic ( Mahon et al, 2021 ) and in papyrus ( Rencoret et al, 2021 ).…”

Different combinations of laccase paralogs non-redundantly control the lignin amount and composition of specific cell types and cell wall layers in Arabidopsis

“…This chemical diversity comprises variation in both phenolic ring (hydroxyphenyl, H ; guaicayl, G ; syringyl, S ) and aliphatic tail (alcohol, X CHOH ; aldehyde, X CHO ) of the canonical phenylpropanoid monomers. In addition to phenylpropanoids, other monomers incorporated into lignin include benzaldehydes (Kim et al 2003), flavonoids (Lan et al 2015; Rencoret et al 2022), stilbenoids (del Río et al 2017; Rencoret et al 2019) as well as other residues containing phenyl ( P ) structures (Faix and Meier 1989; Kawamoto 2017). The main lignified vascular cell types in plants are tracheary elements (TEs), which act as both structural support and sap conduits for long distance water transport.…”