Defence of doctoral thesis: Ioanna Sapouna – Biosynthesis, interactions, and structure of native lignin

Wood biomass is an important resource in the development ofstrategies towards replacement of fossil-based materials. Woodcomprises cellulose, hemicelluloses and lignin, in amounts thatvary between species. Traditionally, the extraction of cellulose hasbeen prioritized for the production of pulp, paper, and novelmaterials, at the expense of the other two thirds of the woodbiomass. Lignin is an important component of wood, comprising acomplex polymer of aromatic compounds. At the moment, mostof this abundant biopolymer is usually burned for energy or simplydiscarded as a huge waste of the pulp industries, after severemodifications in its already complex structure have rendered itdifficult to valorize. A deeper understanding of the lignin polymerand its properties can guide the design of milder extractiontechniques to obtain specific lignin structures as are needed.In this thesis, fundamental aspects of lignin biosynthesis,structure, and interactions with other cell wall components areinvestigated, employing model biological systems and diversechemical methods. Specifically, a Norway spruce tissue culturethat produces lignin extracellularly, without deposition to theplant cell wall, was used to understand the impact of a secondarycell wall hemicellulose on the production and structure of lignin.The main advantage of this model system is that it enables thecollection of lignin without the need for extraction, which is knownto alter the structure of lignin. The same system was studied viatranscriptomic analysis to elucidate the impact of a hemicelluloseon the lignin biosynthetic pathway and other metabolic processes.The effect of extraction on the structure of lignin was addressedwith the development of mild and green extraction protocols forsoftwood and hardwood species. Different conditions led to thecollection of discrete lignin populations. The results could informthe development of lignin-first biorefinery applications, in whichspecific structural properties of the biopolymer are preserved.Finally, native-like lignin fractions extracted using these mildprotocols were used for the preparation of lignin nanoparticles, toshowcase their potential in high-end applications. Furthermore,the importance of unmodified lignin in the properties of woodaerogel applications was also studied, and was included in thisthesis to demonstrate that lignin modification or even removal isnot always necessary to obtain materials with propertiescompetitive to their fossil-based analogues.

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