Industrial crops are grown to produce goods for manufacturing. Rather than food and feed, they supply raw materials for making biofuels, pharmaceuticals, and specialty chemicals, as well as feedstocks for fabricating fiber, biopolymer, and construction materials. Therefore, such crops offer the potential to reduce our dependency on petrochemicals that currently serve as building blocks for manufacturing the majority of our industrial and consumer products. In this review, we are providing examples of metabolites synthesized in plants that can be used as bio-based platform chemicals for partial replacement of their petroleum-derived counterparts. Plant metabolic engineering approaches aiming at increasing the content of these metabolites in biomass are presented. In particular, we emphasize on recent advances in the manipulation of the shikimate and isoprenoid biosynthetic pathways, both of which being the source of multiple valuable compounds. Implementing and optimizing engineered metabolic pathways for accumulation of coproducts in bioenergy crops may represent a valuable option for enhancing the commercial value of biomass and attaining sustainable lignocellulosic biorefineries.

Lignin is considered to represent the second most abundant terrestrial biopolymer next to cellulose. Although quite variable across vascular plants, its content can account for up to 30% of total biomass in certain woody species. Lignin plays important roles in plants, but often represents hurdles to the utilization of cellulosic biomass in different sectors such as pulp industry, forage digestibility, and bioenergy. Certain natural or mutagenesis-induced lignin mutants, which are notorious for exhibiting brown midribs in grasses or red-brown wood in trees, have long been recognized to produce biomass more amenable to animal digestion, biological conversion, and other agro-industrial processes. Advances in molecular biology techniques for DNA sequencing and manipulation have facilitated the identification of genes involved in lignin biosynthesis and enabled the modification of their expression. The first reports on plants modified in lignin via genetic engineering techniques trace back to the mid 90's. Since then, with the emergence of genome sequencing capabilities and the deployment of large-scale transcriptomic, proteomic, and metabolomic approaches, our understanding on lignin metabolism and regulation has expanded. Correspondingly, substantial efforts have been made in tailoring lignin biosynthesis. In this chapter, we cover several bioengineering strategies that leverage recent knowledge obtained on the mechanistic understanding of lignin formation and chemical composition. Modification of lignin content and/or monomeric composition can be achieved not only by tailoring gene expression, but also by exploiting enzyme post-translational modifications, altering enzyme cofactors and co-substrates, and rerouting lignin metabolic precursors.

One of the most significant questions surrounding biofuels is the availability of a sufficient amount of land capable of sustainably producing biofuel feedstocks that do not compete with food production. On Earth, only 29.2% of the surface is above water (149M Km2)-the rest is covered ocean. From this 29.2%, only 59.5% is considered as biologically productive land (86M Km2) and corresponds to forests (39.3M Km2) or agricultural areas (49.3M Km2). Biological productive land corresponds to land that support human demands for food, fiber and timber for infrastructure and energy (FAO definition). The other 40.5% of lands, traditionally considered as non-productive lands, have a very low or no primary productivity since they are covered by ice, human development, or they are located under extreme climate conditions (cold, dry or arid). The productive lands are divided in several biomes mainly classified according to the vegetation types and productivity, which are dictated by the climate and human accessibility. In order to define those land areas suitable for biofuel feedstock production, an evaluation of most of he primary lands has to be conducted. This analysis is presented in this chapter where we evaluate the different types of lands available and discuss biomass availability as a function of land cover type.

The development of alternative transportation fuels that can meet future demands while reducing global warming is critical to the national, environmental, and economic security of the United States. Currently, biofuels are produced largely from starch, but there is a large, untapped resource (more than a billion tons per year) of plant biomass that could be utilized as a renewable, domestic source of carbon-neutral, liquid fuels. However, significant roadblocks hamper the development of cost-effective and energy-efficient processes to convert lignocellulose biomass into fuels. Lignin is a very strong phenolic polymer, which embeds cellulose and hemicellulose, and its recalcitrance to chemical and biological degradations inhibits the conversion of cell wall polysaccharides (cellulose and hemicellulose) into fermentable sugars. Unfortunately, lignin provides such compressive resistance to plant cells that it cannot simply be genetically removed without incurring deleterious consequences on plant productivity. Alternative strategies to significantly reduce lignin recalcitrance would be modifying its composition and deposition. We are currently developing an alternative strategy, which is focusing on the partial replacement of the ?hard bonds? (e.g. ether, carbon bonds) in the lignin polymer with ?easily cleavable? ones (e.g., amide or ester bonds). For this propose, we are rerouting part of the lignin biosynthesis towards the synthesis of phenylpropanoid-derived molecules such as hydroxycinnamic acid amides and esters in order to partially replace conventional lignin monomers in the cell wall. Biosynthetic pathways and preliminary data for de novo synthesis in Arabidopsis of selected phenylpropanoid-derived compounds are presented.

Lignin is one of the most abundant aromatic biopolymers and a major component of plant cell walls. It occurs via oxidative coupling of monolignols, which are synthesized from the phenylpropanoid pathway. Lignin is the primary material responsible for biomass recalcitrance, has almost no industrial utility, and cannot be simply removed from growing plants without causing serious developmental defects. Fortunately, recent studies report that lignin composition and distribution can be manipulated to a certain extent by using tissue-specific promoters to reduce its recalcitrance, change its biophysical properties, and increase its commercial value. Moreover, the emergence of novel synthetic biology tools to achieve biological control using genome bioediting technologies and tight regulation of transgene expression opens new doors for engineering. This review focuses on lignin bioengineering strategies and describes emerging technologies that could be used to generate tomorrow's bioenergy and biochemical crops.

Lignin is a complex polymer assembled from monolignol precursors derived from phenylalanine after several hydroxylation and methylation steps of the aromatic ring and reduction of the lateral chain. Three main monolignols, the p-coumaryl, coniferyl and sinapyl alcohols, give rise, respectively, to the hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units of the polymer. A complete inventory of the genes potentially involved in the monolignol pathway in the model plant, Arabidopsis thaliana, is presented in this review. Genes encoding enzymes implicated in constitutive lignin synthesis were identified on the basis of their homology to monolignol biosynthesis genes of other plants and their high expression in lignified tissues (floral stems, roots). This overview shows that most of these genes belong to multigene families and that some (PAL, 4CL, CAD) are duplicated in this model plant. The genes encoding the cytochrome P450 monooxygenases (C4H, C3H, F5H) are unique except for F5H that has at least one homologue gene present in the complete genome. Mutants and transgenic Arabidopsis lines deregulated in the monolignol biosynthesis pathway are listed and the impact of the target gene deregulation on growth, and lignin content and structure are reported. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Muconic acid (MA) is a dicarboxylic acid used for the production of industrially relevant chemicals such as adipic acid, terephthalic acid, and caprolactam. Because the synthesis of these polymer precursors generates toxic intermediates by utilizing petroleum-derived chemicals and corrosive catalysts, the development of alternative strategies for the bio-based production of MA has garnered significant interest. Plants produce organic carbon skeletons by harvesting carbon dioxide and energy from the sun, and therefore represent advantageous hosts for engineered metabolic pathways towards the manufacturing of chemicals. In this work, we engineered Arabidopsis to demonstrate that plants can serve as green factories for the bio-manufacturing of MA. In particular, dual expression of plastid-targeted bacterial salicylate hydroxylase (NahG) and catechol 1,2-dioxygenase (CatA) resulted in the conversion of the endogenous salicylic acid (SA) pool into MA via catechol. Sequential increase of SA derived from the shikimate pathway was achieved by expressing plastid-targeted versions of bacterial salicylate synthase (Irp9) and feedback-resistant 3-deoxy-D-arabino-heptulosonate synthase (AroG). Introducing this SA over-producing strategy into engineered plants that co-express NahG and CatA resulted in a 50-fold increase in MA titers. Considering that MA was easily recovered from senesced plant biomass after harvest, we envision the phytoproduction of MA as a beneficial option to add value to bioenergy crops.

Naturally, many aerobic organisms degrade lignin-derived aromatics through conserved intermediates including protocatechuate and catechol. Employing this microbial approach offers a potential solution for valorizing lignin into valuable chemicals for a potential lignocellulosic biorefinery and enabling bioeconomy. In this study, two hybrid biochemical routes combining lignin chemical depolymerization, plant metabolic engineering, and synthetic pathway reconstruction were demonstrated for valorizing lignin into value-added products. In the biochemical route 1, alkali lignin was chemically depolymerized into vanillin and syringate as major products, which were further bio-converted into cis, cis-muconic acid (ccMA) and pyrogallol, respectively, using engineered Escherichia coli strains. In the second biochemical route, the shikimate pathway of Tobacco plant was engineered to accumulate protocatechuate (PCA) as a soluble intermediate compound. The PCA extracted from the engineered Tobacco was further converted into ccMA using the engineered E. coli strain. This study reports a direct process for converting lignin into ccMA and pyrogallol as value-added chemicals, and more importantly demonstrates benign methods for valorization of polymeric lignin that is inherently heterogeneous and recalcitrant. Our approach also validates the promising combination of plant engineering with microbial chassis development for the production of value added and speciality chemicals.