Annu. Rev. Plant Biol. 2003. 54:519–46 doi: 10.1146/annurev.arplant.54.031902.134938 Copyright c  2003 by Annual Reviews. All rights reserved L IGNIN BIOSYNTHESIS Wout Boerjan 1 , John Ralph 2 , and Marie Baucher 1,3 1 Department of Plant Systems Biology, Flanders Interuniversity Institute for Biotechnology, Ghent University, K.L. Ledeganckstraat 35, B-9000 Gent, Belgium; email: wout.boerjan@gengenp.rug.ac.be 2 U.S. Dairy Forage Research Center, USDA-Agricultural Research Service, Madison, Wisconsin, 53706; email: jralph@facstaff.wisc.edu 3 Laboratoire de Biotechnologie V´ eg´ etale, Universit´ e Libre de Bruxelles, 1850 Chauss´ ee de Wavre, B-1160 Brussels, Belgium; email: mbaucher@ulb.ac.be Key Words dehydrogenation, nucleation, polymerization, structure, topochemistry, transgenic plants ■ Abstract The lignin biosynthetic pathway has been studied for more than a cen- tury but has undergone major revisions over the past decade. Significant progress has been made in cloning new genes by genetic and combined bioinformatics and bio- chemistry approaches. In vitro enzymatic assays and detailed analyses of mutants and transgenic plants altered in the expression of lignin biosynthesis genes have provided a solid basis for redrawing the monolignol biosynthetic pathway, and structural analy- ses have shown that plant cell walls can tolerate large variations in lignin content and structure. In some cases, the potential value for agriculture of transgenic plants with modified lignin structure has been demonstrated. This review presents a current picture of monolignol biosynthesis, polymerization, and lignin structure. CONTENTS INTRODUCTION ..................................................... 520 LIGNIN COMPOSITION AND STRUCTURE .............................. 520 General Aspects ..................................................... 520 A Broader Definition of Lignins ........................................ 522 VARIABILITY AND TOPOCHEMISTRY ................................. 523 MONOLIGNOL BIOSYNTHESIS ........................................ 524 TRANSGENIC PLANTS ............................................... 528 TRANSPORT OF MONOLIGNOLS ...................................... 531 DEHYDROGENATION ................................................ 532 POLYMERIZATION ................................................... 533 Radical Generation and Radical Coupling ................................ 533 Polymerization Process ............................................... 533 Nucleation Sites ..................................................... 534 BIOTECHNOLOGY ................................................... 535 CONCLUSIONS AND PERSPECTIVES .................................. 536 1040-2519/03/0601-0519$14.00 519