Recent discoveries highlighting the metabolic malleability of plant lignification indicate that lignin can be engineered to dramatically alter its composition and properties. Current plant engineering efforts are primarily aimed at manipulating the biosynthesis of normal monolignols, but in the future apoplastic targeting of phenolics from other metabolic pathways may provide new approaches for designing lignins that are less inhibitory toward polysaccharide fermentation, both with and without biomass pretreatment. To help identify promising new avenues for lignin bioengineering, we are artificially lignifying cell walls from maize cell suspensions with various combinations of normal monolignols (coniferyl and sinapyl alcohols) plus a variety of phenolics (hydroxycinnamate-monolignol esters, hydroxycinnamate-quinic acid esters, diferuloylated compounds, phenylpropanoids, phenolic glucosides, flavonoids, etc) synthesized in the laboratory. Our initial work demonstrated that copolymerzation of coniferyl ferulate with monolignols dramatically improved the alkaline extractability of lignin and the enzymatic hydrolysis of cell walls. In more recent studies, inclusion of feruloylquinic or caffeoylquinic acids with monolignols considerably depressed lignin formation and strikingly improved cell wall fermentability by anaerobic rumen microflora. In contrast, various phenylpropanoids, epicatechin derivatives, and ferulate polyol esters readily formed copolymer-lignins with normal monolignols; cell wall fermentability was somewhat enhanced by greater hydroxylation or 1,2-diol substitution of monolignol substitutes. In ongoing work, we are characterizing the enzymatic saccharification of intact and chemically pretreated cell walls lignified by these and other monolignol substitutes. These and subsequent studies will identify promising bioengineering targets for improving plant fiber utilization in natural and industrial processes.