Editorial overview: Plant biotechnology – lignin engineering Wout Boerjan and John Ralph Current Opinion in Biotechnology 2019, 56:iii–v For a complete overview see the Issue https://doi.org/10.1016/j.copbio.2019.04.001 0958-1669/ã 0001?Elsevier Ltd. All rights reserved. It is particularly timely to publish a special issue on lignin biosynthesis and its engineering giving the depth and breadth of amazing breakthroughs in fundamental understanding and applications that have occurred in recent years. Lignins are polymers that are mainly deposited in secondary-thickened plant cell walls. There, they are produced in a polymerization step (lignifica- tion) from their component monomers by a process of simple combinatorial radical coupling that was once considered an enigma. The monomers that have been known for decades are the p-coumaryl, coniferyl, and sinapyl alcohols. However, the development of new analytical tools along with analyses of lignin structures from a broader range of plant species has led to the discovery of many additional monomers, giving rise to a rich diversity of polymers with different physicochemical properties. As a consequence, the very definition of lignin continues to evolve as new species of plants are characterized in greater detail. The fact that lignin polymerization is a combinatorial radical coupling process, and that so many monomers naturally couple into the lignin polymer, has given rise to the concept that lignin structure is malleable and that plants can be engineered to make lignin structures that are tailored for various end-use applications. Indeed, in industrial operations producing pulp or fermentable sugars from wood, lignin needs to be extracted by expensive chemical treatments, and the efficiency of this process heavily depends on lignin’s amount and composition. On the other hand, lignin is increasingly considered as a valuable source of aromatic building blocks for the chemical industry. As is now becoming evident, particular lignin structures can be designed and engineered into plants to maximize lignin-derived aromatic monomers streams. The acceleration in the interest in lignin and its engineering, caused by the urgent need for a transition from a fossil-based to a biobased economy, is illustrated by the steadily increasing number of papers covering ‘lignin’ with ‘gene’ terms, from 725 in 2010 to 1516 in 2017, with citations escalating at a far greater rate. When we conceived this issue, our objective was to cover lignin research extending from biosynthetic pathway revelations up to its valorization, but always with a focus on biotechnology, and with minimal overlap among the individual reviews. Given that several research groups work on the very same topic, it was, unfortunately, not possible to invite all of the main players in the lignin field and we apologize for this. The following are just a few of the highlights covered in the various contri- butions to this special issue as a testament to the state-of-the-art advances and practical implications of lignin bioengineering. Vanholme et al. [1] review the biosynthetic pathways towards the 35 metabolites from over 10 metabolic classes that have now been identified as lignin building blocks. They examine Wout Boerjan Ghent University, Department of Plant Biotechnology and Bioinformatics, VIB- UGent Center for Plant Systems Biology, Technologiepark 71, B-9052 Ghent, Belgium e-mail: woboe@psb.ugent.be Wout Boerjan is a Professor at the Department of Plant Biotechnology and Bioinformatics of the Faculty of Sciences of Ghent University, and Group Leader at the VIB Center for Plant Systems Biology at Gent (Belgium). He specializes in lignin biosynthesis (gene and pathway discovery) and in lignin engineering in model plants (Arabidopsis), and crops (maize and poplar) to speed up the transition from a fossil-based to a bio-based economy. John Ralph Departments of Biochemistry and Biological Systems Engineering, and the Department of Energy’s Great Lakes Bioenergy Research Center, The Wisconsin Energy Institute, The University of Wisconsin, Madison, WI 53726, USA John Ralph is a Professor in the Departments of Biochemistry and Biological Systems Engineering, and the DOE Great Lakes Bioenergy Research Center, the University of Available online at www.sciencedirect.com ScienceDirect www.sciencedirect.com Current Opinion in Biotechnology 2019, 56:iii–v