Bioengineering of lignin to contain atypical components derived from other metabolic pathways is increasingly being pursued to custom-design lignified cell walls that are inherently more digestible by livestock or more easily pretreated and saccharified for biofuel production. Because plants produce such a diverse array of phenolics that could serve as monolignol precursors for lignin formation, cell wall model studies are invaluable as a screening tool for identifying the most promising genetic engineering targets for forage and biomass crops. Our studies with such models demonstrated that copolymerization of normal monolignols with structurally related phenylpropanoid ester conjugates, such as coniferyl ferulate and rosmarinic acid, dramatically improved cell wall delignification by alkali and their subsequent saccharification by enzymes. Furthermore, copolymerization of normal monolignols with more divergent types of phenolics, such as epigallocatechin gallate (EGCG), yielded lignified cell walls that were intrinsically more fermentable by rumen microflora, as well as extensively saccharified following mild alkaline or acidic pretreatment than conventionally lignified cell walls. As with the aforementioned phenylpropanoid conjugates, oxidative coupling of EGCG with introduced monolignols easily cleaved ester linkages into lignin that facilitated delignification under mild pretreatment conditions. Ortho-OH groups on rosmarinic acid and EGCG also contributed to enhanced pretreatment/saccharification of cell walls and their intrinsic fermentability by blocking the formation of benzyl ether cross-links between lignin and structural carbohydrates. In addition to EGCG, several other flavonoid and gallate derivatives readily formed lignified cell walls that were more inherently fermentable or readily saccharified following mild pretreatment. Overall, less than 25% of the phenolics tested in our studies proved compatible with cell wall lignification or useful for modulating the adverse effects of lignin on cell wall utilization. These findings highlight the value of testing alternate monolignols in biomimetic cell wall models prior to attempting their bioengineering into lignin biosynthesis.

Using biomimetic cell wall models to identify new plant lignin bioengineering targets for improving forage and biomass utilization

246th National Meeting (2013)

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