Unconventional lignin monomers-Extension of the lignin paradigm

Abstract

39 páginas.- 14 figuras.- 134 referencias ; Lignin is largely produced by the oxidative radical coupling of the three monolignols, p-coumaryl, coniferyl, and sinapyl alcohols, which form the p-hydroxyphenyl, guaiacyl, and syringyl lignin units in the lignin polymer. However, it is increasingly appreciated that other "unconventional" phenolic compounds can also behave as lignin monomers participating in oxidative radical reactions during the lignification process and becoming integrated into the lignin polymer in some plants. Various phenolic compounds have been found incorporated into the lignin, including phenolic compounds derived from the monolignol biosynthetic pathway (e.g., monolignol ester conjugates, caffeyl and 5-hydroxyconiferyl alcohols, hydroxycinnamaldehydes, among others). More fascinating still was the discovery in some plants of a series of phenolic compounds arising from other polyphenolic biosynthetic pathways, including flavonoids (tricin, dihydrotricin, naringenin, naringenin chalcone), hydroxystilbenes (resveratrol, piceatannol, and isorhapontigenin, and their O-glucosylated counterparts piceid, astringin, and isorhapontin), and ferulic acid amides (feruloyltyramine, and diferuloylputrescine), that are integrated into the lignin polymer. All of these discoveries indicate that the lignin structure is far more complex than previously thought, that the definition of lignin should not be restricted to a polymer derived solely from the three traditional monolignols, and that other unconventional lignin monomers must be considered, thus expanding the established definition of lignin. The overriding factor remains, however, that lignification, the process of polymerization to the lignin polymer, allows this compositional flexibility because it is a simple chemical process. © 2022 Elsevier Ltd ; JCdR, JRe and AG were funded by the Spanish Project AGL2017-83036-R and PID2020-118968RB-100 (financed by Ministerio de Ciencia e Innovación, MCIN, Agencia Estatal de Investigación, AEI, and European Regional Development Fund, ERDF, "A way of making Europe"), the Regional Andalusian Government (Consejerı ́a de Transformación Económica, Industria, Conocimiento y Universidades, Junta de Andalucıía, project P20-00017), and CSIC (project PIE 202040E185). JRa and HK were funded by the DOE Great Lakes Bioenergy Research Center (DOE Office of Science DE-SC0018409 ; Peer reviewed