Model studies of ferulate-coniferyl alcohol cross-product formation in primary maize walls: Implications for lignification in grasses

Ferulate and diferulates mediate cell wall cross-linking in grasses, but little is known about their cross-coupling reactions with monolignols and their role in lignin formation in primary cell walls. Feruloylated primary walls of maize were artificially lignified and then saponified to release ferulate and diferulates and their cross-products with coniferyl alcohol for analysis by GC-FID, GC-MS, and NMR spectroscopy. Ferulate and 5-5-coupled diferulate had a greater propensity than 8-coupled diferulates to copolymerize with coniferyl alcohol, forming mostly 4-O-beta' and 8-beta' and some 8-O-4' and 8-5' cross-coupled structures. Some 8-beta' structures de-esterified from xylans, but these cross-links were subsequently replaced as 8-coupled diferulates formed stable cross-coupled structures with lignin. Based on the incorporation kinetics of ferulate and diferulates and the predicted growth of lignin, cross-products formed at the onset of lignification acted as nucleation sites for lignin polymerization.