Stacked mutations disrupting syringyl and <i>p</i>‐coumaroylated lignin biosynthesis result in lignin dominated by guaiacyl units in rice

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 genes encoding 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. By 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, along with G-type synthetic lignin, demonstrated that variations in the cell wall environment among these species markedly influence the polymerization patterns of G-type monolignol (coniferyl alcohol). Overall, our findings highlight previously overlooked lineage-specific lignin monomer biosynthesis and polymerization patterns in grasses.