CHAPTER 13: Metabolic Engineering of Plant Allyl/Propenyl Phenol and Lignin Pathways: Future Potential for Biofuels/Bioenergy, Polymer Intermediates, and Specialty Chemicals?

Exciting recent developments in the enzymology and molecular biology of plant phenylpropanoids offer numerous opportunities to re-engineer the composition of plant biomass. Two main targets of such modifications are the optimized production of valuable compounds and reductions in the levels of less desirable products, such as the structural biopolymeric lignins. For example, the amounts of lignin biopolymers in (woody) species might be reduced, with carbon flow concurrently redirected toward production of related nonpolymeric phenylpropanoids, such as the more valuable allyl/propenyl phenols (e.g., eugenol, chavicol). Lignins are monolignol-derived polymeric end-products of the phenylpropanoid pathway (originating from the amino acids phenylalanine and tyrosine). In general, lignins represent a formidable technical challenge, particularly due to their intractable nature, for improved plant biomass utilization, for example, when considering the use of woody biomass for bioethanol production, as well as for wood, pulp, and paper manufacture. Other species-specific outcomes of the phenylpropanoid pathway, however, include metabolites such as lignans, flavonoids, and allyl/propenyl phenols. The recent discovery of the biochemical pathway resulting in the production of the more valuable liquid allyl/propenyl phenols (e.g., eugenol, chavicol, estragole, and anethole), important components of plant spice aromas and flavors, presents one potential approach to the engineering of plant metabolism in new directions. These compounds are synthesized from monolignols in two consecutive enzymatic reactions: (1) acylation of the terminal (C-9) oxygen of the monolignol forming an ester and (2) regiospecific, NAD(P)H-dependent reduction of the phenylpropanoid side chain with displacement of the carboxylate ester as leaving group. The proteins involved in the latter step are homologous to well-characterized phenylpropanoid reductases (pinoresinol-lariciresinol, isoflavone, phenylcoumaran-benzylic ether, and leucoanthocyanidin reductases), with similar catalytic mechanisms being operative. The proteins (and corresponding genes) involved in these transformations have been isolated and characterized and offer the potential of engineering plants to partially redirect carbon flow from lignin (or lignans) into these liquid volatile compounds in oilseeds, leafy or heartwood-forming tissues, or woody stems. The emerging knowledge could also potentially facilitate wood processing in pulp/paper industries and offer sources of renewable plant-derived biofuels, intermediate chemicals in polymer industries, or specialty chemicals in perfume and flavor industries. [ABSTRACT FROM AUTHOR]