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)Hdependent
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.
Key Words: Acetyltransferase, Acyltransferase, Allylphenol, Anethole, Arabidopsis thaliana, Asparagus officinalis, Bacterial cell culture, Basil, Biocatalysis,
Biodiesel, Bioethanol, Biofuel, Biomass, Biosynthesis, C3H, C4H, CAD,
Catalytic hydrogenation, Cationic polymerization, CCOMT, CCR, Cellulose,
Chavicol, Chavicol synthase, Cinnamate 4-hydroxylase, Cinnamoyl CoA oxidoreductase,
Cinnamyl alcohol dehydrogenase, COMT, Coniferyl acetate,
Coniferyl alcohol, Corn, Creosote bush, Cryptomeria japonica, Deoxygenation,
Double-bond reductase, Essential oil, Estragole, Eugenol, Eugenol
synthase, F5H, Ferulate 5-hydroxylase, Flavor,
Flavoring substances, Flax, Flaxseed,
Furanocoumaran, Heartwood, Heat of combustion, Hinokiresinol, Hydrogenation,
Hydroxycinnamoyl CoA ligase, Hydroxycinnamoyl shikimate/quinate
transferase, IFR, Isoeugenol, Isoeugenol synthase, Isoflavone reductase, Larrea
tridentata, Leucoanthocyanidin reductase, Lignan, Lignification, Lignin, Lignin
challenge, Lignin downregulation, Lignin mutant, Lignin problem, Lignin reduction, Lumber, Metabolic engineering,Metabolic optimization, Methylchavicol,
Methyleugenol, Methyltransferase, Miscanthus, Monolignol, Natural flavors,
Natural food additives, NDGA, Nordihydroguaiaretic acid, Ocimum
basilicum, Oilseed, p-anol, p-coumarate 3-hydroxylase, p-coumaryl acetate, p-coumaryl alcohol, p-coumaryl coumarate, PAL, PCBER, Petunia, Petunia
hybrida, Phenylalanine, Phenylalanine ammonia lyase, Phenylcoumaran-benzylic
ether reductase, Phenylpropanoid, Phenylpropanoid pathway, Pinoresinol,
Pinoresinol-lariciresinol reductase, Pinus taeda, Plant cell culture, Plant metabolism,
Plicatic acid, PLR, Polymer, Polymer intermediate, Propenylbenzene,
Propenylphenol, Pulp/paper manufacture, Quinone methide, Reductase,
Regiospecific reduction, Secoisolariciresinol, Spice, Stereospecific reduction,
Styrene polymer, Switchgrass, Syzygium aromaticum, TAL, Thuja plicata,
Vanillin, Vegetable oil, Wood.