Stacked mutations disrupting syringyl and p -coumaroylated lignin biosynthesis in rice result in lignin dominated by guaiacyl units: insights into grass-specific lignin monomer biosynthesis and polymerization mechanisms

The aromatic composition of lignin significantly impacts the usability of lignocellulosic biomass. In eudicots, transgenic and mutant lines with elevated guaiacyl (G) or syringyl (S) lignin units have been successfully generated by manipulating the expression level of CONIFERALDEHYDE 5-HYDROXYLASE (CAld5H). However, this bioengineering approach has proven less effective in grasses, implicating the potential existence of a grass-specific alternative pathway for S lignin biosynthesis.

Through characterization of genome-edited rice mutants, we demonstrated that S lignin in rice can be virtually eliminated by disrupting genes encoding CAld5H along with p - COUMAROYL-COENZYME A:MONOLIGNOL TRANSFERASE (PMT), a grass-specific enzyme essential for the biosynthesis of monolignol p -coumarate conjugates. In contrast, individual mutations in either CAld5H or PMT genes resulted in incomplete elimination of S lignin. These findings provide strong evidence that rice possesses a CAld5H-independent pathway leading to the grass-specific monolignol p -coumarate conjugates.

In-depth structural characterizations of G-dominated lignins from rice and Arabidopsis mutants, natural gymnosperm pine, and G-type synthetic lignin revealed pronounced effects of lineage-dependent cell wall environments on the linkage patterns and molecular weight distributions of the resulting lignin polymers.

Overall, our findings highlight previously overlooked lineage-specific lignin monomer biosynthesis and polymerization patterns in grasses.