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Exploring the plasticity of lignification Mottiar, Yaseen

Abstract

Lignin is a phenolic polymer found predominantly in the secondary cell walls of vascular plants where it contributes to water transport, mechanical support, and plant defence. The occurrence, composition, and structure of lignin vary widely between cell types, throughout development, in response to stress, and across plant lineages. In addition to the hydroxycinnamate-derived monolignols, an array of non-canonical monomers occurs in lignin, clearly illustrating that the molecular processes underpinning lignin formation are highly flexible. This innate plasticity enables the rational design of lignins with predetermined structures and physicochemical properties. Given the biological importance of lignin and the industrial significance of plant biomass, this dissertation explores several promising areas of research in cell wall biology, plant biochemistry, and lignin engineering. The shikimate and aromatic amino acid pathways are crucial metabolic gatekeepers that lie upstream of lignin biosynthesis. In this work, chorismate was studied as an important branchpoint metabolite and evidence was uncovered for redundancy of chorismate mutase isomers in poplar (Populus trichocarpa). Next, a lignin engineering strategy that exploits this important branchpoint was developed. Introduction of a bacterial chorismate pyruvate lyase into transgenic hybrid poplar (Populus alba × grandidentata) was used to divert carbon flux away from phenylpropanoid biosynthesis, leading to lignin with more alkali-labile p-hydroxybenzoate pendent groups. In commelinid monocots, p-coumarate occurs analogously as lignin acylations. The discovery of these moieties in the lignin of kenaf (Hibiscus cannabinus), a eudicot species, represents a new example of convergent evolution and provides a useful tool for lignin engineering. Finally, lignification was studied in eastern leatherwood (Dirca palustris) as the woody stems of this native shrub are exceptionally flexible. In addition to an unusual composition and distribution of lignin, this work uncovered a lignin-deficiency in the middle lamella that asks us to revisit unresolved questions in plant cell wall biology.

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